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Stem hydraulic properties and xylem vulnerability to embolism in three co-occurring Mediterranean shrubs at a natural CO2 spring
Abstract
Seasonal changes in hydraulic properties and vulnerability to xylem embolism
of Erica arborea L.,
Myrtus communis L. and
Juniperus communis L. were analysed by comparing plants
at two locations in a Mediterranean environment. A distinct atmospheric
CO2 concentration
([CO2]) gradient exists between the two sites
with higher [CO2] in the proximity of a
natural CO2 spring (700 mol
mol–1). Changes in native embolism in
E. arborea and M. communis
indicated rather clear seasonal segregation by species and by the growth
[CO2]. J. communis had
constantly lower percentage embolism than the other two species (the effect of
site being not consistent). Differences in summer embolism among species and
between sites were in accordance with vulnerability curves. Volumetric
fractions also showed seasonal and site-dependent changes. Mean specific
hydraulic conductivity was strongly affected by site in
E. arborea and M. communis (in
opposite directions). Hydraulic properties varied as a function of shoot
biomass and leaf area, and the latter increased with increasing sapwood area;
differences between sites were somewhat significant in
M. communis. Foliage biomass increased with stem
biomass; E. arborea had higher values of foliage biomass
at similar values of stem biomass at the control site. Altering branch biomass
allocation may influence or not (depending on the species) hydraulic
adjustment. Plant responses to resource imbalances caused by increasing
[CO2] tend to compensate for the imbalance by
changes in hydraulic properties and biomass allocation. However, the
plasticity or compensation ability of any particular species appears limited,
and effective compensation for large changes in resource balance caused by
environmental forcing factors may require changes in species composition.
... Plants respond simultaneously to changes in environmental conditions through shortterm physiological regulation and long-term anatomical acclimation (Mencuccini 2003). Plant growth performance depends partly on xylem plasticity because the water and carbon balances are closely related to the hydraulic architecture (Tognetti et al. 2001; Breda et al. 2003; Fichot et al. 2011). Leaf water balance, carbon assimilation and stomatal conductance depend on the water supply to foliage, which is also directly linked to the xylem hydraulic architecture (Tyree & Ewers 1991; Hubbard et al. 2001; Breda et al. 2006). ...
... In Mediterranean areas plants need to adapt to two annual periods that limit plant development (Mitrakos 1980; Cherubini et al. 2003 ). Plants need to regulate to withstand embolism produced by winter freeze-thaw events and to low water potential during summer droughts (Sperry et al. 1994; Tognetti et al. 2001; Taneda & Sperry 2008). Many shrubs in the Mediterranean maquis, including E. arborea, are pioneer species and establish after major disturbances, particularly fire events. ...
... If they want to succeed in the race for colonization, plants should exhibit fast growth while regulating their physiology to cope with abiotic stress. Erica arborea has been reported to develop extensive and deep root systems (Gratani & Varone 2004) which may be related to the species' characteristic lower tolerance to low negative potentials inducing cavitation compared to that of co-occurring more drought-tolerant taxa (Tognetti et al. 2001; Quero et al. 2011). In this study we analyzed within a species the acclimation of xylem to climatic variability combining tree-ring growth and xylem anatomical characteristics to better interpret the response to climate of two populations of E. arborea growing in contrasting climatic conditions at the limits of the species distribution: one coastal population under a maritime climate on a Mediterranean island and one population at high altitude on a continental Mediterranean mountain range. ...
Plasticity of xylem architecture can be a species specific strategy to reduce vul-nerability to climate change. To study how the evergreen shrub Erica arborea regulates its xylem at different time scales as a response to climatic variability, we compared time series of annual xylem traits such as ring-width (growth), vessel size (hydraulic diameter and mean vessel area), vessel density, and po-tential conductivity (K h) at two sites characterized by contrasting Mediterranean climates in Italy and Spain. Shrubs regulated their xylem in response to major differences in climate by modifying mostly their growth and vessel density. The different adjustment of xylem observed at the two sites was partly explained by the nonlinear nature of the relationship between the studied traits and tempera-ture. Xylem development was mostly limited by low winter temperature at the cold and moist site, where plants produced more vessels per unit area of xylem and reduced growth in comparison with the warm and dry site. The responses of vessel size and density to climate were opposite. Vessel size and K h were similar at the two sites and exhibited less sensitivity to climate than vessel density and growth. Humid conditions in spring increased growth and vessel size but decreased vessel density at the cold site, whereas the effect on xylem adjustment of high temperatures during the vegetation period was generally contrary to that of high moisture availability. These results likely express within species adapta-tions of the hydraulic function and the safety-efficiency trade-off in response to climate. A decline in growth in response to a recent decrease in precipitation at the dry site could be interpreted as a first sign of vulnerability to increasing drought severity. Similar to other species, climate change may have contrasting effects for E. arborea at its cold and dry distributional limits.
... significantly differed between T W and T D , with T W showing smallest ranges and thus steepest VCs (Table 3.2, Figure 3.5). These differences probably originated from anatomical acclimation to the soil water availability with likely smaller but more vessels in response to drier conditions as has been shown in poplar hybrids by Fichot et al. (2009 (Tognetti et al. 1999, Tognetti et al. 2001, Domec et al. 2010, Warren et al. 2011, none of them investigating interaction effects with soil drought (Table 3.4). In Mediterranean species, a small reduction in vulnerability to drought-induced cavitation was found for trees grown near a natural CO 2 -spring (Table 3.4) (Tognetti et al. 1999, Tognetti et al. 2001). ...
... These differences probably originated from anatomical acclimation to the soil water availability with likely smaller but more vessels in response to drier conditions as has been shown in poplar hybrids by Fichot et al. (2009 (Tognetti et al. 1999, Tognetti et al. 2001, Domec et al. 2010, Warren et al. 2011, none of them investigating interaction effects with soil drought (Table 3.4). In Mediterranean species, a small reduction in vulnerability to drought-induced cavitation was found for trees grown near a natural CO 2 -spring (Table 3.4) (Tognetti et al. 1999, Tognetti et al. 2001). In North-American deciduous forests, FACE-experiments showed no effect or an increase in vulnerability to drought-induced cavitation under elevated [CO 2 ] (Table 3. Upon flushing, we would have artificially increased the number of functional vessels by 50 % (Fig 3.2 , and centrifuge method in Alder et al. (1997). ...
... Hydraulic conductances and conductivities may be made 'specific' by normalizing by the area of supported leaves or the cross-sectional area of transport tissue. Results from the few direct studies of the effects of CO 2 e on xylem hydraulic conductivity and conductance have been contrasting, but in general, ring-porous species such as oaks are more affected than diffuse-porous species such as live oak, trembling aspen, birch or eucalyptus (Tognetti et al. 1999a;Gartner et al. 2003;Atwell et al. 2007;Phillips et al. 2011), and responses of gymnosperms are mixed (Maherali & DeLucia 2000;Tognetti et al. 2001;Domec et al. 2010), with a tendency towards an increase in specific conductivity and a decrease in whole-plant hydraulic conductance under CO 2 e (Fig. 3). For example, found that CO 2 e reduced whole-shoot leaf-specific hydraulic conductance in Q. robur but not in F. sylvatica. ...
... Although we might predict that the overall increase in biomass allocation and conduit size with CO 2 e may result in development of xylem being less resistant to xylem embolism, the results are mixed and sparse. In roughly half of the species studied, resistance to embolism as defined by the water potential at which 50% loss of specific hydraulic conductivity occurred (P 50 in Fig. 3) was unaffected by CO 2 e (Tognetti et al. 1999a(Tognetti et al. , 2001Domec et al. 2010). Very few studies have actually reported that CO 2 e -induced changes in xylem hydraulic characteristics were associated with an increaseinvulnerability to embolism andingeneral, those responses have been shown to vary by plant organs. ...
From 2011 to 2013, Texas experienced its worst drought in recorded history. This event provided a unique natural experiment to assess species-specific responses to extreme drought and mortality of four co-occurring woody species: Quercus fusiformis, Diospyros texana, Prosopis glandulosa and Juniperus ashei. We examined hypothesized mechanisms that could promote these species’ diverse mortality patterns using post-drought measurements on surviving trees coupled to retrospective process modeling. The species exhibited a wide range of gas exchange responses, hydraulic strategies, and mortality rates. Multiple proposed indices of mortality mechanisms were inconsistent with the observed mortality patterns across species, including measures of the degree of iso/anisohydry, photosynthesis, carbohydrate depletion, and hydraulic safety margins. Large losses of spring and summer whole-tree conductance (driven by belowground losses of conductance), and shallower rooting depths, were associated with species that exhibited greater mortality. Based on this retrospective analysis, we suggest that species more vulnerable to drought were more likely to have succumbed to hydraulic failure belowground.
... Hydraulic conductances and conductivities may be made 'specific' by normalizing by the area of supported leaves or the cross-sectional area of transport tissue. Results from the few direct studies of the effects of CO 2 e on xylem hydraulic conductivity and conductance have been contrasting, but in general, ring-porous species such as oaks are more affected than diffuse-porous species such as live oak, trembling aspen, birch or eucalyptus (Tognetti et al. 1999a;Gartner et al. 2003;Atwell et al. 2007;Phillips et al. 2011), and responses of gymnosperms are mixed (Maherali & DeLucia 2000;Tognetti et al. 2001;Domec et al. 2010), with a tendency towards an increase in specific conductivity and a decrease in whole-plant hydraulic conductance under CO 2 e (Fig. 3). For example, found that CO 2 e reduced whole-shoot leaf-specific hydraulic conductance in Q. robur but not in F. sylvatica. ...
... Although we might predict that the overall increase in biomass allocation and conduit size with CO 2 e may result in development of xylem being less resistant to xylem embolism, the results are mixed and sparse. In roughly half of the species studied, resistance to embolism as defined by the water potential at which 50% loss of specific hydraulic conductivity occurred (P 50 in Fig. 3) was unaffected by CO 2 e (Tognetti et al. 1999a(Tognetti et al. , 2001Domec et al. 2010). Very few studies have actually reported that CO 2 e -induced changes in xylem hydraulic characteristics were associated with an increaseinvulnerability to embolism andingeneral, those responses have been shown to vary by plant organs. ...
Here we summarize studies on the effects of elevated [CO2] (CO2e) on the structure and function of plant hydraulic architecture, and explore the implications of those changes using a model. Changes in conduit diameter and hydraulic conductance due to CO2e vary among species. Ring-porous species tend toward an increase in conduit size and consequently conductivity. The effect in diffuse-porous species is much more limited. In conifers the results are mixed, some species showing minor changes in xylem structure, while other studies found increases in tracheid density and diameter. Non-woody plants generally exhibited the reverse pattern with narrower conduits and lower hydraulic conductivity under CO2e. Further, changes in drought-resistance traits suggest that non-woody plants were the most affected by CO2e which may permit them to better resist drought-induced embolism under future conditions. Due to their complexity, acclimation in hydraulic traits in response to CO2e are difficult to interpret when relying solely on measurements. When we examined how the observed tissues-specific trends might alter plant function, our modelling results suggest that these hydraulic changes would lead to reduced conductance and more frequent drought stress in trees that develop under CO2e with a more pronounced effect in isohydric than in anisohydric species.
... One reason for this may be that plants must also tolerate the damaging effects of drought on xylem water transport, but little is known about the interactive effects of [CO 2 ] and water availability on this aspect of plant function. Hydraulic conductance can be increased when plants are grown under elevated vs current (Tognetti et al., 1999(Tognetti et al., , 2001 or current vs glacial [CO 2 ] (Quirk et al., 2013). This may result because lower transpiration rates at elevated [CO 2 ] allow plants to maintain water potential farther from the point where xylem embolism begins. ...
... [CO 2 ] + 1.26, R 2 = 0.87, P = 0.0216). For xeric shrubs (b), however, there appears to be no change in K l with increasing [CO 2 ] (Larrea tridentata (Huxman et al., 1999), Myrtus communis, Juniperus communis and Erica arborea (Tognetti et al., 2001) (Phillips et al., 2011) and Eucalyptus pauciflora (Atwell et al., 2009), Quercus robur (Atkinson & Taylor, 1996;Heath et al., 1997) and Quercus mongolica (Eguchi et al., 2008); the dotted line represents a significant positive relationship for Eucalyptus sp., relative change in K l = 0.0011 [CO 2 ] + 0.66, R 2 = 0.69, P = 0.0104). For studies that measured more than one time point, only the last time point was included. ...
Changes in atmospheric carbon dioxide concentration ([ CO 2 ]) affect plant carbon/water tradeoffs, with implications for drought tolerance. Leaf‐level studies often indicate that drought tolerance may increase with rising [ CO 2 ], but integrated leaf and xylem responses are not well understood in this respect. In addition, the influence of the low [ CO 2 ] of the last glacial period on drought tolerance and xylem properties is not well understood.
We investigated the interactive effects of a broad range of [ CO 2 ] and plant water potentials on leaf function, xylem structure and function and the integration of leaf and xylem function in P haseolus vulgaris .
Elevated [ CO 2 ] decreased vessel implosion strength, reduced conduit‐specific hydraulic conductance, and compromised leaf‐specific xylem hydraulic conductance under moderate drought. By contrast, at glacial [ CO 2 ], transpiration was maintained under moderate drought via greater conduit‐specific and leaf‐specific hydraulic conductance in association with increased vessel implosion strength.
Our study involving the integration of leaf and xylem responses suggests that increasing [ CO 2 ] does not improve drought tolerance. We show that, under glacial conditions, changes in leaf and xylem properties could increase drought tolerance, while under future conditions, greater productivity may only occur when higher water use can be accommodated.
... Similarly, Maherali et al. (2006) also found a strong relationship between vulnerability to cavitation (in branches and roots) and leaf gas exchange rates in 14 co-occurring temperate tree species. Overall, our study supports that there is a functional link between different physiological strategies associated with species performance (i.e., hydraulic function, osmotic properties and carbon economics) and between leaf and stem (Tognetti et al. 2001, 2002, Nardini et al. 2012, thus supporting the single 'fast-slow' plant economics spectrum hypothesis (Reich 2014). We suggest that π tlp may play a central role in generating this spectrum because it is correlated with a suite of coordinated hydraulic and carbon economics traits. ...
Leaf turgor loss point (πtlp) indicates the capacity of a plant to maintain cell turgor pressure during dehydration, which has been proven to be strongly predictive of the plant response to drought. In this study, we compiled a data set of πtlp for 1752 woody plant individuals belonging to 389 species from nine major woody biomes in China, along with reduced sample size of hydraulic and leaf carbon economics data. We aimed to investigate the variation of πtlp across biomes varying in water availability. We also tested two hypotheses: (i) πtlp predicts leaf hydraulic safety margins and (ii) it is correlated with leaf carbon economics traits. Our results showed that there was a positive relationship between πtlp and aridity index: biomes from humid regions had less negative values than those from arid regions. This supports the idea that πtlp may reflect drought tolerance at the scale of woody biomes. As expected, πtlp was significantly positively correlated with leaf hydraulic safety margins that varied significantly across biomes, indicating that this trait may be useful in modelling changes of forest components in response to increasing drought. Moreover, πtlp was correlated with a suite of coordinated hydraulic and economics traits; therefore, it can be used to predict the position of a given species along the 'fast-slow' whole-plant economics spectrum. This study expands our understanding of the biological significance of πtlp not only in drought tolerance, but also in the plant economics spectrum.
... Indeed, despite the convergent behavior with respect to leaf functional and structural traits, Mediterranean shrubs and trees may show species-specific water relationships and hydraulic properties in response to climatic drivers, such as elevated atmospheric CO 2 concentration (Peñuelas et al., 2002;Tognetti et al., 2000Tognetti et al., , 2001; nevertheless, all these species display an intrinsic growth strategy that highly prioritizes water saving over carbon uptake (Tognetti and Peñuelas, 2003). No matter of the strategy, Mediterranean woody plants are highly tolerant of severe water stress and tissue dehydration despite the damage inflicted by extreme drought, and are adapted to warm and dry climates. ...
... Although amounts are small, they are sufficient to initiate reverse water flux throughout large trees and improve the water status of the entire plant, presumably including the root-zone. An impact sufficient to be seen at the whole plant level suggests that foliar uptake could contribute meaningfully to recharge of water stores within plants, repair of cavitated conduits (Tognetti et al. 2001), drive cell expansion and even lead to increased carbon fixation. In addition, the potential for nutrient fluxes into leaves from particulates deposited on leaf surfaces or dissolved in the fog itself (Azevedo & Morgan 1974;Weathers 1999;Weathers et al. 2000) is raised and warrants further study. ...
Fog is a defining feature of the coastal California redwood forest and fog inputs via canopy drip in summer can constitute 30% or more of the total water input each year. A great deal of occult precipitation (fog and light rain) is retained in redwood canopies, which have some of the largest leaf area indices known (Westman & Whittaker, Journal of Ecology 63, 493–520, 1975). An investigation was carried out to determine whether some fraction of intercepted fog water might be directly absorbed through leaf surfaces and if so, the importance of this to the water relations physiology of coast redwood, Sequoia sempervirens. An array of complimentary techniques were adopted to demonstrate that fog is absorbed directly by S. sempervirens foliage. Xylem sap transport reversed direction during heavy fog, with instantaneous flow rates in the direction of the soil peaking at approximately 5–7% of maximum transpiration rate. Isotopic analyses showed that up to 6% of a leaf's water content could be traced to a previous night's fog deposition, but this amount varied considerably depending on the age and water status of the leaves. Old leaves, which appear most able to absorb fog water were able to absorb distilled water when fully submersed at an average rate of 0.90 mmol m2 s−1, or about 80% of transpiration rates measured at the leaf level in the field. Sequoia sempervirens has poor stomatal control in response to a drying atmosphere, with rates of water loss on very dry nights up to 40% of midday summer values and rates above 10% being extremely common. Owing to this profligate water use behaviour of S. sempervirens, it appears that fog has a greater role in suppressing water loss from leaves, and thereby ameliorating daily water stress, than in providing supplemental water to foliar tissues per se. Although direct foliar absorption from fog inputs represents only a small fraction of the water used each day, fog's in reducing transpiration and rehydrating leaf tissues during the most active growth periods in summer may allow for greater seasonal carbon fixation and thus contribute to the very fast growth rates and great size of this species.
... water potentials declined with drought; altered elastic cell-wall properties giving greater capacity for water uptake from the soil; higher turgor potentials during a mid-season drought (soil water content approaching 12%); and higher (less negative) pre-dawn and midday water potentials especially between July and September when drought was severe. Tognetti et al. (2001) additionally found that juniper had a lower percentage of xylem embolism in drought conditions compared to the other two species, suggesting it is the most droughttolerant of the three species. Tognetti et al. (2000b) interpreted these data to indicate that plants growing near the CO 2 vent were either conserving soil water due to direc ...
Summary 1. This account reviews information on all aspects of the biology of Juniperus communis that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles : distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. 2. Juniperus communis (juniper) is an evergreen dioecious gymnosperm shrub with two main population centres in Britain, one on chalk downlands of southern England and the other in northern England and Scotland. British populations are divided into two main subspecies although there is overlap in genetic and morphological features. Subspecies communis varies from a spreading shrub to an erect tree characteristic of calcareous soils in southern England, various soils in the Scottish highlands, while ssp. nana is a small procumbent shrub, restricted to well-drained bogs and, more usually, rocky outcrops, generally as a minor component of upland heaths and montane scrub. Both subspecies are drought and frost tolerant, although sensitive to fire. A third subspecies, hemisphaerica , primarily found in mountains of southern Europe has two small populations on maritime cliffs in the UK. 3. Although not very palatable, J. communis is grazed by small and large mammals when food is short, particularly in winter. Its low palatability is derived from oils found in the needles, cones and wood, dominated by monoterpenes. These have been extensively used in folklore medicine and to flavour alcoholic drinks, and are being investigated for new medicinal uses. 4. Juniperus communis ssp. communis is a characteristic light-demanding invader of pasture but has declined due to agricultural expansion, erosion, overgrazing, fire and poor regeneration, such that it is now rare and threatened across lowland/southern Europe. Although susceptible to overgrazing, some grazing can be beneficial to create the open sward necessary for seedling establishment. Other limits to regeneration are: progressively ageing stands in which male plants predominate; increasing fragmentation of stands that reduces pollination efficacy; and high seed dormancy with consequent variable germinability.
... One possible explanation is that mucilage may play a significant role in regulating water transport in the plant. A reverse transpiration has already been shown in various plant species (Burgess and Dawson, 2004;Koch et al., 2004;Tognetti et al. 2001), and it has been hypothesized that water would be supplied to the plant not only by root uptake, but also by foliar absorption of moisture in the air. The foliar absorption of water would be facilitated by the presence of mucilage on the surface of leaves. ...
Mucilage is thought to play a role in salinity tolerance in certain halophytic species by regulating water ascent and ion transport. The localization and composition of mucilage in the halophyte Kosteletzkya virginica was therefore investigated. Plants were grown in a hydroponic system in the presence or absence of 100mM NaCl and regularly harvested for growth parameter assessment and mucilage analysis with the gas liquid chromatography method. NaCl treatment stimulated shoot growth and biomass accumulation, had little effect on shoot and root water content, and reduced leaf water potential (Psi(w)), osmotic potential (Psi(s)) as well as stomatal conductance (g(s)). Mucilage increased in shoot, stems and roots in response to salt stress. Furthermore, changes were also observed in neutral monosaccharide components. Levels of rhamnose and uronic acid increased with salinity. Staining with a 0.5% alcian blue solution revealed the presence of mucopolyssacharides in xylem vessels and salt-induced mucilaginous precipitates on the leaf abaxial surface. Determination of ion concentrations showed that a significant increase of Na(+) and a decrease of K(+) and Ca(2+) simultaneously occurred in tissues and in mucilage under salt stress. Considering the high proportion of rhamnose and uronic acid in stem mucilage, we suggest that the pectic polysaccharide could be involved in Na(+) fixation, though only a minor fraction of accumulated sodium appeared to be firmly bound to mucilage.
... These effects of fog on transpiration that we observed do not preclude the possibility of other associated phenomena not investigated in our study such as water uptake by foliar absorption, for e.g. repair of cavitated conduits in E. arborea (Tognetti et al. 2001) and other laurel species (Č erma´k et al. 2002). For example, Breazeale et al. (1950) observed cuticular absorption of water from a saturated atmosphere by leaves of tomato plants and Burgess and Dawson (2004) reported that fog droplets deposited on leaves of S. sempervirens trees may be incorporated by the foliar absorption. ...
The ecophysiologic role of fog in the evergreen heath-laurel 'laurisilva' cloud forests of the Canary Islands has not been unequivocally demonstrated, although it is generally assumed that fog water is important for the survival and the distribution of this relict paleoecosystem of the North Atlantic Macaronesian archipelagos. To determine the role of fog in this ecosystem, we combined direct transpiration measurements of heath-laurel tree species, obtained with Granier's heat dissipation probes, with micrometeorological and artificial fog collection measurements carried out in a 43.7-ha watershed located in the Garajonay National Park (La Gomera, Canary Islands, Spain) over a 10-month period. Median ambient temperature spanned from 7 to 15 degrees C under foggy conditions whereas higher values, ranging from 9 to 21 degrees C, were registered during fog-free periods. Additionally, during the periods when fog water was collected, global solar radiation values were linearly related (r2=0.831) to those under fog-free conditions, such that there was a 75+/-1% reduction in median radiation in response to fog. Fog events greatly reduced median diurnal tree transpiration, with rates about 30 times lower than that during fog-free conditions and approximating the nighttime rates in both species studied (the needle-like leaf Erica arborea L. and the broadleaf Myrica faya Ait.). This large decrease in transpiration in response to fog was independent of the time of the day, tree size and species and micrometeorological status, both when expressed on a median basis and in cumulative terms for the entire 10-month measuring period. We conclude that, in contrast to the turbulent deposition of fog water droplets on the heath-laurel species, which may be regarded as a localized hydrological phenomenon that is important for high-altitude wind-exposed E. arborea trees, the cooler, wetter and shaded microenvironment provided by the cloud immersion belt represents a large-scale effect that is crucial for reducing the transpirational water loss of trees that have profligate water use, such as those of the 'laurisilva'.
... Although Ainsworth and Long (2005) observed a 20% average decrease in g s with elevated [CO 2 ] for 40 plant species, this magnitude of response is not generally observed in conifers (Teskey 1995, Tissue et al. 1997, Pataki et al. 1998, Maherali and DeLucia 2000, Medlyn et al. 2001) and is less pronounced under well-watered conditions (Medlyn et al. 2001, Ainsworth and Long 2005), which could explain why a lack of g s response to elevated [CO 2 ] has been noted in experiments in coniferous boreal forests (Saxe et al. 1998, Sigurdsson et al. 2002), where water is generally not a limiting factor. Previous studies have also found no effects of elevated [CO 2 ] on conifer hydraulic characteristics, such as K l , K Ψ and A s :A l (Pataki et al. 1998a, Maherali and Delucia 2000, Tognetti et al. 2001, McCarthy et al. 2007), and only minor effects on tracheid morphology (Kostiainen et al. 2004, McCarthy et al. 2006). We investigated the effects of a 13-year combined fertilization and irrigation treatment and 3 years of exposure to ele- vated [CO 2 ] on stomatal regulation of Norway spruce (Picea abies (L.) Karst.) ...
Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO(2) concentration ([CO(2)]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7 m (67%) and basal diameter by 8 cm (68%); the short-term elevated [CO(2)] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as approximately 7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by approximately 30%. Crown averaged stomatal conductance (g(s)) was described by a Jarvis-type model. The addition of a stomatal response time constant (tau) and total capacitance of stored water (C(tot)) improved the fit of the model. Model estimates for C(tot) scaled with sapwood volume of the bole in fertilized trees. Hydraulic support-defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (K(l)DeltaPsi) -was estimated from height, sapwood-to-leaf area ratio (A(s):A(l)) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g(s) at reference conditions for trees across nutrient and [CO(2)] treatments. Removal of approximately 50% of A(l) from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g(s)) after pruning scales inversely with K(l)DeltaPsi, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A(s):A(l).
Direct transpiration measurements of tree species obtained with Granier’s heat dissipation probes, combined with micrometeorological data were used to derive tree conductance estimates in a cloud-immersed wax myrtle-tree heath forest located in the Garajonay National Park (La Gomera, Canary Islands, Spain). The one-year period time series were analysed distinguishing between foggy and fog-free conditions in order to evaluate the vegetation response to fog. The presence of fog was found to reduce transpiration (T) in both species investigated (E. arborea and M. faya), such that the yearly medians of the hourly T values were 6–15.6 times higher when no fog was present as compared to cloud immersed periods. By contrast the gc estimates exhibit larger values in the afternoon during foggy conditions when compared in terms of the hourly gc medians at each time of the day, and were clearly greater on a daily basis. During foggy conditions, gc followed a monotonically increasing trend. Large variability of conductance estimates was observed across the ranges of micrometeorological conditions explored. The largest conductance values were associated with low solar radiations, air temperatures between 8 and 15 °C, low VPD (<0.5 kPa), and wind velocities of 2–4 m s⁻¹. Laurel forest trees, previously referred as following a profligate water use strategy, may thus profit from the foggy environment by maintaining the stomata opened during the day without significant water losses, while benefiting with the associated carbon gain.
Plants of Vitis vinifera (cv. Sangiovese) were submitted to elevated carbon dioxide concentrations in the field, using a FACE array; the experiment took place in a vineyard in Central Italy, throughout two growth seasons. The effects of fumigation on stem hydraulic properties and leaf water relations were evaluated. A significant decrease in stomatal conductance did not result in marked differences in leaf water potentials of FACE plants. Apparently, embolism formation showed to increase and hydraulic sufficiency to decrease after two years of exposure to elevated CO2. The allometric relationships between stem hydraulic conductivity, leaf area and stem cross-sectional area of xylem imply a physiological origin, though being unclearly affected by elevated CO2. The effect of elevated CO2 on water relations and hydraulic parameters of Sangiovese grapevine displayed variation reliant on fumigation time-span. Under these experimental conditions, changes in hydraulic properties in FACE plants did not provide a direct explanation for variations in leaf water relations in comparison with controls.
We studied leaf fluctuating asymmetry in Myrtus communis occurring around the "I Borboi" natural CO2 spring (Tuscany, Italy) and in a nearby control site, in a Mediterranean environment. Developmental instability, measured as leaf fluctuating asymmetry, is expected to be positively related to environmental stress and negatively to habitat quality. A gradiental decline in leaf width and leaf angle fluctuating asymmetry was found along the decreasing CO2 concentration from double to ambient. The correlation is ascribed to the positive effect of elevated CO2 on developmental stability. Probably, the adaptation process of individuals grown in close proximity to the CO2-enriched area took advantage of improved water use and carbon balance of M. communis plants.
To determine whether there are decreases in hydraulic function of a woody stem when it has increased mechanical loading, Quercus ilex L. seedlings were grown upright or inclined to force the production of large amounts of tension wood (TW). Seedlings were grown in ambient or elevated carbon dioxide concentrations ([CO2]) for 16-17 months to provide two sets of seedlings differing in growth rates and allocation patterns. In both CO2 environments, inclination caused formation of large amounts of TW at the base and mid-section of most stems, but not at the stem tips. Contrary to expectation, there were no significant effects of stem inclination or amount of TW on specific conductivity (k(s)) or vulnerability to embolism. Samples with high amounts of TW had higher vessel frequency, similar average vessel lumen area, similar vessel lumen fraction (6% of the transverse area), elevated frequency of vessels in the smallest diameter class, and higher wood density than samples with very little TW. Samples from seedlings in the elevated [CO2] treatment had similar vessel frequency, larger average vessel lumen area (caused by a higher frequency of large-diameter vessels), similar vessel lumen fraction, and similar wood density as samples from seedlings in the ambient [CO2] treatment. There was a strong position effect: the highest wood density and lowest ks were at the stem base, intermediate values were at the middle, and the lowest density and highest ks were at the stem tip. We conclude that, in a species that uses different cells for mechanical support and water transport, there can be large modifications in performance of the mechanical function through TW formation without impacting the water transport functions-ks and vulnerability to embolism.
Some species of coastal sage and chaparral shrubs of California are extremely tolerant of tissue dehydration, surviving water potentials as low as -9 MPa during dry summer months. Such low water potentials (high tensions on xylem water) are known to cause severe embolism formation in the xylem vessels of woody plants, blocking water transport and potentially causing shoot dieback. Thus drought hardy species of coastal sage and chaparral are either extremely resistant to water stress-induced embolism or they become severely embolized during summer drought. We compared the seasonal changes in xylem water potential and xylem embolism (percent loss in hydraulic conductivity of stem segments due to air emboli) between co-occurring Salvia mellifera (coastal sage) and Ceanothus megacarpus (chaparral) growing in the Santa Monica Mountains of southern California. We also determined the relative sensitivity of each species to water stress-induced embolism by artificially dehydrating branches and measuring percent loss in hydraulic conductivity of xylem tissues at a given water potential. We found that both species experienced the same minimum in seasonal water potentials (-8 MPa) but the xylem of S. mellifera lost 78% in hydraulic conductivity whereas the xylem of C. megacarpus lost only 17% in hydraulic conductivity. These values for a natural plant community were within 10% of those predicted by our artificial dehydration curves. Our estimate of susceptibility to water stress-induced embolism indicated that 50% loss in hydraulic conductivity would occur at -4.5 MPa for S. mellifera but at -11 MPa for C. megacarpus. Irrigation of S. mellifera for one summer reduced loss in conductivity from 78 to 38% and increased leaf areas 10-fold, indicating that xylem embolism and leaf drop were drought induced. Our results show that xylem tissues of S. mellifera are more sensitive to water stress and tissue dehydration than those of co-occurring C. megacarpus. The observed ability of S. mellifera to inhabit drier sites than C. megacarpus may result from drought deciduousness in summer and high growth rates in spring that facilitate rapid construction of new xylem and leaf tissues. It may be that facultative drought deciduousness in coastal sage is tightly coupled to drought-induced embolism of xylem tissues.
Aboveground biomass allocation of Pinus ponderosa on hydrothermally altered andesite in montane and desert climates was measured. Trees from montane climates had higher leaf mass per unit cross-sectional area of sapwood than trees from desert climates, suggesting a function response to differences in climate. Results also indicate that sapwood mass:leaf mass ratios of P. ponderosa may increase [approx]50% with a 5[degrees]C change in mean growing-season temperature. High proportional allocation of biomass to sapwood may improve water relations of P. ponderosa, but because sapwood contains living parenchyma, respiratory costs may be high. Simulated montane trees were 46-52% taller than desert trees, and montane trees 10 cm in dbh had twice the total aboveground mass of desert counterparts. Simulated 50-cm montane and desert trees were almost identical in total mass, even though the montane tree was 46% taller. The predicted proportion of biomass allocated to bole sapwood increased with size for both montane and desert models; however, the 50-cm desert model contained 8% more total sapwood mass than the taller montane model. Biomass of primary and secondary branches differed considerably. The 50-cm desert model had twice as much biomass in primary branches, whereas the montane model had 3 times more biomass in secondary branches than the desert model. For 10-cm trees of the desert and montane models 29 and 33% of the biomass were leaves, respectively. In larger trees, leaf allocation decreased to 5 and 7% for desert and montane models, respectively. The effects of climate on biomass allocation such as reported here, and corresponding changes in whole-plant assimilation rates must be incorporated into growth-response models used to predict future fluctuations in forest productivity due to global climate change. 35 refs., 3 figs., 3 tabs.
The stomatal response to CO2 is important in understanding stomatal physiology, and important in understanding vegetation-atmosphere exchanges at all
scales from the individual plant up to global vegetation. Despite the long history of experiments on stomatal responses to
CO2 there are still considerable uncertainties in both these tasks. The difficulty in understanding differences in stomatal conductance
between plants grown for any length of time in different CO2 atmospheres is stressed because of the many other possible changes in the plants' carbohydrate, nutrient and water relations.
The other key issues that are highlighted are: whether stomata acclimate to CO2 either in parallel with any mesophyll photosynthetic acclimation or independently of changes in the mesophyll; whether stomata
on different leaf surfaces respond to CO2 similarly; and whether reported changes in stomatal frequency are important to leaf gas exchange. The need for direct examination
of stomatal sensitivity of plants grown in different CO2 concentrations is stressed.
Analysis of energy partitioning between defensive investments and growth in woody plants indicates that increasing a tree's life-span should require increased energy investment in protective measures such as thick bark and defensive chemicals. Increased investment in such defenses, however, logically must slow down the growth rate, thereby raising the mortality rate for juveniles in competition for height growth. Early reproduction should also reduce the growth rate. It is hypothesized that rapid growth can substitute for these defenses, but the consequence is rapid decline upon reaching maturity. These predictions are tested with data compiled from the literature for 159 species of North American trees. Data analysis supports predictions. Longevity of angiosperms, but not of gymnosperms was correlated with increased investment in defenses as measured by volumetric heat content of the wood. Wood density was not as good a measure. Longevity of gymnosperms was predicted by resistance to wood decay. For both taxa there was a negative correlation between growth rate and longevity, supporting the hypothesis of growth trade-offs. Age of sexual maturity was closely predicted by longevity in angiosperms. There was no such relationship for conifers as a whole, though there was for pines. The lack of relationship for all conifers might be explained by (i) variation in reproductive opportunities for young trees of different species, or (ii) variation in growth rates of young trees in certain adverse habitats occupied by conifers.
Sites in Northland with mineral springs were examined for their potential as experimental areas to study the effects of elevated carbon dioxide (CO<sub>2</sub>) on grassland. A suitable site was defined as having: (1) grassland species; (2) cold springs; (3) high levels of gas flow; and (4) high concentrations of CO<sub>2</sub>. Two sites were selected for detailed study - Hakanoa Springs near Kamo and Waiare Spring near Kaeo. At Hakanoa, the vegetation was scrubby but at least 10 grassland species were present. Two vents released large volumes of CO<sub>2</sub> resulting in concentrations at 10 cm above ground level that ranged from 5000 μl/litre near the vent to 400 μl/ litre 10 m downwind. At Waiare, the spring was situated in a grazed grass paddock that contained 10 grass species as well as Trifolium repens and Lotus spp. There was little enrichment of CO<sub>2</sub> above the canopy but high concentrations were measured at mid-canopy height with a maximum value exceeding 2000 μl/litre. Because of the nature of the enrichment within, but not above the canopy, it appeared that the enrichment was from the soil. This was confirmed by measurements of soil CO<sub>2</sub> efflux that were consistently very high (greater than 9.9 g CO<sub>2</sub>/m<sup>2</sup> per h in some instances). The springs have existed for decades and the sites offer the potential to study plant material that has been exposed to elevated CO<sub>2</sub> for very long periods.
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.
This study investigated the effect of mechanical support on water transport properties and wood anatomy of stems of western poison oak, Toxicodendron diversilobum (T. & G.) Greene. This plant grows as a vine when support is present but as a shrub when support is absent. I compared vines and shrubs growing naturally in the field and those produced from cuttings of 11 source plants in a common garden. Huber value (xylem transverse area/distal leaf area) was lower but specific conductivity (water volume time-1 xylem transverse area-1 pressure gradient-1) was higher in supported than unsupported plants both in the field and the common garden. The opposing effects of Huber value and mon garden. The opposing effects of Huber value and specific conductivity resulted in the same values of leafspecific conductivity (LSC, water volume time-1 distal leaf area-1 pressure gradient-1) for supported and unsupported shoots at a given site. Therefore, for the same rates of evapotranspiration, supported and unsupported shoots will have the same pressure gradients in their stems. Vessel lumen composed a higher proportion of stem cross-section in supported than unsupported plants (due to slightly wider vessels and not to greater vessel density). These results suggest that the narrow stems of supported plants are compensated hydraulically by the production of wider vessels: at a given site, poison oak plants co-ordinate their leaf and xylem development such that their stems achieve the same overall conductive efficiencies (LSCs), regardless of support conditions.
Drought-deciduous and evergreen species coexist in tropical dry forests. Drought-deciduous species must cope with greater seasonal leaf water-potential fluctuations than evergreen species and this may increase their susceptibility to drought-induced xylem embolism. The relationship between water transport efficiency and leaf life-span were determined for both groups. They differed in seasonal changes of both, wood water content (W
c) and wood specific gravity (G). During the dry season, the W
c in drought-deciduous species declined and the minimum value was recorded when leaf fall was complete. At this time, the volumetric fraction of gas (V
g) increased indicating air entry into xylem vessels. In contrast, W
c, G and V
g changed only slightly throughout the year for evergreen species. Maximum hydraulic conductivity of drought-deciduous species was 2–6 times that of the evergreen species. but was severely reduced at leaf fall. In the evergreen species, similar water conductivities were measured during wet and dry seasons. The trade-off between xylem water transport capacity and leaf lifespan found in species coexisting in this forest reveals the existence of contrasting but successful adaptations to this environment. Drought-deciduous species maximize production in the short term with higher water transport efficiency which leads to the seasonal occurrence of embolisms. Conversely, the behaviour of evergreen species with reduced maximum efficiency is conservative but safe in relation to xylem embolism.
Recent studies on the phytohormonal regulation of seasonal cell-division activity in the cambium, primary-wall radial expansion
of cambial derivatives, differentiation of xylem cells, and growth of the cortex in forest trees of the north temperate zone
are reviewed. Indol-3-ylacetic acid (IAA, auxin) has been characterized by combined gas chromatography–mass spectrometry (GC–MS)
in the cambial region of Abies balsamea, Pinus densiflora, Pinus sylvestris and Quercus robur. All of the evidence supports the hypothesis that developing leaves and extending shoots are primary sources of IAA.
The rate of ethylene emanation varies among conifer species when adjoining phloem and cambial tissues are incubated in vitro.
The cambium from young cuttings of Abies balsamea produces more ethylene than that from older cuttings. Ethylene production by seven-year-old Abies balsamea cambium is substantially increased in vitro when the tissue is provided with exogenous 1-aminocyclopropane-1-carboxylic acid
and IAA. In response to elevated ethylene concentrations, cortex growth is accelerated in both hardwood and conifer seedlings.
Ethrel (2-chloroethylphosphonic acid) increases ray size and ray-cell number and promotes traumatic resin–canal development
in xylem. In Ulmus americana, endogenous ethylene concentrations are inversely correlated with cambial activity. Ethylene decreases vessel diameter in
Acer negundo, Acer platanoides and Ulmus americana. Several studies suggest that ethylene has a role in regulating reaction-wood formation in both conifers and hardwoods.
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
Over two seasons in c. 600 ppm CO2, oak had lower stomatal conductance in CO2-enriched compared to ambient air. Beech showed no response to CO2 concentration on sunny days. Mirroring this pattern, exposure to elevated CO2 reduced whole-shoot hydraulic conductance per unit leaf area in oak, but not in beech.
Several parameters related to the water relations of eight woody hemiepiphytes with different photosynthetic pathways were
studied in situ and in the laboratory on Barro Colorado Island, Panama. As a group, woody hemiepiphytes tended to have less conductive stems
per unit leaf area (lower kL) and invested less wood cross-section per unit leaf area compared to free-standing trees, while their specific conductivity
(Ks) was significantly higher. Among hemiepiphytes, there were significant differences between C3 and CAM (Crassulacean Acid Metabolism) species in respect to leaf characteristics, transpiration rates, diel patterns of
water flow through aerial roots, and in hydraulic architecture parameters. Average transpiration rates of the two Clusia species (C3-CAM) were lower by about an order of magnitude compared to the C3 species. In all C3 species, sap flow through aerial roots (F) closely followed transpiration (E), whereas E and F were decoupled in time in the C3-CAM species Clusia uvitana: considerable long-distance water flow occurred at night. The hydraulic efficiency of the other C3-CAM species, C. minor was the lowest of the five species investigated.
Annual variations in the water relations and stomatal response of Erica arborea, Myrtus communis and Juniperus communis occurring at a natural CO2 vent were analysed under Mediterranean field conditions. A distinct gradient of CO2concentration ([CO2]) exists between two sites near a natural CO2‐emitting vent, with higher [CO2] (700 μmol mol−1) in the proximity of the CO2 spring. Plants at the CO2 spring site have been growing for generations at elevated [CO2]. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits during
the summer drought were severe. Leaf conductance and water potential recovered after major rainfalls in September to predrought
values. Strong relationships between leaf conductance, predawn water potential, and leaf‐specific hydraulic resistance are
consistent with the role of stomata in regulating plant water status. Considerable between‐species variation in sensitivity
of water potentials and stomatal characters to elevated [CO2] were observed. Common to all the shrubs were a reduction in leaf conductance and an increase in water potentials in response
to elevated [CO2]. Elevated [CO2] decreased the sensitivity of leaf conductance to vapour pressure deficit. Morphological characters (including stomatal density
and degree of sclerophylly) showed site‐dependent variations, but degree and sign of such changes varied with the species
and/or the season. Measurements of discrimination against 13C provided evidence for long‐term decreases of water use efficiency in CO2 spring plants. Analysis of C isotope composition suggested that a downward adjustment of photosynthetic capacity may have
occurred under elevated [CO2]. Elevated [CO2] effects on water relations and leaf morphology persisted in the long term, but the three shrubs growing in the same environment
showed species‐specific responses.
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 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.
Potted plants of Ceratonia siliqua L., growing in a greenhouse, were used to detect xylem cavitation (in terms of ultrasound acoustic emissions AE) in internodes and node-to-petiole (N-P) junctions, after different periods of drought (9, 16 and 23 d). Diurnal AE were only 100 in internodes of watered (W) plants but 320, 1250 and 2460 in 9-, 16- and 23-d stressed ones. In N-P junctions, AE were only 15 to 20% with respect to internodes.
Stem perfusion with dye allowed measurement of the percentage of xylem conduit transverse area blocked by cavitation. This was 2% in internodes of W-plants and 5.2, 13.8 and 40.4% in those of 9-, 16- and 23-d stressed ones. In N-P junctions, 18.5% of the xylem conduit transverse area was blocked in the 23-d stressed plants only. The major resistance to cavitation exhibited by the N-P junctions is interpreted in terms of their greater number of narrow xylem conduits. The percentage of blocked xylem conduits within a range of diameters showed that the narrower a xylem conduit, the less likely it was that cavitation would occur. After rewatering, the release of the xylem blockage caused by cavitation occurred within 2 h. Our data suggest that C. siliqua can be considered to be a cavitation avoider, especially in its stem-to-leaf transition zones.
This chapter discusses the patterns of variation in xylem structure found within a woody plant, and emphasizes what is known and what is not known about the functional consequences of this variation for shoot water movement and mechanics. The first section reviews the typical structure of xylem within a tree. There is more information on softwood (gymnosperm) than hardwood (woody angiosperm) anatomy, and therefore many of the paradigms of wood anatomy are based on softwoods. The anatomical variation described here results in systematic variation in efficiency of water transport through the stem. The hydraulic properties discussed here are hydraulic conductivity (kh) and specific conductivity (ks) in the axial direction. Stems experience short- and long-term stress (force per unit area) from a variety of causes, such as gravity, wind, weight of snow or a maturing fruit, removal of a branch, partial failure of the anchorage system, or growth and development. This chapter has emphasized optima for mechanics and hydraulics separately, but the trade-offs between the two must be considered more fully. In the ranges of wood densities and water demands that plants have, we do not even know whether there are trade-offs between mechanics and hydraulics, partly because one must define the hydraulic and mechanical criteria in order to try such analysis.
Ecological trends for occurrence of certain vessel, tracheid and fibre characteristics. have been analysed for 505 species (belonging to 221 genera and 71 families) from Europe, Cyprus, and Madeira. Macroclimatic gradients from boreal, via temperate to mediterranean are strongly related with a decreasing incidence of scalariform perforations, (almost) exclusively solitary vessels, and fibre-tracheids (i. e., fibres with distinctly bordered pits). In this sequence the incidence of different vessel size classes (vessel dimorphism) and vascular tracheids increases. Ring-porous tendencies and spiral vessei thickenings have their peaks in the temperate zone. The subtropical flora of Madeira shows low values for the percentage of species with any of the above attributes.
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 Conducting Units: Tracheids and Vessels.- 2 The Vessel Network in the Stem.- 3 The Cohesion-Tension Theory of Sap Ascent.- 4 Xylem Dysfunction: When Cohesion Breaks Down.- 5 Hydraulic Architecture of Woody Shoots.- 6 Hydraulic Architecture of Whole Plants and Plant Performance.- 7 Other Functional Adaptations.- 8 Failure and "Senescence" of Xylem Function.- 9 Pathology of the Xylem.- References.
Current life history theory is dominated by the r-K continuum, originally developed for animals. Because plants are sessile and plastic in their growth, the r-K model is inadequate. A new model is developed for plants in which the major tradeoff is between fast growth and defense of tissues. Plants at either end of the spectrum may have large numbers of seeds, or not. Interactions between defensive investments and shade tolerance, growth rate, longevity, and age of maturity are explored with data on 158 species of North American trees.
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 vulnerability of xylem to embolism development in Rhododendron maximum L., an evergreen diffuse-porous shrub, was investigated in relation to the frequency of winter freeze–thaw cycles in high and low light sites of the Eastern US. Though the frequency of freeze–thaw cycles during the winter was lower in North Carolina than in Virginia, the hydraulic conductivity of 3-year-old branches was reduced by up to 60% by winter embolism development in North Carolina compared to less than 30% in Virginia. Generally, small vessel diameters and volumes were associated with a significant resistance to embolism formation resulting from repeated freeze–thaws of xylem sap. In stems grown in high light sites (gaps), larger vessel volumes, and greater diameter growth of stems were associated with a significantly higher degree of freeze–thaw embolism development than in those grown in the low light sites. Thus, the growth patterns of R. maximum stems, under conditions of higher light availability, rendered them more susceptible to freeze–thaw-induced embolisms. Vulnerability to drought-induced embolism in stems was not affected by light environment. Rhododendron maximum was relatively sensitive to drought-induced embolism because 50% loss of hydraulic conductivity occurred at a water potential of –2·2 MPa. The distribution and gas exchange of R. maximum are constrained by the dual effects of freeze–thaw cycles and drought on vascular function.
Climatic change may bring about increased aridity to large areas of Europe. Higher temperatures, larger water deficits and high light stress are likely to occur in conjunction with elevated atmospheric CO2. This raises the question whether a high CO2 concentration in the atmosphere can compensate for the decrease in carbon gain in water-stressed plants. The processes which determine dry matter production and the ways they are affected by soil water deficits are discussed. It is now well established that in most species and under most circumstances stomata are the main limiting factor to carbon uptake under water deficit, the photosynthetic machinery being highly resistant to dehydration. However, when other stresses are superimposed, a decline in photosynthetic capacity may be observed. In the short term, under drought conditions, the increase in CO2 in the atmosphere may diminish the importance of stomatal limitation for carbon assimilation, inhibit photorespiration, stimulate carbon partitioning to soluble sugars and increase water-use efficiency. Some recent evidence seems to indicate that under conditions of high irradiance, plants growing at elevated CO2 may develop protection towards photoinhibition, which might otherwise result in significant losses in plant production under stress conditions. In the longer term though, a negative acclimation of photosynthesis appears to occur in many species, an explanation for which still needs to be clearly identified. Similarly, the effects of extended exposure to elevated CO2 under arid conditions are not known. Plant production is more closely related to the integral of photosynthesis over time and total foliage area than to the instantaneous rates of the photosynthetic process. Water deficits result in a decrease in foliage area biomass and, therefore, in productivity. On the other hand, the increase in air temperature may result in more respiratory losses. However, experimental as well as simulatory evidence suggests that doubling CO2 concentration in the air may improve carbon assimilation and compensate partially for the negative effects of water stress even if we assume a down-regulation of the photosynthetic process as a result of acclimation to elevated CO2.
Plants of Quercus robur L. and Prunus avium L. ×P. pseudocerasus Lind, were grown in either ambient (350 vpm) or elevated (700 vpm) CO2. The intention was to examine the effects of elevated CO2 on the morphological and functional development of the stem. The relationships between stem longitudinal transport capacity and development were explored in several ways: stem hydraulic function was related to stem cross-sectional area, supplied leaf area and total stem vessel lumen area.
The mean total vessel number and the total vessel lumen area per stem, for both species, was determined from basal sections of the xylem. In Primus seedlings grown in different CO2 concentrations there was no significant change in the mean vessel size or number of vessels per stem. Quercus seedlings grown at elevated CO2 showed a significant increase in both vessel number and mean vessel size. When total stem vessel area was calculated it had increased twofold for Quercus plants grown at elevated CO2.
Measured stem hydraulic conductivity was shown to increase linearly with supplied leaf area, except in Quercus seedlings grown at elevated CO2. Stem hydraulic conductivity for Quercus seedlings grown at elevated CO2 did not change with the increase in supplied leaf area. This absence of an increase in the stem hydraulic conductivity appeared to relate to changes in total stem vessel area. Despite total stem vessel area being greater at elevated CO2 than that at ambient, it similarly did not increase with supplied leaf area.
The implications of this change in the relationship between leaf area and stem hydraulic conductivity are discussed with respect to the possible effects the change might have on the plant's water balance. The possible causes and significance of the changes in xylem anatomy are also considered in relation to direct effects caused by CO2 or indirect effects on changes in cambial maturity and tree growth.
In this study we analysed how shrub form changes in a broad range of light conditions. The total number of plants was 566, belonging to 8 species (Pistacia lentiscus, Phillyrea latifolia, Myrtus communis, Rhamnus alaternus, Erica arborea, Cistus monspeliensis, C. creticus, C. salvifolius) and 13 different sites. Plants were measured using different morphometric variables: total height, crown diameter, basal diameter, stem height, number of stems and stem diameter. The main trends of variation of shrub form were detected using Principal Component Analysis. The results show that P. lentiscus and M. communis represented a major horizontally oriented growth strategy (resembling a hemisphere or a flattened ellipsoid); E. arborea and P. latifolia developed a similar strategy in open sites but when closure increased they tended to grow in height (tall ellipsoid). Cistus spp. have no possibility of lateral expansion as do the fore-mentioned resprouters and in this case growth in height is favoured over growth in width (resembling a cone).
The potential role of stomatal closure in the control of xylem embolism is investigated by means of a simple model of hydraulic flow in plants. Maintenance of a maximally efficient conducting system requires the stomata to close in an appropriate fashion as evaporative demand increases in order to prevent shoot water potentials falling below the threshold value at which cavitations occur. The model showed that the optimal stomatal behaviour required depends on soil water availability. Further analysis of the model demonstrated that there could be certain circumstances where loss of a proportion of the conducting tissue by embolisms can, perhaps surprisingly, be beneficial in terms of maximizing stomatal aperture and hence short-term productivity. The results are discussed in relation to the signals controlling stomatal aperture, and it is shown that (1) optimal control cannot be obtained using information on leaf water potential alone, and (2) information relating to soil water potential is a necessary requirement for optimal control.
It is estimated that more than 100 geothermal CO2 springs exist in central-western Italy. Eight springs were selected in which the atmospheric CO2 concentrations were consistently observed to be above the current atmospheric average of 354μmol mol-1. CO2 concentration measurements at some of the springs are reported. The springs are described, and their major topographic and vegetational features are reported. Preliminary observations made on natural vegetation growing around the gas vents are then illustrated. An azonal pattern of vegetation distribution occurs around every CO2 spring regardless of soil type and phytoclimatic areas. This is composed of pioneer populations of a Northern Eurasiatic species (Agrostis canina L.) which is often associated with Scirpus lacustris L. The potential of these sites for studying the long-term response of vegetation to rising atmospheric CO2 concentrations is discussed.
Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO2 concentrations in the proximity of a natural CO2 spring. Trees at the spring site have been growing for generations in elevated atmospheric CO2 concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid ‘runaway embolism’. Trees at the CO2 spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO2 was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO2 spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO2 spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO2 spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO2 on plant water use at the organ and whole-tree scale.
The mechanism of water-stress-induced embolism of xylem was investigated in Malosma laurina and Heteromeles arbutifolia, two chaparral shrub species of southern California. We tested the hypothesis that the primary cause of xylem dysfunction in these species during dehydration was the pulling of air through the pores in the cell walls of vessels (pores in pit membranes) as a result of high tensions on xylem water. First, we constructed vulnerability-to-embolism curves for (i) excised branches that were increasingly dehydrated in the laboratory and (ii) hydrated branches exposed to increasing levels of external air pressure. Branches of M. laurina that were dehydrated became 50% embolized at a xylem pressure potential of -1.6 MPa, which is equal in magnitude but opposite in sign to the +1.6 MPa of external air pressure that caused 50% embolism in hydrated stems. Dehydrated and pressurized branches of H. arbutifolia reached a 50% level of embolism at -6.0 and +6.4 MPa, respectively. Secondly, polystyrene spheres ranging in diameter from 20 to 149 nm were perfused through hydrated stem segments to estimate the pore size in the vessel cell walls (pit membranes) of the two species. A 50% or greater reduction in hydraulic conductivity occurred in M. laurina at perfusions of 30, 42, 64 and 82 nm spheres and in H. arbutifolia at perfusions of 20 and 30 nm spheres. Application of the capillary equation to these pore diameters predicted 50% embolism at xylem tensions of -2.2 MPa for M. laurina and -6.7 MPa for H. arbutifolia, which are within 0.7 MPa of the actual values. Our results suggest that the size of pores in pit membranes may be a factor in determining both xylem efficiency and vulnerability to embolism in some chaparral species. H. arbutifolia, with smaller pores and narrower vessels, withstands lower water potentials but has lower transport efficiency. M. laurina, with wider pores and wider vessels, has a greater transport efficiency but requires a deeper root system to help avoid catastro-phically low water potentials.
Terrestrial higher plants exchange large amounts of CO 2 with the atmosphere each year; c. 15% of the atmospheric pool of C is assimilated in terrestrial‐plant photosynthesis each year, with an about equal amount returned to the atmosphere as CO 2 in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO 2 content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO 2 concentration might influence the rate of atmospheric CO 2 increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO 2 concentration may modulate plant and ecosystem responses to CO 2 concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO 2 concentration may be as significant to plant and ecosystem C balance as CO 2 concentration itself. Moreover, human activities such as deforestation and livestock grazing can have impacts on the C balance and structure of individual terrestrial ecosystems that far outweigh effects of increasing CO 2 concentration and climatic change.
In short‐term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO 2 concentration affects terrestrial higher plants in several ways. Elevated CO 2 can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO 2 concentration. Acclimation and adaptation of photosynthesis to increasing CO 2 concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO 2 concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO 2 concentration. Plant respiration may be inhibited by elevated CO 2 concentration, and although a naive C balance perspective would count this as a benefit to a plant, because respiration is essential for plant growth and health, an inhibition of respiration can be detrimental. The net effect on terrestrial plants of elevated atmospheric CO 2 concentration is generally an increase in growth and C accumulation in phytomass. Published estimations, and speculations about, the magnitude of global terrestrial‐plant growth responses to increasing atmospheric CO 2 concentration range from negligible to fantastic. Well‐reasoned analyses point to moderate global plant responses to CO 2 concentration. Transfer of C from plants to soils is likely to increase with elevated CO 2 concentrations because of greater plant growth, but quantitative effects of those increased inputs to soils on soil C pool sizes are unknown.
Whether increases in leaf‐level photosynthesis and short‐term plant growth stimulations caused by elevated atmospheric CO 2 concentration will have, by themselves, significant long‐term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO 2 concentration is a matter for speculation, not firm conclusion. Long‐term field studies of plant responses to elevated atmospheric CO 2 are needed. These will be expensive, difficult, and by definition, results will not be forthcoming for at least decades. Analyses of plants and ecosystems surrounding natural geological CO 2 degassing vents may provide the best surrogates for long‐term controlled experiments, and therefore the most relevant information pertaining to long‐term terrestrial‐plant responses to elevated CO 2 concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO 2 concentrations near vents is limited.
On the whole, terrestrial higher‐plant responses to increasing atmospheric CO 2 concentration probably act as negative feedbacks on atmospheric CO 2 concentration increases, but they cannot by themselves stop the fossil‐fuel‐oxidation‐driven increase in atmospheric CO 2 concentration. And, in the very long‐term, atmospheric CO 2 concentration is controlled by atmosphere‐ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO 2 concentration.
Ring widths of five Mediterranean forest tree species (Arbutus unedo, Fraxinus ornus, Quercus cerris, Quercus ilex and Quercus pubescens) growing close to a natural source of CO2 in Tuscany, Italy and at a nearby control site were compared. At the CO2-enriched site, trees have been growing for decades under elevated CO2 concentrations. They originated from parent trees that also grew under elevated CO2 in natural conditions, and they have been continuously exposed to elevated CO2 throughout their growth. Tree-ring series from each of the species were prepared. Assigning calendar dates to rings was difficult but possible, and ring-width series were built for all species. The ring-width data were analysed using a two-sided t-test to assess if there was a difference between the radial growth at the CO2-enriched site and the control site. The cumulative basal area at the same cambial age at both sites was also compared using a Wilcoxon test. Radial growth of trees at the CO2-enriched site was not significantly different from growth at the control site. For each species, year by year, radial growth at the CO2-enriched site was tested against the control site and significant differences were found in only a few years; these differences were not synchronous with extreme climatic events. The expected increase in above-ground productivity, as one of the ecosystem responses to increasing CO2 during drought stress, was not observed in this Mediterranean woody plant community, despite being water-limited. Other resource limitations, such as low nutrient availability (common in the Mediterranean region), may have counteracted the positive effect of elevated CO2 under drought stress, or trees may have acclimated to the high CO2.
The seasonal patterns of xylem embolism and xylem transport properties in Quercus pubescens Willd. and Quercus ilex L. trees growing in a natural mixed coppice stand in conditions of severe water stress were investigated. Xylem embolism was evaluated in both dehydrating branches and in apical twigs during a whole year. Measurements of xylem water potential were conducted from predawn to sunset on selected sunny days. On the same days, diurnal courses of leaf conductance were monitored. Measurements of half-hourly sap flow were made by the heat-pulse technique throughout the summer. At the onset of summer, a sharp decrease in water potential was observed in both species. Full recovery of water potentials was observed for both species after the first major rainfall event in September. Both experienced serious embolism throughout the year, ranging between minima of c. 60% (expressed as percentage loss of hydraulic conductivity) after the rains in autumn and after bud burst in spring, and maxima of c. 80% during summer and after freezing-thawing events during the winter season. A significant negative linear relationship was found between water potential and xylem embolism in branches dehydrating in air for Q. pubescens and Q. ilex. Q. pubescens had greater efficiency in hydraulic transport (higher specific conductivity and leaf specific conductivity) by the xylem than Q. ilex. In June, leaf conductance was high early in the morning and decreased gradually during the day. Midday depression of leaf conductance, as a result of high evaporative demand combined with water deficit, was observed in both species. In August, leaf conductance of both species was greatly reduced, as water potential dropped to extremely low values, and the stomata were almost completely closed during the afternoon. No hysteresis resulting from plant capacitance was observed in the relationship between shoot water potential and sap flow. Q. pubescens exhibited very high values of whole-tree hydraulic resistance between July and September, whereas Q. ilex generally showed lower values. The effect of soil moisture depletion on the relationship between sap flow and shoot water potential appears as a lowering of water potential at zero flow. A significant decrease of whole-tree hydraulic resistance in both species was observed with the onset of the autumn, preceding the partial recovery of twig hydraulic conductivity. The results demonstrate that both Q. pubescens and Q. ilex, although highly tolerant of severe water stress and tissue dehydration, operate at the limits of safety which are surpassed under severe droughts, and prolonged climatic stress might predispose these Quercus species to decline.
The relative hydraulic conductivity (k) of xylem and resistance (R) to water flow through trunk, primary roots and branches in Picea abies trees growing under contrasting light conditions were investigated. The xylem permeability to water was measured by forcing 10 mM water solution of KC1 through excised wood specimens. From the values of k, the sapwood transverse area and the length of conducting segments, R of the whole trunk, branches and roots was calculated. The relative conductivity of xylem in open-grown trees exceeded that of shade-grown trees by 1.4–3.1 times, while k was closely correlated with the hydraulically effective radius (R
e) of the largest tracheids (R
2 was 0.85–0.94 for open- and 0.51–0.79 for shade-grown trees). Because of both a low k and a smaller sapwood area in shade-grown trees the resistance to water movement through their trunk, roots and branches was many times higher. The distribution of R between single segments of the water-conducting pathway differed considerably in trees from different sites. At high water status the largest share of the total resistance in open- as well as shade-grown trees resides in the apical part of the trunk. The contribution of the branches to total xylem resistance is supposed to increase with developing water deficit.
The leaf-specific hydraulic conductivity (K
L) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (A
L/A
S) and the specific hydraulic conductivity of xylem tissue (K
S). In environments with high atmospheric vapor pressure deficit (VPD), K
L may increase to support higher transpiration rates. We predicted that saplings of Acerrubrum and A.pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher K
L and lower A
L/A
S than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and K
S increased with sapling size for both species. In A. rubrum, K
S did not differ between the two environments but lower A
L/A
S (P=0.05, ANCOVA) led to higher K
L for gap-grown saplings (P < 0.05, ANCOVA). In A.
pensylvanicum, neither K
S, A
L/A
S, nor KL differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and
carbon assimilation of A.pensylvanicum. Maximum stomatal conductance for A.pensylvanicum increased with K
L (r
2=0.75, P < 0.05). A hypothetical large A.
pensylvanicum sapling (2 m tall) had 2.4 times higher K
L and 22 times greater daily carbon assimilation than a small (1 m tall) sapling. Size-related hydraulic limitations in A.pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A.pensylvanicum sapling size (r
2=0.69, P < 0.05). Calculations indicated that small A.pensylvanicum saplings (low K
L) could not transpire at the rate of large saplings (high K
L) without reaching theoretical thresholds for xylem embolism induction. The coordination between K
L and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also
limits transpiration. The K
S of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing
K
L. However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result
of the increase in K
L and K
S with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation
for small saplings in the gap environment.
In this paper, we have reviewed how the hydraulic design of trees influences the movement of water from roots to leaves. The hydraulic architecture of trees can limit their water relations, gas exchange throughout the crown of trees, the distribution of trees over different habitats and, perhaps, even tbe maximum height that a particular species can achieve. Parameters of particular importance include:
(1) tbe vulnerabihty of stems to drought induced cavitation events because cavitation reduces the hydraulic conductance of stems,
(2) the leaf specific conductivity of stems because it determines the pressure gradients and most negative water potentials needed to sustain evaporation from leaves,
(3) the water storage capacity of tissues because this might determine the ability of trees to survive long drought periods.
All of these parameters are determined by the structure and function of anatomical components of trees. Some of the ecological and physiological trade-offs of specific structures are discussed.
A modified version of a method that uses positive air pressures to determine the complete cavitation response of a single axis is presented. Application of the method to Betula occidentalis Hook, gave a cavitation response indistinguishable from that obtained by dehydration, thus verifying the technique and providing additional evidence that cavitation under tension occurs by air entry through interconduit pits. Incidentally, this also verified pressure-bomb estimates of xylem tension and confirmed the existence of large (i.e. >0·4 MPa) tensions in xylem, which have been questioned in recent pressure-probe studies. The air injection method was used to investigate variation within and amongst individuals of B. occidentalis. Within an individual, the average cavitation tension increased from 0·66±0·27 MPa in roots (3·9 to 10·7 mm diameter), to 1·17±0·10 MPa in trunks (12 to 16 mm diameter), to 1·36±0·04 MPa in twigs (3·9 to 5 mm diameter). Cavitation tension was negatively correlated with the hydraulically weighted mean of the vessel diameter, and was negatively correlated with the conductance of the xylem per xylem area. Native cavitation was within the range predicted from the measured cavitation response and in situ maximum xylem tensions: roots were significantly cavitated compared with minimal cavitation in trunks and twigs. Leaf turgor pressure declined to zero at the xylem tensions predicted to initiate cavitation in petiole xylem (1·5 MPa). Amongst individuals within B. occidentalis, average cavitation tension in the main axis varied from 0·90 to 1·90 MPa and showed no correlation with vessel diameter. The main axes of juveniles (2–3 years old) had significantly narrower vessel diameters than those of adults, but there was no difference in the average cavitation tension. However, juvenile xylem retained hydraulic conductance to a much higher xylem tension (3·25 MPa) than did adult xylem (2·25 MPa), which could facilitate drought survival during establishment.
We investigated the occurrence of freezing-induced cavitation in the evergreen desert shrub Larrea tridentata and compared it to co-occurring, winter-deciduous Prosopis velutina. Field measurements indicated that xylem sap in L. tridentata froze at temperatures below c. -5OC, and that this caused no measurable cavitation for minimum temperatures above -7OC. During the same period P. velutina cavitated almost completely. In the laboratory, we cooled stems of L. tridentata to temperatures ranging from -5 to -20OC, held them at temperature for 1 or 12 h, thawed the stems at a constant rate and measured cavitation by the decrease in hydraulic conductivity of stem segments. As observed in the field, freezing exotherms occurred at temperatures between -6.5 and -9OC and as long as temperatures were held above -11OC there was no change in hydraulic conductivity after thawing. However, when stems were cooled to between -11OC and -20OC, stem hydraulic conductivity decreased linearly with minimum temperature. Minimum temperatures between -16 and -20OC were sufficient to completely eliminate hydraulic conductance. Record ( gt 20 year) minimum isotherms in this same range of temperatures corresponded closely with the northern limit of L. tridentata in the Mojave and Sonoran deserts.
Xylem embolism was monitored from mid-winter to mid-summer in four co-occurring species: Betula cordifolia (Reg.) Fern., Fagus grandifolia Ehrh., Abies balsamea (L.) Mill., Picea rubens Sarg. The study site was a west-facing slope in the northern Green Mountains of Vermont, U.S.A.; Betula and conifers were sampled at 914 m; Fagus was collected at 827 m near its local altitudinal limit. Embolism was quantified by the percent the hydraulic conductivity of branch segments was below the maximum obtained following removal of air embolism in xylem conduits. Between early February and early May, the deciduous species averaged 60 to 84% embolism compared to 15 to 60% for the conifers. From April 24 to May 25, embolism in Betula dropped from 81 to 8%; this recovery was associated with root pressures up to 86 kPa as measured with manometers at the lower trunk. Betula trees in which root pressure was eliminated by overlapping saw cuts still showed 75% embolism in June; only 4% was present in control trees cut in a similar fashion after leaf flush. Root pressure was weak (3 kPa) and uncommon in Fagus, and trees remained 80% embolized through June showing considerable dieback. Journal Article
Elevated CO2 may affect litter quality of plants, and subsequently C and N cycling in terrestrial ecosystems, but changes in litter quality associated with elevated CO2 are poorly known. Abscised leaf litter of two oak species (Quercus cerris L., and Q. pubescens Willd.) exposed to long-term elevated CO2 around a natural CO2 spring in Tuscany (Italy) was used to study the impact of increasing concentration of atmospheric CO2 on litter quality and C and N turnover rates in a Mediterranean-type ecosystem. Litter samples were collected in an area with elevated CO2 (>500 ppm) and in an area with ambient CO2 concentration (360 ppm). Leaf samples were analysed for concentrations of total C, N, lignin, cellulose, acid detergent residue (ADR) and polphenol. The decomposition rate of litter was studied using a litter bag experiment (12 months) and laboratory incubations (3 months). In the laboratory incubations, N mineralization in litter samples was measured as well (125 days). Litter quality was expressed in terms of chemical composition and element ratios. None of the litter quality parameters was affected by elevated CO2 for the two Quercus species. Remaining mass in Q. cerris and Q. pubescens litter from elevated CO2 was similar to that from ambient conditions. C. mineralization in Q. pubescens litter from elevated CO2 was lower than that from ambient CO2, but the differnce was insignificant. This effect was not observed for Q. cerris. N. mineralization was higher from litter grown at elevated CO2, but this difference disappeared at the end of the incubation. Litter of Q. pubescens had a higher quality than Q. cerris, and indeed mineralized more rapidly in the laboratory, but not under field conditions.
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.
We investigated how proximity to natural CO2 springs affected the seasonal patterns of xylem embolism in Quercus ilex L., Quercus pubescens Willd., Fraxinus ornus L., Populus tremula L. and Arbutus unedo L., which differ in leaf phenology and wood anatomy. Xylem embolism was evaluated in both artificially dehydrated branches
and in hydrated apical branches collected at monthly intervals during a 20-month sampling period. Initial specific hydraulic
conductivity was also evaluated. We found species-dependent differences in xylem hydraulic properties in response to elevated
CO2 concentration. Populus tremula was the most embolized and A. unedo was the least embolized of the species examined. Effects of elevated CO2 were significant in Q. pubescens, P. tremula and A. unedo, whereas the overall response to elevated CO2 was less evident in F. ornus and Q. ilex. Specific hydraulic conductivity differed among species but not between sites, although the interaction between species and
site was significant. Differences in xylem vulnerability between trees growing near to the CO2 spring and those growing in control areas were small. Although differences in hydraulic properties in response to elevated
CO2 concentration were small, they may be of great importance in determining future community composition in Mediterranean-type
forest ecosystems. The possible causes and ecological significance of such differences are discussed in relation to elevated
CO2 concentration and other environmental conditions.
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.
Northern red oak (Quercus rubra L.) and yellow-poplar (Liriodendron tulipifera L.) were grown for two years in full sunlight (unshaded) or 20% of full sunlight (shaded) under either well-watered or drought conditions. There was a close association between evaporative flux (in situ) and leaf-specific conductivity (LSC) in stem segments of both species. Shaded, drought-stressed seedlings of both species had significantly reduced leaf area, evaporative flux, volume flow rate in xylem, flow velocity, potentially functional xylem area, and LSC than unshaded, well-watered seedlings. Reductions in LSC associated with drought or shade were similar for both species; and within a treatment, both species had similar LSC. Species differed in the manner of LSC adjustment to drought and shade. Reductions in leaf area associated with drought or shade were accompanied primarily by reductions in potentially functional xylem area for L. tulipifera, and by reductions in flow velocity for Q. rubra. These results suggest (1) the existence of a homeostatic balance between evaporative flux and LSC, (2) that species with widely different growth patterns and xylem anatomies may develop similar LSC within the same environment, and (3) a possible hydraulic basis for differences in habitat between ring- and diffuse-porous species.