• Energy Research

Abstract Biodiversity is considered to mitigate detrimental impacts of climate change on the functioning of forest ecosystems, such as drought‐induced decline in forest productivity. However, previous studies produced controversial results and experimental evidence is rare. Specifically, the biological mechanisms underlying mitigation effects remain unclear, as existing work focuses on biodiversity effects related to the community scale. Using trait‐based neighbourhood models, we quantified changes in above‐ground wood productivity of 3,397 trees that were planted in a large‐scale tree diversity experiment in subtropical China across gradients of neighbourhood diversity and climatic conditions over a 6‐year period. This approach allowed us to simultaneously assess to what extent functional traits of a focal tree and biodiversity at the local neighbourhood scale mediate the growth response of individual trees to drought events. We found that neighbourhood tree species richness can mitigate for drought‐induced growth decline of young trees. Overall, positive net biodiversity effects were strongest during drought and increased with increasing taxonomic diversity of neighbours. In particular, drought‐sensitive species (i.e. those with a low cavitation resistance) benefitted the most from growing in diverse neighbourhoods, suggesting that soil water partitioning among local neighbours during drought particularly facilitated most vulnerable individuals. Thus, diverse neighbourhoods may enhance ecosystem resistance to drought by locally supporting drought‐sensitive species in the community. Synthesis. Our findings demonstrate that mechanisms operating at the local neighbourhood scale are a key component for regulating forests responses to drought and improve insights into how local species interactions vary along stress gradients in highly diverse tree communities.

Abstract Biodiversity is considered to mitigate detrimental impacts of climate change on the functioning of forest ecosystems, such as drought‐induced decline in forest productivity. However, previous studies produced controversial results and experimental evidence is rare. Specifically, the biological mechanisms underlying mitigation effects remain unclear, as existing work focuses on biodiversity effects related to the community scale. Using trait‐based neighbourhood models, we quantified changes in above‐ground wood productivity of 3,397 trees that were planted in a large‐scale tree diversity experiment in subtropical China across gradients of neighbourhood diversity and climatic conditions over a 6‐year period. This approach allowed us to simultaneously assess to what extent functional traits of a focal tree and biodiversity at the local neighbourhood scale mediate the growth response of individual trees to drought events. We found that neighbourhood tree species richness can mitigate for drought‐induced growth decline of young trees. Overall, positive net biodiversity effects were strongest during drought and increased with increasing taxonomic diversity of neighbours. In particular, drought‐sensitive species (i.e. those with a low cavitation resistance) benefitted the most from growing in diverse neighbourhoods, suggesting that soil water partitioning among local neighbours during drought particularly facilitated most vulnerable individuals. Thus, diverse neighbourhoods may enhance ecosystem resistance to drought by locally supporting drought‐sensitive species in the community. Synthesis. Our findings demonstrate that mechanisms operating at the local neighbourhood scale are a key component for regulating forests responses to drought and improve insights into how local species interactions vary along stress gradients in highly diverse tree communities.

SummaryVariations in crown forms promote canopy space‐use and productivity in mixed‐species forests. However, we have a limited understanding on how this response is mediated by changes in within‐tree biomass allocation. Here, we explored the role of changes in tree allometry, biomass allocation and architecture in shaping diversity–productivity relationships (DPRs) in the oldest tropical tree diversity experiment.We conducted whole‐tree destructive biomass measurements and terrestrial laser scanning. Spatially explicit models were built at the tree level to investigate the effects of tree size and local neighbourhood conditions. Results were then upscaled to the stand level, and mixture effects were explored using a bootstrapping procedure.Biomass allocation and architecture substantially changed in mixtures, which resulted from both tree‐size effects and neighbourhood‐mediated plasticity. Shifts in biomass allocation among branch orders explained substantial shares of the observed overyielding. By contrast, root‐to‐shoot ratios, as well as the allometric relationships between tree basal area and aboveground biomass, were little affected by the local neighbourhood.Our results suggest that generic allometric equations can be used to estimate forest aboveground biomass overyielding from diameter inventory data. Overall, we demonstrate that shifts in tree biomass allocation are mediated by the local neighbourhood and promote DPRs in tropical forests.

SummaryVariations in crown forms promote canopy space‐use and productivity in mixed‐species forests. However, we have a limited understanding on how this response is mediated by changes in within‐tree biomass allocation. Here, we explored the role of changes in tree allometry, biomass allocation and architecture in shaping diversity–productivity relationships (DPRs) in the oldest tropical tree diversity experiment.We conducted whole‐tree destructive biomass measurements and terrestrial laser scanning. Spatially explicit models were built at the tree level to investigate the effects of tree size and local neighbourhood conditions. Results were then upscaled to the stand level, and mixture effects were explored using a bootstrapping procedure.Biomass allocation and architecture substantially changed in mixtures, which resulted from both tree‐size effects and neighbourhood‐mediated plasticity. Shifts in biomass allocation among branch orders explained substantial shares of the observed overyielding. By contrast, root‐to‐shoot ratios, as well as the allometric relationships between tree basal area and aboveground biomass, were little affected by the local neighbourhood.Our results suggest that generic allometric equations can be used to estimate forest aboveground biomass overyielding from diameter inventory data. Overall, we demonstrate that shifts in tree biomass allocation are mediated by the local neighbourhood and promote DPRs in tropical forests.

AbstractMixed-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.Using tree cores from a large-scale biodiversity experiment, we investigated tree growth and physiological stress responses (i.e. increase in wood carbon isotopic ratio; δ13C) 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 δ13C 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.Tree 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 δ13C did not strengthen from wet to dry years. Instead, richness-growth and richness-δ13C 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.We 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.One-sentence summaryThe drought-tolerance traits of trees and their neighbours determine biodiversity-ecosystem functioning relationships in experimental tree communities.

AbstractMixed-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.Using tree cores from a large-scale biodiversity experiment, we investigated tree growth and physiological stress responses (i.e. increase in wood carbon isotopic ratio; δ13C) 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 δ13C 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.Tree 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 δ13C did not strengthen from wet to dry years. Instead, richness-growth and richness-δ13C 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.We 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.One-sentence summaryThe drought-tolerance traits of trees and their neighbours determine biodiversity-ecosystem functioning relationships in experimental tree communities.