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Previous studies have shown that tree species richness increases forest productivity by allowing for greater spatial complementarity of tree crowns (crown complementarity), which in turn results in more densely packed canopies. However, the mechanisms driving crown complementarity in tree species mixtures remain unclear. Here, we take advantage of a high-resolution, three-dimensional terrestrial laser scanning approach in the context of a large-scale biodiversity-ecosystem functioning experiment in subtropical China (BEF-China) to quantify the extent to which functional dissimilarity and divergences in branch traits between neighbouring trees affect crown complementarity at the scale of tree species pairs (i.e., two adjacent trees). Overall, we found no support that functional dissimilarity (divergence in morphological flexibility, specific leaf area and wood density) promotes crown complementarity. However, we show that the effects of functional dissimilarity (the plasticity of the outer crown structure) on crown complementarity vary in their magnitude and importance depending on branch trait divergences. Firstly, crown complementarity tended to be highest for tree species pairs that strongly differed in their functional traits, but were similar in branch density. In contrast, heterospecific pairs with a low functional trait divergence benefitted the most from a large difference in branch density compared with pairs characterised by a large functional dissimilarity. Secondly, the positive effects of increasing divergence in branching intensity (the plasticity of the inner crown structure) on crown complementarity became most important at low levels of functional dissimilarity, i.e. when species pairs were similar in their branch packing and vice versa. This suggests that species mixing allows trees to occupy canopy space more efficiently mainly due to phenotypic changes associated with crown morphology and branch plasticity. Our findings highlight the importance of considering outer and inner crown structures (e.g. branching architecture) to deepen our understanding of tree-tree interactions in mixed-species communities.

Global change exposes forest ecosystems to many risks including novel climatic conditions, increased frequency of climatic extremes and sudden emergence and spread of pests and pathogens. At the same time, forest landscape restoration has regained global attention as an integral strategy for climate change mitigation. Owing to unpredictable future risks and the need for new forests that provide multiple ecosystem services, mixed-species forests have been advocated for this purpose. However, the successful establishment of mixed forests requires intrinsic knowledge of biodiversity's role for forest ecosystem functioning. In this respect, a better understanding of tree-tree interactions and how they contribute to observed positive tree species richness effects on key ecosystem functions is critical. Here, we review the current knowledge of the underlying mechanisms of tree-tree interactions and argue that positive net biodiversity effects at the community scale may emerge from the dominance of positive over negative interactions at the local neighbourhood scale. In a second step, we demonstrate how tree-tree interactions and the immediate tree neighbourhood's role can be systematically assessed in a tree diversity experiment. The expected results will improve predictions about the effects of tree interactions on ecosystem functioning based on general principles. We argue that this knowledge is urgently required to guide the design of tree species mixtures for the successful establishment of newly planted forests.

Though competition operates distinctly above and below ground, competition with near neighbours explained more variation in crown and root system morphology of young tropical trees than did abiotic conditions. Comparisons of above- and below-ground tree architecture and variation remain scarce in the literature of competition dynamics in forests. Trees compete above and below ground with neighbours, but the extent of such a neighbourhood and the relative importance of above- or below-ground neighbours in determining tree growth rates remain poorly understood. We combined terrestrial laser scans and a root excavation campaign in the tropical planted Sardinilla forest experiment with annual diameter-at-breast height measurements to test how the crowns and root systems of five tropical tree species (N = 128) varied with biotic neighbourhood and abiotic environmental characteristics. Crown and root system extent were compared, and the Hegyi competition index calculated at three spatial scales was regressed on focal tree biomass. We identified which neighbourhood and environmental variables explained most of the variation in mean crown and root system extent and compactness (a measure of morphological homogeneity) at the plot level and for species separately. Root systems were 2.6 ± 1.1 times wider than crowns on average and focal tree biomass was the best predicted by competition with near neighbours. At the plot level, more variation in crown and root system traits was explained by biotic than abiotic characteristics (65.8 and 26.0%, respectively). More intense competition in the near neighbourhood reduced crown extent of one study species (Anacardium excelsum) and root system extent of 4 out of 5 study species, and it also led to more compact crowns and root systems of Tabebuia rosea. After accounting for spatial autocorrelation in focal tree location, soil nitrogen concentration was not found to significantly affect the studied morphological traits. These results emphasize the role of competition in determining tree morphology generally, while also supporting the claim that competition operates distinctly above and below ground.

Background: Species-specific genotypic features, local neighbourhood interactions and resource supply stronglyinfluence the tree stature and growth rate. In mixed-species forests, diversity-mediated biomass allocation has beensuggested to be a fundamental mechanism underlying the positive biodiversity-productivity relationships. Empiricalevidence, however, is rare about the impact of local neighbourhood diversity on tree characteristics analysed at avery high level of detail. To address this issue we analysed these effects on the individual-tree crown architectureand tree productivity in a mature mixed forest in northern Germany. Methods: Our analysis considers multiple target tree species across a local neighbourhood species richnessgradient ranging from 1 to 4. We applied terrestrial laser scanning to quantify a large number of individual maturetrees (N= 920) at very high accuracy. We evaluated two different neighbour inclusion approaches by analysing botha fixed radius selection procedure and a selection based on overlapping crowns. Results and conclusions: We show that local neighbourhood species diversity significantly increases crowndimension and wood volume of target trees. Moreover, we found a size-dependency of diversity effects on treeproductivity (basal area and wood volume increment) with positive effects for large-sized trees (diameter at breastheight (DBH) > 40 cm) and negative effects for small-sized (DBH < 40 cm) trees. In our analysis, the neighbourinclusion approach has a significant impact on the outcome. For scientific studies and the validation of growthmodels we recommend a neighbour selection by overlapping crowns, because this seems to be the relevant scaleat which local neighbourhood interactions occur. Because local neighbourhood diversity promotes individual-treeproductivity in mature European mixed-species forests, we conclude that a small-scale species mixture should beconsidered in management plans. Background: Species-specific genotypic features, local neighbourhood interactions and resource supply strongly influence the tree stature and growth rate. In mixed-species forests, diversity-mediated biomass allocation has been suggested to be a fundamental mechanism underlying the positive biodiversity-productivity relationships. Empirical evidence, however, is rare about the impact of local neighbourhood diversity on tree characteristics analysed at a very high level of detail. To address this issue we analysed these effects on the individual-tree crown architecture and tree productivity in a mature mixed forest in northern Germany. Methods: Our analysis considers multiple target tree species across a local neighbourhood species richness gradient ranging from 1 to 4. We applied terrestrial laser scanning to quantify a large number of individual mature trees (N = 920) at very high accuracy. We evaluated two different neighbour inclusion approaches by analysing both a fixed radius selection procedure and a selection based on overlapping crowns. Results and conclusions: We show that local neighbourhood species diversity significantly increases crown dimension and wood volume of target trees. Moreover, we found a size-dependency of diversity effects on tree productivity (basal area and wood volume increment) with positive effects for large-sized trees (diameter at breast height (DBH) > 40 cm) and negative effects for small-sized (DBH < 40 cm) trees. In our analysis, the neighbour inclusion approach has a significant impact on the outcome. For scientific studies and the validation of growth models we recommend a neighbour selection by overlapping crowns, because this seems to be the relevant scale at which local neighbourhood interactions occur. Because local neighbourhood diversity promotes individual-tree productivity in mature European mixed-species forests, we conclude that a small-scale species mixture should be considered in management plans.

Plant diversity loss can alter higher trophic -level communities via non -random species interactions, which in turn may cascade to affect key ecosystem func-tions. These non -random linkages might be best captured by patterns of phyloge-netic diversity, which take into account co -evolutionary dependencies. However,lack of adequate phylogenetic data of higher trophic levels hampers our mecha-nistic understanding of biodiversity relationships in species-rich ecosystems.2. We used DNA barcoding to generate data on the phylogenetic diversity of lepi-dopteran caterpillars in a large- scale forest biodiversity experiment in subtropicalChina. We analysed how different metrics of lepidopteran phylogenetic diversity(Faith's PD, MPD, MNTD) and taxonomic diversity were influenced by multiple components of tree diversity (taxonomic, functional, phylogenetic).3. Our data from six sampling periods represent 7,204 mitochondrial cytochrome c oxidase subunit I (COI) sequences of lepidopteran larvae, clustered into 461 mo-lecular operational taxonomic units. Lepidopteran abundance, the effective num-ber of species (irrespective of the focus on rare or common species) and Faith's PD and MPD (reflecting basal evolutionary splits), but not MNTD (reflecting re-cent evolutionary splits), significantly increased with experimentally manipulated ree species richness. Lepidopteran MNTD decreased with increasing tree MNTD. Path analyses showed that tree phylogenetic and functional diversity explained part, but not all of the effects of tree species richness on lepidopteran diversity. Importantly, tree diversity effects on lepidopteran diversity were to a large extent indirect, operating via changes in lepidopteran abundance. 4. Synthesis. Our study shows that evolutionary dependencies determine the response of herbivore communities to changes in host plant diversity. Incorporating a wider range of diversity metrics both at the level of producers and consumers can thus help to develop a more comprehensive understanding of the functional consequences of biodiversity change across trophic levels. Moreover, the dependence of trophic linkages on herbivore abundances underlines the need to address the consequences of current declines in insect abundances for ecosystem structure and functioning. peerReviewed

Forest liming is often used as a mitigation tool to counteract soil acidification that is caused by atmospheric deposition. However, our understanding of the long-term effects of liming on forest communities is still incomplete. We examine the effects of repeated liming on the herb layer of spruce stands in the Ore Mountains in East Germany. The area is characterised by acidic bedrock and profound soil acidification caused by high levels of anthropogenic sulphur deposition in the late twentieth century. Lime has been applied as part of forest management since 1988. We created a stratified sample in which we recorded vegetation releves covering spruce stands without liming (L0) and with one to five lime applications (L1–L5). We found a positive correlation between liming intensity and alpha diversity. Most of the indicator species were identified for spruce stands with high liming intensity (L2–L5), but no indicators were found for unlimed stands. From the third liming, the impact of lime application on species diversity and composition saturated. Several of the species promoted by liming are typical for mature stands of the natural vegetation but had declined in the twentieth century. Thus, a slight trend towards the restoration of the natural vegetation can be seen after liming. However, this is contrasted with a strong increase in species that are disturbance indicators (i.e. species of nitrogen-rich soils and open habitats). Continuation of liming as part of regular forestry management will cause these effects to establish permanently over large areas. Based on these findings, we make suggestions to adapt forest management strategies.

Biodiversity is considered to mitigate the adverse effects of changing precipitation patterns. However, our understanding of how tree diversity at the local neighbourhood scale modulates the water use and leaf physiology of individual trees remains unclear. We made use of a large-scale tree diversity experiment in subtropical China to study eight tree species along an experimentally manipulated gradient of local neighbourhood tree species richness. Twig wood carbon isotope composition (δ13Cwood) was used as an indicator for immediate leaf-level responses to water availability in relation to local neighbourhood conditions and a target tree's functional traits. Across species, a target tree's δ13Cwood signatures decreased progressively with increasing neighbourhood species richness, with effects being strongest at high neighbourhood shading intensity. Moreover, the δ13Cwood-shading relationship shifted from positive (thin-leaved species) or neutral (thick-leaved species) in conspecific to negative in heterospecific neighbourhoods, most likely owing to a lower interspecific competition for water and microclimate amelioration. This suggests that promoting tree species richness at the local neighbourhood scale may improve a tree's local water supply with potential effects for an optimized water-use efficiency of tree communities during drought. This assumption, however, requires validation by further studies that focus on mechanisms that regulate the water availability in mixtures. Biodiversity is considered to mitigate the adverse effects of changing precipitation patterns. However, our understanding of how tree diversity at the local neighbourhood scale modulates the water use and leaf physiology of individual trees remains unclear. We made use of a large-scale tree diversity experiment in subtropical China to study eight tree species along an experimentally manipulated gradient of local neighbourhood tree species richness. Twig wood carbon isotope composition (δ13Cwood) was used as an indicator for immediate leaf-level responses to water availability in relation to local neighbourhood conditions and a target tree’s functional traits. Across species, a target tree’s δ13Cwood signatures decreased progressively with increasing neighbourhood species richness, with effects beingstrongest at high neighbourhood shading intensity. Moreover, the δ13Cwood-shading relationship shifted from positive (thin-leaved species) or neutral (thick-leaved species) in conspecific to negative in heterospecific neighbourhoods, most likely owing to a lower interspecific competition for water and microclimate amelioration. This suggests that promoting tree species richness at the local neighbourhood scale may improve a tree’slocal water supply with potential effects for an optimized water-use efficiencyof tree communities during drought. This assumption, however, requires validation by further studies that focus on mechanisms that regulate the water availability in mixtures.

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.

A large proportion of temperate forest plant diversity is found in the herb layer.However, for many of its species, little is known about their autecology, whichmakes it difficult to assess potential threats and efficiently safeguard the diversity of understorey herbaceous communities. This also applies to Gageaspathacea (Liliaceae), a globally rare spring geophyte, which mainly occurs indeciduous forests of northern Central Europe. We investigated the causal relationships between population characteristics of G. spathacea and abiotic siteconditions across different forest communities in the center of its distributionalrange. Leaf length (a surrogate of the species' vegetative propagation) was positively related to soil moisture and soil nitrogen. Consequently, mean leaflength was highest in moist forest communities of the alliance Alno-Ulmion.Moreover, mean variability in leaf length was lowest in those forests, indicatinga higher and more stable vegetative propagation via bulbils. We found no support for a significant relationship between leaf length and leaf density orbetween leaf length and flower formation. Population density varied stronglyamong forest sites, but was not related to soil moisture and hardly influenced by soil nitrogen. Our results suggest that soil water and nutrient supply play avital role in determining the species' vegetative propagation, whereas the duration of habitat continuity is most likely an important determinant of population size and density. Conservation strategies therefore require a betterunderstanding of the complex interrelationships between abiotic site conditionsand the historical context-dependency of habitats. A large proportion of temperate forest plant diversity is found in the herb layer. However, for many of its species, little is known about their autecology, which makes it difficult to assess potential threats and efficiently safeguard the diversity of understorey herbaceous communities. This also applies to Gagea spathacea (Liliaceae), a globally rare spring geophyte, which mainly occurs in deciduous forests of northern Central Europe. We investigated the causal relationships between population characteristics of G. spathacea and abiotic site conditions across different forest communities in the center of its distributional range. Leaf length (a surrogate of the species' vegetative propagation) was positively related to soil moisture and soil nitrogen. Consequently, mean leaf length was highest in moist forest communities of the alliance Alno‐Ulmion. Moreover, mean variability in leaf length was lowest in those forests, indicating a higher and more stable vegetative propagation via bulbils. We found no support for a significant relationship between leaf length and leaf density or between leaf length and flower formation. Population density varied strongly among forest sites, but was not related to soil moisture and hardly influenced by soil nitrogen. Our results suggest that soil water and nutrient supply play a vital role in determining the species' vegetative propagation, whereas the duration of habitat continuity is most likely an important determinant of population size and density. Conservation strategies therefore require a better understanding of the complex interrelationships between abiotic site conditions and the historical context‐dependency of habitats.

1. Diversity of producers (e.g. plants) usually increases the diversity of associated organisms, but the scale (i.e. the spatial area of plant diversity considered) at which plant diversity acts on other taxa has rarely been studied. Most evidence for cross‐taxon diversity relations come from above‐ground consumers that directly interact with plants.2. Experimental tests of plant diversity effects on elusive organisms inhabiting the leaf litter layer, which are important for nutrient cycling and decomposition, are rare.3. Using a large tree diversity experiment, we tested whether tree diversity at the larger plot (i.e. community) or the smaller neighbourhood scale relates to the abundance, species richness, functional and phylogenetic diversity of leaf litter ants, which are dominant organisms in brown food webs.4. Contrary to our expectations of scale‐independent positive tree diversity effects, ant diversity increased only with plot but not neighbourhood tree diversity. While the exact causal mechanisms are unclear, nest relocation or small‐scale competition among ants may explain the stronger tree diversity effects at the plot scale.5. Our results indicate that even for small and less mobile organisms in the leaf litter, effects of tree diversity are stronger at relatively larger scales. The finding emphasizes the importance of diverse forest stands, in which mixing of tree species is not restricted to small patches, for supporting arthropod diversity in the leaf litter. Diversity of producers (e.g. plants) usually increases the diversity of associated organisms, but the scale (i.e. the spatial area of plant diversity considered) at which plant diversity acts on other taxa has rarely been studied. Most evidence for cross-taxon diversity relations come from above-ground consumers that directly interact with plants. Experimental tests of plant diversity effects on elusive organisms inhabiting the leaf litter layer, which are important for nutrient cycling and decomposition, are rare. Using a large tree diversity experiment, we tested whether tree diversity at the larger plot (i.e. community) or the smaller neighbourhood scale relates to the abundance, species richness, functional and phylogenetic diversity of leaf litter ants, which are dominant organisms in brown food webs. Contrary to our expectations of scale-independent positive tree diversity effects, ant diversity increased only with plot but not neighbourhood tree diversity. While the exact causal mechanisms are unclear, nest relocation or small-scale competition among ants may explain the stronger tree diversity effects at the plot scale. Our results indicate that even for small and less mobile organisms in the leaf litter, effects of tree diversity are stronger at relatively larger scales. The finding emphasizes the importance of diverse forest stands, in which mixing of tree species is not restricted to small patches, for supporting arthropod diversity in the leaf litter.

The abandonment of traditional pastoralism as well as the use of heath areas for military purposes has had a major impact on dry heaths in the Continental biogeographical region of Europe, causing severe degradation of its key species Calluna vulgaris (L.) HULL. The reproductive potential of this species in a Continental climate is assumed to be low, although there is yet no observational or experimental evidence for this. More knowledge is also needed about cost-effective and sustainable measures to restore abandoned dry heaths in this biogeographical region, because traditional management options are often too expensive (e.g., sod-cutting) or restricted due to environmental laws and the danger of unexploded ammunition (e.g., burning). Using as an example an 800 ha Continental heathland in Germany that has been abandoned for about two decades, we studied the reproductive potential (seed production, soil seed bank, and germination ability) of degenerate C. vulgaris stands. In addition, we conducted a comprehensive field experiment to test the effects of low-intensity, year-round grazing by Heck cattle and Konik horses as well as one-time mowing and patchy exposure of bare soil on the generative rejuvenation (i.e., recruitment and survival) of degenerate C. vulgaris stands over 3 years. We used generalized linear mixed models for statistical analyses. Seed production of degenerate C. vulgaris stands was high as well as the germination ability of their seeds, being similar to Atlantic heathlands. However, soil seed-bank densities were lower than those found in managed or abandoned Atlantic heaths. Overall seedling recruitment in the field was considerably lower in comparison with Atlantic heaths. Low-intensity grazing or one-time mowing did not induce a substantial increase in C. vulgaris recruitment, whereas an additional one-time creation of bare soil patches or the one-time creation of bare soil without subsequent management significantly facilitated seedling recruitment and survival in the first year. However, from the second year on, the positive effect of the creation of bare soil without subsequent management was no longer present. In the third year, survival of juveniles was significantly supported by low-intensity grazing in combination with shallow soil disturbances as well as in combination with one-time mowing and shallow soil disturbances, whereas mowing alone resulted in marginally significant lower survival. The extremely low seedling recruitment requires a careful choice of suitable management measures to promote the survival of sufficient numbers of Calluna individuals. Therefore, we recommend low-intensity grazing with free-ranging robust breeds and the combination of this with one-time mowing as an effective means of supporting generative rejuvenation of C. vulgaris in degraded heaths. However, at the beginning of the restoration process, the creation of bare soil patches for seedling recruitment is crucial. For implementation into practice, we present different strategies to enhance the proportion of bare soil after long-term abandonment of heaths when traditional management options are no longer feasible.

International audience; Variations 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.

1. Understanding how species diversity affects plant performance is a central question in biodiversity–ecosystem functioning (BEF) research. At the community level, functional trait means and trait dissimilarities have been used to explain biodiversity effects, but with mixed success. To disentangle how functional traits explain community growth and underpin biodiversity–ecosystem functioning relationships, we should elucidate how plant traits affect individual growth across species richness levels, because the role of functional traits on growth depends on the ecological context of the individual.2. We addressed this topic by using detailed data of twelve functional traits and annual growth for 529 individual trees of 31 species at five species‐richness levels in a large forest biodiversity experiment in south‐east China from 4 to 9 years after planting.3. Our analyses show first that individual trait values can change with species richness, indicating that trait values can change due to the biotic context. Secondly, we show that early tree growth is more strongly affected by traits than by species richness. Finally, our data show that growth–trait relationships can change with species richness and with forest age. Trait effects on growth are more pronounced at higher richness levels, indicating that measuring traits on individual trees across richness levels can improve growth predictions and inference of BEF relationships that are shaped by functional traits.4. Synthesis. This study shows that functional trait values and their effect on individual tree growth depend on species richness. Our data support the notion that to elucidate how functional traits shape biodiversity–ecosystem functioning relationships, an important step is to consider the biotic context of individual trees within a community. We have made an initial step by analysing how functional traits affect individual‐tree growth in a diversity‐dependent manner and future research should continue by elucidating the role of traits on tree–tree interactions across diversity levels. 1. Understanding how species diversity affects plant performance is a central question in biodiversity–ecosystem functioning (BEF) research. At the community level, functional trait means and trait dissimilarities have been used to explain biodiversity effects, but with mixed success. To disentangle how functional traits explain community growth and underpin biodiversity–ecosystem functioning relationships, we should elucidate how plant traits affect individual growth across species richness levels, because the role of functional traits on growth depends on the ecological context of the individual.2. We addressed this topic by using detailed data of twelve functional traits and annual growth for 529 individual trees of 31 species at five species‐richness levels in a large forest biodiversity experiment in south‐east China from 4 to 9 years after planting.3. Our analyses show first that individual trait values can change with species richness, indicating that trait values can change due to the biotic context. Secondly, we show that early tree growth is more strongly affected by traits than by species richness. Finally, our data show that growth–trait relationships can change with species richness and with forest age. Trait effects on growth are more pronounced at higher richness levels, indicating that measuring traits on individual trees across richness levels can improve growth predictions and inference of BEF relationships that are shaped by functional traits.4. Synthesis. This study shows that functional trait values and their effect on individual tree growth depend on species richness. Our data support the notion that to elucidate how functional traits shape biodiversity–ecosystem functioning relationships, an important step is to consider the biotic context of individual trees within a community. We have made an initial step by analysing how functional traits affect individual‐tree growth in a diversity‐dependent manner and future research should continue by elucidating the role of traits on tree–tree interactions across diversity levels.

Local neighbourhood interactions are considered a main driver for biodiversity–productivity relationships in forests. Yet, the structural responses of individual trees in species mixtures and their relation to crown complementarity remain poorly understood. Using a large-scale forest experiment, we studied the impact of local tree species richness and structural variability on above-ground wood volume allocation patterns and crown morphology. We applied terrestrial laser scanning to capture the three-dimensional structure of trees and their temporal dynamics. We found that crown complementarity and crown plasticity increased with species richness. Trees growing in species-rich neighbourhoods showed enhanced aboveground wood volume both in trunks and branches. Over time, neighbourhood diversity induced shifts in wood volume allocation in favour of branches, in particular for morphologically flexible species. Our results demonstrate that diversity-mediated shifts in allocation pattern and crown morphology are a fundamental mechanism for crown complementarity and may be an important driver of overyielding. publishedVersion

Light-related interactions can increase productivity in tree-species mixtures compared with monocultures due to higher stand-level absorption of photosynthetically active radiation (APAR) or light-use efficiency (LUE). However, the effects of different light-related interactions, and their relative importance, have rarely been quantified. Here, measurements of vertical leaf-area distributions, tree sizes, and stand density were combined with a tree-level light model (Maestra) to examine how crown architecture and vertical or horizontal canopy structure influence the APAR of 16 monocultures and eight different two-species mixtures with 16 different species in a Chinese subtropical tree diversity experiment. A higher proportion of crown leaf area occurred in the upper crowns of species with higher specific leaf areas. Tree-level APAR depended largely on tree leaf area and also, but to a lesser extent, on relative height (i.e., tree dominance) and leaf-area index (LAI). Stand-level APAR depended on LAI and canopy volume, but not on the vertical stratification or canopy leaf-area density. The mixing effects, in terms of relative differences between mixtures and monocultures, on stand-level APAR were correlated with the mixing effects on basal area growth, indicating that light-related interactions may have been responsible for part of the mixing effects on basal area growth. While species identity influences the vertical distributions of leaf area within tree crowns, this can have a relatively small effect on tree and stand APAR compared with the size and vertical positioning of the crowns, or the LAI and canopy volume.

In the core areas of National Parks (NPs), there might be a conflict between the approaches of process-based conservation, i.e. allowing natural ecological processes to develop without intervention, and of species-based conservation, i.e. active management to protect specific aspects of biodiversity. To better understand this conflict and its consequences, we collected data on non-native plant species (neophytes) managed in 12 German terrestrial NPs and the measures implemented to control these in the period 2006–2016. Based on this, we developed a decision-making process to identify the criteria that should be considered when determining the appropriate management of neophytes. This process considers the distribution of the species in the NP, age and zone of the NP, parameters for success, the degree of threat posed to local conservation goals by the species, and suitability of the measure. The application of this decision-making process to existing neophyte control measures showed that 18 of these measures should be continued, six should be adapted and three terminated. In the examined NPs, there is often no evaluation of performance against their stated goals in neophyte management. For most measures, their efficacy remains unclear. Preventive measures against neophytes are only carried out in four of the 12 NPs investigated here. Given that NPs have the goal of protecting ecological processes, this lack of preventive measures is critical. Especially in this category of protected area, avoiding the introduction of neophytes in the first place can significantly reduce the need for control and eradication measures later. A conflict of objectives between process-based and species-based conservation approaches currently exists in more than half of the NPs investigated here. The decision-making process does not resolve this conflict, but it does make the decisions more transparent and thus more communicable. By collecting and analysing the individual species-specific measures, it will be possible to reduce the number of measures in the future, thereby at least reducing the existing conflict of objectives. Promoting the protection of ecological processes for inherent reasons and following an actualistic interpretation of naturalness can improve the acceptance of neobiota in NPs.

AimsGrowth rates of plants are driven by factors that influence the amount of resources captured and the efficiency of resource use. In trees, the amount of light captured and the efficiency of light use strongly depends on crown characteristics and leaf traits. Although theory predicts that both crown and leaf traits affect tree growth, few studies have yet to integrate these two types of traits to explain species-specific growth rates. Using 37 broad-leaved tree species of subtropical forests in SE China, we investigated how interspecific differences in wood volume growth rates were affected by crown and leaf traits. We tested the hypotheses that (i) larger crown dimensions promote growth rates, (ii) species-specific growth rates are positively related to leaf stomatal conductance, leaf water potential and leaf chemical components, and negatively related to leaf C/N and leaf toughness and (iii) the two sets of traits better explain growth rates in combination than either alone.MethodsOur study was conducted in a large-scale forest Biodiversity and Ecosystem Functioning experiment in China (BEF-China), located in a mountainous region in Jiangxi Province. We related 17 functional traits (two crown dimension and three crown structure traits; six physiological and six morphological leaf traits) to the mean annual growth rate of wood volume of young trees of the studied species. Interrelationships between crown and leaf traits were analyzed using principal component analysis. Simple linear regression analysis was used to test the effect of each trait separately. We used multiple regression analysis to establish the relationship of growth rate to each set of traits (crown traits, physiological and morphological leaf traits) and to the combination of all types of traits. The coefficients of determination (R2adj) of the best multiple regression models were compared to determine the relative explanatory power of crown and leaf traits and a combination of both.Important FindingsThe species-specific growth rates were not related to any of the single crown traits, but were related positively to leaf stomatal conductance and leaf water potential individually, and negatively to leaf toughness, with approximately 13% variance explained by each of the traits. Combinations of different crown traits did not significantly explain the species-specific growth rates, whereas combinations of either physiological or morphological leaf traits explained 24% and 31%, respectively. A combination of both crown and leaf traits explained 42% of variance in species-specific growth rates. We concluded that sets of traits related to carbon assimilation at the leaf-level and to overall amount of leaves exposed at the crown-level jointly explained species-specific growth rates better than either set of traits alone.

Aims Although the net biodiversity effect (NE) can be statistically partitioned into complementarity and selection effects (CE and SE), there are different underlying mechanisms that can cause a certain partitioning. Our objective was to assess the role of resource partitioning and species interactions as two important mechanisms that can bring about CEs by interspecific and intraspecific trait variation. Methods We measured tree height of 2493 living individuals in 57 plots and specific root length (SRL) on first-order roots of 368 of these individuals across different species richness levels (1, 2, 4, 8 species) in a large-scale forest biodiversity and ecosystem functioning experiment in subtropical China (BEF-China) established in 2009. We describe the effects of resource partitioning between species by a fixed component of interspecific functional diversity (RaoQ) and further effects of species interactions by variable components of interspecific and intraspecific functional diversity (community weighted trait similarity and trait dissimilarity, CWS and CWD). Finally, we examined the relationships between biodiversity effects on stand-level tree height and functional diversity (RaoQ, CWS and CWD) in SRL using linear regression and assessed the relative importance of these three components of functional diversity in explaining the diversity effects. Important Findings Our results show that species richness significantly affected SRL in five and tree height in ten out of 16 species. A positive NE was generally brought about by a positive CE on stand-level tree height and related to high values of RaoQ and CWS in SRL. A positive CE was related to high values of all three components of root functional diversity (RaoQ, CWS and CWD). Our study suggests that both resource partitioning and species interactions are the underlying mechanisms of biodiversity effects on stand-level tree growth in subtropical forest.

A better understanding of the linkages between aboveground and belowground biotic communities is needed for more accurate predictions about how ecosystems may be altered by climate change, land management, or biodiversity loss. Soil microbes are strongly affected by multiple factors including local abiotic environmental conditions and plant characteristics. To find out how soil microbial communities respond to multiple facets of the local soil and plant environment, we analysed soil lipid profiles associated with three-year-old monocultures of 29 tree species. These species are native of the diverse subtropical forests of southeast China and greatly vary in functional traits, growth or biomass characteristics, and phylogenetic relatedness. Along with the traits of each tree species, we also determined the soil and plot characteristics in each monoculture plot and tested for phylogenetic signals in tree species-specific microbial indicators. Microbial community structure and biomass were influenced by both soil properties and plant functional traits, but were not related to the phylogenetic distances of tree species. Specifically, total microbial biomass, indicators for fungi, bacteria, and actinomycetes were positively correlated with soil pH, soil organic nitrogen, and soil moisture. Our results also indicate that leaf dry matter content and the leaf carbon to nitrogen ratio influence multivariate soil microbial community structure, and that these factors and tree growth traits (height, crown or basal diameter) positively promote the abundances of specific microbial functional groups. At the same time, a negative correlation between leaf nitrogen content and Gram positive bacterial abundance was detected, indicating plant–microbial competition for nitrogen in our system. In conclusion, even at early stages of tree growth, soil microbial community abundance and structure can be significantly influenced by plant traits, in combination with local soil characteristics.

Climate change and atmospheric deposition of nitrogen affect biodiversity patterns and functions of forest ecosystems worldwide. Many studies have quantified tree growth responses to single global change drivers, but less is known about the interaction effects of these drivers at the plant and ecosystem level. In the present study, we conducted a full-factorial greenhouse experiment to analyse single and combined effects of nitrogen fertilization (N treatment) and drought (D treatment) on 16 morphological and chemical response variables (including tissue δ13C signatures) of one-year-old Fagus sylvatica seedlings originating from eight different seed families from the Cantabrian Mountains (NW Spain). Drought exerted the strongest effect on response variables, reflected by decreasing biomass production and increasing tissue δ13C signatures. However, D and N treatments interacted for some of the response variables, indicating that N fertilization has the potential to strengthen the negative effects of drought (with both antagonistic and amplifying interactions). For example, combined effects of N and D treatments caused a sevenfold increase of necrotic leaf biomass. We hypothesize that increasing drought sensitivity was mainly attributable to a significant reduction of the root biomass in combined N and D treatments, limiting the plants’ capability to satisfy their water demands. Significant seed family effects and interactions of seed family with N and D treatments across response variables suggest a high within-population genetic variability. In conclusion, our findings indicated a high drought sensitivity of Cantabrian beech populations, but also interaction effects of N and D on growth responses of beech seedlings.

Climate extremes are predicted to become more frequent and intense in future. Thus, understanding how trees respond to adverse climatic conditions is crucial for evaluating possible future changes in forest ecosystem functioning.Although much information about climate effects on the growth of temperate trees has been collected in recent decades, our understanding of the influence of forest management legacies on climate-growth relationships is still limited.We used individual tree-ring chronologies from managed and unmanaged European beech forests, located in the same growth district (i.e.with almost identical climatic and soil conditions), to examine how forest managementlegacies (recently managed with selection cutting, N20 years unmanaged, N50 years unmanaged) influence the radial growth of Fagus sylvatica during fluctuating climatic conditions. On average, trees in managed stands had higherradial growth rate than trees in unmanaged stands during the last two decades a 50%. However, the beech trees in the unmanaged stands were less sensitive to drought than those in the managed stands. This effect was most pronouncedin the forest with longest management abandonment (N50 years), indicating that the drought sensitivity of mature beech trees is in these forests the lower, the longer the period since forest management cessation is. Management-mediated modifications in crown size and thus water demand are one likely cause of the observed higher climate sensitivity of beech in the managed stands. Our results indicate a possible trade-off between radial growth rate and drought tolerance of beech. This suggests that reducing stem density for maximizing the radialgrowth of target trees, as is common practice in managed forests, can increase the trees' drought sensitivity. In the prospect of climate change, more information on the impact of forest management practices on the climate growth relationships of trees is urgently needed. Climate extremes are predicted to become more frequent and intense in future. Thus, understanding how trees respond to adverse climatic conditions is crucial for evaluating possible future changes in forest ecosystem functioning. Although much information about climate effects on the growth of temperate trees has been collected in recent decades, our understanding of the influence of forest management legacies on climate-growth relationships is still limited. We used individual tree-ring chronologies from managed and unmanaged European beech forests, located in the same growth district (i.e. with almost identical climatic and soil conditions), to examine how forest management legacies (recently managed with selection cutting, >20 years unmanaged, >50 years unmanaged) influence the radial growth of Fagus sylvatica during fluctuating climatic conditions. On average, trees in managed stands had higher radial growth rate than trees in unmanaged stands during the last two decades a 50%. However, the beech trees in the unmanaged stands were less sensitive to drought than those in the managed stands. This effect was most pronounced in the forest with longest management abandonment (>50 years), indicating that the drought sensitivity of mature beech trees is in these forests the lower, the longer the period since forest management cessation is. Management-mediated modifications in crown size and thus water demand are one likely cause of the observed higher climate sensitivity of beech in the managed stands. Our results indicate a possible trade-off between radial growth rate and drought tolerance of beech. This suggests that reducing stem density for maximizing the radial growth of target trees, as is common practice in managed forests, can increase the trees' drought sensitivity. In the prospect of climate change, more information on the impact of forest management practices on the climate-growth relationships of trees is urgently needed.

Forest canopies and tree crown structures are of high ecological importance.Measuring canopies and crowns by direct inventory methods is time-consuming and of limited accuracy. High-resolution inventory tools, in particular terrestrial laser scanning (TLS), is able to overcome these limitations and obtain three-dimensional (3D) structural information about the canopy with a very high level of detail. The main objective of this study was to introduce a novel method to analyze spatiotemporal dynamics in canopy occupancy at the individual tree and local neighborhood level using high-resolution 3D TLS data. For the analyses, a voxel grid approach was applied. The tree crowns were modeled through the combination of two approaches: the encasement of all crown points with a 3D α-shape, which was then converted into a voxel grid, and the direct voxelization of the crown points. We show that canopy occupancy at individual tree level can be quantified as the crown volume occupied only by the respective tree or shared with neighboring trees. At the local neighborhood level, our method enables the precise determination of the extent of canopy space filling, the identification of tree–tree interactions, and the analysis of complementary space use. Using multitemporal TLS data recordings, this method allows the precise detection and quantification of changes in canopy occupancy through time. The method is applicable to a wide range of investigations in forest ecology research, including the study of tree diversity effects on forest productivityor growing space analyses for optimal tree growth. Due to the high accuracy of this novel method, it facilitates the precise analyses even of highly plastic individual tree crowns and, thus, the realistic representation of forest canopies. Moreover, our voxel grid framework is flexible enough to allow for the inclusion of further biotic and abiotic variables relevant to complex analyses of forest canopy dynamics. Forest canopies and tree crown structures are of high ecological importance. Measuring canopies and crowns by direct inventory methods is time‐consuming and of limited accuracy. High‐resolution inventory tools, in particular terrestrial laser scanning (TLS), is able to overcome these limitations and obtain three‐dimensional (3D) structural information about the canopy with a very high level of detail. The main objective of this study was to introduce a novel method to analyze spatiotemporal dynamics in canopy occupancy at the individual tree and local neighborhood level using high‐resolution 3D TLS data. For the analyses, a voxel grid approach was applied. The tree crowns were modeled through the combination of two approaches: the encasement of all crown points with a 3D α‐shape, which was then converted into a voxel grid, and the direct voxelization of the crown points. We show that canopy occupancy at individual tree level can be quantified as the crown volume occupied only by the respective tree or shared with neighboring trees. At the local neighborhood level, our method enables the precise determination of the extent of canopy space filling, the identification of tree–tree interactions, and the analysis of complementary space use. Using multitemporal TLS data recordings, this method allows the precise detection and quantification of changes in canopy occupancy through time. The method is applicable to a wide range of investigations in forest ecology research, including the study of tree diversity effects on forest productivity or growing space analyses for optimal tree growth. Due to the high accuracy of this novel method, it facilitates the precise analyses even of highly plastic individual tree crowns and, thus, the realistic representation of forest canopies. Moreover, our voxel grid framework is flexible enough to allow for the inclusion of further biotic and abiotic variables relevant to complex analyses of forest canopy dynamics.

Global change affects the functioning of forest ecosystems and the services they provide, but little is known about the interactive effects of co-occurring global change drivers on important functions such as tree growth and vitality. In the present study we quantified the interactive (i.e. synergistic or antagonistic) effects of atmospheric nitrogen (N) deposition and climatic variables (temperature, precipitation) on tree growth (in terms of tree-ring width, TRW), taking forest ecosystems with European beech (Fagus sylvatica L.) as an example. We hypothesised that (i) N deposition and climatic variables can evoke non-additive responses of the radial increment of beech trees, and (ii) N loads have the potential to strengthen the trees' sensitivity to climate change. In young stands, we found a synergistic positive effect of N deposition and annual mean temperature on TRW, possibly linked to the alleviation of an N shortage in young stands. In mature stands, however, high N deposition significantly increased the trees’ sensitivity to increasing annual mean temperatures (antagonistic effect on TRW), possibly due to increased fine root dieback, decreasing mycorrhizal colonization or shifts in biomass allocation patterns (aboveground vs. belowground). Accordingly, N deposition and climatic variables caused both synergistic and antagonistic effects on the radial increment of beech trees, depending on tree age and stand characteristics. Hence, the nature of interactions could mediate the long-term effects of global change drivers (including N deposition) on forest carbon sequestration. In conclusion, our findings illustrate that interaction processes between climatic variables and N deposition are complex and have the potential to impair growth and performance of European beech. This in turn emphasises the importance of multiple-factor studies to foster an integrated understanding and models aiming at improved projections of tree growth responses to co-occurring drivers of global change. Global change affects the functioning of forest ecosystems and the services they provide, but little is known about the interactive effects of co-occurring global change drivers on important functions such as tree growth and vitality. In the present study we quantified the interactive (i.e. synergistic or antagonistic) effects of atmospheric nitrogen (N) deposition and climatic variables (temperature, precipitation) on tree growth (in terms of tree-ring width, TRW), taking forest ecosystems with European beech (Fagus sylvatica L.) as an example. We hypothesised that (i) N deposition and climatic variables can evoke non-additive responses of the radial increment of beech trees, and (ii) N loads have the potential to strengthen the trees' sensitivity to climate change. In young stands, we found a synergistic positive effect of N deposition and annual mean temperature on TRW, possibly linked to the alleviation of an N shortage in young stands. In mature stands, however, high N deposition significantly increased the trees’ sensitivity to increasing annual mean temperatures (antagonistic effect on TRW), possibly due to increased fine root dieback, decreasing mycorrhizal colonization or shifts in biomass allocation patterns (aboveground vs. belowground). Accordingly, N deposition and climatic variables caused both synergistic and antagonistic effects on the radial increment of beech trees, depending on tree age and stand characteristics. Hence, the nature of interactions could mediate the long-term effects of global change drivers (including N deposition) on forest carbon sequestration. In conclusion, our findings illustrate that interaction processes between climatic variables and N deposition are complex and have the potential to impair growth and performance of European beech. This in turn emphasises the importance of multiple-factor studies to foster an integrated understanding and models aiming at improved projections of tree growth responses to co-occurring drivers of global change.

Tree diversity improves forest productivity Experimental studies in grasslands have shown that the loss of species has negative consequences for ecosystem functioning. Is the same true for forests? Huang et al. report the first results from a large biodiversity experiment in a subtropical forest in China. The study combines many replicates, realistic tree densities, and large plot sizes with a wide range of species richness levels. After 8 years of the experiment, the findings suggest strong positive effects of tree diversity on forest productivity and carbon accumulation. Thus, changing from monocultures to more mixed forests could benefit both restoration of biodiversity and mitigation of climate change. Science , this issue p. 80

The spread of competitive grasses, changes in species composition and vegetation structure are direct consequences of grassland and heathland abandonment. As an alternative to more costly management measures such as traditional pastoralism, year-round low-intensity grazing with large herbivores is increasingly used to restore and maintain semi-open habitats. However, the suitability of this grazing regime has not yet been investigated for long-abandoned, highly degraded but nutrient-poor sandy grassland and heathland communities. In particular, it is unclear if year-round grazing is suitable for preventing the further spread of highly competitive grasses such as Calamagrostis epigejos while simultaneously maintaining or improving characteristic species richness and vegetation structure. Hence, we conducted a comprehensive field study on two spatial scales (plot-level: 25 m2, macroplot-level: 1 ha) to analyse the impacts of year-round low-intensity cattle and horse grazing on the development of the highly competitive grass Calamagrostis epigejos, as well as the vegetation structure and plant species richness of long-abandoned but nutrient-poor dry sandy grassland and heathland communities, their mosaics and Calamagrostis stands within an 800 ha heathland between 2008 and 2015. Finally, we assessed the local conservation status of the habitat types after seven years of grazing in comparison to long-abandoned sites. Grazing successfully reduced the coverage of Calamagrostis epigejos, whereby Calamagrostis stands developed towards species-rich sandy grasslands after seven years of grazing. In addition, the quality of the vegetation structure was improved by enhancing the proportion of bare soil, while litter and grass cover, litter thickness and height of the field layer as well as the coverage of ruderal indicators were significantly reduced on grazed sites in comparison to ungrazed sites in 2015. Moreover, we found an overall positive grazing effect on species richness: Total species number, number of target species as well as subordinated target species significantly increased within the vegetation types over time. Thus, year-round low-intensity cattle and horse grazing is a suitable management tool for restoring, maintaining and even improving long-abandoned, nutrient-poor sandy grassland and heathland communities, and thus to enhance the local conservation status of the habitat types. However, if there is a high initial cover of woody species (e.g. shrubs, tree rejuvenation), then an extensive shrub and tree clearance will be necessary, with manual shrub cutting being crucial to reduce the heavy regrowth of the woody species. In addition, a one-time mowing should be implemented in highly degraded heaths to facilitate the vegetative rejuvenation of degenerate stands of Calluna vulgaris, thus improving its attractiveness for the grazing animals.

The effects of atmospheric nitrogen (N) deposition on ecosystem functioning largely depend on the retention of N in different ecosystem compartments, but accumulation and partitioning processes have rarely been quantified in long-term field experiments. In the present study we analysed for the first time decadal-scale flows and allocation patterns of N in a heathland ecosystem that has been subject to airborne N inputs over decades. Using a long-term 15N tracer experiment, we quantified N retention and flows to and between ecosystem compartments (above-ground/below-ground vascular biomass, moss layer, soil horizons, leachate). After 9 years, about 60% of the added 15N-tracer remained in the N cycle of the ecosystem. The moss layer proved to be a crucial link between incoming N and its allocation to different ecosystem compartments (in terms of a short-term capture, but long-term release function). However, about 50% of the 15N captured and released by the moss layer was not compensated for by a corresponding increase in recovery rates in any other compartment, probably due to denitrification losses from the moss layer in the case of water saturation after rain events. The O-horizon proved to be the most important long-term sink for added 15N, as reflected by an increase in recovery rates from 18 to 40% within 8 years. Less than 2.1% of 15N were recovered in the podzol-B-horizon, suggesting that only negligible amounts of N were withdrawn from the N cycle of the ecosystem. Moreover, 15N recovery was low in the dwarf shrub above-ground biomass (

Biodiversity-ecosystem functioning (BEF) research has extended its scope from communities that are short-lived or reshape their structure annually to structurally complex forest ecosystems. The establishment of tree diversity experiments poses specific methodological challenges for assessing the multiple functions provided by forest ecosystems. In particular, methodological inconsistencies and nonstandardized protocols impede the analysis of multifunctionality within, and comparability across the increasing number of tree diversity experiments. By providing an overview on key methods currently applied in one of the largest forest biodiversity experiments, we show how methods differing in scale and simplicity can be combined to retrieve consistent data allowing novel insights into forest ecosystem functioning. Furthermore, we discuss and develop recommendations for the integration and transferability of diverse methodical approaches to present and future forest biodiversity experiments. We identified four principles that should guide basic decisions concerning method selection for tree diversity experiments and forest BEF research: (1) method selection should be directed toward maximizing data density to increase the number of measured variables in each plot. (2) Methods should cover all relevant scales of the experiment to consider scale dependencies of biodiversity effects. (3) The same variable should be evaluated with the same method across space and time for adequate larger-scale and longer-time data analysis and to reduce errors due to changing measurement protocols. (4) Standardized, practical and rapid methods for assessing biodiversity and ecosystem functions should be promoted to increase comparability among forest BEF experiments. We demonstrate that currently available methods provide us with a sophisticated toolbox to improve a synergistic understanding of forest multifunctionality. However, these methods require further adjustment to the specific requirements of structurally complex and long-lived forest ecosystems. By applying methods connecting relevant scales, trophic levels, and above- and belowground ecosystem compartments, knowledge gain from large tree diversity experiments can be optimized. Biodiversity–ecosystem functioning (BEF) research has extended its scope from communities that are short‐lived or reshape their structure annually to structurally complex forest ecosystems. The establishment of tree diversity experiments poses specific methodological challenges for assessing the multiple functions provided by forest ecosystems. In particular, methodological inconsistencies and nonstandardized protocols impede the analysis of multifunctionality within, and comparability across the increasing number of tree diversity experiments. By providing an overview on key methods currently applied in one of the largest forest biodiversity experiments, we show how methods differing in scale and simplicity can be combined to retrieve consistent data allowing novel insights into forest ecosystem functioning. Furthermore, we discuss and develop recommendations for the integration and transferability of diverse methodical approaches to present and future forest biodiversity experiments. We identified four principles that should guide basic decisions concerning method selection for tree diversity experiments and forest BEF research: (1) method selection should be directed toward maximizing data density to increase the number of measured variables in each plot. (2) Methods should cover all relevant scales of the experiment to consider scale dependencies of biodiversity effects. (3) The same variable should be evaluated with the same method across space and time for adequate larger‐scale and longer‐time data analysis and to reduce errors due to changing measurement protocols. (4) Standardized, practical and rapid methods for assessing biodiversity and ecosystem functions should be promoted to increase comparability among forest BEF experiments. We demonstrate that currently available methods provide us with a sophisticated toolbox to improve a synergistic understanding of forest multifunctionality. However, these methods require further adjustment to the specific requirements of structurally complex and long‐lived forest ecosystems. By applying methods connecting relevant scales, trophic levels, and above‐ and belowground ecosystem compartments, knowledge gain from large tree diversity experiments can be optimized.

Forest ecosystems contribute substantially to global terrestrial primary productivity and climate regulation, but, in contrast to grasslands, experimental evidence for a positive biodiversity-productivity relationship in highly diverse forests is still lacking1. Here, we provide such evidence from a large forest biodiversity experiment with a novel design2 in subtropical China. Productivity (stand-level tree basal area, aboveground volume and carbon and their annual increment) increased linearly with the logarithm of tree species richness. Additive partitioning3 showed that increasing positive complementarity effects combined with weakening negative selection effects caused a strengthening of the relationship over time. In 2-species mixed stands, complementary effects increased with functional distance and selection effects with vertical crown dissimilarity between species. Understorey shrubs reduced stand-level tree productivity, but this effect of competition was attenuated by shrub species richness, indicating that a diverse understorey may facilitate overall ecosystem functioning. Identical biodiversity-productivity relationships were found in plots of different size, suggesting that extrapolation to larger scales is possible. Our results highlight the potential of multi-species afforestation strategies to simultaneously contribute to mitigation of climate change and biodiversity restoration.

Studies on tree communities have demonstrated that species diversity can enhance forest productivity, but the driving mechanisms at the local neighbourhood level remain poorly understood. Here, we use data from a large-scale biodiversity experiment with 24 subtropical tree species to show that neighbourhood tree species richness generally promotes individual tree productivity. We found that the underlying mechanisms depend on a focal tree's functional traits: For species with a conservative resource-use strategy diversity effects were brought about by facilitation, and for species with acquisitive traits by competitive reduction. Moreover, positive diversity effects were strongest under low competition intensity (quantified as the total basal area of neighbours) for acquisitive species, and under high competition intensity for conservative species. Our findings demonstrate that net biodiversity effects in tree communities can vary over small spatial scales, emphasising the need to consider variation in local neighbourhood interactions to better understand effects at the community level.

Baumann, M., Dittrich, S. & Oheimb, G. v. 2019. Effects of periodic liming on the bryophyte layer in Norway spruce (Picea abies) stands in the Erzgebirge (Ore Mountains). – Herzogia 32: 269–287.Severe changes in forest soil conditions, tree health and growth as well as plant species composition have been observed as a consequence of atmospheric acid deposition. The most important measure to counteract soil acidification is forest liming. The long-term effects of repeated liming on the bryophyte layer of forests, however, have rarely been analysed. We used a stratified design to sample the bryophyte layer of spruce stands located in the Erzgebirge (Ore Mountains, eastern Germany). Study sites were unlimed or limed at different intensities between one and five times since 1988. We found a significant effect of liming intensity on the composition and diversity of the bryophyte layer. The liming gradient is reflected by numerous indicator species, by indicator values for pH and nitrogen, and the by alpha and beta diversity of the species present. Liming strongly reduces the cover of many acidophytes typical for the habitat. By contrast, indicator species of disturbance and some rare, nutrient-demanding species appear to benefit from liming. These effects increase with liming intensity. Our results suggest that larger liming exclusion zones should be created to protect the typical bryophyte species composition.

AimsThe aim of our research was to understand small-scale effects of topography and soil fertility on tree growth in a forest biodiversity and ecosystem functioning (BEF) experiment in subtropical SE China.MethodsGeomorphometric terrain analyses were carried out at a spatial resolution of 5×5 m. Soil samples of different depth increments and data on tree height were collected from a total of 566 plots (667 m2 each). The soils were analyzed for carbon (soil organic carbon [SOC]), nitrogen, acidity, cation exchange capacity (CEC), exchangeable cations and base saturation as soil fertility attributes. All plots were classified into geomorphological units. Analyses of variance and linear regressions were applied to all terrain, soil fertility and tree growth attributes.Important FindingsIn general, young and shallow soils and relatively small differences in stable soil properties suggest that soil erosion has truncated the soils to a large extent over the whole area of the experiment. This explains the concurrently increasing CEC and SOC stocks downslope, in hollows and in valleys. However, colluvial, carbon-rich sediments are missing widely due to the convexity of the footslopes caused by uplift and removal of eroded sediments by adjacent waterways. The results showed that soil fertility is mainly influenced by topography. Monte–Carlo flow accumulation (MCCA), curvature, slope and aspect significantly affected soil fertility. Furthermore, soil fertility was affected by the different geomorphological positions on the experimental sites with ridge and spur positions showing lower exchangeable base cation contents, especially potassium (K), due to leaching. This geomorphological effect of soil fertility is most pronounced in the topsoil and decreases when considering the subsoil down to 50cm depth. Few soil fertility attributes affect tree height after 1–2 years of growth, among which C stocks proved to be most important while pHKCl and CEC only played minor roles. Nevertheless, soil acidity and a high proportion of Al on the exchange complex affected tree height even after only 1–2 years growth. Hence, our study showed that forest nutrition is coupled to a recycling of litter nutrients, and does not only depend on subsequent supply of nutrients from the mineral soil. Besides soil fertility, topography affected tree height. We found that especially MCCA as indicator of water availability affected tree growth at small-scale, as well as aspect. Overall, our synthesis on the interrelation between fertility, topography and tree growth in a subtropical forest ecosystem in SE China showed that topographic heterogeneity lead to ecological gradients across geomorphological positions. In this respect, small-scale soil–plant interactions in a young forest can serve as a driver for the future development of vegetation and biodiversity control on soil fertility. In addition, it shows that terrain attributes should be accounted for in ecological research.

tAnthropogenic deposition of reactive nitrogen (N) has increased during the 20th century, and is con-sidered an important driver of shifts in ecosystem functions and biodiversity loss. The objective of thepresent study was to identify those ecosystem functions that best evidence a target ecosystem’s sensitiv-ity to N deposition, taking coastal heathlands as an example. We conducted a three-year field experimentin heathlands of the island Fehmarn (Baltic Sea, North Germany), which currently are subject to a back-ground deposition of 9 kg N ha−1yr−1. We experimentally applied six levels of N fertilisation (applicationof 0, 2.5, 5, 10, 20, and 50 kg N ha−1yr−1), and quantified the growth responses of different plant speciesof different life forms (dwarf shrubs, graminoids, bryophytes, lichens) as well as shifts in the C:N ratios ofplant tissue and humus horizons. For an applicability of the experimental findings (in terms of heathlandmanagement and critical load assessment) fertilisation effects on response variables were visualised bycalculating the treatment ‘effect sizes’. The current year’s shoot increment of the dominant dwarf shrubCalluna vulgaris proved to be the most sensitive indicator to N fertilisation. Shoot increment significantlyresponded to additions of ≥ 5 kg N ha−1yr−1already in the first year, whereas flower formation of Callunavulgaris increased only in the high-N treatments. Similarly, tissue C:N ratios of vascular plants (Callunavulgaris and the graminoids Carex arenaria and Festuca ovina agg.) only decreased in the highest N treat-ments (50 and 20 kg N ha−1yr−1, respectively). In contrast, tissue C:N ratios of cryptogams respondedmore quickly and sensitively than vascular plants. For example, Cladonia spp. tissue C:N ratios respondedto N additions ≥ 5 kg N ha−1yr−1in the second study year. After three years we observed an increase incover of graminoids and a corresponding decrease of cryptogams at N fertilisation rates of ≥ 10 kg N ha−1yr−1. Soil C:N ratios proved to be an inappropriate indicator for N fertilisation at least within our three-year study period. Although current critical N loads for heathlands (10−20 kg N ha−1yr−1) were confirmedin our experiment, the immediate and highly sensitive response of the current year’s shoots of Callunavulgaris suggests that at least some ecosystem functions (e.g. dwarf shrub growth) also might respond tolow (i.e. < 10 kg N ha−1yr−1) but chronic inputs of N. Anthropogenic deposition of reactive nitrogen (N) has increased during the 20th century, and is considered an important driver of shifts in ecosystem functions and biodiversity loss. The objective of the present study was to identify those ecosystem functions that best evidence a target ecosystem’s sensitivity to N deposition, taking coastal heathlands as an example. We conducted a three-year field experiment in heathlands of the island Fehmarn (Baltic Sea, North Germany), which currently are subject to a background deposition of 9 kg N ha−1 yr−1. We experimentally applied six levels of N fertilisation (application of 0, 2.5, 5, 10, 20, and 50 kg N ha−1 yr−1), and quantified the growth responses of different plant species of different life forms (dwarf shrubs, graminoids, bryophytes, lichens) as well as shifts in the C:N ratios of plant tissue and humus horizons. For an applicability of the experimental findings (in terms of heathland management and critical load assessment) fertilisation effects on response variables were visualised by calculating the treatment ‘effect sizes’. The current year’s shoot increment of the dominant dwarf shrub Calluna vulgaris proved to be the most sensitive indicator to N fertilisation. Shoot increment significantly responded to additions of ≥ 5 kg N ha−1 yr−1 already in the first year, whereas flower formation of Calluna vulgaris increased only in the high-N treatments. Similarly, tissue C:N ratios of vascular plants (Calluna vulgaris and the graminoids Carex arenaria and Festuca ovina agg.) only decreased in the highest N treatments (50 and 20 kg N ha−1 yr−1, respectively). In contrast, tissue C:N ratios of cryptogams responded more quickly and sensitively than vascular plants. For example, Cladonia spp. tissue C:N ratios responded to N additions ≥ 5 kg N ha−1 yr−1 in the second study year. After three years we observed an increase in cover of graminoids and a corresponding decrease of cryptogams at N fertilisation rates of ≥ 10 kg N ha−1 yr−1. Soil C:N ratios proved to be an inappropriate indicator for N fertilisation at least within our three-year study period. Although current critical N loads for heathlands (10−20 kg N ha−1 yr−1) were confirmed in our experiment, the immediate and highly sensitive response of the current year’s shoots of Calluna vulgaris suggests that at least some ecosystem functions (e.g. dwarf shrub growth) also might respond to low (i.e. < 10 kg N ha−1 yr−1) but chronic inputs of N.

Understanding how trees respond to global change drivers is central to predict changes in forest structure and functions. Although there is evidence on the mode of nitrogen (N) and drought (D) effects on tree growth, our understanding of the interplay of these factors is still limited. Simultaneously, as mixtures are expected to be less sensitive toglobal change as compared to monocultures, we aimed to investigate the combined effects of N addition and D on the productivity of three tree species (Fagus sylvatica, Quercus petraea, Pseudotsuga menziesii) in relation to functional diverse species mixtures using data from a 4-year field experiment in Northwest Germany. Here we show that species mixing can mitigate the negative effects of combined N fertilizationand D events, but the community response is mainly driven by the combination of certain traits rather than the tree species richness of a community. For beech, we found that negative effects of D on growth rates were amplified by N fertilization (i.e., combined treatment effects were non-additive), while for oak and fir, the simultaneous effects of Nand D were additive. Beech and oak were identified as most sensitive to combined NCD effects with a strong size-dependency observed for beech, suggesting that the negative impact of NCD becomes stronger with time as beech grows larger. As a consequence, the net biodiversity effect declined at the community level, which can be mainly assignedto a distinct loss of complementarity in beech-oak mixtures. This pattern, however, was not evident in the other species-mixtures, indicating that neighborhood composition (i.e., trait combination), but not tree species richness mediated the relationship between tree diversity and treatment effects on tree growth. Our findings point to the importance of the qualitative role (‘trait portfolio’) that biodiversity play in determining resistance of diverse tree communities to environmental changes. As such, they provide further understanding for adaptive management strategies in the context of global change. Understanding how trees respond to global change drivers is central to predict changes in forest structure and functions. Although there is evidence on the mode of nitrogen (N) and drought (D) effects on tree growth, our understanding of the interplay of these factors is still limited. Simultaneously, as mixtures are expected to be less sensitive to global change as compared to monocultures, we aimed to investigate the combined effects of N addition and D on the productivity of three tree species (Fagus sylvatica, Quercus petraea, Pseudotsuga menziesii) in relation to functional diverse speciesmixtures using data from a 4-year field experiment in Northwest Germany. Here we show that species mixing can mitigate the negative effects of combined N fertilization and D events, but the community response is mainly driven by the combination of certain traits rather than the tree species richness of a community. For beech, we found that negative effects of D on growth rates were amplified by N fertilization (i.e., combined treatment effects were non-additive), while for oak and fir, the simultaneous effects of N and D were additive. Beech and oak were identified as most sensitive to combined NCDeffects with a strong size-dependency observed for beech, suggesting that the negative impact of NCD becomes stronger with time as beech grows larger. As a consequence, the net biodiversity effect declined at the community level, which can be mainly assigned to a distinct loss of complementarity in beech-oak mixtures. This pattern, however, was not evident in the other species-mixtures, indicating that neighborhood composition (i.e., trait combination), but not tree species richness mediated the relationship between tree diversity and treatment effects on tree growth. Our findings point to the importance of the qualitative role (‘trait portfolio’) that biodiversity play in determining resistance of diverse tree communities to environmental changes. As such, they provide further understanding for adaptive management strategies in the context of global change.

Sclerophyllous oak forests are an important ecosystem type of the natural vegetation in the Mediterranean region. As a part of the mosaic-like landscape, old-growth oak forests, in particular, provide a wide range of ecosystem functions and services. However, due to the general scarcity of older forests in the Mediterranean region, studies on structure and dynamics of oak forests are mostly restricted to younger stands. The aim of this study was to analyze stand structure, dead wood, and natural regeneration of an old Quercus calliprinos forest in northern Israel (nature reserve Ya'ar Bar'am) and to compare the results with those reported from other evergreen Mediterranean oak forests. The mean density of trees was 735 ha-1, and the mean basal area amounted to 25.0 m2 ha-1. The mean tree height was 6.6 m, and the maximum tree height was 13.2 m. The mean dead wood volume was 11 m3 ha-1. The sapling density ranged from 1,050 to 6,250 ha-1. We conclude that Bar'am Forest is of great value for conservation bi...

The dominant plant species of European heathlands Calluna vulgaris is considered vulnerable to drought and enhanced nitrogen (N) loads. However, impacts may vary across the distribution range of Calluna heathlands. We tested the hypothesis that Calluna of southern and eastern marginal populations (MP) are more resistant to drought events than plants of central populations (CP), and that this is mainly due to trait differences such as biomass allocation patterns. Furthermore, we hypothesised that N fertilisation can offset differences in drought susceptibility between CP and MP. We conducted a full-factorial 2-year greenhouse experiment with Calluna plants of CP and MP and quantified growth responses in terms of biomass production, allocation and tissue δ13C signatures. Biomass production, shoot–root ratios and tissue δ13C values of 1-year-old plants were higher for CP than for MP, indicating a higher drought susceptibility of CP. These trait differences were not observed for 2-year-old plants. N fertilisation increased shoot–root ratios of 1- and 2-year-old plants and across populations due to a stimulation of the aboveground biomass allocation. As a consequence, population-related differences in drought susceptibility were offset for N-fertilised plants. We concluded that Calluna plants originating from different populations developed adaptive traits to local climates, which determined their drought sensitivity. However, the higher drought resistance of MP can be attenuated by an N-induced increase in shoot–root ratios. This suggests that analyses on plant growth responses to global change should include multi-factor approaches with a focus on different populations throughout a species’ distribution range.

International audience; & Context Terrestrial laser scanning (TLS) provides a valuable tool for the retrieval of detailed individual-tree structural pa-rameters, but has never previously been applied to young stands under field conditions. & Aims The aim was to explore the performance of TLS in a young tree plantation located in a heterogeneous environment in subtropical China. & Methods We investigated 438 young trees for congruence between direct field and TLS measurements of total tree height, stem diameter at ground height, and length and height of the longest branch using correlation tests. We applied generalized linear models to examine whether congruence was affected by the observed structural parameter or extrinsic factors (e.g., potential occlusion, point cloud quality). & Results TLS made it possible to detect trees higher than 40 cm. The TLS-retrieved data were highly congruent with the data obtained from direct measurements. The poor descrip-tions of stems and branches of some individuals of small-sized and leaf-on tree species were due to occlusion by ground vegetation and leaf-on branches. Observed structural parame-ter and extrinsic factors did not explain the variance between the two approaches. & Conclusion TLS proved to be a promising tool for high-resolution, non-destructive analyses of tree dendrometrics in young regenerating plantations.

Questions: The maintenance of a diverse sapling pool is of particular importance for the regeneration and persistance of species-rich forest ecosystems. However, the mechanisms of co-existence of saplings have rarely been studied experimentally. Do species richness, species composition, species identity and stand density affect the co-existence, growth patterns and crown architecture of tree saplings? Location: Jiangxi Province, southeast China. Methods: In a field experiment, we manipulated the local neighbourhood of saplings of four early-successional subtropical species (Schima superba, Elaeocarpus decipiens, Quercus serrata and Castanea henryi) with regard to species richness (one, two and four species), species composition (monocultures, six-two-species combinations and one-four-species combination) and stand density (low, intermediate and high). We tested for treatment effects and impact of species identity on growth variables, biomass allocation, crown architectural traits and branch demography. Results: Species richness was a poor predictor of all response variables, but enhanced pruning and branch turnover. In contrast, species composition proved to be of great importance for growth, biomass allocation, crown architecture and branch demography. Local neighbourhood interactions of saplings were characterized by complementary or facilitative, as well as by competitive mechanisms. Intra-specific competition was higher than inter-specific competition for two species (C. henryi, Q. serrata) depending on the respective species combination. To a high degree, the competitive ability of species can be explained by species identity. Competition for light likely played a major role in our experiment, as evidenced by the strong response of crown architecture and branch demography to the manipulated predictor variables. Conclusions: Effects of species composition and species identity on growth rates and crown architecture variables of tree saplings point to niche separation as a mechanism of species co-existence, while effects of species richness were not yet prominent at the sapling life stage. In a field experiment, we manipulated the local neighbourhood of saplings of four species and surveyed tree sapling growth and crown architecture. Effects of species composition and species identity on growth rates and crown architectural variables point to niche separation as a mechanism of species coexistence, while effects of species richness were not yet prominent at the sapling life stage. © 2012 International Association for Vegetation Science.

The study presents an analysis of the diameter distributions of nine virgin beech forests ( Fagus sylvatica ) in south-eastern Europe. Data were collected from published and unpublished sources. We included predominantly full calliperings of coherent areas between 3.6 and 13.0 ha of size. The objective of the study was to compare and characterize the curve forms and to test systematically which mathematical function provides a better fit: the negative exponential function, third or seventh degree Weibull functions. The parameters were estimated applying the maximum likelihood method. To evaluate the goodness of fit, the absolute discrepancy was used. The residuals were examined with respect to systematic deviation. The nine virgin forests displayed a great variety of structures, only four out of 36 possible pairs of diameter distributions were found to be from the same population. The negative exponential model produced a good fit for four of the nine empirical distributions. However, the analysis of the residuals produced by the models exhibited systematic errors. Both the negative exponential and the third degree Weibull function underpredict to some extent the number of stems in the midsize diameter range in all nine stands. The higher number of trees frequently found in the midsize range of the empirical distributions as compared to the models indicates a common trend towards a rotated sigmoid diameter distribution. For some stands even a tendency towards two peaks in the distribution can be found. These results reveal that the reverse J-shaped curve form is not the only applicable model for describing diameter distributions in virgin beech forests. The systematic nature of the deviations from the negative exponential curve leads us to conclude that we might have found a general trend typical for virgin beech forests in south-eastern Europe.