Biology subjects

Schnabel, F.

Publications and source records attributed to Schnabel, F..

13 recordsLinked to original sources

bioRxiv2026

Stomatal sensitivity to VPD across 38 tree species reflects past drought responses but is little explained by stomatal and leaf economic traits

Climate change is exposing trees to rising atmospheric water demand. Because stomata regulate plant water loss, understanding stomatal behaviour is critical. However, few comparative studies examine species-specific stomatal responses to atmospheric demand. Here, we quantify stomatal behaviour in 38 temperate tree species (27 broadleaved, 11 conifers) in a German research arboretum during a warm summer without soil drought. We measured the diurnal stomatal conductance (gs) of 787 leaves and modelled gs species-specific responses to leaf-to-air vapour pressure deficit (VPDL). Four descriptors of the gs-VPDL relationship were derived: maximum stomatal conductance (gmax), the VPDL that maximises gs, the VPDL associated with a 50% reduction in gs, and the stomatal closure rate. We relate these metrics to drought-induced shifts of carbon isotope ratios and to leaf functional traits. Large variation in stomatal behaviour exists, ranging from early-closure to VPDL-tolerant strategies. Species maintaining gs at higher VPDL also show weaker physiological responses to past drought. gmax is partly explained by leaf economic traits; other gs-VPDL descriptors are not related to stomatal anatomy and leaf economy. Our interspecific comparison reveals substantial variation in stomatal responses to atmospheric demand that is relevant to drought responses, but cannot be readily inferred from commonly measured leaf traits.

ecology↗

bioRxiv2025

Above- and below-ground trait coordination across 90 angiosperm and gymnosperm tree species

Quantifying the variation in plant traits reveals the trade-offs involved in plant ecological strategies and is fundamental to understanding underlying plant fitness mechanisms. Thus, the ecological success of plant species in a certain habitat may depend on the coordinated performance of both leaves and roots. However, despite the growing interest in trait variation, it is still uncertain i) to what extent the leaf economics spectrum (LES) and root economics space (RES) hold across locally coexisting tree species and ii) whether leaf and fine-root traits are correlated. In a research arboretum, we simultaneously measured eight key traits in leaves and fine-roots on 270 individuals belonging to 90 tree species, encompassing both angiosperm and gymnosperm species. We find varied plant resource strategies associated with leaves and fine-roots for angiosperms and gymnosperms. We observe a clear LES for gymnosperms and a clear RES for angiosperms. Our results support the existence of a correlation between analogous leaf and fine-root traits across all species. However, varying trait coordination across clades indicates varying resource acquisition strategies above- and belowground, highlighting the need to consider large-scale phylogenetic relatedness to better understand plant fitness.

ecology↗

bioRxiv2024

Multi-year drought strengthens positive and negative functional diversity effects on tree growth response

O_LIMixed-species forests are proposed as strategy to increase the resistance and resilience of forests to drought stress. However, evidence suggest that increasing tree species richness does not consistently enhance tree growth responses to drought. Moreover, tree diversity effects under unprecedented multiyear droughts remain uncertain, calling for a better understanding of the underlying processes. C_LIO_LIHere, we used a network of planted tree diversity experiments to investigate how drought-induced growth responses of individual trees are influenced by neighborhood tree diversity and the functional traits of the focal tree species. We analyzed tree cores (948 trees across 16 species) from nine experiments across Europe featuring gradients of tree species richness (1-6 species), which experienced severe droughts in recent years. Radial growth response to drought was quantified as tree-ring biomass increment using X-ray computed tomography. We applied hydraulic trait-based growth models to analyze single-year drought responses across all sites and site-specific responses during consecutive drought years for six sites as a function of neighborhood tree diversity. C_LIO_LIThe large variability in tree growth responses to a single-year drought was partially explained by the focal species hydraulic safety margin (representing species drought tolerance) and drought intensity, but independent of neighborhood species richness or functional trait diversity. However, tree diversity effects on growth responses strengthened during consecutive drought years and were site-specific, with contrasting direction (both positive and negative). This indicated opposing pathways of diversity effects under consecutive drought events, possibly resulting from competitive release or greater water consumption in diverse mixtures. C_LIO_LIWe conclude that tree diversity effects on growth responses to single-year droughts may differ considerably from responses to consecutive drought years. Our study highlights the need to consider trait-based approaches (specifically, hydraulic traits) and tree neighborhood scale processes to understand the multifaceted growth responses of tree mixtures under prolonged drought stress. C_LI

ecology↗

bioRxiv2024

Large-scale remote sensing reveals that tree mortality in Germany appears to be greater than previously expected

Global warming poses a major threat to forests and events of increased tree mortality are observed globally. Studying tree mortality often relies on local-level observations of dieback while large-scale analyses are lacking. Satellite remote sensing provides the spatial coverage and sufficiently high temporal and spatial resolution needed to investigate tree mortality at landscape-scale. However, adequate reference data for training satellite-based models are scarce. In this study, we employed the first maps of standing deadwood in Germany for the years 2018-2022 with 10 m spatial resolution that were created by using tree mortality observations spotted in hundreds of drone images as the reference. We use these maps to study spatial and temporal patterns of tree mortality in Germany and analyse their biotic and abiotic environmental drivers using random forest regression. In 2019, the second consecutive hotter drought year in a row, standing deadwood increased steeply to 334 {+/-} 189 kilohectar (kha) which corresponds to 2.5 {+/-} 1.4% of the total forested area in Germany. Picea abies, Pinus sylvestris, and Fagus sylvatica showed highest shares of standing deadwood. During 2018-2021 978 {+/-} 529 kha (7.9 {+/-} 4.4%) of standing dead trees accumulated. The higher mortality estimates that we report compared to other surveys (such as the ground-based forest condition survey) can be partially attributed to the fact that remote sensing captures mortality from a birds eye perspective and that the high spatial detail (10 m) in this study also captures scattered occurrences of tree mortality. Atmospheric drought (i.e., climatic water balance and vapor pressure deficit) and temperature extremes (i.e., number of hot days and frosts after vegetation onset) were the most important predictors of tree mortality. We found increased tree mortality for smaller and younger stands and also on less productive sites. Monospecific stands were generally not more affected by mortality, but only when interactions with damaging insects (e.g., bark beetles) occurred. Because excess tree mortality rates threaten many forests across the globe, similar analyses of tree mortality are warranted and technically feasible at the global scale. We encourage the international scientific community to share and compile local data on deadwood occurrences (see example: www.deadtrees.earth) as such a collaborative effort is required to help understand mortality events on a global scale.

ecology↗

bioRxiv2024

Forest dieback in drinking water protection areas - a hidden threat to water quality

For centuries, forests have been considered a natural safeguard for drinking water quality. We challenge this view in light of the rising frequency of climate extremes, such as droughts coinciding with high temperatures, which have caused unprecedented pulses of forest dieback globally. Drought-induced forest diebacks may jeopardize the crucial role of forests in protecting water quality, potentially even turning forests into sources of contamination. To underscore the critical importance of the topic, here we provide the first comprehensive assessment of forest cover, type, and dieback (assessed as canopy cover loss) across drinking Water Protection Areas (WPAs) in Germany, one of the countries hit most severely by the unprecedented Central European drought of 2018-2020. Our findings reveal a high forest cover of 43% in WPAs, from which a substantial amount of 5% canopy cover got lost within only three years as a direct or indirect consequence of the drought. Spruce-dominated forests, constituting 28% of all forests in WPAs, were particularly susceptible, but other dominant tree species also experienced anomalously high mortality rates. Combining this assessment with exemplary records of nitrate concentrations in the groundwater of WPAs revealed that forest dieback can significantly impair drinking quality. On average, nitrate concentrations more than doubled in WPAs with severe forest dieback, whereas nitrate concentrations did not significantly change in undisturbed WPAs. However, we also found pronounced differences between WPAs affected by forest dieback, underlining the need for further data and research to derive a generalizable understanding of the underlying mechanisms and controls. Based on this assessment, we deduce critical data and knowledge gaps essential to developing well-informed prediction, adaptation, and mitigation strategies. We call for interdisciplinary research addressing the hidden threat forest dieback poses for our drinking water resources.

biochemistry↗

bioRxiv2024

Functional traits explain growth resistance to successive hotter droughts across a wide set of common and future tree species in Europe

O_LIIn many regions worldwide, forests suffer from climate change-induced droughts. The hotter drought in Europe in 2018 with the consecutive drought years 2019 and 2020 caused large-scale growth declines and forest dieback. We investigated if tree growth responses to the 2018-2020 drought can be explained by tree functional traits related to drought tolerance, growth, and resource acquisition. C_LIO_LIWe assessed growth resistance, that is, growth during drought compared to pre-drought-conditions, in 71 planted tree species using branch shoot increments. We leveraged gap-filled trait data related to drought tolerance (P50, stomata density and conductivity), growth and resource acquisition (SLA, LNC, C:N, Amax) and wood density from the TRY database to explain growth resistance for gymnosperms and angiosperms. C_LIO_LIWe found significantly reduced growth during drought across all species. Legacy effects further decreased growth resistance in 2019 and 2020. Gymnosperms showed decreasing growth resistance with increasing P50 and acquisitiveness, such as high SLA, LNC, and Amax. Similar results were found for angiosperms, however, with less clear pattern. Four distinct response types emerged: Sufferer, Late sufferer, Recoverer and Resisters, with gymnosperms predominately falling into the Sufferer and Late sufferer categories. C_LIO_LIOur study provides evidence for significant growth reductions and legacy effects in response to consecutive hotter droughts, which can be explained by functional traits across a wide set of tree species. The a posteriori classification into response types revealed the diversity of temporal responses to a prolonged drought. We conclude that high drought tolerance bolsters growth resistance, while acquisitive species suffer stronger under drought. C_LI

ecology↗

bioRxiv2024

Tree diversity increases carbon stocks and fluxes above- but not belowground in a tropical forest experiment

International commitments advocate large-scale forest restoration as a nature-based solution to climate change mitigation through carbon (C) sequestration. Mounting evidence suggests that mixed compared to monospecific planted forests may sequester more C, exhibit lower susceptibility to climate extremes and offer a broader range of ecosystem services. However, experimental studies comprehensively examining the control of tree diversity on multiple C stocks and fluxes above- and belowground are lacking. To address this gap, we leverage data from the Sardinilla experiment in Panama, the oldest tropical tree diversity experiment which features a gradient of one-, two-, three- and five-species mixtures of native tree species. Over 16 years, we measured multiple above- and belowground C stocks and fluxes, ranging from tree aboveground C, over leaf litter C production, to soil organic carbon (SOC). We show that tree diversity significantly increased aboveground C stocks and fluxes, with a 57% higher gain in aboveground tree C in five-species mixtures compared to monocultures (35.7{+/-}1.8 vs 22.8{+/-}3.4 Mg C ha-1) 16 years after planting. In contrast, we observed a net reduction in SOC (on average -11.2{+/-}1.1 Mg C ha-1) and no significant difference in SOC3 stocks (the predominantly tree-derived, i.e., C3 plant-derived SOC fraction) between five-species mixtures and monocultures (13.0{+/-}0.9 vs 15.1{+/-}1.3 Mg C ha-1). Positive tree diversity effects persisted despite repeated climate extremes and strengthened over time for aboveground tree growth. Structural equation models showed that higher tree growth in mixtures enhanced leaf litter and coarse woody debris C fluxes to the soil, resulting in a tightly linked C cycle aboveground. However, we did not observe significant links between above- and belowground C stocks and fluxes. Our study elucidates the mechanisms through which higher tree diversity bolsters the climate mitigation potential of tropical forest restoration. Restoration schemes should prioritize mixed over monospecific planted forests.

ecology↗

bioRxiv2024

Diversity-enhanced canopy space occupation and leaf functional diversity jointly promote overyielding in tropical tree communities

Understanding the mechanisms that drive biodiversity-productivity relationships is critical for guiding forest restoration. Although complementarity among trees in the canopy space has been suggested as a key mechanism for greater productivity in mixed-species tree communities, empirical evidence remains limited. Here, we used data from a tropical tree diversity experiment to disentangle the effects of tree species richness and community functional characteristics (community-weighted mean and functional diversity of leaf traits) on canopy space filling, and how these effects are related to overyielding. We found that canopy space filling was largely explained by species identity effects rather than tree diversity effects. Communities with a high abundance of species with conservative leaf traits were those with most densely packed canopies. Overall, a higher canopy space filling translated into an enhanced wood productivity, with communities associated with a high taxonomic and functional diversity being the most productive. Importantly, most communities (83%) produced more wood volume than the average of their constituent species in monoculture (i.e. most communities overyielded). Our results show that overyielding increased with leaf functional diversity and positive net biodiversity effects on canopy space filling, which mainly arose due to a high taxonomic diversity. These findings suggest that both taxonomic diversity-enhanced canopy space filling and canopy leaf diversity are important drivers for overyielding in mixed-species forests. Consequently, restoration initiatives should promote stands with functionally diverse canopies by selecting tree species with large interspecific differences in leaf nutrition, as well as leaf and branch morphology to optimize carbon capture in young forest stands.

ecology↗

bioRxiv2024

Forest growth resistance and resilience to the 2018-2020 drought depend on tree diversity and mycorrhizal type

O_LIThe frequency of consecutive drought years is predicted to increase due to climate change. These droughts have strong negative impacts on forest ecosystems. Mixing tree species is proposed to increase the drought resistance and resilience of tree communities. However, this promising diversity effect has not yet been investigated under extreme drought conditions and in the context of complementary mycorrhizal associations and their potential role in enhancing water uptake. C_LIO_LIHere, we investigate whether tree diversity promotes growth resistance and resilience to extreme drought and whether drought responses are modulated by mycorrhizal associations. We used inventory data (2015 - 2021) of a young tree diversity experiment in Germany, manipulating tree species richness (1, 2, 4 species) and mycorrhizal type (communities containing arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) tree species, or both). For all tree communities, we calculated basal area increment (BAI) in the periods before, during, and after the drought and used the concepts of resistance and resilience to quantify growth responses to drought. C_LIO_LIWe found strong growth declines during the extreme 2018-2020 drought for most of the tree communities. Contrary to our hypothesis, we did not find that tree species richness per se can buffer the negative impacts of extreme drought on tree growth. However, while for EM communities, drought resistance, and resilience decreased with tree species richness, they increased for AM communities and communities comprising both mycorrhizal types. We highlight that among various tree species mixtures, only those with mixed mycorrhizal types outperformed their respective monocultures during and after drought. Further, under extreme drought, the community tends to segregate into "winner" and "loser" tree species in terms of diversity, indicating a potential intensification of competition. C_LIO_LIWhile we cannot disentangle the underlying mechanisms, or clarify the role of mycorrhiza during drought, our findings suggest that mixtures of mycorrhizal types within tree communities could help safeguard forests against increasing drought frequency. C_LIO_LISynthesis: Drought resistance and resilience of tree communities depend on tree diversity and mycorrhizal association types. Mixing tree species with diverse mycorrhizal types holds promise for forest restoration in the face of climate change. C_LI

ecology↗

bioRxiv2023

Tree diversity increases forest temperature buffering

Global warming is increasing the frequency and intensity of climate extremes. Forests may buffer such extreme events by creating their own microclimate below their canopy via cooling hot and insulating against cold macroclimate air temperatures. This buffering capacity of forests may be increased by tree diversity and may itself maintain forest functioning and biodiversity. However, despite its relevance for many ecosystem processes, the effect of tree diversity on temperature buffering is largely unexplored. Here, we show that tree species richness consistently increases forest temperature buffering across daily, monthly, and annual scales over six years. This finding is based on data from a large-scale tree diversity experiment covering a species richness gradient of 1 to 24 tree species. We found that species richness strengthened both components of forest temperature buffering: the attenuation of hot and of cold macroclimate air temperatures, with the cooling effect being more pronounced. The buffering effect of tree species richness was mediated by canopy density and structural diversity, assessed as leaf area index and stand structural complexity index, respectively. Safeguarding and planting diverse forests may thus mitigate negative effects of global warming and climate extremes on ecosystem functions and communities below the tree canopy.

ecology↗

bioRxiv2022

Neighbourhood species richness and drought-tolerance traits modulate tree growth and δ13C responses to drought

O_LIMixed-species forests are promoted as a forest management strategy for climate change adaptation, but whether they are more resistant to drought than monospecific forests remains contested. Particularly, the trait-based mechanisms driving the role of tree diversity under drought remain elusive. C_LIO_LIUsing tree cores from a large-scale biodiversity experiment, we investigated tree growth and physiological stress responses (i.e. increase in wood carbon isotopic ratio; {delta}13C) to changes in climate-induced water availability (wet to dry years) along gradients in neighbourhood tree species richness and drought-tolerance traits. We hypothesized that neighbourhood species richness increases growth and decreases {delta}13C and that these relationships are modulated by the abiotic (i.e. climatic conditions) and the biotic context. We characterized the biotic context using drought-tolerance traits of focal trees and their neighbours. These traits are related to cavitation resistance vs resource acquisition and stomatal control. C_LIO_LITree growth increased with neighbourhood species richness. However, we did not observe a universal relief of water stress in species-rich neighbourhoods. Neighbourhood species richness effects on growth and {delta}13C did not strengthen from wet to dry years. Instead, richness-growth and richness-{delta}13C relationships were modulated by climatic conditions and the traits of trees and their neighbours. At either end of each drought-tolerance gradient, species responded in opposing directions during drought and non-drought years. C_LIO_LIWe show that species drought-tolerance traits can explain the strength and nature of biodiversity-ecosystem functioning relationships in experimental tree communities experiencing drought. Mixing tree species can increase growth but may not universally relieve drought stress. C_LI One-sentence summaryThe drought-tolerance traits of trees and their neighbours determine biodiversity-ecosystem functioning relationships in experimental tree communities.

ecology↗

bioRxiv2021

Cumulative growth and stress responses to the 2018-2019 drought in a European floodplain forest

Droughts increasingly threaten the worlds forests and their potential to mitigate climate change. In 2018-2019, Central European forests were hit by two consecutive hotter drought years, an unprecedented phenomenon that is likely to occur more frequently with climate change. Here, we examine tree growth and physiological stress responses (increase in carbon isotope composition; {Delta}{delta}13C) to this consecutive drought based on tree-rings of dominant tree species in a Central European floodplain forest. Tree growth was not reduced for most species in 2018, indicating that water supply in floodplain forests can partly buffer meteorological water deficits. Drought stress responses in 2018 were comparable to former single drought years but the hotter drought in 2018 induced drought legacies in tree growth while former droughts did not. We observed strong decreases in tree growth and increases in {Delta}{delta}13C across all tree species in 2019, which are likely driven by the cumulative stress both consecutive hotter droughts exerted. Our results show that consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with comparably high levels of water supply.

ecology↗

bioRxiv2021

Hydraulic diversity stabilizes productivity in a large-scale subtropical tree biodiversity experiment

Extreme climatic events threaten forests and their climate mitigation potential globally. Understanding the drivers promoting ecosystem stability is therefore considered crucial to mitigate adverse climate change effects on forests. Here, we use structural equation models to explain how tree species richness, asynchronous species dynamics and diversity in hydraulic traits affect the stability of forest productivity along an experimentally manipulated biodiversity gradient ranging from 1 to 24 tree species. Tree species richness improved stability by increasing species asynchrony. That is, at higher species richness, inter-annual variation in productivity among tree species buffered the community against stress-related productivity declines. This effect was mediated by the diversity of species hydraulic traits regarding drought tolerance and stomatal control, but not by the community-weighted means of these traits. The identified mechanisms by which tree species richness stabilizes forest productivity emphasize the importance of hydraulically diverse, mixed-species forests to adapt to climate change.

ecology↗