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Tamm review: Forest understorey and overstorey interactions: So much more than just light interception by trees
Abstract
Plant interactions play a key role in forest ecosystem dynamics. The tallest plants, namely the overstorey trees, are obvious major drivers, particularly in competition for light. This process has already been amply described. However, the role played by lower strata has often been underestimated. In this review, we first briefly recall the role of over- and understoreys in structuring forest microclimate, mostly through light sharing. We then focus on belowground interactions between over- and understorey, where knowledge is more piecemeal, partly because of measurement difficulties. Even so, some studies show that competition for water and nutrients by the overstorey controls the development of understorey vegetation much more than competition for light. The reverse (overstorey limitation by the understorey) has also been encountered, but has been much less well researched. We also address the involvement of mycorrhizae, specifically their role in alleviating overstorey drought stress and contributing to nutrient cycling. We go on to show how another example of key ecosystem engineers, large mammalian herbivores, shape above- and belowground resources and intervene in over- and understorey interactions. In conclusion, for a better understanding of forest dynamics and adapted management, particularly in the context of global climate change, we advocate taking account not only of trees but of all forest components. Belowground processes need more research. The roles of mycorrhizal networks, root exudates, microbiota, and chemical cues need to be further explored to gain a finer understanding of the interactions between over- and understorey.
... Some studies have demonstrated negative effects of plantations on biodiversity, mainly when compared to primary forests (Potton 1994;Fitzerbert et al. 2008;Gomes-Gonzales et al. 2020). However, it is generally understood that management interventions can enhance stand development by improving light, nutrients, and water availability (Balandier et al. 2006(Balandier et al. , 2022b. Therefore, plantations, if appropriately managed through manipulation of spatiotemporal heterogeneity and by mitigating natural disturbances, could help enhance biodiversity at a landscape scale (Lindenmayer et al. 2010;Horák et al. 2019). ...
... While herbicide applications, or other methods used to control competing vegetation, simultaneously inhibit under-story plants and enhance planted tree survival and growth in the early stage of stand development, light availability may also limit understory development after crown closure (Wagner et al. 2011;Valladares et al. 2016). In addition, overstory trees increase their ability to access soil water and nutrients relative to understory vegetation following crown closure (Balandier et al. 2022b). In this study, the crown closure at both sites occurred at about age 12 when pre-commercial thinning was conducted in HI and HFI plots. ...
We address the concerns that managed forest plantations may cause reductions of diversity of understory vegetation. We performed inventories of two mature ponderosa pine plantations in which multiple treatments were applied during plantation establishment. At stand age 35, we measured tree heights and diameters, understory plant cover and diversity, soil nutrients and chemical properties, and soil microbial biomass and diversity with phospholipid fatty acid. We found a significant, positive effect from both herbicide (H) and fertilizer (F) applications on subsequent overstory tree growth and development (P < 0.05); insecticide (I) effect was minimal. We observed negative effects on understory plant diversity decades later when herbicide was applied during stand establishment. However, lower plant diversity and ground cover appeared to have been caused primarily from overstory canopy closure, supported by the increased understory cover and diversity observed in the HI and HFI plots that had been thinned at age 12. Similarly, while fertilizer increased tree growth at both sites, it only negatively affected understory plants at the higher quality site. We did not find significant influence of treatment effects on soil nutrients and microbial communities. Therefore, to mitigate the potential loss of understory biodiversity in plantations, foresters can manage overstory trees with traditional pre-commercial thinning techniques and early tending.
... So far, past studies have yielded contradictory conclusions regarding climatic controls on the growth of understory versus overstory trees, but causal mechanisms remain poorly understood (Balandier et al., 2022;He et al., 2005;Orwig & Abrams, 1997). ...
... Specifically, the interception of solar radiation by the overstory and greater competition with neighboring trees have been reported to reduce the beneficial effects of warming on understory growth (Chen et al., 1999;Davis et al., 2019;Onoda et al., 2014). To the contrary, the greater positive warming effects observed for understory jack pine may be caused by their faster growth rates at younger ages and interplay with positive diversity effect from the overstory (Balandier et al., 2022;Davis et al., 2019). However, we acknowledge that the possible underlying mechanisms driving the observed trends in our study remain unclear, ...
Reports of forest sensitivity to climate change are based largely on the study of overstory trees, which contribute significantly to forest growth and wood supply. However, juveniles in the understory are also critical to predict future forest dynamics and demographics, but their sensitivity to climate remains less known. In this study, we applied boosted regression tree analysis to compare the sensitivity of understory and overstory trees for the 10 most common tree species in eastern North America using growth information from an unprecedented network of nearly 1.5 million tree records from 20,174 widely distributed, permanent sample plots across Canada and the United States. Fitted models were then used to project the near-term (2041-2070) growth for each canopy and tree species. We observed an overall positive effect of warming on tree growth for both canopies and most species, leading to an average of 7.8%-12.2% projected growth gains with climate change under RCP 4.5 and 8.5. The magnitude of these gains peaked in colder, northern areas for both canopies, while growth declines are projected for overstory trees in warmer, southern regions. Relative to overstory trees, understory tree growth was less positively affected by warming in northern regions, while displaying more positive responses in southern areas, likely driven by the buffering effect of the canopy from warming and climate extremes. Observed differences in climatic sensitivity between canopy positions underscore the importance of accounting for differential growth responses to climate between forest strata in future studies to improve ecological forecasts. Furthermore, latitudinal variation in the differential sensitivity of forest strata to climate reported here may help refine our comprehension of species range shift and changes in suitable habitat under climate change.
... The composition of vegetation within managed forests is influenced by a myriad of factors, ranging from climatic conditions and soil characteristics to human activities such as silvicultural practices and land management (Ameray et al., 2021). Different tree species have varying growth rates, competitive abilities, and interactions with understory vegetation, resulting in distinct ecological traits that influence ecosystem dynamics (Balandier et al., 2022). The success of these plantations is measured not only in terms of economic productivity but also in terms of their ability to maintain or enhance ecological integrity (Karr et al., 2022). ...
Forest vegetation is an important component of forest ecosystems, contributing to terrestrial plant diversity while also providing a variety of ecological services. In managed landscapes, plantations emerge as dominant kinds after stand-replacing disturbances. However, the dynamics of vegetation cover, diversity, and composition in plantation forests remains poorly understood in the subtropical region. Our study recorded a rich floral diversity with 173 angiosperm species, characterized by varying life forms and distinct flowering phenology. The uneven distribution of species across families demonstrated the complexity of the ecosystem, with Poaceae being dominant. Diversity patterns among different plantation types varied, with Dalbergia sissoo and Populus nigra plantations exhibiting higher species richness and diversity. Conversely, Eucalyptus camaldulensis and Morus alba plantations displayed lower diversity, emphasizing the influence of plantation type on biodiversity. Non-metric multidimensional scaling (nMDS) and PERMANOVA analyses revealed significant dissimilarity patterns of vegetation composition. Indicator species analysis identified unique compositions within each plantation type, emphasizing the importance of conserving specific types to protect indicator species and maintained ecological distinctiveness. Canonical Correspondence Analysis (CCA) demonstrated that road accessibility, stem cutting, and fire significantly influenced plant distribution patterns. The present research underscored the importance of considering plantation type in forest management for biodiversity conservation and highlighted the environmental variables' influence on the formation of plant communities. These results provided major implications for sustainable forest management and conservation efforts in tropical regions.
... However, most previous studies focused on the impacts of drought on the overstory tree leaf phenology but overlooked the understory plants Smith et al., 2019). The understory phenology is influenced by a more intricate interplay of climatic and biotic factors due to its disadvantage in competing with the overstory (Balandier et al., 2022;De Frenne et al., 2013). For instance, understory plants may undergo phenological escape due to changes in the light environment caused by the canopy (Fridley, 2012). ...
Subtropical forests, recognized for their intricate vertical canopy stratification, exhibit high resistance to extreme drought. However, the response of leaf phenology to drought in the species‐rich understory remains poorly understood. In this study, we constructed a digital camera system, amassing over 360,000 images through a 70% throughfall exclusion experiment, to explore the drought response of understory leaf phenology. The results revealed a significant advancement in understory leaf senescence phenology under drought, with 11.75 and 15.76 days for the start and end of the leaf‐falling event, respectively. Pre‐season temperature primarily regulated leaf development phenology, whereas soil water dominated the variability in leaf senescence phenology. Under drought conditions, temperature sensitivities for the end of leaf emergence decreased from −13.72 to −11.06 days °C ⁻¹ , with insignificance observed for the start of leaf emergence. Consequently, drought treatment shortened both the length of the growing season (15.69 days) and the peak growth season (9.80 days) for understory plants. Moreover, this study identified diverse responses among intraspecies and interspecies to drought, particularly during the leaf development phase. These findings underscore the pivotal role of water availability in shaping understory phenology patterns, especially in subtropical forests.
... the composition and distribution of understory vegetation (Kumar et al., 2018). Nevertheless, understory vegetation has a detrimental impact on sharing environmental resources (De Lombaerde et al., 2020) or poses an interference competition against target overstory trees (Balandier et al., 2022) when it covers more forest land area. Moreover, overstory management, mainly thinning and pruning practices, can directly and indirectly affect the formation of understory vegetation . ...
Introduction
Understory removal is a traditional practice in forest management to reduce fire risk and promote seedling regeneration. However, its effect on understory diversity, biomass and soil nutrients in temperate forest ecosystems is less known, which limits our assessment of the effectiveness of understory vegetation management.
Methods
We quantified the composition of the understory species, their diversity, and the biomass of the understory and factors driving changes in these parameters in primary mixed broad-leaved Pinus koraiensis forest (BKF), secondary Betula platyphylla forest (BF), and Larix gmelinii plantation (LF) in northeast China after a 5-year understory removal.
Results
After understory removal, the number of shrub and herb species in BKF and LF decreased, while the number of shrub species in BF increased significantly and that of herb species decreased; the species with strong light preference, Equisetum hyemale, Impatiens noli-tangere , and Filipendula Palmata , were dominant in the herb layer of the three forest types; Shannon–Wiener diversity, Pielou evenness, and Simpson diversity of the herb layer in LF increased significantly ( P < 0.05), while those of the shrub and herb layers in BF and LF showed no significant changes ( P > 0.05). The total understory biomass of understory of BKF and LF decreased by 0.94 t·hm ⁻² and 1.32 t·hm ⁻² , respectively, while that of BF increased by 1.31 t·hm ⁻² ; soil NH 4 + -N and total phosphorus (TP) were the key factors regulating understory vegetation diversity and biomass, respectively.
Conclusion
These results suggest that understory removal is a beneficial management strategy for increasing shrub biomass and diversity in secondary forests, while it should be avoided in primary forests and plantations to prevent the reduction of understory plant diversity and soil nutrient loss.
... This multi-pronged impact can stimulate change in understorey community composition that is less noticeable in the tree community's response to fire (Kasel et al., 2017;Swan et al., 2018). Understanding how changing fire regimes impact the understorey is critical given the importance of this community in regulating forest regeneration, nutrient and water cycling, and providing habitat and biodiversity (Gupta et al., 2015;Landuyt et al., 2019;Balandier et al., 2022). Floristic or structural simplification of the understorey may cause a loss of many of these roles, and threaten the stability and resilience held by a functionally diverse ecosystem (Coverdale and Davies, 2023). ...
... In temperate forests, understory plants beneath the tree canopy typically comprise the majority of plant diversity and fulfil many essential ecosystem services and roles (Balandier et al., 2022). In addition to providing food and habitat for animal species, a robust understory can be more resistant to non-native plant invasions (Johnstone et al., 2016;McGlone et al., 2011), promote native fire regimes (Moore et al., 1999) and control tree regeneration (Allen et al., 2002). ...
A combination of forest thinning followed by prescribed burning is widely applied in the western United States to increase ecosystem resistance and resilience to disturbances. Understory plant community responses may be driven by both management treatments and climatic factors. Thus, responses to treatments during a 20‐year megadrought have implications for the role of management in fostering adaptive capacity to climate change.
We used a network of five sites (600 plots) spanning an environmental gradient in ponderosa pine ( Pinus ponderosa ) forests of the American Southwest, an ecosystem that is broadly distributed and actively managed throughout the western United States. We used repeated long‐term monitoring data to quantify plant community responses to treatment 1–5‐, 6–10‐ and >10‐year post‐implementation. Specifically, we focussed on the effects of treatment and abiotic conditions on native and non‐native plant cover and species richness and the proportion of native species with northern (cool‐mesic) biogeographic affinities.
Overall, thinning and prescribed burning nearly doubled native cover and increased native species richness by about 50% relative to untreated controls. These effects persisted for over a decade after treatment, even under the influence of significant and persistent drought. Cover and richness were also greater on intermediate to wet sites. Finally, native species with northern biogeographic affinities were reduced for up to 5 years after treatment relative to those with southern (warm‐xeric) affinities, and in dry years, indicating that both management and interannual climate variability may foster shifts to plant communities that are more resilient to a warming climate.
Synthesis and applications . In ponderosa pine forests of the American Southwest, tree thinning followed by prescribed burning will generally promote restoration goals of increasing resilience to climate change by enhancing the diversity and abundance of native understory plant species, even during a persistent 20‐year megadrought.
... For example, variation in water availability influences nutrient filtration, storage and washing from the soil. This fact affects plant species distribution and modifying plant traits such as nutritional quality to herbivores (Chapin et al. 2002;Balandier et al. 2022), plant performance (i.e., growth, survival and reproduction) (Landsman and Thiel 2021) and the ecological interactions with canopy arthropods (Huberty and Denno 2004). In addition, arthropods actively select favorable microclimates to avoid desiccation and maximize their metabolic efficiency and optimize foraging activity Price 1988, 1992;Landsman and Thiel 2021). ...
The understanding of ecological mechanisms shaping canopy arthropods community assembly, remains as one of the main targets of ecology. It is necessary to elucidate how abiotic factors such as precipitation shape arthropod communities in oak canopy. We evaluated the changes of canopy arthropod diversity assemblages associated to Quercus castanea along a precipitation gradient in four study sites, where Q. castanea occurs. Five mature Q. castanea trees were selected per site to collect canopy arthropods using fogging techniques. We collected 7,447 arthropods representing 754 morphospecies grouped into 16 different orders associated to Q. castanea along the precipitation gradient. We found differences in arthropod richness and abundance as well as in guild composition along the precipitation gradient. The general pattern showed higher canopy arthropod diversity in dry sites compared to wet sites. Also, arthropod guild composition differed along the precipitation gradient; dry sites harbored higher abundance of phytophages and detritivores, while wet sites had higher richness of phytophages and predators. Local climatic variables such as temperature were related to the canopy of Q. castanea arthropod community in dry sites. Our findings confirm that at a local scale, water availability is an important abiotic factor that generates changes in the assemblies of canopy arthropod community of Q. castanea, harboring more arthropod diversity in dry sites. We highlight the importance of conserving oak species because Mexico is the main center of richness, diversification and endemism of oaks in the northern hemisphere, and they represent key species in temperate forest harboring a great diversity of unique canopy arthropods.
... Furthermore, in our studies we focused solely on the above-ground biomass of the ericaceous shrubs, due to our a priori focus on the shrubs' functionality as cervid forage. However, the shrubs are also subject to competition with trees and other plants below ground (Balandier et al., 2022;Mielke et al., 2022), and may adapt to canopy layer influences by altering the allocation of resources below ground (Nielsen et al., 2007;Jäderlund et al., 1997;Tolvanen, 1994). Such processes and responses were not addressed by our design. ...
Shrubs of the family Ericaceae, including bilberry (Vaccinium myrtillus), cowberry (V. vitisidaea) and heather (Calluna vulgaris), often dominate the understory in Eurasian boreal and cold-temperate forests. These ericaceous shrubs play crucial roles in forest ecosystems, acting as foundation species for forest communities, as well as influencing carbon cycles and the delivery of other ecosystem services. They also provide a key food resource for various wildlife, including cervids. However, over recent decades, forestry induced changes to the tree layer is thought to have reduced habitat suitability and thus the abundance of these shrubs in Sweden. Nevertheless, the precise nature of how the overstory impacts ericaceous shrubs remains largely unquantified. This thesis investigated the impact of the forest overstory on three species of ericaceous shrubs, focusing on the tree species composition and density of forest stands distributed along a large latitudinal gradient in Sweden. We carried out biomass collections and measurements, surveys, and a three-year browsing exclosure experiment. We quantified how forest stand characteristics shaped the above-ground biomass, growth, morphology, and macro-nutritional composition of these shrubs. We determined that Norway spruce (Picea abies) dominated stands had comparatively less above-ground biomass of the three focal species than Scots pine (Pinus sylvestris) stands; their plants were also shorter, and a larger proportion of their biomass provided suitable forage for cervids (I). These shorter shrubs also provided a larger proportion of new shoot biomass (II) and had a different macronutrient composition (IV) compared to shrubs in more open stands. These findings indicate that forest owners’ decisions regarding tree species composition and density have a strong influence on the biomass, morphology, and nutritional composition of these plant species in Sweden. Three years of exclusion to large cervids’ browsing did not reveal differences in the plants’ growth and morphology in pine dominated stands (III). In addition to providing new insights into the ecology of ericaceous shrubs, we also provide biometric functions to predict the above-ground and forage biomass of the three shrubs (I), and models to predict their production of annual shoot biomass (II). In summary, our results provide a better understanding of canopy influences on ericaceous shrubs growing in conifer forests. This thesis provides tools that can help improve the management of these shrub species, and greatly increase our ability to determine stand and landscape scale availability of this food resource for cervids. We consider our findings within the larger context of landscape scale management, and highlight the importance of ensuring the continued maintenance of these ericaceous shrubs and the diverse range of ecosystem services they provide.
... Understory vegetation was considered to be an important driver of forest ecological processes due to their faster nutrient turnover rate than woody plants Dhar et al. 2020), which makes understory vegetation crucial in forest ecosystems. However, although we can recognize this, it is often removed in plantation management to prevent it from competing for resources with target trees (Balandier et al. 2022). In contrast, the impact of understory vegetation on ecosystem processes remains poorly understood. ...
Background and aimsIncreased nitrogen (N) deposition may exacerbate soil phosphorus (P) deficiency, which alters soil greenhouse gas (GHG) emissions by changing soil properties and understory vegetation in subtropical forests. However, the effects of N addition, P addition, and understory vegetation interactions on soil carbon dioxide (CO2) and nitrous oxide (N2O) emissions in forest ecosystems and their underlying mechanisms remain unclear.Methods
We conducted a continuous N and P addition experiment in a subtropical Chinese fir plantation. The effects of N and P addition on soil CO2 and N2O emissions with different understory vegetation were examined using a structural equation model.ResultsThe addition of N and P did not directly affect soil CO2 emission of the understory, which was affected by understory species (F = 2.86; p = 0.05). N indirectly and positively affects soil CO2 emission through plant elements, and directly affect N2O emission of understory soil. Compared with CK, the addition of N2 promoted the N2O cumulative emission by 74.3%. N has a direct positive effect on soil N2O emissions, while P addition indirectly exerts a negative influence on N2O emissions through its impact on soil properties. Plant elements and soil properties were explained in 26.3% of soil CO2 emissions, of which 17.6% and 16.0% variations were explained by plant elements and soil properties, respectively. Leaf total N was the most important factor for predicting CO2 emissions.Conclusions
Our results suggested that soil CO2 emission was more dependent on plant elements, and soil N2O emission was directly dependent on N addition rather than plant elements. Thus, different patterns of soil GHG emissions and associated controls following N and P addition provided novel insights into predicting the effects of understory vegetation on climate change mitigation outcomes.
... Jumlah pohon yang ada pada tutupan lahan HHBK dan pinus pun lebih banyak dibandingkan dengan HHK dan semak. Balandier et al. (2022) menyatakan bahwa vegetasi tumbuhan bawah sebagai komponen penyimpan karbon dapat ditekan keberadaannya oleh vegetasi pohon di atas permukaan tanah karena persaingan dalam mendapatkan cahaya matahari dan kompetisi bawah tanah (air dan nutrisi). Keberadaan tumbuhan bawah juga dapat disebabkan oleh senyawa kimia yang dikeluarkan oleh suatu tanaman agar dapat bersaing untuk bertahan hidup seperti pinus. ...
Human activities and global warming cause forest fires that impact increasing carbon emissions, destroy forest biodiversity and destroy soil properties. This research is needed to examine carbon storage and soil physical properties on various types of land cover in the Cempaka Forest area, which will later be used as a basis for carrying out restoration and conservation actions. This study used four observation points: Timber Forest Products, Non-Timber Forest Products, Pine, and Shrubs land cover. Carbon storage was measured in the Cempaka forest using the standard RaCSA procedure. The results showed that land cover differences significantly affect the value of carbon storage, with the highest value for storing carbon being the land cover of HHK. However, the difference in land cover did not cause different results on the physical properties of the soil. Correlation and simple regression between the value of carbon storage and other parameters show that carbon storage is strongly influenced by tree biomass and litter, with respective values of 94% and 21%.
... Mangroves are the second-largest ecosystem service providers for the dependent coastal communities, next only to coral reef ecosystems. Located in a transition zone between marine and terrestrial ecosystems (Balandier et al. 2022), mangroves provide various ecosystem goods and services (Hilmi et al. 2021). Particularly, the scientific community and policymakers highly appreciated mangroves' carbon storage, fishery production, and coastal protection services in the recent decade (Kathiresan 2018;Ragavan et al. 2019;Salminah and Alviya 2019). ...
Natsir M, Ulya Z, Fitriani R. 2022. Mangrove forest utilization policies reconceptualized with a view to improving the regional economy in Aceh Tamiang District, Indonesia. Biodiversitas 23: 6570-6578. One of the Aceh's (Indonesia) districts with mangrove forests is Aceh Tamiang District. Manyak Payeud, Bendahara, Seureuwey, and Banda Mulia are the four sub-district in the Aceh Tamiang region that have the most potential tourist sites. However, due to the widespread illegal logging of mangroves carried out by the community to suit their daily requirements, the preservation of mangrove forests in the four sub-districts above is seriously damaged. As a result of illegal logging, mangroves are becoming less common in Aceh Tamiang District. Therefore, a unique policy, which can incorporate both community needs and economic development, is needed to manage the mangroves of Aceh Taminga District. The expectation is that mangrove management will enable Aceh Tamiang District to develop tourist attractions in this study. The normative juridical research methodology with SWOT analysis was applied in this study to understand the potential of mangrove management in improving the regional Economy in Aceh Tamiang District. The findings of this study are based on a SWOT analysis of four subdistricts in Aceh Tamiang District. The result suggests that the notion of forestry policy can support both local community interests and regional economic development. Additionally, it can be carried out through the Village Qanun to raise community awareness of the need to maintain and manage mangroves to enable the implementation of regional plans. To harmonize the understanding of the significance of mangroves as one of the regional tourist destinations, institutional coordination between the Aceh Tamiang district government and the provincial and central governments can also be used to realize the development of Aceh Tamiang tourism policies.
Trees and stands store large amounts of biomass, but this storage is dynamic in time and space. It depends on the species, stand structure, silvicultural systems, and silvicultural practices. Furthermore, interactions between the trees in the stands and forests and disturbances result in biomass variability. The forest systems biomass estimation sometimes does take into account this variability. Additionally, all harvests remove biomass to a smaller or larger extent from the forest systems. Their sustainability is dependent on the amount and biomass components removed. The biomass exports are related to the management goals and the harvest type. Overall, stem biomass exports have smaller impacts than whole tree harvest on the sustainability and resilience of forest systems. However, forest residues removal can be done to maintain the forest system sustainability as long as biomass components richer in nutrients are maintained, at least partially, in the forest systems.
Understory vegetation accounts for a large proportion of floral diversity. It provides various ecosystem functions and services, such as productivity, nutrient cycling, organic matter decomposition and ecosystem self-regeneration. This review summarizes the available literature on the current status and progress of the ten most studied branches of understory vegetation on both its structural and functional aspects based on global climate change and forest management practice. Future research directions and priorities for each branch is suggested, where understory vegetation in response to the interplay of multiple environmental factors and its long-term monitoring using ground-based surveys combined with more efficient modern techniques is highlighted, although the critical role of understory vegetation in ecosystem processes individually verified in the context of management practices or climate changes have been extensively investigated. In sum, this review provides insights into the effective management of the regeneration and restoration of forest ecosystems, as well as the maintenance of ecosystem multilevel structures, spatial patterns, and ecological functions.
In the context of increasing heat periods and recurrence of droughts, and thus higher soil water depletion, we explored and quantified the role of understorey vegetation in ecosystem evapotranspiration in boreal and temperate forests. We reviewed and analysed about 200 papers that explicitly gave figures of understorey vegetation evapotranspiration relative to different stand features and traits. Understorey vegetation accounted on average for one-third of total ecosystem evapotranspiration during the growing season. Overstorey leaf area index (LAI) is the main variable that drives understorey evapotranspiration through radiation interception. Most data show that below an overstorey LAI of 2–3, the contribution of the understorey vegetation to ecosystem evapotranspiration increases exponentially, following the exponential increase of the climatic demand, i.e. potential evapotranspiration. Different factors have the potential to modulate this effect such as species composition and phenology, root distribution, and interaction with droughts. Consequently, managers must be aware that depending on understorey species present on site and stand structure, understorey vegetation can contribute significantly to a negative stand water balance.
Oak regeneration is jeopardized by purple moor grass, a well-known competitive perennial grass in the temperate forests of Western Europe. Belowground interactions regarding resource acquisition and interference have been demonstrated and have led to new questions about the negative impact of purple moor grass on ectomycorrhizal colonization.
The objective was to examine the effects of moor grass on root system size and ectomycorrhization rate of oak seedlings as well as consequences on nitrogen content in oak and soil.
Oak seedlings and moor grass tufts were planted together or separately in pots under semi-controlled conditions (irrigated and natural light) and harvested one year after planting. Biomass, N content in shoot and root in oak and moor grass as well as number of lateral roots and ectomycorrhizal rate in oak were measured.
Biomass in both oak shoot and root was reduced when planting with moor grass. Concurrently, oak lateral roots number and ectomycorrhization rate decreased, along with a reduction in N content in mixed-grown oak.
An interference mechanism of moor grass is affecting oak seedlings performance through reduction in oak lateral roots number and its ectomycorrhization, observed in conjunction with a lower growth and N content in oak. By altering both oak roots and mycorrhizas, moor grass appears to be a species with a high allelopathic potential. More broadly, these results show the complexity of interspecific interactions that involve various ecological processes involving the soil microbial community and need to be explored in situ.
The nitrogen (N) economics of plants are generally described in terms of functional traits and how these affect N availability in a given environment. However, recent studies have shown that plant symbionts play a crucial role in plant N economics. A plant together with its symbiont can be considered as a meta‐organism, the holobiont. Plant‐associated symbionts are shaped by the plant, thereby extending the plant's phenotype. Decomposers also play an important role in plant N economics, yet are usually not included in the plant holobiont.
In this review, we show the important roles that both symbionts and decomposers play in plant N economics. We focus on how plants respond to fluctuating N availability in a complex interaction network, which includes the plant's strategies and its interactions and feedback loops with the soil biota and with neighbouring plants, through competition for N by exploitation and interference.
Synthesis. Plant N economics and the outcome of plant–plant interactions in a community cannot be fully described solely through the functional traits of plant individuals. Properties emerging from the interaction network bring new insights into plant N economics. Further research is now needed to gain a deeper understanding of plant N economics and resource economics in plant communities by integrating a broader extended plant phenotype.
Mature temperate woodlands are commonly dominated by ectomycorrhizal trees, whereas understory plants predominantly form arbuscular mycorrhizal associations. Due to differences in plant–fungus compatibility between canopy and ground layer vegetation the ‘mycorrhizal mediation hypothesis’ predicts that herbaceous plant establishment may be limited by a lack of suitable mycorrhizal fungal inoculum.
We examined plant species data for 103 woodlands across Great Britain recorded in 1971 and in 2000 to test whether herbaceous plant species richness was related to the proportion of arbuscular mycorrhizal woody plants. We compared the effect of mycorrhizal type with other important drivers of woodland plant species richness.
We found a positive effect of the relative abundance of arbuscular mycorrhizal woody plants on herbaceous plant species richness. The size of the observed effect was smaller than that of pH. Moreover, the effect persisted over time, despite many woodlands undergoing marked successional change and increased understorey shading.
This work supports the mycorrhizal mediation hypothesis in British woodlands and suggests that increased abundance of arbuscular mycorrhizal woody plants is associated with greater understory plant species richness.
Conventional conservation policies in Europe notably rely on the passive restoration of natural forest dynamics by setting aside forest areas to preserve forest biodiversity. However, since forest reserves cover only a small proportion of the territory, conservation policies also require complementary conservation efforts in managed forests in order to achieve the biodiversity targets set up in the Convention on Biological Diversity. Conservation measures also raise the question of large herbivore management in and around set‐asides, particularly regarding their impact on understory vegetation. Although many studies have separately analyzed the effects of forest management, management abandonment, and ungulate pressure on forest biodiversity, their joint effects have rarely been studied in a correlative framework. We studied 212 plots located in 15 strict forest reserves paired with adjacent managed forests in European France. We applied structural equation models to test the effects of management abandonment, stand structure, and ungulate pressure on the abundance, species richness, and diversity of herbaceous vascular plants and terricolous bryophytes. We showed that stand structure indices and plot‐level browsing pressure had direct and opposite effects on herbaceous vascular plant species diversity; these effects were linked with the light tolerance of the different species groups. Increasing canopy cover had an overall negative effect on herbaceous vascular plant abundance and species diversity. The effect was two to three times greater in magnitude than the positive effects of browsing pressure on herbaceous plants diversity. On the other hand, a high stand density index had a positive effect on the species richness and diversity of bryophytes, while browsing had no effect. Forest management abandonment had few direct effects on understory plant communities, and mainly indirectly affected herbaceous vascular plant and bryophyte abundance and species richness and diversity through changes in vertical stand structure. Our results show that conservation biologists should rely on foresters and hunters to lead the preservation of understory vegetation communities in managed forests since, respectively, they manipulate stand structure and regulate ungulate pressure. Their management actions should be adapted to the taxa at stake, since bryophytes and vascular plants respond differently to stand and ungulate factors.
Biological control of nutrient cycles is well documented in aquatic ecosystems, where consumer‐driven recycling by herbivores can significantly impact ecosystem stoichiometry. In contrast, little is known in terrestrial ecosystems, where there is evidence that herbivores can also impact ecosystem stoichiometry. I studied a stoichiometric model of the soil‐plant‐herbivore system. The model shows that herbivores influence the ecosystem stoichiometry mainly through the direct and indirect controls of ecosystem inputs and losses, in a more complex way than predicted by the classic consumer‐driven recycling theory. Overall, it shows that herbivores affect nutrient ratios in terrestrial ecosystems mostly independently of their own stoichiometric ratios, and that their impact may be different in forest versus grassland. The results highlight the sensitivity of terrestrial ecosystems to elusive actors, negligible in biomass but capable of modifying nutrient loss rates with major impacts on nutrient cycles and ecosystem stoichiometry. I addressed the impact of herbivores on nitrogen and phosphorus recycling in terrestrial ecosystems (forest and grassland) with a stoichiometric model. My results suggest that herbivores impact ecosystem stoichiometry mostly independently of their own stoichiometry, and may promote either N or P limitation depending on the propensity for loss of soil organic and mineral nitrogen relative to phosphorus, and on the N:P ratio exported by the herbivores from the ecosystem.
Plant–fungal associations strongly influence forest carbon and nitrogen cycling. The prevailing framework for understanding these relationships is through the relative abundance of arbuscular (AM) versus ectomycorrhizal (EcM) trees. Ericoid mycorrhizal (ErM) shrubs are also common in forests and interactions between co‐occurring ErM shrubs and AM and EcM trees could shift soil biogeochemical responses. Here we test hypotheses that the effects of ErM shrubs on soil carbon and nitrogen either extend or are redundant with those of EcM trees.
Using regional vegetation inventory data (>3,500 plot observations) we evaluated the frequency, richness and relative abundance of ErM plants in temperate forests in the eastern United States and examined their relationship with EcM plant cover. We then used surface soil (7 cm) data from 414 plots within a single forest to analyse relationships between ErM plant cover, relative EcM tree basal area and soil carbon and nitrogen concentrations while accounting for other biogeochemical controls, such as soil moisture.
At both scales, we found a positive relationship between ErM and EcM plants, and the majority of ErM plants were in the shrub layer. Within the forest site, ErM plants strongly modulated tree mycorrhizal dominance effects. We found negative relationships between EcM relative basal area and soil carbon and nitrogen concentrations, but these relationships were weak to negligible in the absence of ErM plants. Both EcM relative basal area and ErM plant cover were positively associated with the soil carbon‐to‐nitrogen ratio. However, this relationship was driven by relatively lower nitrogen for EcM trees and higher carbon for ErM plants. As such, the functional effects of ErM plants on soil biogeochemistry neither extended nor were redundant with those of EcM trees.
Synthesis. We found that ErM shrubs strongly influenced the relationship between tree mycorrhizal associations and soil biogeochemistry, and the effects of ErM shrubs and EcM trees on carbon and nitrogen were functionally distinct. Our findings suggest that ErM shrubs could confound interpretation of AM versus EcM tree effects in ecosystems where they co‐occur but also bolster growing calls to consider mycorrhizal functional types as variables that strongly influence forest biogeochemistry.
Large herbivores play a key role in terrestrial ecosystems, and their populations alter the plant composition in many ecosystems. One example is deer, whose populations in the Northern Hemisphere have increased over the last decades. This has resulted in increased browsing pressure on saplings and, as a consequence, has hampered forest regeneration in some areas. Such herbivore-induced changes in the regeneration of forest stands have changed their composition and biodiversity. Although there is an established link between the deer population density and the level of damage caused by browsing, the question remains open: What is the effect of increasing deer density over years on the regeneration of palatable and less browsing-resistant tree species? In this study, we investigated the relationships between the regeneration dynamics of five tree species (European beech Fagus sylvatica, silver fir Abies alba, sycamore Acer pseudoplatanus, hornbeam Carpinus betulus, and oak Quercus spp.) and browsing pressure in two years with low and high red deer (Cervus elaphus) density and at similar roe deer (Capreolus capreolus) density. We set up experimental plots in Roztocze National Park (central-eastern Poland), a protected area where natural predators of deer are constantly present. In addition to deer density, we analysed parameters that may affect browsing intensity: height of saplings, forage availability (sapling density), sapling diversity and light intensity (canopy openness). We found that increasing red deer density significantly increased browsing on all tree species, with saplings taller than 50 cm being under the strongest pressure. Moreover, higher deer density altered deer forage selection-it increased their selection for unpalatable European beech and decreased for highly preferred sycamore. Additionally, more species-diverse patches and closed canopy attracted deer and increased the probability of browsing on saplings. Contrary to our predictions, sapling density decreased browsing intensity. The study confirms that high ungulate density has significant effects on tree recruitment and may alter tree species composition of the forest stand. We provide clear evidence that deer herbivory plays a crucial role in tree regeneration in top-down processes in natural ecosystems.
Animals that disperse plant or fungal propagules following forest disturbances, including timber harvests, play an important role during forest regeneration by dispersing seeds or spores from intact communities to disturbed sites. Determining how complementary or redundant animal species are as dispersers of propagules is key to understanding the stability of dispersal dynamics and informing management practices.
Here we examine the functional, temporal and spatial components of mycorrhizal fungal spore dispersal by small mammals (rodents and shrews) following timber harvest. We tracked the interactions of seven mammal species and 34 fungal taxa composed of two mycorrhizal functional types, arbuscular mycorrhizae (AM) and ectomycorrhizae (ECM), in 11 patch cuts (0.4 ha) distributed across hardwood and mixedwood forest in the north‐eastern United States. Over a 2‐year period directly following harvests, we measured the relative contribution of mammal species to spore dispersal using networks that integrated mammal abundance with spore loads in scat. We also measured species‐specific microhabitat associations.
Mammals were complementary in their dispersal of AM and ECM fungal spores. However, within AM dispersal networks, mammals were redundant, with asynchrony in mammal population fluctuations among years leading to changes in the relative importance of mammal species. Comparatively, ECM fungal spore consumption and dispersal was primarily provided by one rodent species (Myodes gapperi), resulting in dispersal networks that were highly specialized. This indicates that AM spore dispersal is more robust to changes in mammal community turnover compared to ECM spore dispersal.
Mammal species varied from having no microhabitat associations to associating with a variety of different forest structure and ground cover conditions, indicating that species play different roles from broadcasting spores widely to depositing spores in discrete locations. By preserving microhabitat characteristics that are associated with small mammals in harvested areas, particularly downed wood and patches of pre‐harvest vegetation, forest managers can help maintain dispersal of mycorrhizal fungi.
A free Plain Language Summary can be found within the Supporting Information of this article.
Context
Accurately predicting microclimate is considered a high priority for understanding organismal responses to climate change at biologically relevant scales. However, approaches to developing robust microclimate datasets and understanding of the biophysical processes altering microclimatic regimes are limited.
Objectives
We developed and evaluated an approach for predicting microclimatic temperatures in montane forests that incorporates the influence of complex vegetation structure and landscape physiography. Additionally, we determined spatiotemporal mismatches between free-air and microclimatic temperatures to highlight the location, phenology, and magnitude of differences in predicted temperature.
Methods
We combined temperature datalogger measurements with LiDAR-derived vegetation and GIS-derived landscape physiographic characteristics to downscale free-air temperatures to microclimatic (3 m² spatial resolution) temperatures in the Great Smoky Mountains. We assessed the contribution of forest vegetation layers in altering microclimatic temperatures and model accuracy, and compared coarse-grain temperature maps with microclimatic temperature maps.
Results
Understory vegetation structure contributes to microclimatic buffering of near-surface, forest temperatures and enhances the accuracy of maximum temperature predictions during the growing season by altering the effects of solar insolation and topographic convergence index on microclimatic temperatures. Elevation and solar insolation covaried with spatiotemporal mismatches between free-air and microclimatic temperatures, suggesting that these landscape physiographic characteristics may contribute to deviations between macro- and micro-scale temperature.
Conclusions
Our findings demonstrate the importance of including complex vegetation characteristics and biophysical interactions as climate forcing factors in microclimate modeling. We also demonstrate the plausibility of accurately predicting microclimatic temperatures over broad extents, an important step in predicting potential organismal responses to climate change.
The Janzen–Connell Hypothesis (JCH) predicts that density‐responsive and host‐specific natural enemies limit the population sizes of abundant species. Importantly, these interactions help to maintain local community diversity through time. While ample evidence exists for the demographic predictions of the JCH, it remains unclear which natural enemies drive these dynamics across different plant communities.
While large mammalian herbivores are often assumed to lack the specialized diet needed to drive Janzen–Connell effects, they do show a degree of host‐preference that could drive density‐dependent plant demography. However, the potential role of large mammalian herbivores in Janzen–Connell interactions has only rarely been investigated.
Using 204 seedling transects (1 m × 10 m) at 51 sites across a 900‐ha forested reserve in southwestern Pennsylvania (USA), we examined the role that large mammals play in driving conspecific negative density dependence (CNDD) in temperate tree seedlings. Individual fences were erected around half of the transects (n = 102) to exclude large mammals, and were paired with adjacent unfenced transects. Within transects, a total of ~15,000 individual seedlings were monitored over three growing seasons.
Demographic neighbourhood models were constructed to examine the influence of neighbourhood composition and density on seedling survival and growth. An interaction term between conspecific neighbour density and fencing treatment was included to test the hypothesis that large herbivores cause CNDD.
We found that seedling survival was influenced by both conspecific neighbour density and fencing. CNDD was strongest when large mammals were allowed access to seedlings, and these results were driven by two abundant taxa (Prunus serotina and Fraxinus spp). Despite evidence that large mammals mediate CNDD, we found no effect of fencing on rarified species richness or evenness in seedling transects during the study.
Synthesis. Understanding the specific natural enemies driving conspecific negative density dependence remains vital for understanding the maintenance of forest diversity across the globe. Our results indicate that large mammalian herbivores are capable of driving CNDD in temperate tree species. These results suggest that large mammals may be an important and generally overlooked agent contributing to Janzen–Connell interactions in forest communities. We expect that further research examining large mammals in other systems will be important in the future.
The mechanisms causing invasive species impact are rarely empirically tested, limiting our ability to understand and predict subsequent changes in invaded plant communities. Invader disruption of native mutualistic interactions is a mechanism expected to have negative effects on native plant species. Specifically, disruption of native plant‐fungal mutualisms may provide non‐mycorrhizal plant invaders an advantage over mycorrhizal native plants. Invasive Alliaria petiolata (garlic mustard) produces secondary chemicals toxic to soil microorganisms including mycorrhizal fungi, and is known to induce physiological stress and reduce population growth rates of native forest understory plant species. Here, we report on a 11‐yr manipulative field experiment in replicated forest plots testing if the effects of removal of garlic mustard on the plant community support the mutualism disruption hypothesis within the entire understory herbaceous community. We compare community responses for two functional groups: the mycorrhizal vs. the non‐mycorrhizal plant communities. Our results show that garlic mustard weeding alters the community composition, decreases community evenness, and increases the abundance of understory herbs that associate with mycorrhizal fungi. Conversely, garlic mustard has no significant effects on the non‐mycorrhizal plant community. Consistent with the mutualism disruption hypothesis, our results demonstrate that allelochemical producing invaders modify the plant community by disproportionately impacting mycorrhizal plant species. We also demonstrate the importance of incorporating causal mechanisms of biological invasion to elucidate patterns and predict community‐level responses.
Plant-to-plant signaling is a key mediator of interactions among plant species. Plants can perceive and respond to chemical cues emitted from their neighbors, altering survival and performance, impacting plant coexistence and community assembly. An increasing number of studies indicate root exudates as key players in plant-to-plant signaling. Root exudates mediate root detection and behavior, kin recognition, flowering and production, driving inter- and intra-specific facilitation in cropping systems and mixed-species plantations. Altered interactions may be attributed to the signaling components within root exudates. Root ethylene, strigolactones, jasmonic acid, (-)-loliolide and allantoin are signaling chemicals that convey information on local conditions in plant-plant interactions. These root-secreted signaling chemicals appear ubiquitous in plants and trigger a series of belowground responses inter- and intra-specifically, involving molecular events in biosynthesis, secretion and action. The secretion of root signals, mainly mediated by ATP-binding cassette transporters, is critical. Root-secreted signaling chemicals and their molecular mechanisms are rapidly revealing a multitude of fascinating plant-plant interactions. However, many root signals, particularly species-specific signals and their underlying mechanisms, remain to be uncovered due to methodological limitations and root-soil interactions. A thorough understanding of root-secreted chemical signals and their mechanisms will offer many ecological implications and potential applications for sustainable agriculture. This article is protected by copyright. All rights reserved.
The success of tree recruitment in Mediterranean holm oak (Quercus ilex) forests is threatened by the increasing intensity, duration and frequency of drought periods. Seedling germination and growth are modulated by complex interactions between abiotic (microhabitat conditions) and biotic factors (mycorrhiza association) that may mitigate the impacts of climate change on tree recruitment. To better understand and anticipate these effects, we conducted a germination experiment in a long-term precipitation reduction (PR) field experiment where we monitored seedling establishment and survival, micro-habitat conditions and ectomycorrhizal (ECM) colonization by different mycelia exploration types during the first year of seedling growth. We hypothesized that (i) the PR treatment decreases seedling survival relative to the control with ambient conditions, (ii) microhabitat conditions of water and light availability are better predictors of seedling survival than the PR treatment, (iii) the PR treatment will favour the development of ECM exploration types with drought-resistance traits such as differentiated rhizomorphs. Contrary to our first hypothesis, seedling survival was lower in control plots with overall higher soil moisture. Micro-habitat light and soil moisture conditions were better predictors of seedling survival and growth than the plot-level PR treatment, confirming our second hypothesis. Furthermore, in line with our third hypothesis, we found that ECM with longer extramatrical mycelia were more abundant in the PR treatment plots and were positively correlated to survival, which suggests a potential role of this ECM exploration type in seedling survival and recruitment. Although summer drought was the main cause of seedling mortality, our study indicates that drier conditions in spring can increase seedling survival, presumably through a synergistic effect of drought adapted ECM species and less favourable conditions for root pathogens.
Ungulates have become abundant in many temperate forests, shifting tree species composition by browsing and altering soil physical conditions by trampling. Whether these effects cascade down to other trophic levels and ecosystem processes is poorly understood. Here, we assess the paths through which ungulates have cascading effects on other trophic levels (regeneration, litter, invertebrates, rodents and organic matter decomposition). We compared ungulate effects by comparing 15 response variables related to different trophic levels between paired fenced and unfenced plots in twelve temperate forest sites across the Netherlands, and used pathway analysis model to identify the (in)direct pathways through which ungulates have influenced these variables. We found that plots with ungulates (that is, unfenced) compared to plots without (that is, fenced) had lower litter depth, sapling diversity, sapling density, rodent activity, macro-invertebrate biomass, decomposition rate of tea bags, pine and birch litter and higher soil compaction. These findings were used in a path analysis to establish potential causal relationships, which showed that ungulate presence: decreased sapling density, which indirectly decreased rodent activity; decreased litter depth, which indirectly reduced invertebrate diversity; increased soil compaction, which also decreased invertebrate diversity. Soil pH decreased invertebrate biomass, which also increased nitrogen mineralization. Yet, we did not find cascading effects of ungulates on decomposition rates. Importantly, an increase in ungulate abundance strengthens the cascading effects in this system. Our results suggest that ungulates can trigger cascading effects on lower trophic levels, yet decomposition and mineralization rates are resilient to ungulate browsing and trampling. Therefore, temperate forests conservation could benefit by limiting ungulate abundance.
Arbuscular mycorrhizal (AM) plants and fungi associate with lower soil organic matter, higher pH, lower phosphorus and higher nitrogen than ectomycorrhizal (EM) ones. However, soil conditions correlate with climatic factors, and we suggest that temperature and humidity have also direct roles in the success of mycorrhiza types. The hypothesis here is that EM perform better at low temperatures than AM, and AM resist drought better than EM.
Narrowleaf cottonwood (Populus angustifolia E. James) forms both AM and EM. We grew seedlings in soil at 14, 20 and 26 °C in factorial combinations with adequate watering and a cyclic mild drought for 4 and 7 weeks.
As hypothesized, the percent of EM root tips was largest at 14 °C, while the proportional root length with AM was largest at the two higher temperatures. However, unlike expectations, drought increased EM formation slightly, while the AM colonization was lower in the dry treatment. Plant growth was reduced more by low temperature than drought. Root branching was more prominent at low temperature and root length and mass growth at higher temperatures.
Soil nutrient availability did not provide a direct explanation to the results, as both soluble soil N and P were the same in 14 and 20 °C, while the change in mycorrhiza colonization took place between these temperatures. Differences in root morphology (root branching vs length) may affect the proportions of the mycorrhiza types at different temperature regimes. The most likely explanation to the differential colonization is that temperature affects AM and EM fungi in a different way. In nature, temperature and humidity regimes are tightly correlated, and temperature as such may be a stronger determinant for the success of mycorrhiza types than has been previously considered. The poorer performance of AM in low-temperature and drought conditions may reflect stress avoidance rather than stress tolerance by AM fungi.
AimsBelowground carbon transfer from plant to plant has been extensively described, but such transfer for nitrogen has been less thoroughly investigated when the donor is a non-N2-fixing species. This study, applied to forest regeneration, aimed to determine whether tree seedlings facilitated neighbouring grass growth through nitrogen transfer at an early stage of development, thus facilitating nitrogen acquisition by understory species.Methods
Quercus petraea seedlings were planted in pots either sole-grown or mixed-grown with Molinia caerulea tufts or another oak seedling. 15N-urea pulse-chase labelling (cotton wick method) was performed in oak shoots and the fate of 15N in each soil and plant compartment was tracked for one year. N transfer pathways were investigated using two degrees of physical separation between root systems.ResultsMolinia dry weight was higher when mixed-grown with oak seedlings than when sole-grown. Increase in grass dry weight correlated with N transfer from donor oak to receiver Molinia. Interestingly, the presence of Molinia increased N rhizodeposition of oak. N allocation in donor oak towards root in winter and shoot in spring was enhanced.Conclusions
Oak seedlings facilitated Molinia growth through rapid N transfer, underlining the ability of non-N2-fixing species to supply N to neighbours. 15N allocation within donor oak and its rhizodeposition depended on neighbour identity.
Irregular precipitation and drought caused an increase in tree mortality rates in multiple forest biomes with alterations in both ecosystem services and carbon balance. Carob (Ceratonia siliqua) growth and production in arid and semi-arid ecosystems are likely affected by climate change-induced droughts. Understanding the physiological responses of drought-induced early- stage tree death and strategies to enhance drought tolerance and optimize growth will help tree improvement programs. Mycorrhizal inoculation has a pronounced impact on plant growth, water absorption, mineral nutrition, and protection from abiotic stresses. However, a better understanding of these complex interconnected cellular processes and arbuscular mycorrhizal fungi (AMF)- mediated mechanisms regulating drought tolerance in plants will enhance its potential application as an efficient approach for bio-amelioration of stresses. The objectives of this work were to elucidate the different effects of autochthone AMF on inorganic solute and water content uptakes, organic adjustments (sugar and proteins content), leaf gas exchange (stomatal conductance and efficiency of photosystems I and II), and oxidative damage of two contrasting ecotypes of carob seedlings: coastal (southern ecotype (SE)) and in-land (northern ecotype (NE)) under control (C), drought (by cessation of irrigation for 15 days (15D)), and recovery (R) conditions. Our findings showed that AMF promoted growth, nutrient content, and physiological and biochemical parameters in plants of both ecotypes during C, 15D, and R conditions. After four days of recovery, stomatal conductance (gs), the maximum photochemical efficiency of PSII (Fv/Fm ), water content, and plant uptake of mineral nutrients (P, K, Na, and Ca) were significantly higher in shoots of mycorrhizal (AM) than non-mycorrhizal (NM) control plants. Consequently, AMF reduced to a greater degree the accumulation of hydrogen peroxide (H2O2 ) and oxidative damage to lipid (malondialdehyde (MDA)) content in AM than NM plants in NE and SE, after recovery. Altogether, our findings suggest that AMF can play a role in drought resistance of carob trees at an early stage by increasing the inorganic solutes (P, K, Na, and Ca), water content uptake, organic solutes (soluble sugars and protein content), stomatal conductance, and defense response against oxidative damage during re- watering after drought stress.
Irregular precipitation and drought caused an increase in tree mortality rates in multiple forest biomes with alterations in both ecosystem services and carbon balance. Carob (Ceratonia siliqua) growth and production in arid and semi-arid ecosystems are likely affected by climate change-induced droughts. Understanding the physiological responses of drought-induced early-stage tree death and strategies to enhance drought tolerance and optimize growth will help tree improvement programs. Mycorrhizal inoculation has a pronounced impact on plant growth, water absorption, mineral nutrition, and protection from abiotic stresses. However, a better understanding of these complex interconnected cellular processes and arbuscular mycorrhizal fungi (AMF)-mediated mechanisms regulating drought tolerance in plants will enhance its potential application as an efficient approach for bio-amelioration of stresses. The objectives of this work were to elucidate the different effects of autochthone AMF on inorganic solute and water content uptakes, organic adjustments (sugar and proteins content), leaf gas exchange (stomatal conductance and efficiency of photosystems I and II), and oxidative damage of two contrasting ecotypes of carob seedlings: coastal (southern ecotype (SE)) and inland (northern ecotype (NE)) under control (C), drought (by cessation of irrigation for 15 days (15D)), and recovery (R) conditions. Our findings showed that AMF promoted growth, nutrient content, and physiological and biochemical parameters in plants of both ecotypes during C, 15D, and R conditions. After four days of recovery, stomatal conductance (gs), the maximum photochemical efficiency of PSII (Fv/Fm), water content, and plant uptake of mineral nutrients (P, K, Na, and Ca) were significantly higher in shoots of mycorrhizal (AM) than non-mycorrhizal (NM) control plants. Consequently, AMF reduced to a greater degree the accumulation of hydrogen peroxide (H2O2) and oxidative damage to lipid (malondialdehyde Plants 2020, 9, 80 2 of 19 (MDA)) content in AM than NM plants in NE and SE, after recovery. Altogether, our findings suggest that AMF can play a role in drought resistance of carob trees at an early stage by increasing the inorganic solutes (P, K, Na, and Ca), water content uptake, organic solutes (soluble sugars and protein content), stomatal conductance, and defense response against oxidative damage during re-watering after drought stress.
Arbuscular mycorrhiza represents an ubiquitous nutritional symbiosis between the roots of most terrestrial plant species and fungi of the subphylum Glomeromycotina (Spatafora et al., 2016). Terrestrial habitats are unlikely to be limited in propagules of arbuscular mycorrhizal fungi (AMF), because AMF propagule densities build up fast in vegetated soil (e.g. Gould et al., 1996). We start to appreciate, however, that shortages in AMF propagules are common in some habitats, such as agricultural fields subject to intensive farming (Schnoor et al., 2011; Manoharan et al., 2017). Forest habitats in the temperate region might also be occasionally AMF propagule limited (Veresoglou et al., 2017), but to the best of our understanding this has not been shown with empirical data.
Insect herbivory can have important consequences for the functioning of terrestrial ecosystems. Despite a growing recognition of the role of herbivores in above‐ground–below‐ground interactions, our current understanding is mainly restricted to studies of vertebrates in grassland and tundra ecosystems, while ecosystems with tree‐like canopies (termed forests below) and invertebrates remain understudied.
Here, we assess the current state of knowledge of one key aspect of plant–herbivore interactions by conducting a meta‐analysis of the peer‐reviewed literature on the below‐ground consequences of above‐ground insect herbivory in forest ecosystems. Main results are reported as aggregated relative effect sizes (Cohen's d).
We find that above‐ground insect herbivory reduced below‐ground carbon (C) allocation by plants to roots (−0.56) and root exudation (−0.85), causing shifts in root–symbiont communities, for example, a decrease (−0.67) in the abundance of ectomycorrhizal fungi. Microbial decomposer abundances showed no significant responses, while soil faunal abundances increased (0.50). C and nitrogen (N) mineralization rates (C: 0.48, N: 0.48) along with nutrient leaching (C: 0.30, N: 0.77) increased, with a stronger response to outbreak relative to background insect densities. The negative responses increased in strength in colder and dryer biomes while positive responses were reinforced in warmer and wetter biomes, thus extending previously shown effects for vertebrate herbivores to also include insect herbivory. The positive response by soil fauna to insect herbivory was the notable exception. This may be associated with the limited physical soil disturbance caused by insects compared to ungulates. Furthermore, we identified an under‐representation in the literature of large areas of boreal and tropical biomes calling for research priorities to fill these knowledge gaps. We present three recommendations for future research: addressing (a) biological drivers of biogeochemistry and response pathways, (b) knowledge gap from boreal and tropical forests, and (c) heterogeneity of herbivore disturbances.
Synthesis. Insect herbivores significantly accelerate soil C and N cycling during outbreaks in forest ecosystems, but we lack knowledge on the underlying biological drivers. Overall, below‐ground responses to insect herbivory are similar to vertebrate herbivory responses, which may simplify implementing herbivory effects into ecosystem models. Nonetheless, we identify a few important differences and general knowledge gaps on which we base recommendations for future research.
Arbuscular mycorrhiza (AM) is a symbiosis between plants and Glomeromycotina fungi, and is distributed throughout the plant kingdom and all terrestrial ecosystems. Colonization in plant roots usually takes structural forms of either Paris‐ or Arum‐type, distinguished by intracellular hyphal coils and arbuscules and exemplified by Paris quadrifolia and Arum maculatum (Gallaud, 1905; Smith & Smith, 1997; Dickson et al., 2007), respectively, with a near 1:1 distribution among plant species (Dickson et al., 2007).
Root exudates are a pathway for plant–microbial communication and play a key role in ecosystem response to environmental change. Here, we collate recent evidence that shows that plants of different growth strategies differ in their root exudation, that root exudates can select for beneficial soil microbial communities, and that drought affects the quantity and quality of root exudation. We use this evidence to argue for a central involvement of root exudates in plant and microbial response to drought and propose a framework for understanding how root exudates influence ecosystem form and function during and after drought. Specifically, we propose that fast‐growing plants modify their root exudates to recruit beneficial microbes that facilitate their regrowth after drought, with cascading impacts on their abundance and ecosystem functioning. We identify outstanding questions and methodological challenges that need to be addressed to advance and solidify our comprehension of the importance of root exudates in ecosystem response to drought.
Dual‐mycorrhizal plants are capable of associating with fungi that form characteristic arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) structures. Here, we address the following questions: (1) How many dual‐mycorrhizal plant species are there? (2) What are the advantages for a plant to host two, rather than one, mycorrhizal types? (3) Which factors can provoke shifts in mycorrhizal dominance (i.e. mycorrhizal switching)? We identify a large number (89 genera within 32 families) of confirmed dual‐mycorrhizal plants based on observing arbuscules or coils for AM status and Hartig net or similar structures for EM status within the same plant species. We then review the possible nutritional benefits and discuss the possible mechanisms leading to net costs and benefits. Cost and benefits of dual‐mycorrhizal status appear to be context dependent, particularly with respect to the life stage of the host plant. Mycorrhizal switching occurs under a wide range of abiotic and biotic factors, including soil moisture and nutrient status. The relevance of dual‐mycorrhizal plants in the ecological restoration of adverse sites where plants are not carbon limited is discussed. We conclude that dual‐mycorrhizal plants are underutilized in ecophysiological‐based experiments, yet are powerful model plant–fungal systems to better understand mycorrhizal symbioses without confounding host effects.
The tendency of closely related plant species to share natural enemies has been suggested to limit their co‐occurrence and performance, but we lack a deep understanding on how mutualistic interactions such as the mycorrhizal symbiosis affect plant–plant interactions depending on the phylogenetic relatedness of the interacting plants. We hypothesise that the effect of the mycorrhizal symbiosis on plant−plant facilitative interactions depends on the phylogenetic distance between the nurse and facilitated plants.
A recently published meta‐analysis compiled the strength of plant facilitative interactions in the presence or absence (or reduced abundance) of mycorrhizal fungi. We use phylogenetically informed Bayesian linear models to test whether the effect size is influenced by the phylogenetic distance between the plant species involved in each plant facilitative interaction.
Conspecific facilitative interactions are more strongly enhanced by mycorrhizal fungi than interactions between closely related species. In heterospecific interactions, the effect of the mycorrhizal symbiosis on plant facilitation increases with the phylogenetic distance between the nurse and facilitated plant species.
Our result showing that the effect of mycorrhizal symbiosis on the facilitation interactions between plants depends on their phylogenetic relatedness provides new mechanisms to understand how facilitation is assembling ecological communities.
A spatially explicit global map of tree symbioses with nitrogen-fixing bacteria and mycorrhizal fungi reveals that climate variables are the primary drivers of the distribution of different types of symbiosis.
The Janzen‐Connell (JC) hypothesis is a major ecological explanation for high species richness, in particular in tropical forest ecosystems. Central components of the JC hypothesis are noncompetitive effects of distance and density dependence, two drivers that contribute independently to species coexistence, but are ultimately linked in the field. However, although numerous studies provide evidence for either distance‐ or density‐dependent effects based on observational data, experimental testing of simultaneous and interactive effects of distance and density has rarely been conducted, especially in a comprehensive multispecies approach. Here, we make use of the forest Biodiversity‐Ecosystem Functioning project (BEF) –China to estimate distance‐ and density‐dependent effects in a reciprocal tree seedling transplant experiment of 11 tree species. We deployed 13,490 juveniles of all 11 species in their own (home) and in all foreign monocultures (away), as well as at three different levels of planting density, thereby testing for distance and density effects, respectively. In addition, to quantify the amount to which density effects were brought about by potential additional effects of intraspecific competition, we set up a common garden experiment with different levels of planting density, where an additional “shadow” treatment controlled for effects of canopy shading. Although the “away” and “high‐density” treatments significantly impaired the performance and productivity of seedlings, leaf damage and survival was exclusively affected by either the home/away or the density treatment, respectively. Negative density‐dependent effects on leaf damage were less pronounced in the “home” treatment, showing that the effects were not additive. In addition, results obtained in the Common Garden Experiment showed that negative effects of high density may be also brought about by intraspecific competition as an alternative density‐responsive mechanism and less by true JC effects. Overall, our results provide strong support on a multispecies basis for the influence of host‐specific effects already operating in early stages of a forest plantation. However, they also emphasize the need to account appropriately for potential additional density‐responsive mechanisms such as intraspecific competition or microenvironmental conditions when addressing the role of JC effects for species coexistence.
Macroclimate warming is often assumed to occur within forests despite the potential for tree cover to modify microclimates. Here, using paired measurements, we compared the temperatures under the canopy versus in the open at 98 sites across 5 continents. We show that forests function as a thermal insulator, cooling the understory when ambient temperatures are hot and warming the understory when ambient temperatures are cold. The understory versus open temperature offset is magnified as temperatures become more extreme and is of greater magnitude than the warming of land temperatures over the past century. Tree canopies may thus reduce the severity of warming impacts on forest biodiversity and functioning.
Aims
In this study, we investigated the effects of reduced snow depth on plant phenology, productivity, nitrogen (N) cycling, and N use in canopy and understory vegetation. We hypothesized that decreased snow depth would hasten the timing of leaf flushing and N uptake in understory vegetation, increasing its N competitive advantage over canopy trees.
Results
Snow removal did not directly affect the phenology of either canopy or understory vegetation. Understory vegetation took up more N in the snow removal plots than in the control plots, particularly in the mid- to late-growing season. Leaf production and N uptake in canopy trees also did not differ between the control and snow removal plots, but N resorption efficiency in the snow removal plots (57.6%) was significantly higher than those in control plots (50.0%).
Conclusions
Increased N uptake by understory plants may induce N limitation in canopy trees, which in turn may cause canopy trees to increase their N use efficiency. Such competitive advantage of understory vegetation over canopy trees against snow reduction may affect N cycling via litter quality and quantity not only just after the growing season but also in subsequent seasons.
We here describe the multiple mechanisms by which ungulates distribute diaspores across landscapes. There are three primary and three secondary seed dispersal mechanisms by which ungulate dispersal agents contribute to the spread of plant diaspores, both with and without the intervention of other biotic and abiotic agents. These dispersal mechanisms may be combined in successive inter-dependent steps. Native, introduced and domestic ungulates co-occur in many ecosystems and frequently interact with numerous plant species, which facilitates long-distance dispersal of both native and exotic plants. However, ungulate taxonomic diversity conceals a much higher diversity in terms of the functional traits involved in ungulate-mediated dispersal (e.g., feeding regime, fur morphology). These traits may strongly affect emigration, transfer and immigration in the animal-mediated plant dispersal, and consequently; they may also impact overall seed dispersal effectiveness, both quantitatively and qualitatively. In this review, we compare internal mechanisms, where seeds must survive digestive treatments (regurgitation, endozoochory), with external mechanisms, where diaspores are carried on the outside of the vectors (epizoochory). We include both primary epizoochory (direct adhesion to fur essentially) and secondary epizoochory (diaspore-laden mud adhering to hooves or the body and, transfer through contact with a conspecific). We addressed the overlap/complementarity of ungulates for the plant species they disperse through a systematic literature review. When two ungulate species co-occur, there is always an overlap in the plant species dispersed by endozoochory or by fur-epizoochory. Further, when we consider the proportion of plant species dispersed both internally and externally by an ungulate, the overlap is higher for grazing than browsing ungulates. We identify two challenges for the field of dispersal ecology: the proportion of all diaspores produced that are carried over long distances by ungulates, and the relative importance of ungulates on the whole as the main dispersal agent for plants. Furthermore, the fact that numerous plants dispersed by fur-epizoochory do not feature any specific adaptations is intriguing. We discuss unsolved methodological challenges and stress research perspectives related to ungulate-mediated dispersal: for example, taking animal behavior and cognition into account and studying how ungulates contribute to the spread of invasive exotic plants and altitudinal plant dispersal.
Root exudation is an important process determining plant interactions with the soil environment. Many studies have linked this process to soil nutrient mobilization. Yet, it remains unresolved how exudation is controlled and how exactly and under what circumstances plants benefit from exudation. The majority of root exudates including primary metabolites (sugars, amino acids, and organic acids) are believed to be passively lost from the root and used by rhizosphere-dwelling microbes. In this review, we synthetize recent advances in ecology and plant biology to explain and propose mechanisms by which root exudation of primary metabolites is controlled, and what role their exudation plays in plant nutrient acquisition strategies. Specifically, we propose a novel conceptual framework for root exudates. This framework is built upon two main concepts: (1) root exudation of primary metabolites is driven by diffusion, with plants and microbes both modulating concentration gradients and therefore diffusion rates to soil depending on their nutritional status; (2) exuded metabolite concentrations can be sensed at the root tip and signals are translated to modify root architecture. The flux of primary metabolites through root exudation is mostly located at the root tip, where the lack of cell differentiation favors diffusion of metabolites to the soil. We show examples of how the root tip senses concentration changes of exuded metabolites and translates that into signals to modify root growth. Plants can modify the concentration of metabolites either by controlling source/sink processes or by expressing and regulating efflux carriers, therefore challenging the idea of root exudation as a purely unregulated passive process. Through root exudate flux, plants can locally enhance concentrations of many common metabolites, which can serve as sensors and integrators of the plant nutritional status and of the nutrient availability in the surrounding environment. Plant-associated micro-organisms also constitute a strong sink for plant carbon, thereby increasing concentration gradients of metabolites and affecting root exudation. Understanding the mechanisms of and the effects that environmental stimuli have on the magnitude and type of root exudation will ultimately improve our knowledge of processes determining soil CO2 emissions, ecosystem functioning, and how to improve the sustainability of agricultural production.
Abstract
Olive trees are often subjected to a long dry season with low water availability which induces oxidative stress. The present study was conducted to evaluate the effectiveness of native Rhizophagus manihotis (Rma) and non-native Funneliformis mosseae (Fmo) arbuscular mycorrhizal (AM) fungi (AMF) in enhancing olive protection against oxidative
stress induced by water deficit. Olive plantlets, cv. Picholine marocaine, were inoculated (AM-plants) or not (NMplants) with Rma or Fmo and subjected to well-watered (75% of field capacity) or water-stressed (25% of field capacity) conditions. After two months, obtained results showed that water stress significantly decreased growth and biomass
production of NM-plants, but AMF alleviated the detrimental effects of water deficit on the growth of olive plants. Inoculation with Rma increased shoot height by 120%, root length by 56%, fresh weight by 170% (shoot) and 210% (root), and dry weight by 220% (shoot) and 220% (root) compared to NM-plants. AM colonization enhanced drought tolerance in terms of protection against oxidative stress. Mycorrhizal plants showed lower levels of hydrogen peroxide (H2O2), malondialdehyde (MDA) and electrolyte leakage (EL) than NM-plants. Rma colonization decreased two times H2O2, 2.6 times MDA and two times EL levels compared to NM-plants. This protective effect seems to be due to the enhanced activities of superoxide dismutase (534 U mg–1 protein), catalase (298 U mg–1 protein), guaiacol peroxidase (47 U mg–1 protein), and ascorbate peroxidase (305 U mg–1 protein) which were highest in Rma-plants. Moreover, Rmaplants showed the lowest oxidative damage to lipid and highest soluble protein content. Thus, the native AMF Rma ought to be considered as a biological tool for enhancing olive tolerance to drought.
key words: mycorrhizal colonization; Olea europaea; water deficit; antioxidant activity; drought tolerance.
The formation of a common mycorrhizal network (CMN) between roots of different plant species enables nutrient transfers from one plant to another and their coexistence. However, almost all studies on nutrient transfers between CMN-connected plants have separately, but not simultaneously, been demonstrated under the same experimentation. Both conspecific and heterospecific seedlings of Cinnamomum camphora, Bidens pilosa, and Broussonetia papyrifera native to a karst habitat in southwest China were concurrently grown in a growth microcosm that had seven hollowed compartments (six around one in the center) being covered by 35.0-μm and/or 0.45-μm nylon mesh. The Ci. camphora in the central compartment was supplied with or without Glomus etunicatum and ¹⁵N to track N transfers between CMN-connected conspecific and heterospecific seedlings. The results showed as follows: significant greater nitrogen accumulations, biomass productions, ¹⁵N content, % Ntransfer, and the Ntransfer amount between receiver plant species ranked as Br. papyrifera≈Bi. pilosa > Ci. camphora under both M⁺ and M⁻, and as under M⁺ than under M⁻ for Ci. camphora but not for both Bi. Pilosa and Br. papyrifera; the CMN transferred more nitrogen (¹⁵N content, % Ntransfer, and Ntransfer amount) from the donor Ci. camphora to the heterospecific Br. papyrifera and Bi. pilosa, with a lower percentage of nitrogen derived from transfer (%NDFT). These findings suggest that the CMN may potentially regulate the nitrogen transfer from a donor plant to individual heterospecific receiver plants, where the ratio of nitrogen derived from transfer depends on the biomass strength of the individual plants.
Most large herbivores require some type of management within their habitats. Some populations of large herbivores are at the brink of extinction, some are under discussion for reintroduction, whilst others already occur in dense populations causing conflicts with other land use. Large herbivores are the major drivers for forming the shape and function of terrestrial ecosystems. This 2006 book addresses the scientifically based action plans to manage both the large herbivore populations and their habitats worldwide. It covers the processes by which large herbivores not only affect their environment (e.g. grazing) but are affected by it (e.g. nutrient cycling) and the management strategies required. Also discussed are new modeling techniques, which help assess integration processes in a landscape context, as well as assessing the consequences of new developments in the processes of conservation. This book will be essential reading for all involved in the management of both large herbivores and natural resources.
Oak regeneration in temperate forests often fails in the presence of understorey grass. Competition by resource exploitation between plants has been extensively studied. By contrast, competition by interference, especially chemical interference (allelopathy), has been much less thoroughly examined and its relative importance remains unclear. We investigated the influence of allelopathic interaction on plant performance (biomass production) in a pot experiment with sessile oak (Quercus petraea) and purple moor grass (Molinia caerulea), either sole-or mixed-grown. Plants were watered with either Quercus root exudates or Molinia root exudates. After 6 months of growth, oak biomass increment was significantly lowered by Molinia root exudates. The oak's root system was more strongly affected than its aerial part. Quercus root exudates favoured oak growth but did not affect moor grass. Conversely, Molinia root exudates had a small depressive effect on its own growth, but its biomass was favoured by the presence of oak grown in the same pot. Resource exploitation had a more detrimental effect than allelopathy and both processes together decreasing oak biomass by 50%. Although untargeted metabolomic analysis by UHPLC failed to identify any potentially allelopathic substances involved, our study demonstrates a lower but critical contribution of chemical interference on oak seedling-moor grass competition compared to exploitation processes. To ensure oak regeneration, management of forest ecosystems should thus first focus on reducing moor grass close to oak seedlings to help decrease its allelopathic effect and ease resource competition.
Root exudates are part of the rhizodeposition process, which is the major source of soil organic carbon (C) released by plant roots. This flux of C is believed to have profound effects on C and nutrient cycling in ecosystems. The quantity of root exudates depends on the plant species, the period throughout the year, and external biotic and abiotic factors. Since root exudates of mature trees are difficult to collect in field conditions, very little is known about their flux, especially in water-limited ecosystems, such as the seasonally hot and dry Mediterranean maquis. Here, we collected exudates from DNA-identified roots in the forest from the gymnosperm Cupressus sempervirens L. and the evergreen angiosperm Pistacia lentiscus L. by 48-h incubations on a monthly temporal resolution throughout the year. We examined relationships of the root exudate C flux to abiotic parameters of the soil (water content, water potential, temperature) and atmosphere (vapor pressure deficit, temperature). We also studied relationships to C fluxes through the leaves as indicators of tree C balance. Root exudation rates varied significantly along the year, increasing from 6 μg C cm −2 root day−1 in both species in the wet season to 4- and 11-fold rates in Pistacia and Cupressus, respectively, in the dry season. A stepwise linear mixed-effects model showed that the three soil parameters were the most influential on exudation rates. Among biotic factors, there was a significant negative correlation of exudation rate with leaf assimilation in Cupressus and a significant negative correlation with leaf respiration in Pistacia. Our observation of enhanced exudation flux during the dry season indicates that exudation dynamics in the field are less sensitive to the low tree C availability in the dry season. The two key Mediterranean forest species seem to respond to seasonal changes in the rhizosphere such as drying and warming, and therefore invest C in the rhizosphere under seasonal drought.
In tree-based intercropping systems, roots of trees and crops are interacting and could influence ecosystem services provided by soil microorganisms. Here, the analysis of diversity of arbuscular mycorrhiza fungi (AMF) associated with roots of walnut and maize reveals differences. Of interest, Funneliformis genus is mainly associated with maize roots, and one OTU related to an uncultured Glomus, might form a common mycorrhizal network linking roots of both plants. In addition, the analysis of ¹³C of mycelium living in the surrounding environment of roots, suggest that part of the carbon derived from walnut trees could be transferred to maize plants. Our results suggest that in temperate agroforestry ecosystems, AMF could participle in the redistribution of nutrients between connected plants.
Forest cover creates a specific microclimate by buffering most environmental variables. If the influence of the overstory on microclimatic variables has been well studied, the role of the understory has received less attention. In this study we investigated how the shrub layer modifies solar radiation, air temperature (T), relative air humidity (RH), vapor pressure deficit (VPD) and soil moisture under different thinning treatments in an Aleppo pine forest (Pinus halepensis Mill.). Microclimatic variables were measured along a vegetation cover gradient made up of three pine densities (dense, medium, low) and open conditions, with or without the presence of shrubs. The results were analyzed with a focus on the summer period which represents a major bottleneck for plant development in the Mediterranean area. Average T and VPD values increased with decreasing vegetation cover (+1.38°C and +0.21 kPa for the whole year) while RH decreased (-2.34%). Along the same gradient, daily amplitude of T, RH, VPD increased while the buffering capacity decreased. These patterns were more pronounced during the summer period compared to the whole year and were primarily driven by overstory transmittance. However, the shrub layer played a significant role in the low pine cover treatment where it was developed and in open conditions. Soil water content in the forest area was higher under low pine cover without shrubs than it was in the other treatments, though differences were less marked during summer drought episodes. In open conditions, soil moisture was always significantly lower beneath the shrub canopy than outside it. Despite a reduction in soil moisture, shrubs may represent safe sites for woody seedling development in sparse pine forests and in treeless areas by buffering the microclimate during the summer period.
Key message
Wildfire danger and burnt areas should increase over the century in southern Europe, owing to climate warming. Fire-prone area expansion to the north and to Mediterranean mountains is a concern, while climate-induced burnt area increase might be limited by fuel availability in the most arid areas. Further studies are needed to both assess and reduce uncertainties on future trends.
Context
Wildfire is the main disturbance in forested ecosystems of southern Europe. Warmer and drier conditions projected in this region are expected to profoundly affect wildfire regimes.
Aims
In this review, we pursue a twofold objective: (i) report the trends in wildfire danger and activity projected under warming climate in southern Europe and (ii) discuss the limitations of these projections.
Methods
We reviewed 23 projection studies that examined future wildfire danger or wildfire activity at local, regional or continental scale.
Results
Under the scenarios with the highest greenhouse gas emissions, we found that projection studies estimate an increase in future fire danger and burnt areas varying, on average, from 2 to 4% and from 5 to 50% per decade, respectively. Further comparisons on the magnitude of increase remained challenging because of heterogeneous methodological choices between projection studies. We then described three main sources of uncertainty that may affect the reliability of wildfire projections: climate projections, climate-fire models and the influences of fuels, fire-vegetation feedbacks and human-related factors on the climate-fire relationships.
Conclusion
We suggest research directions to address some of these issues for the purpose of refining fire danger and fire activity projections in southern Europe.
Pinus pinea is an important species for ecological and economic reasons in the Mediterranean area and especially in Lebanon. However, conditions of its natural regeneration have received little attention. Our study aimed to test the influence of soil disturbances, predation and autotoxicity on pine regeneration. A seed sowing field experiment was conducted in a mature stand in which two types of soil disturbances were tested (soil scar-ification and litter burning) in presence or absence of predation. In addition, a laboratory experiment evaluating the influence of litter (present, absent or burned) and green needle leachates (i.e. in order to mimic potential autotoxicity) on seed germination and seedling growth was conducted. Results showed a strong effect of soil disturbance, in particular soil scarification which promoted seedling emergence and early survival, whereas the role of predation was low. Forty to sixty days after seed installation, seedling density peaked at 14.9 ± 0.4 seedling.m in the scarification treatment, 13.1 ± 0.4 in the burning treatment and 8.4 ± 0.4 in the control treatment. However, an unexpected high mortality rate was recorded at the end of the field experiment in all treatments, as after seven months seedling density dropped to 0.2 ± 0.02. In the laboratory experiment, we emphasized an autotoxic effect of green needle aqueous extract on seed germination and seedling growth in the presence of litter. This allelopathic effect could be potentially linked to the presence of quinic acid which was measured as the major metabolite detected in needle aqueous extracts. In conclusion, we recommend soil preparation treatments to favor seedling emergence in combination with thinning to achieve seedling development on the long term.
The mutualistic symbiosis between forest trees and ectomycorrhizal fungi (EMF) is among the most ubiquitous and successful interactions in terrestrial ecosystems. Specific species of EMF are known to colonize specific tree species, benefitting from their carbon source, and in turn, improving their access to soil water and nutrients. EMF also form extensive mycelial networks that can link multiple root‐tips of different trees. Yet the number of tree species connected by such mycelial networks, and the traffic of material across them, are just now under study. Recently we reported substantial belowground carbon transfer between Picea, Pinus, Larix and Fagus trees in a mature forest. Here we analyze the EMF community of these same individual trees and identify the most likely taxa responsible for the observed carbon transfer. Among the nearly 1200 EMF root‐tips examined, 50‐70% belong to operational taxonomic units (OTUs) that were associated with 3 or 4 tree host species, and 90% of all OTUs were associated with at least two tree species. Sporocarp 13C signals indicated that carbon originating from labeled Picea trees was transferred among trees through EMF networks. Interestingly, phylogenetically more closely related tree species exhibited more similar EMF communities and exchanged more carbon. Our results show that belowground carbon transfer is well orchestrated by the evolution of EMFs and tree symbiosis.
Temperate forests cover 16% of the global forest area. Within these forests, the understorey is an important biodiversity reservoir that can influence ecosystem processes and functions in multiple ways. However, we still lack a thorough understanding of the relative importance of the understorey for temperate forest functioning. As a result, understoreys are often ignored during assessments of forest functioning and changes thereof under global change. We here compiled studies that quantify the relative importance of the understorey for temperate forest functioning, focussing on litter production, nutrient cycling, evapotranspiration, tree regeneration, pollination and pathogen dynamics. We describe the mechanisms driving understorey functioning and develop a conceptual framework synthesizing possible effects of multiple global‐change drivers on understorey‐mediated forest ecosystem functioning. Our review illustrates that the understorey's contribution to temperate forest functioning is significant but varies depending on the ecosystem function and the environmental context, and more importantly, the characteristics of the overstorey. To predict changes in understorey functioning and its relative importance for temperate forest functioning under global change, we argue that a simultaneous investigation of both overstorey and understorey functional responses to global change will be crucial. Our review shows that such studies are still very scarce, only available for a limited set of ecosystem functions and limited to quantification, providing little data to forecast functional responses to global change. This article is protected by copyright. All rights reserved.
Mycorrhizal fungi benefit plants by improved mineral nutrition and protection against stress, yet information about fundamental differences among mycorrhizal types in fungi and trees and their relative importance in biogeochemical processes is only beginning to accumulate. We critically review and synthesize the ecophysiological differences in ectomycorrhizal, ericoid mycorrhizal and arbuscular mycorrhizal symbioses and the effect of these mycorrhizal types on soil processes from local to global scales. We demonstrate that guilds of mycorrhizal fungi display substantial differences in genome‐encoded capacity for mineral nutrition, particularly acquisition of nitrogen and phosphorus from organic material. Mycorrhizal associations alter the trade‐off between allocation to roots or mycelium, ecophysiological traits such as root exudation, weathering, enzyme production, plant protection, and community assembly as well as response to climate change. Mycorrhizal types exhibit differential effects on ecosystem carbon and nutrient cycling that affect global elemental fluxes and may mediate biome shifts in response to global change. We also note that most studies performed to date have not been properly replicated and collectively suffer from strong geographical sampling bias towards temperate biomes. We advocate that combining carefully replicated field experiments and controlled laboratory experiments with isotope labelling and ‐omics techniques offers great promise towards understanding differences in ecophysiology and ecosystem services among mycorrhizal types.
In this review, we consider a traditional conceptual model of nutrient cycling in forests and evaluate 1) assumptions and issues with existing methods for measuring and calculating nutrient pools and fluxes, including the estimation of errors; 2) how various elements of the conceptual model vary with geographic and climatic region, and gaps in knowledge about certain regions; 3) predictions from nutrient cycling data for the effects of harvesting, burning, fertilization, and elevated CO2, including the effects of nutrient cycling on productivity and the effects of productivity on nutrient cycling. As is true of all models, traditional models of forest nutrient cycling are all incorrect in the sense that they are approximations and do not capture all features the real world. For example, none of these traditional models include the important effects of catastrophic events such as wildfire, insect attack, hurricanes, etc. Nonetheless, traditional nutrient cycling models have allowed us to explore the collective implications of our current understanding of nutrient cycling processes. While the methods apply to plantations the focus of this review has been on natural forest studies.
Despite much effort, reliable estimates of some transfers such as soil weathering and nitrogen fixation remain elusive. Soluble exports on a watershed level are not reliable representatives of exports from terrestrial nutrient cycles because correct conditions for such measurements are relatively rare and also because such estimates are subject to deep soil weathering and stream spiraling beyond the rooting zone. Soluble exports by lysimetery are subject to errors in the estimation of water flux and the delineation of the depth of rooting.
The current versions of these traditional models will no doubt require modifications in the future to account for new information becomes available, for example, the delays between root uptake and the appearance of nutrients in aboveground biomass, the importance of soil nutrient hotpots for uptake, and the unforeseen ability of nitrogen-limited trees to extract additional nitrogen from soils when root growth in stimulated by elevated CO2.
L’échec de régénération naturelle ou par plantation est un problème
récurrent dans les chênaies à molinie où la compétition pour les
ressources entre les deux espèces est particulièrement forte.
Un mode de gestion visant à contenir la molinie grâce à l’ombrage des
arbres adultes est un moyen de réduire la compétition interspécifique.
Cependant, si la compétition avec la molinie diminue, les jeunes chênes sont alors confrontés à une plus forte compétition avec les arbres adultes, notamment pour l’eau dans le sol. Il est donc nécessaire de doser la lumière par le couvert des arbres adultes, pour réduire la croissance de la molinie tout en veillant à ne pas induire une trop forte compétition pour l’eau entre les arbres adultes et les plants
de chêne. Selon notre étude expérimentale, une gamme de lumière
transmise comprise entre 35 à 45%, correspondant à une surface
terrière de 8 à 10 m² par ha, semble permettre ce compromis.
Chemical interactions in forested ecosystems play a role in driving biodiversity and ecosystem dynamics. Plant phenolics released by leaching can influence surrounding plants and soil organisms such as bacteria, fungi or arthropods. However, our knowledge about such chemically-mediated biotic interactions in Mediterranean oak forests is still limited, in particular whether they play a role in the limited forest regeneration. In this study, we analyzed how phenolics of Cotinus coggygria, a dominant shrub of Mediterranean downy oak (Quercus pubescens) forests, influence understory herbaceous plant species, downy oak regeneration and soil organisms in order to obtain a more integrative view of possible direct and indirect interactions triggered by this shrub species. We performed a series of experiments testing the effect of aqueous extracts of C. coggygria, mimicking natural lea-chates, on these organisms. Cotinus coggygria contained a high quantity of phenolics in green and senescent leaves but much less in leaf litter. Extracts from C. coggygria leaves stimulated bacterial communities, exhibited few effects on both saprophytic and symbiotic fungi, and negatively affected Collembola. Herbaceous species growth was particularly impaired by extracts from green and senescent leaves, although these effects were alleviated in the presence of soil microorganisms. In both greenhouse and field experiments, C. coggygria affected early oak seedling establishment in particular through a reduced root growth, but exhibited no effect on later seedling and sapling growth. We discussed the implication of these results for the balance between competition and facilitation in oak forests and concluded that C. coggygria has the potential to strongly alter biotic interactions , understory plant diversity and oak forest dynamics.
Abscisic acid (ABA) is the key phytohormone modeling the stomata behavior under drought conditions. However, stomata closure is not correlated with leaf ABA content in mycorrhizal olive plant. The aim of the present study is to evaluate the impact of arbuscular mycorrhizal (AM) symbiosis on the control of stomata functioning by ABA in two olive tree (Olea europaea L.) cultivars ‘Zarrazi’ and ‘Meski’ subjected to dehydration–rehydration treatment. AM-inoculated (Myc +) and non-inoculated (Myc-) olive plants were subjected to water stress and then rewatered (recovery). Leaf ABA content, stomatal conductance and transpiration rate were measured in: (1) irrigated control, (2) moderately and severely stressed and (3) recovered plants. In both Zarrazi and Meski Myc– plants, ABA content increased in parallel with drought intensity and stomatal closure. However, an intra-specific variability appeared in inoculated plants; in Meski Myc + plants ABA content was not influenced by drought and recovery treatments, whereas in Zarrazi Myc + plants the ABA amount increased under moderate water stress and even further after water relief, but decreased under severe water stress. However, in Myc + plants of both cultivars, stomatal closure was not correlated with leaf ABA content. The results reveal that the ABA is not the key factor controlling the stomatal closure in AM-inoculated olive plants under drought conditions. In fact, other AM-related factors are involved in the control of stomata regulation in mycorrhizal olive plants exposed to severe drought. These factors act specifically in the drought-resistant cultivar ‘Zarrazi’ permitting a suitable stomata behavior.
Forest conversion into agricultural land has resulted in a continuous decline in forest cover and in a reduced size and increased edge-to-core ratio of the remaining fragments. Forest edges are more directly exposed to sunlight, wind and pollutants and the resulting changes in habitat quality might have a large impact on plant and animal communities. Few studies, however, have focused on forest edge effects on mycorrhizal fungus communities. Here, we used high-throughput sequencing to study how communities of arbuscular mycorrhizal (AMF) and ectomycorrhizal fungi (EcMF), present in both the roots of the dual mycorrhizal tree Alnus glutinosa and in the soil, changed with increasing distance from the forest edge within fragmented forests embedded in an intensively managed agricultural matrix. Overall, we found 158 AMF OTUs and 275 EcMF OTUs. Soil moisture content increased with increasing distance from the forest edge, whereas soil nitrate concentration increased with increasing distance in south-facing and decreased in north-facing edges. Distance to the forest edge had a significant effect on EcMF community composition that largely overlapped with the observed changes in soil variables, especially soil moisture content. Apart from this distance effect, there were also clear effects of edge orientation on mycorrhizal diversity and community composition. While AMF OTU richness was higher at south- than at north-facing edges, the opposite pattern was found for EcMF. Community composition of both mycorrhiza types also differed significantly between south- and north-facing edges. We conclude that altered environmental conditions at forest edges cause significant changes in mycorrhizal communities, which could subsequently affect ecosystem functioning.