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Interpretation of seasonal changes of xylem embolism and plant hydraulic resistance in Fagus sylvatica
Abstract
The annual course of xylem embolism in twigs of adult beech trees was monitored, and compared to concurrent changes of tree water status and hydraulic resistances. Xylem embolism was quantified in 1-year-old apical twigs by the hydraulic conductivity as a percentage of the maximum measured after removal of air emboli. Tree and root hydraulic resistances were estimated from water potential differences and sap flux measurements. The considerable degree of twig embolism found in winter (up to 90% loss of hydraulic conductivity) may be attributed to the effect of freeze-thaw cycles in the xylem. A partial recovery from winter embolism occurred in spring, probably because of the production of new functional xylem. Xylem embolism fluctuated around 50% throughout the summer, without significant changes. Almost complete refilling of apical twigs was observed early in autumn. A significant negative correlation was found between xylem embolism and precipitation; thus, an active role of rainfall in embolism reversion is hypothesized. Tree and root hydraulic resistances were found to change throughout the growing period. A marked decrease of hydraulic resistance preceded the refilling of apical twigs in the autumn. Most of the decrease in total tree resistance was estimated to be located in the root compartment.
... Wang montre également qu'il existe une très forte corrélation entre la perte de conductivité à la fin de l'hiver et la date de débourrement des bourgeons, il note que les espèces à pores diffus comme le hêtre (plus résistantes à l'embolie) débourrent plus précocement que les espèces à zone poreuse comme le chêne. Il existe donc une relation entre perte de conductivité et phénologie foliaire et peut être même avec le taux de croissance au printemps (Magnani et Borghetti, 1995 ...
... Le hêtre n'a pas une régulation stomatique suffisamment efficace pour limiter le risque de cavitation (cf. paragraphe précédent), c'est une espèce sensible à l'embolie estivale (Magnani et Borghetti, 1995 ;Rust et Hüttl, 1999). ...
... Several studies have reported effects of freezing on xylem embolism in Fagus sylvatica L. Magnani and Borghetti 1995;Hacke and Sauter 1996). The patterns of embolism development were typical of a diffuse-porous species with a graduated increase over winter ). ...
A l'aide d'un fluxmètre haute pression (hpfm), nous avons déterminé la répartition des résistances hydrauliques dans les houppiers de hêtre. Nous avons observé un gradient de résistance dans les houppiers avec les plus fortes résistances dans les branches d'ombre. Ce gradient est fortement corrélé avec l'éclairement reçu par les branches au cours de leur croissance. C'est la structure morphologique (rameaux courts et rameaux longs) et anatomique (diamètre et densité des vaisseaux) des branches modulées par l'éclairement qui induisent ce gradient. Ainsi, nous avons montré que la taille réduite des vaisseaux et l'empilement des nu?ds dans les branches d'ombre augmentaient les résistances hydrauliques. L'architecture hydraulique est acclimatée aux conditions microclimatiques et permet de soutenir la demande évaporatoire en limitant les risques de dysfonctionnement (cavitation et embolie). Lorsqu'il y a modification des conditions de croissance (contrainte hydrique, modification de l'éclairement), nous avons montré que des mécanismes de régulation et d'acclimatation se mettent en place. La régulation stomatique permet une réponse rapide et à court terme en limitant fortement les flux d'eau, alors que des modifications anatomiques (modification de la taille des vaisseaux et de la densité) permettent à l'arbre de s'acclimater aux nouvelles conditions de croissance. Cependant le système conducteur du hêtre est une structure vulnérable. Au cours de l'année, nous avons observé des phases de développement de l'embolie. Chaque hiver, l'embolie se développe progressivement suite à l'alternance de périodes de gel et de dégel et au cours de l'été lorsque s'installe une contrainte hydrique précoce et intense (cf. 1998). Une fois les vaisseaux emboisés, la conductivité hydraulique est réduite et chez le hêtre, elle n'est restaurée qu'au printemps suivant par la formation d'un nouveau cerne.
... This observation reinforces the notion that such conditions filter out wide vessels to reduce vulnerability to frost-induced cavitation, especially those above the 30 μm threshold. Indeed, this disturbance can be particularly harmful if it takes place early in spring when leaves are already transpiring (Magnani andBorghetti 1995, Lemoine 2001). For instance, frost-induced cavitation in spring has been reported to cause conductivity losses up to 60% in beech (Magnani and Borghetti 1995), which can significantly increase decay and the mortality risk. ...
... Indeed, this disturbance can be particularly harmful if it takes place early in spring when leaves are already transpiring (Magnani andBorghetti 1995, Lemoine 2001). For instance, frost-induced cavitation in spring has been reported to cause conductivity losses up to 60% in beech (Magnani and Borghetti 1995), which can significantly increase decay and the mortality risk. These results are consistent with reported patterns of reduced community variances of wood anatomy traits in response to low temperatures Enquist 2007, Swenson et al. 2012). ...
Understanding plant trait coordination and variance across climatic gradients is critical for assessing forests’ adaptive potential to climate change. We measured eleven hydraulic, anatomical and leaf-level physiological traits in European beech (Fagus sylvatica L.) along a moisture and temperature gradient in the French Alps. We assessed how traits covaried, and how their population-level variances shifted along the gradient. The intrapopulation variances of vessel size and xylem-specific conductivity reduced in colder locations as narrow vessels were observed in response to low temperature. This decreased individual-level water transport capacity compared to the warmer and more xeric sites. Conversely, the maximum stomatal conductance and Huber value variances were constrained in the arid and warm locations, where trees showed restricted gas exchange and higher xylem-specific conductivity. The populations growing under drier and warmer conditions presented wide variance for the xylem anatomical and hydraulic traits. Our results suggest that short-term physiological acclimation to raising aridity and heat in southern beech populations may occur mainly at the leaf level. Furthermore, the wide variance of the xylem anatomical and hydraulic traits at these sites may be advantageous since more heterogeneous hydraulic conductivity could imply populations’ greater tree-tree complementarity and resilience against climatic variability. Our study highlights that both intrapopulation trait variance and trait network analysis are key approaches for understanding species adaptation and the acclimation potential to a shifting environment.
... Those variations are mainly shaped by fluctuations in environmental factors (Yin et al., 2004;Körner, 2007;Jump et al., 2009;Wang et al., 2014). For example, the lower temperatures found at higher elevations can restrict the extension of leaves and reduce their size (Magnani and Borghetti, 1995). Stronger solar radiation at higher elevations may lead to lower leaf dry matter, smaller overall sizes, and shorter petioles (Niinemets et al., 2004;Pan et al., 2009). ...
... The light-saturated photosynthetic rate (Amax, μmol m -2 s -1 ), stomatal conductance (Gs, mmol m -2 s -1 ), leaf mass per area (LMA, g m -2 ), and leaf nitrogen concentration (Narea, g m -2 ) were extracted from Zhang et al. (2005). Based on these sets of measurements, leaf nitrogen use efficiency (PNUE, μmol g -1 s -1 ) was calculated as Amax divided by Narea physiological water stress (Magnani and Borghetti, 1995;López-Bernal et al., 2015). We also noted significant bivariate correlations between MAT and leaf morphological traits (Table 3). ...
Leaf morphological traits vary along elevational gradients so that plants can adapt to their surrounding habitat. However, the abiotic factors that shape those traits are still debatable, especially for plants at high elevations where the environment can be very harsh. Quercus guyavifolia H. Léveillé (Fagaceae) is a woody plant found along the southeastern boundary of the Qinghai-Tibet Plateau. Distribution elevations of the species range from 2000 m to 4500 m approximately, providing an excellent opportunity to explore correlations among leaf traits and environmental factors. We used multiple-regression models to investigate spatial trends in leaf morphology and their environmental determinants. As elevation increased, values for leaf area, length, width, the ratio of leaf length to width, shape factor, and petiole length and width decreased significantly and all were positively correlated with mean annual precipitation. Leaf length, the ratio of leaf length to width, and petiole length were negatively correlated with the daily mean maximum intensity of ultraviolet (UV)-B irradiation during the growing season. Our results indicated that the amounts of both precipitation and UV irradiation largely shape the leaf morphology in Q. guyavifolia along the elevation gradient. This study provides evidence for the adaptive plasticity of leaves in response to environments in regions at high elevations.
... The uncoupling between stomatal density and conductance (that resulted in increase in stomatal limitations) in HE suggests a decrease in plant hydraulic conductivity, or other adjustments linked to the characteristics of the site of origin, that resulted in limited stomatal opening, and led to the observed increase in intrinsic water use efficiency in both substrates. Indeed, the higher elevation location can represent a more challenging environment, due to a combination of multiple factors, from changes in CO 2 partial pressure, temperatures (e.g., a few degree drop in temperature would likely lead to cold-induced increases in soil resistance in the water flow from the soil to the leaves, (Magnani and Borghetti, 1995), strong winds and high solar and UV radiations (Cotado and Munné-Bosch, 2020). For instance, substantial difference in atmospheric pressure can act as an evolutionary selective agent for different leaf functional traits and stomatal density and Vcmax have often been reported to increase with increasing elevation (Wang et al., 2017). ...
... Además de la sensibilidad estomática a la sequía edáfica o atmosférica, la especie desarrolla un sistema hidráulico, que pese a mostrar cierta plasticidad en términos de aclimatación a distintas condiciones micro-climáticas en lo relativo a la luz y el agua (Cochard et al. 1999;Herbette et al. 2010), muestra una progresiva pérdida de su funcionalidad ante niveles crecientes de estrés hídrico asociada a la cavitación de los vasos que constituyen el xilema (figura 2, y Magnani and Borghetti 1995;Lemoine et al. 2002;Aranda et al. 2005). Este fenómeno afecta tanto a individuos adultos como a los brinzales (Cochard et al. 1999;Caquet et al. 2009), y a distintos elementos de la planta: desde las raíces a las hojas, pasando por las ramas y ramillos que conforman la copa. ...
... The decrease in StWC and the increase in StWC fluctuations (Figures 4, 6 and 7) indicate that the distribution and transport of water in the stem is disrupted and the stem's ability to store and transport water is reduced [37], at a time when we know that the tree is suffering from pest and disease infestation. Under conditions such as plant senescence and pest and disease infestation, plant vigor decreases and normal metabolism is disturbed. ...
Plant vitality is an important indicator of plant health. Previous studies have often assessed plant vitality using related physiological parameters, but few studies have examined the effects of changes in plant vitality on stem water content (StWC), which can be measured online, in real time, and nondestructively using a novel fringing impedance sensor. In the present study, the sensor calibration results showed a linear fitting relationship between the sensor output voltage and StWC, with coefficients reaching 0.96. The coefficients of correlations between StWC and four plant physiological parameters related to plant vitality (net photosynthetic rate, transpiration rate, stomatal conductance, and intercellular carbon dioxide concentration) were more than 0.8, indicating that StWC can be used to characterize plant vitality to a certain extent. A comparison between plants with normal vitality and weakened vitality showed that the self-regulation ability of plants gradually weakened as the plant vitality decreased, the diurnal mean of StWC lowered, and the diurnal range of StWC increased. In conclusion, StWC can be used as a new parameter to assess plant vitality.
... The leaf morphological traits of many species vary with elevation (Cordell et al., 1998;Bresson et al., 2011). For example, the lower temperatures found at higher elevations can restrict the extension of leaves and reduce their size (Magnani and Borghetti, 1995). However, the affirmation of leaf morphological plasticity and its regulations in different environments are still required. ...
Article Skvortsovia ISSN 2309-6497 (Print) ISSN 2309-6500 (Online) Skvortsovia: 7(2): 41-53 (2021) http://skvortsovia.uran.ru/ Abstract The variations of leaf morphology and functional traits of Quercus castaneifolia C.A.Mey. in various regions of Azerbaijan with different gradients of water availability, elevation, mean annual precipitation and mean annual temperature have been studied. The differentiation in morphological and functional traits of 300 leaves from 30 trees from 3 populations were measured and analyzed using univariate and multivariate analyses. The aim of the study was to evaluate inter-and intra-population differentiation. As a result, we determined that morphological characteristics such as leaf size, leaf area, leaf width, and leaf perimeter decreased with increasing precipitation. The foliar moisture parameters, leaf mass per unit area, water content, relative water content, and succulence, positively correlated with annual precipitation and elevation but negatively correlated with mean annual temperature. This study provides evidence for the adaptive plasticity-polymorphism of leaves in response to environmental changes.
... water potential) difference between an embolised and a sap-filled conduit (Sperry & Tyree, 1988;Zwieniecki & Secchi, 2015). Such conditions occur during high transpiration rates Zufferey et al., 2011) or during long-term exposure to soil drought when transpiration is minimal and organs undergo continuous desiccation (Sperry & Tyree, 1988, Magnani & Borghetti, 1995. Once the pressure difference exceeds a critical level, the gas from an embolised conduit suddenly penetrates into a neighbouring sap-filled conduit (Zwieniecki & Secchi, 2015). ...
During soil drought (i.e., limited soil water availability to plants), woody species may adjust the structure of their vessel network to improve their resistance against future soil drought stress. Impacts of soil drought on intervessel lateral contact remain poorly understood despite of its significance to xylem transport efficiency and safety. Here, we analysed drought‐induced modifications in xylem structures of temperate angiosperm trees with a focus on intervessel lateral contact. Anatomical analyses were performed both in stems of seedlings cultivated under different substrate water availability and annual rings of mature individuals developed during years of low and high soil drought intensities. In response to limited water availability, a decrease in vessel diameter (up to ‐20%) and simultaneous increase in vessel density (up to +60%) were observed both in seedlings and mature trees. Conversely, there were only small and inconsistent drought‐induced changes in intervessel contact frequency and intervessel contact fraction (typically up to ±15%) observed across species, indicating that intervessel lateral contact is a conservative trait. The small adjustments in intervessel lateral contacts were primarily driven by changes in the contact frequencies between neighbouring vessels (i.e., vessel grouping) rather than by changes in proportions of shared cell walls. Our results demonstrate that angiosperm tree species, despite remarkable adjustments in vessel dimensions and densities upon soil drought, exhibit surprisingly invariant intervessel lateral contact architecture.
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... The efficiency-safety tradeoff often occurs in different tissues and structures, changes with the season and plant growth (Holmlund et al., 2016;Prendin et al., 2018), maintains xylem function (Aguilar-Romero et al., 2017;van der Sande et al., 2019). The efficiency-safety tradeoff has been proved in hydraulic conductivity, which is comparatively higher during the growing season (Magnani and Borghetti, 1995;Jaquish and Ewers, 2001;Aranda et al., 2005), while the plant embolism resistance capacity exhibits the opposite trend (Varela et al., 2018;Zhang et al., 2018; but see Gleason et al., 2016). The efficiency-safety tradeoff is driven by environmental factors and contributes to habitat partitioning and niche divergence in sympatric species (Aguilar-Romero et al., 2017;Santiago et al., 2018;Rosas et al., 2019). ...
Plants may maintain long-term xylem function via efficiency-safety tradeoff and segmentation. Most studies focus on the growing season and community level. We studied species with different efficiency-safety tradeoff strategies, Quercus acutissima, Robinia pseudoacacia, Vitex negundo var. heterophylla, and Rhus typhina, to determine the seasonality of this mechanism. We separated their branches into perennial shoots and terminal twigs and monitored their midday water potential (Ψmd), relative water content (RWC), stem-specific hydraulic conductivity (Ks), loss of 12, 50, and 88% of maximum efficiency (i.e., P12, P50, P88) for 2 years. There were no correlations between water relations (Ψmd, RWC, Ks) and embolism resistance traits (P12, P50, P88) but they significantly differed between the perennial shoots and terminal twigs. All species had weak annual hydraulic efficiency-safety tradeoff but strong segmentation between the perennial shoots and the terminal twigs. R. pseudoacacia used a high-efficiency, low-safety strategy, whereas R. typhina used a high-safety, low-efficiency strategy. Q. acutissima and V. negundo var. heterophylla alternated these strategies. This mechanism provides a potential basis for habitat partitioning and niche divergence in the changing warm temperate zone environment.
... Rather, a majority of these species may be outcompeted in optimal growing conditions by other groups thus forcing gymnosperms to elevations where wind and ice blasting can destroy leaf cuticles and lead to drought stress (Li et al. 2004). In addition, colder soils and air temperatures also reduce the water uptake ability of the root system and induce drought stress (Magnani and Borghetti 1995). On top of this, the very thin soils and steep slopes at higher elevations may significantly reduce the availability of water to trees. ...
Phylogenetic constraints on ecophysiological adaptations and specific resource requirements are likely to explain why some taxonomic/functional groups exhibit different richness patterns along climatic gradients. We used interpolated species elevational distribution data and climatic data to describe gymnosperm species richness variation along elevational and climatic gradients in the Himalayas. We compared the climatic and elevational distributions of gymnosperms to those previously found for bryophytes, ferns, and angiosperm tree lineages to understand the respective drivers of species richness. We divided our study location into three regions: Eastern; Central; and Western Himalayas, in each calculating gymnosperm species richness per 100-m band elevational interval by determining the sum of species with overlapping elevational distributions. Using linear regression, we analyzed the relationship between species’ elevational mid-point and species’ elevational range size to test the Rapoport’s rule for gymnosperms in the Himalayas. Generalized linear models were used to test if potential evapotranspiration, growing degree days, and the number of rainy days could predict the observed patterns of gymnosperm species richness. We used the non-linear least squares method to examine if species richness optima differed among the four taxa. We found supporting evidence for the elevational Rapoport’s rule in the distribution of gymnosperms, and we found a unimodal pattern in gymnosperm species richness with elevation, with the highest species richness observed at ca. 3000 m. We also found a unimodal pattern of gymnosperm species richness along both the potential evapotranspiration and growing degree day gradients, while the relationship between species richness and the number of rainy days per year was non-significant. Gymnosperm species richness peaked at higher elevations than for any other plant functional group. Our results are consistent with the view that differences in response of contrasting plant taxonomic groups with elevation can be explained by differences in energy requirements and competitive interactions.
... Hydraulic performance of (secondary) xylem under drought and its recovery from drought depends on (I) hydraulic vulnerability to embolism, expressed as, e.g., P 50 (i.e., the water potential resulting in 50% conductivity loss), (II) hydraulic safety margins, which is the P 50 in relation to the minimum water potential measured in the field Sperry 1988, Choat et al. 2012;Delzon and Cochard 2014;Choat et al. 2018), and on (III) the capability of refilling emptied tracheids, which is presumably related to the hydraulic capacitance (Johnson et al. 2012;McCulloh et al. 2014;Trifilò et al. 2015, or (IV) the potential to produce new xylem tissue (Magnani and Borghetti 1995;Cochard et al. 2001). Water storage capacitance is the amount of water released from a tissue due to a defined decline in water potential (Tyree and Yang 1990;Meinzer et al. 2003;Domec et al. 2006). ...
Key message
The relationship between relative water loss (RWL) and hydraulic conductivity loss (PLC) in sapwood is robust across conifer species. We provide an empirical model ( conifer-curve ) for predicting PLC from simple RWL measurements. The approach is regarded as a new relevant phenotyping tool for drought sensitivity and offers reliable and fast prediction of diurnal, seasonal, or drought-induced changes in PLC.
Context
For conifer species drought is one of the main climate risks related to loss of hydraulic capacity in sapwood inducing dieback or mortality. More frequently occurring drought waves call for fast and easily applicable methods to predict drought sensitivity.
Aims
We aimed at developing a fast and reliable method for determination of the percent loss of hydraulic conductivity (PLC) and eventually the drought sensitivity trait P50, i.e., the water potential that causes 50% conductivity loss.
Methods
We measured the loss of water transport capacity, defined as the relative water loss (RWL) together with PLC in trunk wood, branches, and saplings of eight different conifer species. Air injection was used to induce specific water potentials.
Results
The relationship between RWL and PLC was robust across species, organs, and age classes. The equation established allows fast prediction of PLC from simple gravimetrical measurements and thus post hoc calculation of P50 (r² = 0.94).
Conclusion
The approach is regarded as a relevant new phenotyping tool. Future potential applications are screening conifers for drought sensitivity and a fast interpretation of diurnal, seasonal, or drought-induced changes in xylem water content upon their impact on conductivity loss.
... D'un point de vue écophysiologique, le hêtre présente le paradoxe d'avoir une croissance radiale très sensible au déficit hydrique du sol (Magnani et Borghetti, 1995 ;Backes et Leuschner, 2000 ;Lebourgeois et al., 2005) mais de ne présenter que rarement de la mortalité (0,3% de mortalité annuelle en moyenne en France depuis 20 ans, communication personnelle Louis-Michel Nageleisen, DSF). Cet arbre, appelé aussi fayard, atteint sa limite sud d'aire de répartition en France dans la région PACA. ...
Une augmentation des dépérissements forestiers a été observée ces dernières décennies et les mécanismes écophysiologiques sous-jacents à un phénomène de mortalité sont aujourd’hui mal connus. La multiplicité des études a permis de dégager plusieurs hypothèses sur les mécanismes fonctionnels mis à l’œuvre lors d’un événement de dépérissement menant à la mortalité dont deux se dégagent : un dysfonctionnement du système hydraulique ou un épuisement des réserves carbonées. Néanmoins, ces hypothèses se sont révélées être non exclusives, ni exhaustives. D’autres hypothèses ont alors été proposées, notamment la contribution d’un dysfonctionnement azoté. De nombreuses observations font état d’un risque potentiel sur l’état écologique du hêtre face à ces changements climatiques globaux par notamment une baisse de la disponibilité en azote du sol et une augmentation des événements de sécheresse. Le travail au sein de cette thèse a donc cherché à évaluer la contribution des métabolismes azoté et carboné aux dysfonctionnements observés lors d’un épisode de dépérissement menant à une mortalité. Durant les 3 ans du projet, nous avons étudié comment le métabolisme azoté et carboné pouvaient être impactés par des défoliations annuelles successives ou une sécheresse longue et intense. Notre étude a permis de montrer que face à une contrainte hydrique sévère ou une défoliation, le cycle azoté interne à l’arbre est conservé avec une forte allocation de l’azote vers le compartiment foliaire au printemps et un recyclage efficace vers les organes pérennes à l’automne. Nous avons pu estimer que cet azote recyclé à l’automne contribue fortement à la mise en place du nouveau compartiment foliaire au printemps suivant et ce, même face à des contraintes importantes. Nous avons pu également mettre en évidence que la quantité de réserves carbonées est maintenue face à une défoliation et, au moins dans un premier temps, face à une sécheresse. Néanmoins, la demande proportionnelle pour des besoins osmotiques a mené à des changements de la composition des sucres de jeunes branches et, face à une sécheresse longue et intense, à une baisse de la quantité de réserves carbonées jusqu’à la mort de l’arbre où les réserves carbonées sont fortement diminuées mais pas totalement épuisées. Finalement, le taux de mortalité dans notre expérimentation fut très faible indiquant la résistance du hêtre lorrain à des contraintes extrêmes. Nos résultats soulignent le caractère de résistance du hêtre face à une contrainte via des ajustements des métabolismes internes mais cette résistance pourrait être perdue si la contrainte est plus longue et plus récurrente. Ces éléments peuvent questionner sur le possible maintien du hêtre face aux changements climatiques
... Hence, PLC measured in well-watered plants before the beginning of the second drought cycle (i.e., in the 'Pre-drought' campaign, Figure 4) might suggest that the production of new functional conduits during spring contributed to the recovery of stem hydraulics. Indeed, after winter embolism, an increase in hydraulic conductivity of young terminal branches of mature European beech trees was observed in spring at the onset of cambial activity (Magnani andBorghetti 1995, Cochard et al. 2001). ...
Severe drought events threaten tree water transport system, productivity and survival. Woody angiosperms generally die when embolism-induced loss of hydraulic conductance (PLC) surpasses 80–90% under intense water shortage. However, the recovery capability and possible long-term carry-over effects of repeated drought events could dictate the fate of species’ population under climate change scenarios. Potted saplings of European beech (Fagus sylvatica L.) were subjected to two drought cycles in two consecutive growing seasons, aiming to induce minimum leaf water potentials (Ψmd) of about −4 MPa, corresponding to hydraulic thresholds for survival of this species. In the first cycle, a well-irrigated (C) and a drought-stressed group (S) were formed, and, in the following summer, each group was divided in a well-irrigated and a drought-stressed one (four groups in total). The impact of the multiple drought events was assessed by measuring wood anatomical traits, biomass production, water relations, stem hydraulics and non-structural carbohydrate (NSC) content. We also investigated possible connections between stem hydraulics and carbon dynamics during the second drought event and following re-irrigation. S plants had lower Ψmd and maximum specific hydraulic conductivity (Ks) than C plants in the following growing season. Additionally, aboveground biomass production and leaf number were lower compared to C trees, resulting in lower water consumption. However, PLC was similar between groups, probably due to the production of new functional xylem in spring. The second drought event induced 85% PLC and promoted conversion of starch-to soluble sugars. Nevertheless, 1 week after re-irrigation, no embolism repair was observed and soluble sugars were reconverted to starch. The previous drought cycle did not influence the hydraulic performance during the second drought, and after re-irrigation S plants had 40% higher wood NSC content. Our data suggest that beech cannot recover from high embolism levels but multiple droughts might enhance stem NSC availability.
... month for SPEI calculation, since short time-scales are mainly related to soil water content in Mediterranean montane conditions. Magnani and Borghetti (1995) found that xylem embolism of apical twigs of mature beech trees (northern Apennines) did not have a major effect on hydraulic resistance observed during the growing season in which predawn water potential did not point at soil moisture stress. However, a considerable hydraulic resistance at the interface between soil and root is expected when soil dries out (Tognetti et al., 1998). ...
Due to the recent spread of forest die-off worldwide, concerns arise about the relative influence of specific climate parameters on tree growth decline in semi-arid environments, such as the Mediterranean mountain forests. As temperatures increase, drought may reduce tree productivity and survival across these forest ecosystems. Drought-induced tree growth decline can be interpreted as an early-warning signal of forest vulnerability. Here, we modeled the relationship between tree-ring width index (RWI) of beech populations in mountain forests of south-central Italy and the standardized precipitation evapotranspiration index (SPEI) derived from local weather stations. The aim of the research was to propose a procedure to determine the tipping point of drought severity triggering tree decline in beech forests. We focused on the cumulative water balance over the previous 1 month, which was particularly appropriate in relationship with the soil water conditions of these Mediterranean mountains. Under drier conditions, the correlation between RWI and SPEI was stronger, soil water supply (early fall of current year) and atmospheric evaporative demand (late spring of current year) being the dominant factors limiting tree growth of southern beech populations.
... The strong negative correlations found by us between tree line ele- & Cochard, 1999;Magnani & Borghetti, 1995) which is proved to affect plant success in cold environments (Tyree, Davis, & Cochard, 1994). The increasing height of the snowpack with elevation should act as thermal insulation upon layering F. sylvatica multistemmed dwarfs, whose size decreases with elevation in the more gradual natural transition tree line. ...
Aim
High‐elevation forest line or tree line is an ecological ecotone representing the upper elevation thermal limit for forest development. The current tree line position is the result of the past human activity interacting with climatic and topographic conditions. In this study, we investigate how climate, local topographic factors and anthropogenic disturbance currently affect tree line distribution.
Location
Apennine Mountains, 900 km latitudinal gradient along the Italian Peninsula.
Methods
Overall, 302 mountain peaks were studied, comprising 3,622 km of measured tree lines. The position of the Fagus sylvatica tree line in all peaks was assessed and correlated with 58 selected variables representing climate, topography and human disturbance.
Results
The mean tree line elevation was 1,589 m a.s.l., with considerable variability among peaks. Contrary to our expectations, the tree line elevation was lower in the warmer southerly exposed slopes compared to north‐facing aspects, where we found the highest tree line (2,141 m a.s.l.). Correlation analysis indicates that both climatic and human density variables are associated with tree line elevation, with the climate having more influence in high elevation mountains, while human impact plays a prominent role in low elevation mountain peaks. Specifically, we found negative correlations between density of the resident population around each peak and tree line elevation at all examined dates (1861, 1921, and 2011), suggesting a pervasive negative impact of human activity on tree lines. As regards climatic variables, tree line elevation showed a stronger negative correlation with winter and spring months temperature than with mean annual temperature. Noteworthy, climatic variables had stronger effect on high elevation peaks (>1,900 m a.s.l.) compared with low elevation ones (<1,900 m a.s.l.).
Main Conclusion
Our data provide evidence that the current position of the F. sylvatica tree line in the Apennines is heavily depressed as a result of a complex interaction between climatic factors and the past human pressure.
... Hydraulic traits of beech xylem have been studied by both techniques. However, almost all studies were performed on branches (Magnani and Borghetti, 1995;Cochard et al., 2001;Cochard et al., 2005;Herbette et al., 2010;Hajek et al., 2016;Stojnić et al., 2018) or very young saplings (Steppe and Lemeur, 2004;Steppe and Lemeur, 2007;Caquet et al., 2009;Aranda et al., 2014) and research on mature tree trunks is comparatively few (Lemoine et al., 2002;Nikolova et al., 2009;Jonard et al., 2011;Oladi et al., 2014). These studies were conducted in different disciplines with different objectives but it can be concluded that the hydraulic system of beech has a plastic and adaptive potential to respond to meteorological and especially precipitation variations. ...
Global climate change might cause changes in tree growth and functionality. Beech (Fagus sp.) forms a group of important forest species which are sensitive to these changes and affected by them. Fagus sylvatica L. (European beech) and F. orientalis Lipski (Oriental beech) are chief members of this genus, widespread in Central Europe and temperate regions of the Mediterranean and Middle East. Oriental beech is mainly distributed in the narrow strip of the Hyrcanian forests along the Caspian Sea in north Iran, where a strong thermal and moisture gradient from west to east is developed. We used this gradient as an opportunity to study the sensitivity of Oriental beech to meteorological variables manifested in annual radial growth, vessel traits, hydraulic conductivity, and wood density. Three study sites along the Oriental beech forests of Iran, i.e. Asalem (in the west), Nowshahr (in the center), and Gorgan (in the east) were selected. Precipitation decreases from west to east while the average temperature increases. In a separate approach of this study, radial growth and xylem anatomical features of different populations of Oriental beech trees were compared with European ones, planted side by side in a common garden in northern Germany. Since Oriental beech is generally found in warmer and drier climates than European beech, we hypothesized that its climatic responses to temperature and precipitation may genetically be different in strength than those of F. sylvatica. For the detailed analysis of the differences in growth behavior and wood anatomy of Oriental and European beech, ~ 30 years old trees of both species with different origins (four different provenances of Turkey and Germany) were felled. Cores and/or disks were prepared from trees and xylem functional traits including tree-ring width (TRW), average vessel lumen area (AVLA), total vessel lumen area (TVLA), maximum vessel size (MaxVS), and vessel density (VD) were measured on microscopic cross sections for each tree ring. Chronologies of each trait were prepared using established dendrochronological approaches. The potential specific and total hydraulic conductivity (Ks and Kj) as well as the vulnerability index (VINX) were calculated from measured anatomical variables. HF-densitometry was used to provide annual wood density profiles; however due to technical problems, these results were only valid for the Oriental beech trees in Gorgan. The average ring width in Nowshahr was higher than at the two other sites, while Asalem had the smallest TRW Size of vessels and porosity of tree rings were higher in Nowshahr and Gorgan than in Asalem It can be concluded that the observed increase in temperature at Gorgan is not accompanied by water stress and precipitation is not a limiting growth factor in this site. Gorgan also had the highest VINX suggesting a high degree of mesomorphy. Lower annual precipitation in Gorgan is mostly a result of lower precipitation in the non-growing seasons and hence has not yet influenced vessel features profoundly. The strongest climatic signals were observed in AVLA and VD of Asalem trees. Correlations of these factors with climate variables were much stronger than those in tree–ring widths. However, the promoting effect of early spring temperature and July precipitation on radial growth of beech trees in Asalem scales down towards the east of the Caspian forests (in Nowshahr and Gorgan). These contrasting climate-growth relations from west to east can also be seen in some xylem anatomical variables like AVLA and VD. It seems that beech trees in Asalem are the most sensitive among our study sites, which contrasts our hypothesis that Gorgan beeches experience a more stressful climate and hence will show a stronger relation with precipitation. Moreover, the strategy of Asalem trees to compensate exogenous factors is somewhat different from trees growing in Nowshahr and Gorgan. In Asalem, TRW is highly related to VD while in the two other sites, TRW is clearly separated from VD and its variations are highly related to TVLA. Comparing Oriental and European beech in a common garden showed that phenotypic plasticity of the European beech populations is greater than that of Oriental beech. Among the studied traits, TRW and VD showed a high plasticity both between and within species. June was the most determinant month for radial growth and vascular properties of Oriental and European beech trees. However, Oriental beech growth was less responsive to precipitation while its vessel size was more temperature–dependent than for European beech. Oriental beech may genetically be more drought-tolerant or less water-dependent compared with European beech. Gorgan beech trees have faced a simultaneous reduction in growth and wood density in pointer years. However, the relationship between TRW and wood density is nonlinear. We conclude that Oriental beech trees growing in the west of Iran in Asalem behave more similar like European beech concerning their sensitivity to drought. However, Oriental beech is more adaptable to drier climate.
... Seasonal patterns of xylem embolism and conductivity in ring-porous or diffuse-porous trees are well-documented (Magnani & Borghetti 1995;Jaquish & Ewers 2001). In general, usually the greatest degree of embolism occurs in winter. ...
Although cavitation is common in plants, it is unknown whether the cavitation resistance of xylem is seasonally constant or variable. We tested the changes in cavitation resistance of Acer mono before and after a controlled cavitation-refilling and freeze-thaw cycles for a whole year. Cavitation resistance was determined from ‘vulnerability curves’ showing the percent loss of conductivity (PLC) versus xylem tension. Cavitation fatigue was defined as a reduction of cavitation resistance following a cavitation-refilling cycle while frost fatigue was caused by a freeze-thaw cycle. A. mono developed seasonal changes in native embolisms; values were relatively high during winter but relatively low and constant throughout the growing season. Cavitation fatigue occurred and changed seasonally during the 12-month cycle; the greatest fatigue response occurred during summer and the weakest during winter, and the transitions occurred during spring and autumn. A. mono was highly resistant to frost damage during the relatively mild winter months; however, a quite different situation occurred during the growing season, as the seasonal trend of frost fatigue was strikingly similar to that of cavitation fatigue. Seasonality changes in cavitation resistance may be caused by seasonal changes in the mechanical properties of the pit membranes.
... The standard approach is to determine the conducting capacity of the ensemble of vascular conduits within the measured segment and thus the ability of the measured segment to supply downstream regions of the plant with water (Sperry et al., 1988a). The hydraulic conductivity of an individual segment may change with time due to the cavitation of xylem conduits and any subsequent re®lling (Waring and Running, 1978;Milburn, 1979;Sperry, 1986;Sperry et al., 1988b;Pickard, 1989;Tyree and Yang, 1992;Edwards et al., 1994;Cochard et al., 1994;Magnani and Borghetti, 1995;Zwieniecki and Holbrook, 1998). Because cavitation and embolism repair take place at the level of individual vessels, understanding dynamic changes in conductivity requires methods that address the hydraulic properties of individual conduits. ...
Studies of the hydraulic properties of xylem vessels have been limited to measurements of whole plant or whole stem segments. This approach allows the longitudinal transport properties of the ensemble of vessels within a stem to be determined, but provides little information on radial transport. Here the xylem of Fraxinus americana L. has been examined using a new method that allows the transport properties of individual vessels to be examined. One goal of this study was to quantify transport parameters relevant to embolism repair. The longitudinal conductivity of vessel segments open at both ends (i.e. no end walls) agreed with values predicted by the Poiseuille equation. Radial specific conductance (conductance per unit area) was approximately six orders of magnitude lower than the longitudinal conductance of the vessel segment normalized by the cross‐sectional area of the vessel lumen. There was a step increase in the radial specific conductance of previously gas‐filled vessels when the delivery pressure exceeded 0.4 MPa. This is consistent with the idea that positive pressure, required for embolism repair, can be compartmentalized within a vessel if the bordered pit chambers are gas‐filled. The diffusion coefficient for the movement of gas from a pressurized air‐filled vessel was of the same order of magnitude as that for air diffusing through water (1.95 e ⁻⁹ m ² s ⁻¹ ). Estimates of the time needed to displace all of the gas from an air‐filled vessel were in the order of 20 min, suggesting that gas removal may not be a major limitation in embolism repair.
... If temperature is the main driver of phenological manifestations in a species, then damage from late spring and early autumn frost is minimal for this species (Hänninen 1990;Häkkinen et al. 1998), but if the main driver is water availability in the soil, then damage from drought, such as embolism in the xylem, will be avoided (Magnani and Borghetti 1995). Higher precipitation quantities before the vegetation season may increase the need for temperature sums (Fu et al. 2014), which confirms that precipitation can also indirectly contribute to the beginning of leaf unfolding. ...
Narrow-leaved ash (Fraxinus angustifolia Vahl) is a fast growing and light demanding forest tree species. In Croatia, the largest complexes of this species can be found along the Sava River. The main objective of research was to determine the influence of environmental drivers on budburst dates. Research was carried out in the clonal seed orchard of Nova Gradiška through four vegetation seasons (2012, 2014, 2015 and 2016). In total, 42 clones were involved in this study. The target phase of phenology monitoring was the phase with distinct budburst and partial segregation of bud scales. Three environmental drivers were selected (temperature, precipitation and insolation) with three different summing dates (1st November, 1st December and 1st January). Statistical data processing included the following analyses: Spearman Rank Correlation, Linear Regression, Multiple Regression and Stepwise Multiple Regression. Among three tested environmental drivers in all of the analyses performed in the study, insolation proved to be the most important factor in the description of budburst dates in narrow-leaved ash.
... European forest managers are concerned by the response of beech (Fagus sylvatica L.) to water shortage because this species, which usually grows in temperate and humid environments, is sensitive to drought (Epron and Dreyer, 1993;Magnani and Borghetti, 1995;Backes and Leuschner, 2000;Cochard et al., 2001;Leuschner et al., 2001) and is thus potentially vulnerable to future climatic changes. Studies on the effect of soil drought on beech transpiration, as estimated from sap flow measurements (T), and on stand evapotranspiration (E T ), as measured by eddy covariance, have already been performed (e.g., Irvine et al., 1998;Granier et al., 2000;Barbour et al., 2005). ...
... Previous studies found that the increase in δ 13 C values with altitude was correlated with variation in structural and physiological leaf traits. A possible explanation was that although there seemed to be more precipitation at high altitudes, colder air temperatures could reduce the stomatal conductance and the diffusion of CO 2 , and colder soils could reduce the water uptake of the root system and lead to the water stress (Magnani and Borghetti 1995;Körner 1998). As a consequence, 13 C in leaf was enriched with increasing altitude. ...
To understand the effects of leaf physiological and morphological characteristics on δ
13C of alpine trees, we examined leaf δ
13C value, LA, SD, LNC, LPC, LKC, Chla+b, LDMC, LMA and Narea in one-year-old needles of Picea schrenkiana var. tianschanica at ten points along an altitudinal gradient from 1420 m to 2300 m a.s.l. on the northern slopes of the Tianshan Mountains in northwest China. Our results indicated that all the leaf traits differed significantly among sampling sites along the altitudinal gradient (P<0.001). LA, SD, LPC, LKC increased linearly with increasing elevation, whereas leaf δ
13C, LNC, Chla+b, LDMC, LMA and Narea varied non-linearly with changes in altitude. Stepwise multiple regression analyses showed that four controlled physiological and morphological characteristics influenced the variation of δ
13C. Among these four controlled factors, LKC was the most profound physiological factor that affected δ
13C values, LA was the secondary morphological factor, SD was the third morphological factor, LNC was the last physiological factor. This suggested that leaf δ
13C was directly controlled by physiological and morphological adjustments with changing environmental conditions due to the elevation.
... This is, however, decoupled from later desiccation and mortality dynamics (Delzon and Cochard, 2014). More than 90% loss of hydraulic conductivity of beech seedlings and young stands is reputed to be reached between −2.2 MPa (Magnani and Borghetti, 1995) and −4.0 MPa (Cochard et al., 1999). Advanced mortality in young beech was found at mean xylem water potentials of −4.5 MPa (Barigah et al., 2013, and this study). ...
European beech (Fagus sylvatica L., hereafter beech), one of the major native tree species in Europe, is known to be drought sensitive. Thus, the identification of critical thresholds of drought impact intensity and duration are of high interest for assessing the adaptive potential of European beech to climate change in its native range. In a common garden experiment with one-year-old seedlings originating from central and marginal origins in six European countries (Denmark, Germany, France, Romania, Bosnia-Herzegovina, and Spain), we applied extreme drought stress and observed desiccation and mortality processes among the different populations and related them to plant water status (predawn water potential, ΨPD) and soil hydraulic traits. For the lethal drought assessment, we used a critical threshold of soil water availability that is reached when 50% mortality in seedling populations occurs (LD50SWA). We found significant population differences in LD50SWA (10.5–17.8%), and mortality dynamics that suggest a genetic difference in drought resistance between populations. The LD50SWA values correlate significantly with the mean growing season precipitation at population origins, but not with the geographic margins of beech range. Thus, beech range marginality may be more due to climatic conditions than to geographic range. The outcome of this study suggests the genetic variation has a major influence on the varying adaptive potential of the investigated populations.
... Besides, embolism is one of the most important threats to vascular plants, causing catastrophic xylem dysfunction when a threshold of water stress is passed (Tyree and Sperry, 1988). So far, embolism is commonplace in plants (Milburn, 1991) and native steady state embolism and the related percent loss conductivity (PLC) are assessed by inserting excised stem segments in conductivity apparatus (Cochard 1992; Magnani and Borghetti 1995; Sperry et al. 1987). ...
... First, this major forest species has a very widespread distribution in Europe and thus occurs in contrasting climatic areas (Figure 1). It is also known to be relatively sensitive to drought (Magnani andBorghetti 1995, Bréda et al. 2006) and therefore vulnerable to cavitation (Cochard et al. 1999). This may have favored the selection of ecotypes adapted to the local pedo-climatic conditions. ...
... European forest managers are concerned by the response of beech (Fagus sylvatica L.) to water shortage because this species, which usually grows in temperate and humid environments, is sensitive to drought (Epron and Dreyer, 1993; Magnani and Borghetti, 1995; Backes and Leuschner, 2000; Cochard et al., 2001; Leuschner et al., 2001) and is thus potentially vulnerable to future climatic changes. Studies on the effect of soil drought on beech transpiration , as estimated from sap flow measurements (T), and on stand evapotranspiration (E T ), as measured by eddy covariance, have already been performed (e.g., Irvine et al., 1998; Granier et al., 2000; Barbour et al., 2005). ...
... These authors showed that 'cavitation fatigue' is linked with increased xylem permeability to air, consequently to successive cavitation events. As it has been shown that many woody species appeared to operate with relatively high PLC values throughout the growing season (Hargrave et al. 1994;Sperry et al. 1994;Tognetti & Borghetti 1994;Magnani & Borghetti 1995). This seasonality of PLC values restricts the usage of petiole as an embolism-proxy, as leaves could experience several events of water deficit-induced loss of conductivity. ...
The latest climate change scenarios predict a rise in mean temperature in Europe of 2 to 4°C for 2099 (IPCC, 2007), associated with extreme climatic events such as severe droughts. Knowing adaptation capabilities of tree species is crucial for understanding their responses and forest ecosystem fate in the near future. Our study object is a species complex inside the Fraxinus genus (ash, Oleaceae). In France, F. excelsior and F. angustifolia are autochthonous, form natural hybrid populations and show remarkable phenotypic and ecological plasticity. This could promote the emergence of new individuals (genotypes) more able to deal with fluctuating environments. Our objective is to characterise the capability of adaptation of different Fraxinus populations, representing the three statuses (F.excelsior, F.angustifolia and hybrids) under abiotic constraints (water constraint). To solve this issue, we examine in a low water constraint experiment (-0.9 MPa) ecophysiological and genetic response, using saplings. A second and more severe water constraint experiment (-4 MPa) was used to investigate ash response to the loss of hydraulic conductivity. The most noticeable result was an intermediate and highly variable behaviour of hybrid ash populations in the two experiments (A, gs, WUEi, PLC) linked with they respective introgression degree (closer to F.excelsior or F.angustifolia). This hybrid trees could be used for foresters as a resource and insurance against catastrophic forest stand decline, for a future climate.
... Hydraulic conductivity of stems during winter, and the associated problems of cavitation, xylem embolism (which blocks water transport) and xylem re®lling after embolism have been largely studied in temperate deciduous trees (Tyree, 1983;Cochard and Tyree, 1990;Just and Sauter, 1991;Ame glio and Cruziat, 1992;Sperry et al., 1994;Ame glio et al., 1995;Magnani and Borghetti, 1995;Hacke and Sauter, 1996;Lemoine et al., 1999). In contrast, little attention has been paid to water availability in buds of these trees. ...
This paper investigates the uptake, transport, state and self-diffusion of water in twigs and buds ofPicea abies L. (Karst.) trees in winter until reactivation in spring. The presence or absence of xylem in embryonic shoots, as well as
the intensity and type of bud dormancy were also studied. Three clones of P. abies were grown in a forest in north-eastern France. The clones differed in their timing of bud-burst, with the two clones from
the Vosges showing different degrees of early bud-burst and the clone from Poland showing late bud-burst. One-bud cuttings
grown in standard forcing conditions showed a great difference in dormancy between the two provenances, but only a small difference
between the two Vosges clones. Earliness of bud-burst was not strongly linked to the intensity of dormancy. A dye experiment
combined with light microscopy indicated increased velocity of water transport in stems up to a maximum in April, initial
entry of water into embryonic shoots, protoxylem differentiation in embryonic shoots from April, and then, shortly before
bud-burst, water transport through the newly-formed protoxylem almost up to the meristem. Nuclear magnetic resonance measurements
of the transverse relaxation time (T2) and the self-diffusion coefficient of water confirmed these observations and showed water availability in embryonic shoots.
The sequence of water-related phenomena occurring in early spring was the same in the three clones, but was earliest in the
Vosges clone with the earliest bud-burst and latest in the Polish clone with late bud-burst. The results imply that this sequence
of water-related phenomena leads to bud-burst. Copyright 2000 Annals of Botany Company
... Similarly, Palmroth et al. (1999) reported conifers from higher altitudes growing under lower annual mean temperatures used water more effi ciently as indicated by lower Δ values. Magnani and Borghetti (1995) found that cold (nonfrozen) soil induced high resistance to water fl ux from soil to leaves, which according to Roberts (1977) can have signifi cant implications for freezing tolerance. Therefore, it would be useful to examine the eff ects of WI on freezing tolerance and physiological traits associated with WUE of a coldsensitive species, such as perennial ryegrass. ...
Perennial ryegrass (Lolium perenne L.) is a coolseason turfgrass susceptible to low temperature injury. Wilt-based irrigation (WI) is a common practice for scheduling irrigation to turf by allowing a mild, negative water imbalance to occur as an alternative to maintaining turf under nonlimiting soil moisture. Moisture stress has been shown to promote freezing tolerance but this has not been investigated in response to WI. Objectives of this study were to examine perennial ryegrass in response to WI (applied at 50% visual leaf roll) on freezing tolerance (median lethal temperature at 50% survival [LT 50]), 13C discrimination (Δ), internal leaf-to-ambient CO 2 concentration (c i:c a) ratio, leaf water, rooting, water use efficiency (WUE), and wilting tendency (days to wilt and number of wilt events) among three top-performing (TP) and three bottomperforming (BP) perennial ryegrass cultivars. Cultivars were selected based on turf quality trials from a northern location (Orono, ME). Plant measurements were made in the greenhouse in 2007 and 2008 comparing the first (single) and last (multiple) wilt cycle after 68 d of irrigation between well watered and WI. Multiple wilt exposures significantly enhanced freezing tolerance compared to a single wilt event for TP cultivars only, but gains in freezing tolerance were small (<1.0°C). Freezing tolerance (lower LT 50 values) was generally associated with lower yield, WUE, and leaf water content. In some TP cultivars, wilt-induced freezing tolerance was associated with greater wilting tendency and drought sensitivity indicated by lower Δ values and ci:ca ratio. Repeated wilt exposures increased rooting over a single wilt event.
... Plants decrease the size of their parts to reduce water loss through transpiration. Another possibility is that colder soils reduce the water uptake of the root system and induce water stress (Magnani and Borghetti 1995). Fruit morphological characteristics such as length, width and weight also decreased significantly with increased in altitude. ...
Genome scan was done with 439 dominant marker loci to identify outlier loci in three populations of Morus alba from the trans‐Himalayan region. Employing finite and hierarchical island population genetic models, we detected 30 (6.85%) loci under selection, of which 3.64% were under directional selection and 3.19% under balancing selection. The hierarchical island model was efficient in detecting signatures of natural selection with low occurrence of false positives. Data showed significant spatial genetic structure (SGS), especially in population from Nubra valley with significantly high SGS. Bayesian approach revealed high genetic differentiation and inbreeding in Nubra population. Indus and Suru valleys populations showed comparatively low SGS, genetic differentiation, and inbreeding. The study showed that M. alba falls in the range of mixed mating and outcrossing species. However, the population of Nubra valley showed selfing with high SGS. Significant SGS strengthens the presence of natural selection in M. alba natural populations.
... The declining trend in height growth of Smith fir points to a stress gradient along the altitudinal transect, which is characterized by the dominant temperature gradient. Given the dominant role of water stress in limiting tree height growth [37][38][39]42,43], low temperatureinduced water stress or water uptake [44][45][46][47][48], together with gravity, appears to play an important role in limiting tree height of Smith fir with increasing elevation. It was supported by an increasing trend of leaf d 13 C values and a decreasing trend of the specific leaf area for Smith fir and Rhododendron aganniphum var. ...
Little is known about tree height and height growth (as annual shoot elongation of the apical part of vertical stems) of coniferous trees growing at various altitudes on the Tibetan Plateau, which provides a high-elevation natural platform for assessing tree growth performance in relation to future climate change. We here investigated the variation of maximum tree height and annual height increment of Smith fir (Abies georgei var. smithii) in seven forest plots (30 m640 m) along two
altitudinal transects between 3,800 m and 4,200/4,390 m above sea level (a.s.l.) in the Sygera Mountains, southeastern Tibetan Plateau. Four plots were located on north-facing slopes and three plots on southeast-facing slopes. At each site, annual shoot growth was obtained by measuring the distance between successive terminal bud scars along the main stem of 25 trees that were between 2 and 4 m high. Maximum/mean tree height and mean annual height increment of Smith fir
decreased with increasing altitude up to the tree line, indicative of a stress gradient (the dominant temperature gradient) along the altitudinal transect. Above-average mean minimum summer (particularly July) temperatures affected height increment positively, whereas precipitation had no significant effect on shoot growth. The time series of annual height increments of Smith fir can be used for the reconstruction of past climate on the southeastern Tibetan Plateau. In addition, it can be expected that the rising summer temperatures observed in the recent past and anticipated for the future will enhance Smith fir’s growth throughout its altitudinal distribution range.
... Plants decrease the size of their parts to reduce water loss through transpiration. Another possibility is that colder soils reduce the water uptake of the root system and induce water stress (Magnani and Borghetti 1995). Fruit morphological characteristics such as length, width and weight also decreased significantly with increased in altitude. ...
Ten quantitative morphological characters were studied in 56 Morus alba L. trees representing three natural populations from the trans-Himalayan Ladakh region. The altitude of collection sites ranged from 2815 to 3177 m above the sea level (asl). Coefficient of variation (CV) showed high phenotypic variation in M. alba. Linear regression analysis revealed that leaf and fruit size decreases with an increase in altitude. High CV was observed for leaf length, leaf width, petiole length, leaf area, inter-nodal distance, number of nodes, bud length, fruit length, fruit width and fruit weight. Similarly, a high phenotypic plasticity index was observed for bud length, leaf length, leaf width, petiole length, leaf area, inter-nodal distance, number of nodes, fruit length, fruit width and fruit weight. For every 100 m increase in altitude, leaf length, leaf width and leaf area decreased by 1 cm, 0.8 cm and 16.6 cm2, respectively. Analysis of covariance showed a predominant altitudinal effect on the morphological characters in comparison to the population effect. A small change in the altitude caused significant change in the plant morphological characteristics. The present investigation represents to our knowledge the first study addressing phenotypic variation in mulberry along an altitudinal gradient.
Climate change is one of the most prominent factors influencing the spatial distribution of plants in China, including gymnosperms. Climatic factors influence gymnosperm distribution along elevational gradients on the Qinghai-Xizang (Tibet) Plateau (QTP), and understanding how species adapt to these factors is important for identifying the impacts of global climate change. For the first time, we examined the county-level distribution of gymnosperm species on QTP using data from field surveys, published works, monographs, and internet sources. We used simulated distribution data of gymnosperms (N = 79) along the elevational gradients to investigate the overall impact of environmental variables in explaining the richness pattern of gymnosperms. Eighteen environmental variables were classified into three key variable sets (climatic seasonality, energy-water, and physical tolerance). We employed principal component analysis and generalized linear models to assess the impact of climatic variables on the gymnosperm's richness pattern. Gymnosperm species are unevenly distributed across the plateau and decline gradually from the southeast to the northwest. The altitudinal gradients have a unimodal relationship with the richness of gymnosperms, with the maximum species richness at an elevation of 3200 m. The joint effects of physical tolerance and energy-water predictors have explained the highest diversity of gymnosperms at mid-elevation. Because the richness peak correlates significantly with the wettest month's precipitation and moisture index, this confirms the significance of moisture on gymnosperm distributions due to increased precipitation during the wet season. Furthermore, our results provide evidence that climatic sea-sonality factors are involved in the decline of gymnosperm richness at high elevations. A total of 37% of gymnosperm species on QTP are listed as vulnerable, nearly threatened, or endangered, with elevations ranging from 600 m to 5300 m. As a result, we conclude that gymnosperms are at high risk of extinction because of the current climate fluctuations caused by global climate change. Our research offers fundamental data for the study and protection of gymnosperm species along the steepest elevation gradients.
The raising concentration of atmospheric CO2 resulted in global warming. The forest ecosystem in Tibet played an irreplaceable role in maintaining global carbon balance and mitigating climate change for its abundant original forest resources with powerful action of carbon sink. In the present study, the samples of soil and vegetation were collected at a total of 137 sites from 2001 to 2018 in Tibet. Based on the field survey of Tibet’s forest resources and 8th forest inventory data, we estimated the carbon storage and carbon density of forest vegetation (tree layer, shrub, grass, litter and dead wood) and soil (0–50 cm) in Tibet. Geostatistical methods combined with Kriging spatial interpolation and Moran’s I were applied to reveal their spatial distribution patterns and variation characteristics. The carbon density of forest vegetation and soil in Tibet were 74.57 t ha−1 and 96.24 t ha−1, respectively. The carbon storage of forest vegetation and soil in Tibet were 344.35 Tg C and 440.53 Tg C, respectively. Carbon density of fir (Abies forest) was 144.80 t ha−1 with the highest value among all the forest types. Carbon storage of spruce (Picea forest) was the highest with 99.09 Tg C compared with other forest types. The carbon density of fir forest and spruce forest both increased with the rising temperature and precipitation. Temperature was the main influential factor. The spatial distribution of carbon density of forest vegetation, soil, and ecosystem in Tibet generally showed declining trends from western Tibet to eastern Tibet. Our results facilitated the understanding of the carbon sequestration role of forest ecosystem in the Tibet. It also implied that as the carbon storage potential of Tibet’s forests are expected to increase, these forests are likely to serve as huge carbon sinks in the current era of global warming and climate change.
Phylogenetic constraints on ecophysiological adaptations and specific resource requirements are likely to explain why some taxonomic/functional groups exhibit different richness patterns along climatic gradients. We used interpolated species elevational distribution data and climatic data to describe gymnosperm species richness variation along elevational and climatic gradients in the Himalayas. We compared the climatic and elevational distributions of gymnosperms to those previously found for bryophytes, ferns and angiosperm tree lineages to understand the respective drivers of species richness. We divided our study location into three regions: the Eastern; Central; and Western Himalayas, in each calculating gymnosperm species richness per 100-m band elevational interval by determining the sum of species with overlapping elevational distributions. Using linear regression, we analyzed the relationship between species’ elevational mid-point and species’ elevational range size to test the Rapoport’s rule for gymnosperms in the Himalayas. Generalized linear models were used to test if potential evapotranspiration, growing degree days and the number of rainy days could predict the observed patterns of gymnosperm species richness. We used the non-linear least squares method to examine if species richness optima differed among the four taxa. We found supporting evidence for the elevational Rapoport’s rule in the distribution of gymnosperms and we found a unimodal pattern in gymnosperm species richness with elevation, with the highest species richness observed at ca. 3000 m. We also found a unimodal pattern of gymnosperm species richness along both the potential evapotranspiration and growing degree day gradients, while the relationship between species richness and the number of rainy days per year was non-significant. Gymnosperm species richness peaked at higher elevations than any other plant functional group. Our results are consistent with the view that differences in response of contrasting plant taxonomic groups with elevation can be explained by differences in energy requirements and competitive interactions.
Although water is the most abundant molecule on the Earth’s surface, the availability of water is the factor that most strongly restricts terrestrial plant production on a global scale. Low water availability limits the productivity of many natural ecosystems at different time scales (Fig. 5.1). In addition, losses in crop yield due to water stress exceed losses due to all other biotic and environmental factors combined (Boyer 1985). Water availability is also a major determinant of plant and biome distribution. Regions where rainfall is abundant and fairly evenly distributed over the growing season, such as in the wet tropics, have lush vegetation. Where seasonal droughts are frequent and severe, forests are replaced by grasslands or savannas, as in the Asian steppes, North American prairies and tropical savannas (Hirota et al. 2011). Further decrease in rainfall results in semideserts, with scattered shrubs, and finally deserts. Even the effects of temperature are partly exerted through water relations, because rates of evaporation and transpiration are correlated with temperature. Thus, if we want to explain natural patterns of productivity or to increase crop productivity, it is crucial that we understand the controls over plant water relations and the consequences for plant growth of an inadequate water supply. Understanding plant water relations is also important to improve our ability to predict the effects of more frequent extreme climatic events such as droughts and floods on the future distribution and functioning of natural ecosystems.
Understanding the effects of simulated warming on photosynthetic performance of aquatic plants may provide strong supports for predicting future dynamics of wetland ecosystems in the context of climate change. The plateau wetlands located in Yunnan province are highly sensitive to climate warming due to their high altitude and cold temperature. Here, we conducted a temperaturecontrolled experiment using two temperature manipulations (ambient temperature as the control and 2°C higher than ambient temperature as the warmed treatment) to determine the photosynthetic characteristics of two lakeside dominant species (Scirpus validus Vahl and Typha orientalis C. Presl.) in Dianchi Lake. Net photosynthetic rate (Pn), stomatal conductance (Gs), intercellular CO2 concentration (Ci), and transpiration rate of S. validus that grew under warmed treatment were all significantly higher than those under the control. Gs and Ci of T. orientalis showed similar patterns as S. validus did. For the response curves of Pn to photosynthetic active radiation (Pn-PAR) and intercellular CO2 concentration (Pn-Ci), S. validus had higher Pn values under elevated temperatures than the control, while Pn-PAR and Pn-Ci curves of T. orientalis did not separate clearly under two temperature scenarios. Both S. validus and T. orientalis had higher maximum net photosynthetic rate, light saturation point, dark respiration rate, the maximum rate of RuBP carboxylation (Vcmax), maximum electron transport rate driving RuBP regeneration (Jmax), the ratio of Vcmax to Jmax, triosephosphate utilization, and 1, 5-bishosphate carboxylase ribulose content under warmed treatment than those under the control. This study provides a preliminary step for predicting the future primary production and vegetation dynamics of plateau wetlands in Yunnan province.
Conifers show a biogeographical distribution across a wide range of contrasting
environmental conditions, stretching from the Arctic Circle to the equator and
Southern Hemisphere. In mountainous ecosystems, conifers can dominate at high
elevations with low temperatures severely limiting tree growth and survival.
Conifers growing at sites with temperature limiting conditions are highly sensitive
to ongoing climatic change, where warmer and drier conditions will impact their
growth. Understanding how high-elevation conifers will respond to these changes
in climate is critical, as they play a role in regulating terrestrial carbon storage
(facilitated by the formation of woody tissue) and water balance (by releasing
water to the atmosphere via transpiration). The environmental regulation of wood
formation (i.e., tracheid development in conifers), which dictates annual ringwidth
patterns, is commonly associated with the tree’s photosynthetic activity,
while other growth-limiting factors might also be relevant. For example, tree
growth requires turgidity in the cambium to exert the pressure necessary for cell
expansion, assimilates to lengthen and thicken cell walls, warmth to allow the
metabolic reactions to take place, and time for these processes to be completed.
Yet, an in-depth study on how important tree hydraulics (i.e., transpiration
dynamics) are in regulating “turgor-driven” growth in high elevation forests is
lacking.
As part of the LOTFOR project, the general objective of this work is to develop a
better mechanistic understanding on how tree hydraulics and environmental
factors interact in regulating wood formation and shaping tree rings in highelevation
conifer trees. More specifically, the coupling between stem hydrological
cycles and structural carbon dynamics is investigated in the context of increasing
temperature and water scarcity. This thesis combines multi-annual records of both
intra-annual wood formation data and high-resolution hydraulic measurements
within a mechanistic growth model to explain inter- and intra-annual tree growth
patterns. To simulate the impact from recent climate change on these mechanisms,
a space-for-time experimental setting is applied within the Lötschental, located in
the Swiss Alps, where we collected data of two commonly occurring conifer
species (Larix decidua Mill. and Picea abies Karst. L.) along an elevation/thermal
gradient and contrasting wet and dry sites. Additionally, evaluations are
performed on existing methodologies for measuring sap flow and handling large
wood anatomical datasets.
BACKGROUND:: In dioecious plants, morphological adjustment to climate change may differ between male and female individuals due to greater reproductive effort in females. Not accounting for sexual variation could lead to incorrect assessment of a species response to climate change. OBJECTIVES: The aim of this study was to assess how important gender-specific responses are to Hippophae rhamnoides in changing trans-Himalayan environments. METHOD: Leaf morphological characters of male and female Hippophae rhamnoides individuals along an altitudinal gradient (2797-4117 m) and plants raised in 'common-garden' experiment was measured. RESULTS: Leaves become smaller in length and area, but became thicker with decreasing specific leaf area (SLA) with increasing altitude in both the gender. Leaf size, area, thickness, chlorophyll and petiole length were found to be higher in males than in females, while female had a higher SLA. When cuttings from the plants were grown in a common-garden experiment, the altitudinal effect disappeared for all morphological variables suggesting that most leaf morphological variation in H. rhamnoides is environmentally determined. In the event of climate change, our study showed that phenotypic plasticity would be a crucial determinant of plant response in mountainous region. Effect of altitudinal gradient on leaf morphology was more conspicuous in males suggesting that males are more responsive to change in environmental conditions.. CONCLUSION: The results suggested that males will adapt better to the changing climate and may lead to a male-biased population in the event of climate change. Stressful environments cause added detrimental impact on female than on male.
The narrow-leaved ash (Fraxinus angustifolia Vahl) is one of the three indigenous ash species in Europe. The largest continuous complexes of the narrow-leaved ash in the Republic of Croatia are located along the Sava river (80% of the total area). Over the recent years, the dieback of the narrow-leaved ash had a decreasing trend on natural regeneration, both in Europe and in the Republic of Croatia. Research into the genotypic and phenology variability was conducted in two clonal seed orchards (Čazma, Nova Gradiška) on a total of 92 genotypes belonging to two seed regions (seed region Upper Posavina, Middle Croatia and Pokuplje, and seed region Middle Posavina). Genetic variability was tested with eight microsatellite markers (Femsatl 4, Femsatl 8, Femsatl 10, Femsatl 11, Femsatl 16, M2-30, FR639485, FR646655). Phenological observations of the flushing and flowering were conducted over three consecutive years (2014, 2015, 2016), and performed in regular seven-day intervals from the beginning of February until the end of June. Flushing and flowering were classified into six development phases (six phases of flushing and six phases of flowering). Based on the development phases of flushing, quantitative genetic parameters were estimated, such as genotypic, environmental and phenotypic variance, and broad-sense heritability. Ecotypic forms and the impact of climate factors (temperature, precipitation, insolation) on the beginning of flushing were studied. Phenological observations of flowering were used to study the dynamics of flowering and the forms of sexuality of the narrow-leaved ash. The aim of the study was to determine the level of intra and interpopulation variability, assess the values of heritability for the quantitative trait of flushing, establish the existence of ecotypic forms and impact of climate factors on the beginning of flushing, and the flowering dynamics through three-year phenological observations. Based on microsatellite markers, the studied genotypes do not express a statistically significant interpopulation genetic variability (FST = 0.007; p = 0.018). The highest genetic variability belongs to the intrapopulation level of structure (99.30 %). The estimated quantitative trait of the beginning of flushing suggest moderate to high values of heritability (H2 = 0.20 to 0.63), and an increase in genotypic and decrease in environmental variance in proportion with the age of the studied genotypes. The property of the beginning of flushing of the narrow-leaved ash expressed a tendency of separation into two ecotypic forms (early and late), where the forms differ/separate by 12 days on average. The Čazma clonal seed orchard contains a considerably larger share of genotypes expressing a tendency for later flushing (69 %), unlike the clonal seed orchard of Nova Gradiška, where the majority are early flushing genotypes (64 %). Of the studied climate variables, insolation expressed the best statistical significance with regard to the variable of the beginning of flushing (CSO Čazma, r = 0.89; CSO Nova Gradiška, r = 0.96). The possibilities of applying the models of Growing Degree Days and Dynamic Model Chilling Portions were studied, and the temperature sum threshold for seed orchards was 13°C (CSO Čazma) and 15°C (CSO Nova Gradiška), with the measurements beginning on November 1, whereas for negative temperature sums no statistically significant difference was obtained (p = 0.854). Research into the sexuality and flowering in clonal seed orchards resulted in the establishment of the existence of male and hermaphrodite inflorescences, whereas female inflorescences were not found. Also observed was a change of sex in the same genotypes over the studied years.
Leaf carbon isotope composition (δ¹³C) of both vascular and non-vascular plants were investigated in order to assess their variability along an altitude gradient (414, 620, 850, 1086,1286 and 1462 m) from a subtropical monsoon forest located at Mt. Tianmu Reserve, eastern China. Leaf δ¹³C values of all plant species ranged from -34.4 to -26.6‰, with an average of -29.8‰. There is no significant difference in leaf δ¹³C between vascular plants and mosses, however, trees had significantly higher δ¹³C values than herbs. For pooled data, leaf δ¹³C was positively correlated with altitude. Leaf δ¹³C was significantly and negatively correlated with annual mean temperature and atmospheric pressure, while it was significantly and positively correlated with soil water content. Furthermore, there was no relationship between leaf δ¹³C and soil nitrogen content or soil phosphorus content. The altitudinal trend in leaf δ¹³C is the consequence of the interaction between temperature, atmospheric pressure and soil water content.
Leaf carbon isotope composition (delta C-13) of both vascular and non-vascular plants were investigated in order to assess their variability along an altitude gradient (414, 620, 850, 1086, 1286 and 1462 m) from a subtropical monsoon forest located at Mt. Tianmu Reserve, eastern China. Leaf delta C-13 values of all plant species ranged from -34.4 to -26.6 parts per thousand, with an average of -29.8 parts per thousand. There is no significant difference in leaf delta C-13 between vascular plants and mosses, however, trees had significantly higher delta C-13 values than herbs. For pooled data, leaf delta C-13 was positively correlated with altitude. Leaf delta C-13 was significantly and negatively correlated with annual mean temperature and atmospheric pressure, while it was significantly and positively correlated with soil water content. Furthermore, there was no relationship between leaf delta C-13 and soil nitrogen content or soil phosphorus content. The altitudinal trend in leaf delta C-13 is the consequence of the interaction between temperature, atmospheric pressure and soil water content.
Uptake of water by the root system is critical for plant functioning, as it balances aboveground water losses by transpiration and facilitates movement towards the roots of nutrients and other chemicals. The coupled processes in the soil-plant-atmosphere continuum are controlled at the plant, the cellular, or even molecular level (Kramer and Boyer 1995; Steudle and Peterson 1998). A basic understanding is needed of the process of root water uptake, combining soil physical and plant physiological perspectives in models of plant behaviour. This can be related to the water status in their environment to determine accurate plot-level soil water balances, evaluate plant adaptation to drought, and analyse below-ground competition in mixed vegetation systems.
Understanding how photosynthesis responds to the environment is crucial for improving plant production and maintaining biodiversity in the context of global change. Covering all aspects of photosynthesis, from basic concepts to methodologies, from the organelle to whole ecosystem levels, this is an integrated guide to photosynthesis in an environmentally dynamic context. Focusing on the ecophysiology of photosynthesis how photosynthesis varies in time and space, responds and adapts to environmental conditions and differs among species within an evolutionary context the book features contributions from leaders in the field. The approach is interdisciplinary and the topics covered have applications for ecology, environmental sciences, agronomy, forestry and meteorology. It also addresses applied fields such as climate change, biomass and biofuel production and genetic engineering, making a valuable contribution to our understanding of the impacts of climate change on the primary productivity of the globe and on ecosystem stability.
For an examination of the progression of cavitation in large-diameter earlywood vessels of a deciduous ring-porous tree, potted saplings of Fraxinus mandshurica var japonica Maxim. were frozen and then thawed. The changes in the amount and distribution of water in the lumina of the current year's earlywood vessels during the course of the freezing and thawing were visualized by cryo-scanning electron microscopy. When samples were frozen, most of the current year's earlywood vessels were filled with water. After the subsequent thawing, the percentage of cavitated current-year earlywood vessels gradually increased with time. All of the current year's earlywood vessels were cavitated within 24 h, and only limited amounts of water remained in the lumina of earlywood vessels. Similar cavitation of earlywood vessels was observed after thawing of frozen, excised stem pieces. In contrast, many vessels of the current year's latewood retained water in the lumina during freezing and thawing. These observations indicate that the cavita-tion of the current year's earlywood vessels is not produced during freezing but progresses during rewarming after freezing in F. mand-shurica var japonica.
Evidence is fast accumulating that some form of refilling mechanism does operate in the xylem of some species. If it is a common phenomenon, refilling under tension has important implications for our understanding of long distance transport in plants. The involvement of living cells suggests that some plants can exert a degree of active control over the hydraulic properties of their xylem. Instead of being an irreversible dysfunction, variation in conductance caused by cavitation may be part of important regulatory mechanisms. The hydraulic conductance of the xylem in species exhibiting refilling may be a dynamic balance between embolism formation and repair that varies throughout the day in response to changes in xylem tension and the vigor of the refilling mechanism. In many species stomatal conductance is closely coordinated with the hydraulic conductance of the soil to leaf pathway. Transpiration is usually regulated in a way that allows maximum xylem tension to approach the threshold at which significant cavitation occurs. This fine balance between normal xylem tension and the loss of conductivity suggests a functional role for cavitation as part of a feedback mechanism linking stomatal regulation to hydraulic conductance and plant water status. A sensitive signaling mechanism linked directly to the onset of hydraulic failure in the xylem would integrate aspects of both the supply and demand for water at the leaf surface, allowing the plant to maintain maximum rates of photosynthesis and respond quickly to short-term changes in evaporative demand.
Physiological and ecological adaptations of altitudinal gradients reveal alpine plants' ecological and evolutionary responses to environmental changes. Here we quantitatively investigated the variation in the foliar physiological and morphological traits of alpine tree species (Abies fargesii) along the altitudinal gradient in the Taibai Mountains, China. We collected the needle samples of Taibai fir (A. fargesii) from seven sites at altitudes of 2550, 2650, 2750, 2850, 2950, 3050 and 3150 m, respectively, and measured the 12 foliar physiological and morphological traits. Each set of needle sample (100 needles) was randomly selected from the upper-third of A. fargesii canopies.
The results showed that leaf mass per unit area (LMA), stable carbon isotope composition (delta C-13), stomatal rows (SR), leaf carbon concentration per unit area (C-area), leaf nitrogen concentration per unit leaf mass (N-mass) and area (N-area) linearly increase significantly while stomatal density (SD), number of stomata per unit nitrogen concentration (S-t/N) and per unit leaf mass (S-t/LM) decrease with the altitudes raise. Moreover, all measured traits presented both strong correlations and significantly linear relationships with the main climate factors such as the mean temperature, rainfall and relative humidity during the growing season as well as the altitudes, except for leaf free water concentration (LWC), leaf carbon concentration per unit leaf mass (C-mass) and C: N ratio.
The patterns of foliar traits in response to altitudes imply that the alpine plants need higher cost (e.g. higher nutrient concentration) to adapt to the harsher environments along altitudinal gradient. Moreover, our results show that the variation patterns of the leaf traits for A. fargesii plants should be driven by the interactions of multi-climate factors because the abiotic factors that directly influence the growth of plants covary with the increasing altitudes.
Xylem cavitation in winter and recovery from cavitation in the spring were visualized in two species of diffuse-porous trees, Betula platyphylla var. japonica Hara and Salix sachalinensis Fr. Schm., by cryo-scanning electron microscopy after freeze-fixation of living twigs. Water in the vessel lumina of the outer three annual rings of twigs of B. platyphylla var. japonica and of S. sachalinensis gradually disappeared during the period from January to March, an indication that cavitation occurs gradually in these species during the winter. In April, when no leaves had yet expanded, the lumina of most of the vessels of both species were filled with water. Many vessel lumina in twigs of both species were filled with water during the period from the subsequent growth season to the beginning of the next winter. These observations indicate that recovery in spring occurs before the onset of transpiration and that water transport through twigs occurs during the subsequent growing season. We found, moreover, that vessels repeat an annual cycle of winter cavitation and spring recovery from cavitation for several years until irreversible cavitation occurs.
Pressurized air (3.5 MPa) produced massive embolization and loss of hydraulic conductivity in detached willow twigs (Salix viminalis L.) enclosed in a pressure collar. A state of tension in the xylem water column was not a necessary precondition for this embolization. Ultrasound acoustic emissions on the distal side and close to the collar were recorded only when air-saturated water was leaving the collar, whether moved by transpiration or by an overpressure applied to a reservoir at the base of the twig. Water released from embolized xylem elements increased the water potential of the leaves. The results show that air entry into xylem elements inside the pressure collar is the main mechanism responsible for the loss of hydraulic conductivity with the pressure collar technique, and that ultrasound signals originating within the collar are not sensed on the outside.
The hydraulic conductivity and gas exchange parameters of holm oak trees (Quercus ilex ssp. ballota (Desf.) Samp.) from an open woodland ecosystem in southwestern Spain were determined in young shoots (≤1 year old) over the course of 3 years, at 6–7 week intervals. Acorn production (AP) was also determined. Specific hydraulic conductivity (K
s), leaf specific hydraulic conductivity (K
l), loss of hydraulic conductivity (PLC), and xylem water potential (Ψ) were assessed. In addition, histological cuttings of stem cross sections were examined to determine the density and diameter of xylem vessels. Acorn production was related to mid-summer K
s and PLC values, but not for other dates or to parameters associated with instantaneous gas exchange. Although AP may depend on several physiological and climatic variables, these results indicated a close non-linear relation between hydraulic conductivity during periods of summer water stress and AP. It was difficult to recover xylem conductivity after a rain event when the PLC of twigs exceeded a threshold of approximately 68 %, which therefore had a negative effect on the AP. However, if the PLC ≤ 55 %, then the hydraulic conductivity and Ψ could be quickly recovered after a rain event and the effect of water stress would have less importance. Because holm oak usually operates at the limits of safety for hydraulic conductivity, which are surpassed under severe water stress (Ψ
predawn ≤ −3.0 MPa), a global change scenario leading to drier conditions in the Mediterranean basin will increase the negative effects of summer drought on AP, leading to negative consequences for the ecosystem dynamics. The right choice of drought-adapted ecotypes and silvicultural practices that increase soil water retention and reduce plant competition should be taken into account for forest management.
Stem samples were collected at Abruzzo National Park. A tree-ring chronology was developed from the original ring-width measurements. Response functions were computed between the tree-ring chronology and monthly climatic variables for 1950-1985, using a 12-month dendroclimatic window, from the current August to the previous September. The bootstrap method was used to assess significance of relations between tree growth and climate. The final tree-ring chronology, derived from a total of 8554 ring widths, spans the period from 1672-1987. Radial growth of mature beech trees at the study sites is limited by summer drought, and relies heavily on accumulation and melting of winter snowpack to meet its moisture demand in the growing period. -from Author
Hydraulic resistance to water flow was measured in the xylem and leaves of above-ground portions of Acer saccharum and Acer rubrum for trees with trunk diameters (D) ranging from 0.02 to 0.2 m. Resistance (area basis) to water flow in leaves and petioles was 24 and 13 × 10 MPa m2s kg−1 for A. saccharum and A. rubrum, respectively. Leaf area of whole trees was proportional to D1.52. Absolute xylem resistance (MPa s kg−1 was proportional to D−1.54. So xylem resistance (area basis) was proportional to D0.2, which was not significantly different from a zero dependence (D0) for D≪0.2 m. On a leaf area basis, the resistance to water flow in the whole shoots was found to be approximately 50% in
leaves and petioles, 35% in crown xylem, and 15% In trunk xylem for D≪0.2 m.
Abstract Hydraulic conductivity of the xylem is computed as the quotient of mass flow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of time-dependent reductions in flow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh. stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m−3 oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after flow-impeding air emboli have been removed by repeated high-pressure (175 kPa) flushes. Correlation between microbial contamination and declining conductivity suggests that long-term (> 4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (< 4h) measurements may be caused by movements of air emboli in vessels and/or participate matter.
The transfer of pollutants from the atmosphere to the internal tissues of a leaf may involve several routes and physical processes. Wind and abrasive damage, which are components of weathering, cause changes in leaf surface characteristics and cuticular integrity which influence gaseous diffusion and the uptake of solutes.Wind-induced components of weathering of individual leaves of Fagus sylvatica were simulated by the use of a miniature wind tunnel and surface abrasion. The consequence of exposure to wind was widespread disruption of trichomes, and evidence of smoothing of cuticular wax was obtained using low-temperature scanning electron microscopy (SEM). An abrasive treatment also damaged trichomes, created cuticular lesion visible by SEM and significantly increased total surface conductance, to water vapour, on both adaxial (astomatous) and abaxial (stomatous) surfaces. The structural integrity of the cuticle and the spatial distribution of cuticular lesions was investigated by using ruthenium red as a convenient dye tracer.Leaves were found to be most susceptible to abrasive damage on their abaxial surfaces, where the spatial distribution of lesions was significantly clustered. The occurrence of these lesions may be related to a natural structural and conductive heterogeneity across the leaf surface. Sites most vulnerable to damage were trichomes and protruding veins.Exposure of F. sylvatica leaves to natural weathering processes in an exposed upland site increased uptake of (35S) sulphate from droplets placed on the adaxial cuticle by 30-fold on average compared with leaves grown in a sheltered location at the same altitude. The spatial heterogeneity of sulphate uptake measured at discrete points over a single leaf was also greater in leaves from the exposed site.Consequently, weathering may increase the heterogeneity of gas exchange and the uptake of solutes, by increasing leakiness at localised sites across the leaf surface.
The reactions of sessile oak (Quercus petraea (Mattuschka) Liebl.) to drought were studied under natural conditions in a 32-year-old stand near Nancy (northeastern France) during the summers of 1989 (strongly rain deficient) and 1990. A plot of five trees was subjected to imposed water shortage, while a group of irrigated trees was used as a control. Measurements of xylem sap flows and water potential enabled the computation of plot transpiration, canopy conductance, and specific hydraulic conductance in the soil-tree continuum. Stomatal conductance was measured directly with a porometer. Specific hydraulic conductance of our oaks was of the same order of magnitude as that reported for other species. It decreased significantly during spring because of a time lag between cambial growth and leaf area expansion. Measured transpiration was close to potential evapotranspiration, except during days with high vapor pressure deficits, which promoted stomatal closure in the absence of soil water deficits. Imposed drought caused predawn leaf water potentials to reach values as low as -2.0 MPa and a progressive decline in hydraulic conductance, which was probably attributable to modifications in hydraulic properties at the soil-root interface. This gradual decline in conductance was attributed to their deep rooting (1.40 m). This study revealed that Q. petraea may be considered as drought tolerant because of adaptations like deep rooting, efficient and safe xylem sap transport, maintenance of significant stomatal conductance, and significant transpiration, even during strong drought stress.
Xylem embolism, the reduction of water flow by air-filled vessels, was measured in a stand of 5- to 8-year-old sugar maple (Acer saccharum Marsh.) saplings growing in a nursery bed in northwestern Vermont. Embolism was quantified as percentage loss in hydraulic conductivity of trunk and branch segments relative to maximum values obtained by removing air from vessels by repeated high pressure (173 kPa) perfusions. Ten segments per tree were cut from 6 trees for each of 11 measurement periods spaced at roughly monthly intervals from May 1986 to June 1987. Journal Article
A new method is presented for measuring whole-shoot hydraulic conductance, KT (kg s⁻¹ MPa⁻¹). The method was also used to determine other conductance values in maple (Acer saccharum Marsh.) stem segments of differing diameter including: Kh (absolute conductance or conductance per unit pressure gradient, kg s⁻¹ m MPa⁻¹), Ks (specific conductance or Kh per unit wood area, kg s⁻¹ m⁻¹ MPa⁻¹), and LSC (leaf specific conductance or Kh per unit leaf area, kg s⁻¹ m⁻¹ MPa⁻¹). A regression of KT versus stem basal diameter, D (m), gave KT = 5.998 × 10⁻²D1.402 (R² = 0.986 for D from 0.001 to 0.1 m) and a regression for leaf area, AL (m²), gave AL = 4.667 × 10³D2.007 (R² = 0.981 for D from 0.001 to 0.3 m). More than 50% of the resistance to water flow in large shoots (0.1 m in diameter and 8 to 10 m long) was contained in branches less than 0.012 m in diameter, i.e., in the distal 1.5 m of branches.
We used the regressions to predict the steady state difference in pressure potential, P, between the base of a shoot of diameter D and the average pressure potential at the apices of the shoot; the relation is given by P = 7.781 × 10⁴ED0.605, where E is the average evaporative flux density (kg s⁻¹ m⁻²) in the leaves attached to the shoot. After comparing the predictions of this equation to field observations of E and leaf water potential and stomatal conductance, we concluded that the hydraulic conductance of large maple shoots is sufficiently low to prevent maximum stomatal conductance in maple leaves.
The seasonal progression of xylem dysfunction from tyloses and embolism induced both by cavitation and frost was studied in Quercus rubra L. and Quercus alba L. branches. Vessel lengths and diameters were measured in current-year rings of branches of various ages. Vessels in current-year shoots are about the same size as those in many diffuse porous trees, but vessels in older branches are two to six times larger in diameter and typically more than 10 times longer. Large Quercus vessels were more vulnerable to cavitation than small vessels. The small vessels in current-year shoots were more vulnerable to cavitation than vessels of comparable size in diffuse porous species. Earlywood vessels are completely blocked by tyloses within a year of their formation. Tylose growth starts in winter, but the vessels are not fully blocked until the next summer. Many latewood vessels, by contrast, remain free of complete blockage for several years. In Q. rubra, loss of hydraulic conductivity in current-year shoots due to cavitation reaches 20% by August and > 90% after the first hard frost. Both laboratory and field observations confirm that the role of frost in causing loss of hydraulic conduction by embolism is much more dramatic in Quercus than in conifers and diffuse porous hardwoods.
Flow of water in a single tree was modeled in terms of the Darcy equation using a catena of four compartments: root, stem (further divided into discs), branches and leaves. Within each compartment or disc, water content was related to both water potential and conductivity of the xylem tissue using power or logarithmic functions, thus introducing both capacitance and variable resistance to flow in the model. Transpiration from the leaves to the atmosphere was used as the upper boundary to the model, and the soil–root interface as the lower boundary. Parameters for the water content, water potential and conductivity functions, together with physical dimensions were obtained by direct measurement or from the literature.
A sensitivity analysis showed that the largest changes in simulated water potential and flow were associated with changes in the parameters directly controlling conductivity. Simulation of both smoothed diurnal changes and stepwise changes showed a phase lag down the tree, with flow tending to approach a steady state, but with changes in the gradients of water potential, water content and conductivity.
A preliminary test of the model was made against field data using the Penman-Monteith equation to estimate the transpiration rate in a well-watered Pinus contorta Dougl. stand. Stem flow, water potential and water content were measured directly on a representative tree, which was subsequently harvested to provide dimensions and laboratory estimations of the parameters in the functions by direct measurement.
Despite many studies of the percent loss of hydraulic conductivity in excised branches, there is doubt as to whether cutting
stems in air introduces unnatural embolism into the xylem at the cut surface. To address this question, hydraulic conductivity
was measured in seedlings of northern red oak (Quercus rubra L.) and rooted scions of eastern cottonwood (Populus deltoides Bartr. ex Marsh.) that had been droughted in pots. Results indicate that in situ dehydration produced a very similar vulnerability
curve (% loss of conductivity versus water potential) to those previously obtained by bench-top dehydration of excised branches
of eastern cottonwood and red oak. In eastern cottonwood cuttings, conductivity loss increased sharply below water potentials
of –1.0 MPa, with 100% loss of conductivity occurring by –2.0 MPa, whereas conductivity loss in red oak seedlings was more
gradual, i.e., increasing below –1.5 MPa and sustaining 100% loss of conductivity by about –4.0 MPa.
Hydraulic properties of xylem in seven species of conifer were studied during late winter and early spring 1991. Vulnerability to cavitation and air embolism was investigated using hydraulic conductivity and acoustic techniques. Embolisms were induced in branches excised from mature trees by air-drying them in the laboratory. Both techniques gave comparable results indicating that they both assess the same phenomenon. Within a tree, vulnerability was related to the permeability of the xylem, the largest stems tended to cavitate before the smallest ones when water deficits developed in a branch. Interspecific comparisons showed large differences in the xylem water potential needed to induce significant embolism, values ranged from -2.5 MPa in Pinus sylvestris to -4 MPa in Cedrus atlantica, but these differences did not correlate with differences in the xylem permeability of the species. The vulnerability of a species to air embolism was found to be consistent with its ecophysiological behavior in the presence of water stress, drought-tolerant species being less vulnerable than drought-avoiding species.
A pressure chamber was used to establish the vertical gradients of leaf water potential (Psi(Leaf)) and stem water potential (Psi(Stem)) in field-grown tobacco (Nicotiana tabacum L. var. Havanna seed 211) at three different times of day. Leaves enclosed in polyethylene bags and aluminum foil the previous afternoon and left to equilibrate overnight were used to determine Psi(Stem). The greatest difference between Psi(Leaf) and Psi(Stem) occurred in the upper part of the plant at 1100 hours Eastern Standard Time and was 5.5 bars. The largest vertical gradient in Psi(Stem) occurred at 1300 hours. The soil water potential (Psi(Soil)), extrapolated from the potential of leaves on a completely enclosed plant, was higher than -1 bar. The vertical gradient in Psi(Stem) and the difference between Psi(Leaf) and Psi(Stem) showed the existence of a resistance to water movement within the stem (r(stem)) and a further resistance between the stem and leaf (r(petiole)). The r(petiole) and root resistance (r(root)) were estimated to be 931 and 102 bars seconds per cubic centimeter, respectively. The r(stem) was low (94 bars seconds per cubic centimeter) at 1100 hours but increased to 689 bars seconds per cubic centimeter at 1300 hours.
The prospect of future climate change has stimulated research into the physiological responses of plants to stress. Water is a key factor controlling the distribution and abundance of plants in nature and the efficient uptake and subsequent transport of water within the plant is critical in hot, dry regions. This book, based on a meeting which focused on the failure of the hydraulic pathway within the xylem, brings together contributions from a range of experts who have worked on the cavitation of water in the transport system. The phenomenon of cavitation, discovered only in the 1960s, is now becoming recognised as being widespread and, whilst its ecological significance is a matter for further research, many scientists consider than embolism in the xylem predisposes plants to further water stress. Cavitation and refilling may, therefore, hold the key to vegetational response to climatic warming and drying. This book will provide a valuable compendium of information for those working in the plant and environmental sciences as well as for those whose interests lie in the more applied disciplines of agriculture and forestry.
Xylem embolism, the reduction of water flow by air-filled vessels, was measured in a stand of 5- to 8-year-old sugar maple (Acer saccharum Marsh.) saplings growing in a nursery bed in northwestern Vermont. Embolism was quantified as percentage loss in hydraulic conductivity of trunk and branch segments relative to maximum values obtained by removing air from vessels by repeated high pressure (173 kPa) perfusions. Ten segments per tree were cut from 6 trees for each of 11 measurement periods spaced at roughly monthly intervals from May 1986 to June 1987. During the 1986 growing season, embolism increased significantly from 11 to 31% in the larger branches and trunk (segment diameter #8805;0.5 cm), but remained at about 10% in twigs (segment diameter <0.5 cm). This was unexpected because the greatest water stess and thus potential for embolism occurs in twigs. During the winter, embolism increased throughout the trees and the trend with diameter was reversed; by February, small twigs were 84% embolized vs. 69% for larger branches and trunk. Dye perfusions showed that winter embolism in trunks was localized on the south side; this may have resulted from water loss by sublimation or evaporation in the absence of water uptake. Beginning in late March, embolism decreased throughout the trees to approximately 20% in June. This decrease was associated with positive xylem pressure of at least 16 kPa which may have originated in the roots, because weather conditions at the time were unfavorable for the generation of stem pressures characteristic of Acer species in early spring.
The loss of water by evaporation from the leaves of plants (transpiration) is an inevitable accompaniment of the photosynthetic absorption of carbon dioxide from the atmosphere. Water is absorbed from the soil by the roots to make good this transpirational loss. There is thus a flow of water through the plant from the soil to the atmosphere — the so-called transpiration stream — and it is the purpose of this chapter to focus attention on one part of this stream, namely the flow of water from the soil into, and through the roots. However, to abstract this partial process from the whole root-plant-atmosphere system can be misleading and a note of caution is necessary.
Long-termed investigations were performed from 1957 to 1969 at the Eberswalde root laboratory to analyse root and shoot growth of 17 tree species. The present paper demonstrates the growth rhythms of a number of representative species. The maxima of root development occur in the period of May – September, but correspondence between different tree species could not be observed. Only once a stagnation of root growth within the vegetation period could be found in Larix leptolepis during a dry year. In September, the intensity of root degrowth creases and growth stops in October-November under the climatic conditions of Eberswalde.
When the heat pulse velocity method of measuring sap flux in trees is based on idealized heat transport theory, it seriously
underestimates actual water flow. Two-dimensional numerical solution of the physical heat flow system demonstrates that both
sensor thermal properties and sap flow interruption in the vicinity of implanted probes cause significant departure from the
idealized theory. New numerical solutions to the practical heat pulse problem are given and applied to earlier experimental
data. Transpiration rates thus determined in conifers agree within 5% of those measured by weighing lysimetry or in a diffuse
porous hardwood within 10% of those calculated from a climatized cuvette. A table of numerical solutions applicable to the
most widely published heat pulse sensing configuration is given.
The relationship between the foliar phenology of 43 north temperate tree species and the winter impairment of hydraulic conductivity was investigated. Among the deciduous hardwoods, there was a highly significant correlation between the loss of hydraulic conductivity by late winter and the timing of bud burst in the spring. The diffuse-porous species, which incur less loss of hydraulic conductivity than the ring- and semi-ring-porous species, leafed out significantly earlier; the diffuse-porous species also tended to senesce later in the autumn. -from Authors
1. Variation in late-winter xylem embolism, phenological traits, growth parameters, xylem anatomy and allozyme characteristics were examined in four populations of European beech (Fagus sylvatica) from different geographical origins in Italy. 2. Ultrasonic acoustic emissions from plant stems were measured during winter. Late-winter xylem embolism was quantified before budburst. The timing of budburst was recorded and plant growth parameters were monitored during the spring. The dimension and density of xylem conduits were measured. The genetic variability was investigated using isoenzymes as genetical markers. 3. Significant differences between populations in the rate of acoustic emissions, late-winter embolism, phenological traits and spring growth parameters were observed. Xylem embolism was higher in populations that displayed a higher rate of acoustic emissions during winter. The most embolized population displayed later budburst and slower growth in the spring. No significant differences in xylem anatomy were found. 4. Six out of 10 isoenzymatic loci showed significantly different allele frequencies between populations, but no clear association was found between the genetic variation and the variation of the other traits investigated.
Water, solute and pressure potentials have been investigated in shoots in the canopy of a Sitka spruce stand using the pressure chamber. Water potentials of - 15 bar or lower frequently occurred in the upper parts of the canopy on warm, sunny days of low vapour pressure deficit although there was never a shortage of water in the soil. Root water potentials ranged between -0.5 and -4.5 bar. Gradients of water potential of up to 2 bar m-1 occurred in the trunk and larger gradients occurred in the primary and secondary branches. On overcast or wet days the water potentials in the canopy were higher and the gradients smaller. It was concluded that the resistance to flow in the trunk was largely responsible for the large drop in potential between roots and leaves. The resistance per unit length was estimated as 107 bar s m-4 in the trunk and was considerably higher than in the angiosperms or pines for which comparable data are available. The solute potential of shoots in the canopy varied between -13 and -24 bar depending on the time of year. There was some tendency for solute potentials to be lower higher up in the canopy. From typical Hofler diagrams constructed from the water and solute potentials, the change in pressure potential (turgor pressure) with water content was determined, and a pressure dependent bulk modulus of elasticity derived. A water potential of -15 bar was equivalent to a fall in pressure potential to about 40% of that at full turgor. Empirical equations relating solute, pressure and water potential to water content were determined.
Several aspects of the secondary xylem anatomy of oak (Quercus robur L.) branches, trunks and roots are investigated. These are. growth ring width and definition, vessel size (diameter), shape (eccentricity) and distribution, fihre type (ge1atinous or normal), and fibre, axial parenchyma and ray distribution. These observations are put in perspective by comparison with beech (Fagus sylvatica L.), and discussed in relation to their implications in wood identification, archaeology, dendrochronology and water relations of the tree.
Quercus ilex L. growing in the southern Mediterranean Basin region is exposed to xylem embolism induced by both winter freezing and summer drought. The distribution of the species in Sicily could be explained in terms of the different vulnerability to embolism of its xylem conduits. Naturally occurring climatic conditions were simulated by:
(1) maintaining plants for 3h at ambient temperatures of 0, - 1-5, - 2 5, - 5 0 and - 11 °C; and
(2) allowing plants to dry out to ratios of their minimum diurnal leaf water potentials (Yl) to that at the turgor loss point (Ytlp) of 0 6, 0 9,1 05, 1 20 and 1-33.
The loss of hydraulic conductivity of one-year-old twigs reached 40% at - 1 5°C and at Yl/Ytlp = 1 05. Recovery from these strains was almost complete 24 h after the release of thermal stress or after one irrigation, respectively. More severe stresses reduced recovery consistently. The percentages of xylem conduits embolized following application of the two stresses, were positively related to xylem conduit diameter. The capability of the xylem conduits to recover from stress was positively related to the conduit diameter in plants subjected to summer drought, but not in the plants subjected to winter freezing stress. The ecological significance of the different vulnerabilities to embolism of xylem conduits under naturally occurring climatic conditions is discussed.
The cuticular conductance (gc) of the astomatous adaxial surface of Fagus sylvatica L. leaves was determined under varying vapour pressure deficits at a constant temperature of 20 °C. Cuticular conductance
was determined from the weight loss of detached leaves after both the stomatous abaxial surface and the cut end of the petiole
had been sealed using low melting-point paraffin wax. Cuticular conductance was found to decrease as the water vapour pressure
was increased in steps. No response was observed when vapour pressure deficit was decreased from an initially high value.
It is concluded that these results are consistent with the hypothesis that cuticular conductance is influenced by the water
content of the cuticle and that the apparent hysteresis is likely to be a result of the long time-constant for the process
of cuticle rehydration in intact leaves.
Sap flow (F) and leaf water potential (LWP) were followed diurnally in mature Valencia and Shamouti orange trees in an orchard. The hydraulic conductance of these trees
was computed from the diurnal relationship between the LWP and F. The driving force for water movement was estimated from a weighted average of sunlit and shaded LWP, assuming that leaves in the shade transpire to some extent. LWP of covered, non-transpiring leaves was also measured hourly. It was assumed to represent the xylem water potential within
the axial conduit of the trunk. Relating covered LWP to F on an hourly basis enables the computation of the hydraulic conductance of the root system, including axial conductances.
The hydraulic conductance of the transpiring crown was computed. Its magnitude was comparable to the root system hydraulic
conductance.
Large variations in resistance to water flow in plants may be caused by cavitation in root xylem. A relationship is found between resistance to air flow through frozen cotton root segments and stress in the plant from which the segment was taken, thus suggesting a possible method of deducing cavitation and its effect on plant resistance.
Hydraulic conductivity (Lp), radial conductivity (LR), axial conductance (Kh), and related anatomical characteristics for lateral roots of Agave deserti were investigated during root growth and drought-induced abscission. The elongation rate of lateral roots averaged 5 mm d−1 under wet conditions and was reduced 95% by 17 d of drought (ψsoil−0.5 MPa); 46% of such roots abscised at 21 d of drought. Under wet conditions, Lp decreased with age up to 30 d, mainly reflecting
suberization of endodermal cells. Lp and LR decreased basipetally along the roots under wet conditions and after 18 d of drought and were lower in the case of drought,
both responses reflecting increases in the number of suberized endodermal cells. In contrast, Lp and LR increased basipetally along abscised roots at 35 d of drought, presumably due to the collapse of cortical cells. Kh increased with distance from the root tip; the greatest increase, which coincided with the loss of cross walls in late metaxylem
vessels, occurred closer to the tip for roots from droughted plants compared with well-watered plants. Kh was reduced during drought by embolism of the xylem vessels, similar to the results for other studies.
A scanning electron microscope equipped with a freezing stage was used to examine the effects of slow freezing on pit aspiration
and bubble formation in living tree stems.
The size (approximately 2.0 μm diameter) and the spherical or ellipsoidal shape of the bubbles found in the centre of frozen
lumens indicated freezing rates greater than 25 μm s−1. Both unaspirated and aspirated bordered pits were found in the frozen xylem. The technique used did not reveal enough pits
to determine whether unaspirated pits were more prevalent than aspirated pits.
These results are compared with hypotheses and results from previous work on freezing in conifer xylem.
Trees of Juglans regia L. shed leaves when subjected to drought. Before shedding (when leaves are yellow), the petioles have lost 87% of their maximum hydraulic conductivity, but stems have lost only 14% of their conductivity. This is caused by the higher vulnerability of petioles than stems to water-stress induced cavitation. These data are discussed in the context of the plant segmentation hypothesis.
The hypothesis that ray parenchyma cells are actively involved in the refilling of embolized xylem of Pinus sylvestris L. was tested by killing the ray parenchyma and comparing rehydration of killed stems with that of control material. Killing of ray parenchyma was achieved using hot water or sodium azide. In most experiments, the available water for refilling was at negative water potential. Experiments were done on three kinds of plant material: small branch segments, potted seedlings and small potted trees. In all experiments, there was no indication that the azide-killed xylem was slower to refill than the control material and it was concluded that the parenchyma has no role in the refilling process, which therefore must be purely physical or physicochemical. Stems treated with hot water did not refill; we suggest that this may be caused by high temperatures decreasing the water permeability of the tracheid wall. The refilling of small branch segments may be explained by surface tension forces (capillary action), which inside the tracheid lumen may lower the water potential down to -9.7kPa; this may be enough to draw in water from the available water that in experiment one was at -2kPa. In the case of seedlings or saplings, capillary action cannot explain refilling, because the xylem water potentials were always lower than those estimated from tracheid radii. Condensation of water during diurnal cycles of warming and cooling is also unlikely to contribute to refilling significantly. To account for refilling in these cases, it is supposed that the tracheid wall may be chemically active and able to lower the water potential below the value expected by capillarity.
The problem of the putative nocturnal refilling of diurnally embolized tracheary elements is considered. It is shown that: (i) an embolus should be close to compositional equilibrium with the ambient atmosphere by the end of the day; and (ii) if an embolus is compressed to as little as 2% above atmospheric pressure, it should be resorbed in 1000 sec or less. Such a compression could arise from nocturnally developed root pressure; however, it could also result from the release of intracellular water by temperature-associated osmosis, especially at the plant's apex.
Daily patterns of stomatal conductance (gs), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one-year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured gs was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (gc) was determined for hourly intervals using gs measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of gc and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly-aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter-time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of gs to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.
Uptake of water and magnesium chloride solution was investigated through the outer surface of twigs of Picea abies (L.) Karst. Water uptake was determined by using pressure/volume (P/V) curves of the twigs as a basis for calculation to avoid problems of superficial extraneous water. When water was sprayed on bark and needles of 3- to 7-year-old twigs at a xylem water potential of -1.00 MPa, they absorbed as much as 80 mm3 water in 200 min/g twig dry weight as the twig water potential recovered to -0.15 MPa. With fluorescent dyes, pathways for absorption of water and solutes through the twig bark were found, particularly through the radially orientated ray tissue. In addition to uptake by mass flow, magnesium could also diffuse along a concentration gradient from the twig surface into the xylem. In the field, the magnitude of these uptake processes would depend on the concentration of elements deposited by atmospheric precipitation, the concentration gradient between the plant surface and the xylem sap, the xylem water potential and the intensity and duration of each precipitation event.
Sap flow rate (Qw) and leaf water potential (Ψw.leaf) in adult specimens of birch (Betula) and oak (Quercus) were measured under contrasting soil moisture conditions (Ψw.sofl). With sufficient soil moisture Qw reached about 250 cm3h−1 calculated per unit tree-trunk segment as given by 1 cm length of its circumference. In soil water-stress conditions (when
Ψw.leaf = = −15 × 105Pa), birch stopped transpiration and wilted. Oak transpired even when Ψw.leaf fell below −20 × 105Pa. The relation between Qw and Ψw.leaf was always linear and with various Ψw.soil differed in the slopes of regression lines only. Hydraulic conductance (Kwcu) with nonlimiting moisture conditions reached about 6 × 10−9m3 10−5Pa−1s−1 and “conductivity” (“kwa”) when calculated per leaf area unit reached about 23 m 10−5Pa−1s−1. Kwcu and “kwa” were of about one half to nine times greater in birch than in oak. On the basis of relations between Ψw.soil at various depths, Ψw.leaf and Qw (resp. Kw) it is possible to assess the maximal rooting depth and the effective depth where the maximum of absorption of roots occurs.
It is to be seen that the root system macrostructure substantially participates in the drought avoidance of adult trees in
a forest stand.
The extent to which stomatal conductance (gs) was capable of responding to reduced hydraulic conductance (k) and preventing cavitation-induced xylem pressures was evaluated in the small riparian tree, Betula occidentalis Hook. We decreased k by inducing xylem cavitation in shoots using an air-injection technique. from 1 to 18d after shoot injection we measured midday transpiration rate (E), gs, and xylem pressure (Yp-xylem) on individual leaves of the crown. We then harvested the shoot and made direct measurements of k from the trunk (2-3 cm diameter) to the distal tip of the petioles of the same leaves measured for E and gs. The k measurement was expressed per unit leaf area (k-1, leaf-specific conductance). Leaves measured within 2d of shoot injection showed reduced gs and E relative to non-injected controls, and both parameters were strongly correlated with k-1. At this time, there was no difference in leaf Yp-xylem between injected shoots and controls, and leaf Yp-xylem was not significantly different from the highest cavitation-induced pressure (Yp-cav) in the branch xylem (-.43 +- 0.029 MPa, n = 8). Leaves measured 7-18d after shoots were injected exhibited a partial return of g-s and E values to the control range. This was associated with a decrease in leaf Yp-xylem below Yp-cav and loss of foliage. The results suggest the stomata were incapable of long-term regulation of E below control values and that reversion to higher E caused dieback via cavitation.
Xylem embolism was measured in nine tree species for one or more years. Species were ring-porous (Quercus sp.), diffuse-porous (Alnus, Betula, Populus spp.) or coniferous (Picea, Larix, Abies spp.). Intraspecific (Populus tremuloides) and intrageneric (Betula, Alnus) comparisons were made between sites in northern Utah and interior Alaska. Most embolism, gt 90% in some dicot species, occurred in winter. Within sites, dicot trees embolized more than conifers. Between sites, Alaskan dicot trees embolized less than their Utah counterparts. Differences were explained by vulnerability to embolism caused by freeze-thaw cycles. Most conifers were entirely resistant, whereas dicot trees were vulnerable. Less embolism in Alaskan dicot trees was associated with fewer freeze-thaw events in Alaska vs. Utah. Vulnerability was positively correlated with conduit volume and hydraulic conductance per unit xylem area (ks). Trachaeids were superior to vessels in avoiding freeze-thaw-induced embolism, and had lower ks. At the other extreme, ring-porous xylem had the highest ks but lost gt 90% of hydraulic conductance after a single freeze-thaw event. Vulnerability to water-stress-induced cavitation was not correlated with conduit volume or ks. Dicot species either reversed winter embolism by refilling vessels with positive root pressures during spring (Betula, Alnus spp.), or tolerated it and relied on new xylem production to restore hydraulic conductance (Quercus sp.). Conifers reversed embolism by refilling tracheids in the absence of positive pressure. Populus species behaved inconsistently, showing some reversal one year but none the next. Even without embolism reversal, Populus species had hydraulic conductances per unit leaf area equal to other diffuse-porous species.
The vulnerability of xylem conduits to cavitation theoretically determines the maximum flow rate of water through plants, and hence maximum transpiration (E), stomatal conductance (gs), and leaf area (A1). Field-grown Betula occidentalis with a favourable water supply exhibit midday xylem pressures approaching the cavitation-inducing range (- 1.42 to - 2.2 MPa). We studied the ability of the stomata to prevent cavitation-inducing pressures when whole-plant hydraulic conductance per leaf area (k1) was reduced by making overlapping transverse cuts in the main stem. Controls were intact, or had only the phloem cut in the same pattern. Reducing k1 caused two responses:
(1) variable gs, or E with Ypx falling into the cavitation range causing up to 98% embolism and 100% leaf death,
(2) decreased gs and E with Ypx remaining above the cavitation point and no leaf death or induction of cavitation. Shoots avoiding cavitation either produced new xylem and returned to control values of k1, gs, and E (experiments in June and July), or they showed continued decline in gs, and E associated with increasing Ypx and eventual premature senescence of leaves (experiments in August). Whether embolism occurred after reducing k, probably depended on the response time of stomata, and the proximity of Ypx to the cavitation range when the xylem was cut. Stomata probably responded indirectly to reduced k via small changes in leaf Y; root signalling was unlikely because of the constant rooting environment.
Attempts to correlate values of stomatal conductance and leaf water potential with particular environmental variables in the field are generally of only limited success because they are simultaneously affected by a number of environmental variables. For example, correlations between leaf water potential and either flux of radiant energy or vapour pressure deficit show a diurnal hysteresis which leads to a scatter diagram if many values are plotted. However, a simple model may be adequate to relate leaf water potential to the flow of water through the plant. The stomatal conductance of illuminated leaves is a function of current levels of temperature, vapour pressure deficit, leaf water potential (really turgor pressure) and ambient CO 2 concentration. Consequently, when plotted against any one of these variables a scatter diagram results. Physiological knowledge of stomatal functioning is not adequate to provide a mechanistic model linking stomatal conductance to all these variables. None the less, the parameters describing the relationships with the variables can be conveniently estimated from field data by a technique of non-linear least squares, for predictive purposes and to describe variations in response from season to season and plant to plant.
Air-embolism formation in xylem vessels of Populus tremuloides Michx. was quantified by its reduction of hydraulic conductivity in branch segments. Embolism was induced by increasing xylem
tension in drying stems, or by inserting one end of a hydrated stem in a pressure bomb and increasing air pressure in the
bomb. Both treatments produced the same response suggesting that embolism by water stress was caused by air entering water-filled
vessels, presumably through inter-vessel pits. In rapidly-growing P. tremuloides branches, the vessels of the outer growth ring were functional whereas vessels in older xylem were mostly embolized. This
selective embolizing of older vessels was associated with a marked increase in permeability of their inter-vessel pits to
air, relative to pits of younger vessels. Air-injection pressures less than 1·0 MPa completely embolized older vessels that
had been re-filled in the laboratory, whereas pressures over 4·0 MPa were required to embolize young vessels. Greater permeability
of old vessels was due to degradation of their pit membranes as seen in the scanning electron microscope; large openings were
present that were not seen in pit membranes of young vessels. These holes would allow air to penetrate vessel ends at low
pressure differences causing embolism. Degradation of pit membranes causing the selective dysfunction of older sapwood may
be a general phenomenon initiating heartwood formation in many species.
Resistance to water uptake in an irrigated and a water-stressed 22-year-old citrus tree was examined during 7 weeks. Frequent
irrigation maintained the soil water potential of one citrus tree between 0.0 and −0.016 MPa in the upper 0.6 m, while the
other tree received no water for 44 d. Sap flow in the trunks was measured once a week from dawn to dusk at hourly intervals
with a calibrated heat pulse technique. At the same time leaf water potential was measured on sunlit and shaded leaves.
Sap flow in the stressed tree decreased with time at a higher rate than the water potential difference between soil and leaf,
which implies an increase in the resistance of the transport system. Diurnal sap flow and diurnal leaf water potential of
the irrigated and stressed trees were linearly related throughout the experimental period. The slope of the linear relationship
between flow and potential for both trees remained nearly constant, indicating that the resistance within the tree remained
constant. The effect of drying appeared to be a progressive decline of the leaf water potential at zero flow (computed by
linear regression). This was ascribed to a reduction of the transport of water in the soil toward the roots.
A root excision technique was used to estimate the proportion of total resistance to water flux residing in the soil, the
root, and the xylem of lodgepole pine (Pinus contorta Dougl ex. Loud.) trees in the field. Root excision at mid-day always resulted in rapid recovery of leaf water potential when
water was supplied to the cut stem, suggesting a high soil-root resistance. Transpiration was unaffected if leaf water potential
before cutting was not limiting leaf conductance. By mid-June water uptake by the excised stem always exceeded calculated
crown transpiration indicating recharge of internal sapwood storage. Predawn leaf water potential before root excision was
highly correlated with total soil-plant resistance (r2 = 0·89) and calculated root water uptake (r2 = 0·92).
A theoretical treatment is given of isothermal potential losses due to frictional resistances against water flow in plants.
It becomes apparent that in the past two main difficulties have been widely ignored: the proper choice of dimensions for fluxes
and resistances deserves consideration, and the original van den Honert concept seems to be especially prone to misinterpretation.
A revised equation is presented for total water potential at a certain point in the plant, and some implications of this approach
are outlined.
We investigated the vulnerability of xylem to embolism and the seasonal occurrence of xylem embolism in Italian alder (Alnus cordata Loisel.) by acoustic and hydraulic methods. Wood anatomy was also studied. More than eighty percent of the vessels were less than 50 mm long and no vessels were longer than 120 mm. Mean vessel diameter was 48 micro m. Ultrasound acoustic emissions from root and branch segments dehydrating in air followed a similar pattern: in both tissues, emission peaks were recorded when the relative water content of the xylem was around 0.2. In branches dehydrating in air, xylem embolism increased linearly as water potential decreased. In trees in the field, more than 80 percent of hydraulic conductivity was lost in the tree crowns during winter. Recovery from winter embolism occurred mostly before bud burst. In summer, xylem embolism was low (< 30%) and acoustic emissions from roots, stem and branches of trees in the field were also low.
Vulnerability to xylem embolism by freeze-thaw cycles and water stress was quantified in ring-porous (Quercus gambelii Nutt.), diffuse-porous (Populus tremuloides Michx., Betula occidentalis Hook.), and conifer species (Abies lasiocarpa Nutt., Juniperus scopulorum Sarg.). Embolism was measured by its reduction of xylem hydraulic conductivity; it was induced by xylem tension (water-stress response) and by a tension plus a freeze-thaw cycle (freeze response). Conifers showed little (Juniperus) or no (Abies) freeze response even to repeated cycles. In contrast, Quercus embolized more than 90% by freezing at tensions below 0.2 MPa, whereas similar embolism without freezing required tensions above 4.5 MPa. Diffuse-porous trees (Betula, Populus) showed an intermediate freeze response. The magnitude of the freeze response was correlated with conduit volume but occurred at higher tensions than predicted from theory. Large early-wood vessels (2.8 x 10(-9) m(3)) in oak were most vulnerable to embolism by freezing, small vessels in Populus and Betula were intermediate (approximately 7 x 10(-11) m(3)), and tracheids in conifers (about 3 x 10(-13) m(3)) were most resistant. The same trend was found within a stem: embolism by freeze-thawing occurred preferentially in wider conduits. The water-stress response was not correlated with conduit volume; previous work indicates it is a function of interconduit pit membrane structure. Native embolism levels during winter corroborated laboratory results on freezing: Quercus embolized 95% with the first fall freeze, Populus and Betula showed gradual increases to more than 90% embolism by winter's end, and Abies remained below 30%.
Freezing of stem sections and entire twigs of hemlock (Tsuga canadensis) has been demonstrated to occur without increasing the resistance to the movement of water through the frozen part after rewarming. This was interpreted to mean that freezing did not produce cavitation in the xylem sap even though A) the sap was unquestionably frozen; B) it contained dissolved gases; and C) it was under tension before freezing and after. Freezing stem sections of some other evergreen gymnosperms during the summer again produced no evidence for cavitation of the xylem sap. On the other hand, freezing stem sections of some angiosperms invariably increased the resistance to sap flow leading to wilting and death in a few hours when the sap tension was at normal daytime values at the time of freezing. These results were interpreted to mean that the bordered pits on the tracheids of gymnosperms function to isolate the freezing sap in each tracheid so that the expansion of water upon freezing not only eliminates any existing tension but also develops positive pressure in the sap. Dissolved gases frozen out of solution may then be redissolved under this positive pressure as melting occurs. As the bubbles are reduced in size by this ice pressure developed in an isolated tracheid, further pressure is applied by the surface tension of the water against air. If the bubbles are redissolved or are reduced to sufficient small size by the time the tension returns to the sap as the last ice crystals melt, then the internal pressure from surface tension in any existing small bubbles may exceed the hydrostatic tension of the melted sap and the bubbles cannot expand and will continue to dissolve.