Neighbour species richness and local structural variability modulate aboveground allocation patterns and crown morphology of individual trees

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.     

Stabilizing effects of diversity on aboveground wood production in forest ecosystems: linking patterns and processes

Both theory and evidence suggest that diversity stabilises productivity in herbaceous plant communities through a combination of overyielding, species asynchrony and favourable species interactions. However, whether these same processes also promote stability in forest ecosystems has never been tested. Using tree ring data from permanent forest plots across Europe, we show that aboveground wood production is inherently more stable through time in mixed‐species forests. Faster rates of wood production (i.e. overyielding), decreased year‐to‐year variation in productivity through asynchronous responses of species to climate, and greater temporal stability in the growth rates of individual tree species all contributed strongly to stabilising productivity in mixed stands. Together, these findings reveal the central role of diversity in stabilising productivity in forests, and bring us closer to understanding the processes which enable diverse forests to remain productive under a wide range of environmental conditions.     

Soil microbiome mediates positive plant diversity‐productivity relationships in late successional grassland species

Which processes drive the productivity benefits of biodiversity remain a critical, but unanswered question in ecology. We tested whether the soil microbiome mediates the diversity‐productivity relationships among late successional plant species. We found that productivity increased with plant richness in diverse soil communities, but not with low‐diversity mixtures of arbuscular mycorrhizal fungi or in pasteurised soils. Diversity‐interaction modelling revealed that pairwise interactions among species best explained the positive diversity‐productivity relationships, and that transgressive overyielding resulting from positive complementarity was only observed with the late successional soil microbiome, which was both the most diverse and exhibited the strongest community differentiation among plant species. We found evidence that both dilution/suppression from host‐specific pathogens and microbiome‐mediated resource partitioning contributed to positive diversity‐productivity relationships and overyielding. Our results suggest that re‐establishment of a diverse, late successional soil microbiome may be critical to the restoration of the functional benefits of plant diversity following anthropogenic disturbance.     

Links between tree species, symbiotic fungal diversity and ecosystem functioning in simplified tropical ecosystems

We studied the relationships among plant and arbuscular mycorrhizal (AM) fungal diversity, and their effects on ecosystem function, in a series of replicate tropical forestry plots in the La Selva Biological Station, Costa Rica. Forestry plots were 12 yr old and were either monocultures of three tree species, or polycultures of the tree species with two additional understory species. Relationships among the AM fungal spore community, host species, plant community diversity and ecosystem phosphorus-use efficiency (PUE) and net primary productivity (NPP) were assessed. Analysis of the relative abundance of AM fungal spores found that host tree species had a significant effect on the AM fungal community, as did host plant community diversity (monocultures vs polycultures). The Shannon diversity index of the AM fungal spore community differed significantly among the three host tree species, but was not significantly different between monoculture and polyculture plots. Over all the plots, significant positive relationships were found between AM fungal diversity and ecosystem NPP, and between AM fungal community evenness and PUE. Relative abundance of two of the dominant AM fungal species also showed significant correlations with NPP and PUE. We conclude that the AM fungal community composition in tropical forests is sensitive to host species, and provide evidence supporting the hypothesis that the diversity of AM fungi in tropical forests and ecosystem NPP covaries.     

Greater than the sum of the parts: how the species composition in different forest strata influence ecosystem function

The mechanisms underpinning forest biodiversity‐ecosystem function relationships remain unresolved. Yet, in heterogeneous forests, ecosystem function of different strata could be associated with traits or evolutionary relationships differently. Here, we integrate phylogenies and traits to evaluate the effects of elevational diversity on above‐ground biomass across forest strata and spatial scales. Community‐weighted means of height and leaf phosphorous concentration and functional diversity in specific leaf area exhibited positive correlations with tree biomass, suggesting that both positive selection effects and complementarity occur. However, high shrub biomass is associated with greater dissimilarity in seed mass and multidimensional trait space, while species richness or phylogenetic diversity is the most important predictor for herbaceous biomass, indicating that species complementarity is especially important for understory function. The strength of diversity‐biomass relationships increases at larger spatial scales. We conclude that strata‐ and scale‐ dependent assessments of community structure and function are needed to fully understand how biodiversity influences ecosystem function.     

Partitioning the biodiversity effects on productivity into density and size components

Plant density and size — two factors that represent plant survival and growth — are key determinants of yield but have rarely been analysed explicitly in the context of biodiversity–productivity relationships. Here, we derive equations to partition the net, complementarity and selection effects of biodiversity into additive components that reflect diversity-induced changes in plant density and size. Applications of the new method to empirical datasets reveal contrasting ways in which plant density and size regulate yield in species mixtures. In an annual plant diversity experiment, overyielding is largely explained by selection effects associated with increased size of highly productive plant species. In a tree diversity experiment, the cause of overyielding shifts from enhanced growth in tree size to reduced mortality by complementary use of canopy space during stand development. These results highlight the capability of the new method to resolve crucial, yet understudied, demographic links between biodiversity and productivity.     

Tree species richness affects litter production and decomposition rates in a tropical biodiversity experiment

We report data on leaf litter production and decomposition from a manipulative biodiversity experiment with trees in tropical Panama, which has been designed to explore the relationship between tree diversity and ecosystem functioning. A total of 24 plots (2025 m²) were established in 2001 using six native tree species, with 1‐, 3‐, and 6‐species mixtures. We estimated litter production during the dry season 2005 with litter traps; decomposition was assessed with a litter bag approach during the following wet season. Litter production during the course of the dry season was highly variable among the tree species. Tree diversity significantly affected litter production, and the majority of the intermediate diverse mixtures had higher litter yields than expected based on yields in monoculture. In contrast, high diverse mixtures did not show such overyielding in litter production. Litter decomposition rates were also highly species‐specific, and were related to various measures of litter quality (C/N, lignin/N, fibre content). We found no overall effect of litter diversity if the entire litter mixtures were analyzed, i.e. mixing species resulted in pure additive effects and observed decomposition rates were not different from expected rates. However, the individual species changed their decomposition pattern depending on the diversity of the litter mixture, i.e. there were species‐specific responses to mixing litter. The analysis of temporal C and N dynamics within litter mixtures gave only limited evidence for nutrient transfer among litters of different quality. At this early stage of our tree diversity experiment, there are no coherent and general effects of tree species richness on both litter production and decomposition. Within the scope of the biodiversity‐ecosystem functioning relationship, our results therefore highlight the process‐specific effects diversity may have. Additionally, species‐specific effects on ecosystem processes and their temporal dynamics are important, but such effects may change along the gradient of tree diversity.     

Functional identity is the main driver of diversity effects in young tree communities

Two main effects are proposed to explain biodiversity–ecosystem functioning relationships: niche complementarity and selection effects. Both can be functionally defined using the functional diversity (FD) and functional identity (FI) of the community respectively. Herein, we present results from the first tree diversity experiment that separated the effect of selection from that of complementarity by varying community composition in high‐density plots along a gradient of FD, independent of species richness and testing for the effects of FD and community weighted means of traits (a proxy for FI) on stem biomass increment (a proxy for productivity). After 4 years of growth, most mixtures did not differ in productivity from the averages of their respective monocultures, but some did overyield significantly. Those positive diversity effects resulted mostly from selection effects, primarily driven by fast‐growing deciduous species and associated traits. Net diversity effect did not increase with time over 4 years.     

Tree Diversity,Environmental Heterogeneity,and Productivity in a Mexican Tropical Dry Forest1

Species diversity–environmental heterogeneity (D–EH) and species diversity–productivity (D–P) relationships have seldom been analyzed simultaneously even though such analyses could help to understand the processes underlying contrasts in species diversity among sites. Here we analyzed both relationships at a local scale for a highly diverse tropical dry forest of Mexico. We posed the following questions: (1) are environmental heterogeneity and productivity related?; (2) what are the shapes of D–EH and D–P relationships?; (3) what are individual, and interactive, contributions of these two variables to the observed variance in species diversity?; and (4) are patterns affected by sample size, or by partitioning into average local diversity and spatial species turnover? All trees (diameter at breast height ≥5 cm) within twenty‐six 0.2‐ha transects were censused; four environmental variables associated with water availability were combined into an environmental heterogeneity index; aboveground standing biomass was used as a productivity estimator. Simple and multiple linear and nonlinear regression models were run. Environmental heterogeneity and productivity were not correlated. We found consistently positive log‐linear D–EH and D–P relationships. Productivity explained a larger fraction of among‐transect variance in species diversity than did environmental heterogeneity. No effects of sample size were found. Different components of diversity varied in sensitivity to environmental heterogeneity and productivity. Our results suggest that species' differentiation along water availability gradients and species exclusion at the lowest productivity (driest) sites occur simultaneously, independently, and in a scale‐dependent fashion on the tree community of this forest.     

Tree communities of different aged logged areas in an Afrotropical rainforest

Predicting the recovery processes in tree communities after logging is critical when developing conservation strategies. We assessed the patterns in tree communities in logged and primary forests in Kibale National Park, Uganda, representing 9‐ to 19‐year‐old clear‐cuts of former conifer plantations, 42‐ to 43‐year‐old logged forests and primary forests. Species density and diversity were lower and dominance higher in the 9‐ to 19‐year‐old forests compared to the 42‐ to 43‐year‐old forests or primary forests. The tree species density, diversity and dominance of 42‐ to 43‐year‐old forests did not differ significantly from primary forests. However, they had a lower stem density, and higher cover of Acanthus pubescens, a shrub known to arrest the succession in Kibale. The tree community compositions of 9‐ to 19‐year‐old, 42‐ to 43‐year‐old and primary forests differed from each other. A large group of tree species (21) were primary forest indicators, that is, they were either missing or relatively rare in logged forests. The results of this study show that even after four decades of natural recovery, logged Afrotropical forests can still be distinguished from primary forests in their tree community compositions, emphasizing the slow community recovery and the important role of primary forests when preserving the tree communities in tropical rainforests.     

Biodiversity, productivity and the temporal stability of productivity: patterns and processes

Theory predicts that the temporal stability of productivity, measured as the ratio of the mean to the standard deviation of community biomass, increases with species richness and evenness. We used experimental species mixtures of grassland plants to test this hypothesis and identified the mechanisms involved. Additionally, we tested whether biodiversity, productivity and temporal stability were similarly influenced by particular types of species interactions. We found that productivity was less variable among years in plots planted with more species. Temporal stability did not depend on whether the species were planted equally abundant (high evenness) or not (realistically low evenness). Greater richness increased temporal stability by increasing overyielding, asynchrony of species fluctuations and statistical averaging. Species interactions that favoured unproductive species increased both biodiversity and temporal stability. Species interactions that resulted in niche partitioning or facilitation increased both productivity and temporal stability. Thus, species interactions can promote biodiversity and ecosystem services.     

Grazing weakens temporal stabilizing effects of diversity in the Eurasian steppe

Many biodiversity experiments have demonstrated that plant diversity can stabilize productivity in experimental grasslands. However, less is known about how diversity–stability relationships are mediated by grazing. Grazing is known for causing species losses, but its effects on plant functional groups (PFGs) composition and species asynchrony, which are closely correlated with ecosystem stability, remain unclear. We conducted a six‐year grazing experiment in a semi‐arid steppe, using seven levels of grazing intensity (0, 1.5, 3.0, 4.5, 6.0, 7.5, and 9.0 sheep per hectare) and two grazing systems (i.e., a traditional, continuous grazing system during the growing period (TGS), and a mixed one rotating grazing and mowing annually (MGS)), to examine the effects of grazing system and grazing intensity on the abundance and composition of PFGs and diversity–stability relationships. Ecosystem stability was similar between mixed and continuous grazing treatments. However, within the two grazing systems, stability was maintained through different pathways, that is, along with grazing intensity, persistence biomass variations in MGS, and compensatory interactions of PFGs in their biomass variations in TGS. Ecosystem temporal stability was not decreased by species loss but rather remain unchanged by the strong compensatory effects between PFGs, or a higher grazing‐induced decrease in species asynchrony at higher diversity, and a higher grazing‐induced increase in the temporal variation of productivity in diverse communities. Ecosystem stability of aboveground net primary production was not related to species richness in both grazing systems. High grazing intensity weakened the temporal stabilizing effects of diversity in this semi‐arid grassland. Our results demonstrate that the productivity of dominant PFGs is more important than species richness for maximizing stability in this system. This study distinguishes grazing intensity and grazing system from diversity effects on the temporal stability, highlighting the need to better understand how grazing regulates ecosystem stability, plant diversity, and their synergic relationships.     

Tree functional diversity affects litter decomposition and arthropod community composition in a tropical forest

Disturbance can alter tree species and functional diversity in tropical forests, which in turn could affect carbon and nutrient cycling via the decomposition of plant litter. However, the influence of tropical tree diversity on forest floor organisms and the processes they mediate are far from clear. We investigated the influence of different litter mixtures on arthropod communities and decomposition processes in a 60‐year‐old lowland tropical forest in Panama, Central America. We used litter mixtures representing pioneer and old growth tree species in experimental mesocosms to assess the links between litter types, decomposition rates, and litter arthropod communities. Overall, pioneer species litter decomposed most rapidly and old growth species litter decomposed the slowest but there were clear non‐additive effects of litter mixtures containing both functional groups. We observed distinct arthropod communities in different litter mixtures at 6 mo, with greater arthropod diversity and abundance in litter from old growth forest species. By comparing the decay of different litter mixtures in mesocosms and conventional litterbags, we demonstrated that our mesocosms represent an effective approach to link studies of litter decomposition and arthropod communities. Our results indicate that changes in the functional diversity of litter could have wider implications for arthropod communities and ecosystem functioning in tropical forests.     

Intraspecific plant–soil feedback and intraspecific overyielding in Arabidopsis thaliana

Understanding the mechanisms of community coexistence and ecosystem functioning may help to counteract the current biodiversity loss and its potentially harmful consequences. In recent years, plant–soil feedback that can, for example, be caused by below‐ground microorganisms has been suggested to play a role in maintaining plant coexistence and to be a potential driver of the positive relationship between plant diversity and ecosystem functioning. Most of the studies addressing these topics have focused on the species level. However, in addition to interspecific interactions, intraspecific interactions might be important for the structure of natural communities. Here, we examine intraspecific coexistence and intraspecific diversity effects using 10 natural accessions of the model species Arabidopsis thaliana (L.) Heynh. We assessed morphological intraspecific diversity by measuring several above‐ and below‐ground traits. We performed a plant–soil feedback experiment that was based on these trait differences between the accessions in order to determine whether A. thaliana experiences feedback at intraspecific level as a result of trait differences. We also experimentally tested the diversity–productivity relationship at intraspecific level. We found strong differences in above‐ and below‐ground traits between the A. thaliana accessions. Overall, plant–soil feedback occurred at intraspecific level. However, accessions differed in the direction and strength of this feedback: Some accessions grew better on their own soils, some on soils from other accessions. Furthermore, we found positive diversity effects within A. thaliana: Accession mixtures produced a higher total above‐ground biomass than accession monocultures. Differences between accessions in their feedback response could not be explained by morphological traits. Therefore, we suggest that they might have been caused by accession‐specific accumulated soil communities, by root exudates, or by accession‐specific resource use based on genetic differences that are not expressed in morphological traits. Synthesis. Our results provide some of the first evidence for intraspecific plant–soil feedback and intraspecific overyielding. These findings may have wider implications for the maintenance of variation within species and the importance of this variation for ecosystem functioning. Our results highlight the need for an increased focus on intraspecific processes in plant diversity research to fully understand the mechanisms of coexistence and ecosystem functioning.     

Species‐time‐area and phylogenetic‐time‐area relationships in tropical tree communities

The species‐area relationship (SAR) has proven to be one of the few strong generalities in ecology. The temporal analog of the SAR, the species‐time relationship (STR), has received considerably less attention. Recent work primarily from the temperate zone has aimed to merge the SAR and the STR into a synthetic and unified species‐time‐area relationship (STAR) as originally envisioned by Preston (1960). Here we test this framework using two tropical tree communities and extend it by deriving a phylogenetic‐time‐area relationship (PTAR). The work finds some support for Preston's prediction that diversity‐time relationships, both species and phylogenetic, are sensitive to the spatial scale of the sampling. Contrary to the Preston's predictions we find a decoupling of diversity‐area and diversity‐time relationships in both forests as the time period used to quantify the diversity‐area relationship changes. In particular, diversity‐area and diversity‐time relationships are positively correlated using the initial census to quantify the diversity‐area relationship, but weakly or even negatively correlated when using the most recent census. Thus, diversity‐area relationships could forecast the temporal accumulation of biodiversity of the forests, but they failed to “back‐cast” the temporal accumulation of biodiversity suggesting a decoupling of space and time.     

Functional diversity predicts overyielding effect of species combination on primary productivity

The effect of biodiversity on ecosystem functioning has proven variable both within and among manipulative studies. Species richness is the most commonly used measure of biodiversity in such studies, but the range of species’ functional traits (functional diversity), not the number of species per se, likely underpins a key mechanistic link between species richness and ecosystem functioning. However, the majority of experiments that have examined the effect of functional diversity have manipulated functional group richness, an approach recognised to suffer numerous limitations. Continuous measures of functional diversity avoid many of these limitations, but the relationship between continuous functional diversity and the magnitude of ecosystem processes has been largely untested. Using one vs two‐species mixtures of rock pool macroalgae as a model, we conducted a field experiment to determine the effect of a continuous measure of functional diversity (functional attribute diversity, FAD, the degree of functional differentiation based on four functional traits) on the magnitude of net primary productivity and overyielding, based upon two alternative null‐models. The total magnitude of productivity was largely determined by the identity of species present, not FAD. However, FAD proved to be a good predictor of overyielding (variation in productivity after the dominant effects of species identity had been accounted for). Furthermore, despite differences in the mean magnitude of the effect of combining species, the positive relationship between FAD and overyielding was consistent according to both additive and substitutive null‐models. Our findings imply that whilst knowledge of species’ independent contributions remains indispensable in the prediction of biotic effects on ecosystem functioning within a trophic level, continuous measures of functional diversity should be used as a supplementary tool to predict the magnitude of overyielding, thereby refining predictions.     

Tree species identity determines wood decomposition via microclimatic effects

Empirical evidence suggests that the rich set of ecosystem functions and nature's contributions to people provided by forests depends on tree diversity. Biodiversity–ecosystem functioning research revealed that not only species richness per se but also other facets of tree diversity, such as tree identity, have to be considered to understand the underlying mechanisms. One important ecosystem function in forests is the decomposition of deadwood that plays a vital role in carbon and nutrient cycling and is assumed to be determined by above‐ and belowground interactions. However, the actual influence of tree diversity on wood decay in forests remains inconclusive. Recent studies suggest an important role of microclimate and advocate a systematical consideration of small‐scale environmental conditions. We studied the influence of tree species richness, tree species identity, and microclimatic conditions on wood decomposition in a 12‐year‐old tree diversity experiment in Germany, containing six native species within a tree species richness gradient. We assessed wood mass loss, soil microbial properties, and soil surface temperature in high temporal resolution. Our study shows a significant influence of tree species identity on all three variables. The presence of Scots pine strongly increased wood mass loss, while the presence of Norway spruce decreased it. This could be attributed to structural differences in the litter layer that were modifying the capability of plots to hold the soil surface temperature at night, consequently leading to enhanced decomposition rates in plots with higher nighttime surface temperatures. Therefore, our study confirmed the critical role of microclimate for wood decomposition in forests and showed that soil microbial properties alone were not sufficient to predict wood decay. We conclude that tree diversity effects on ecosystem functions may include different biodiversity facets, such as tree identity, tree traits, and functional and structural diversity, in influencing the abiotic and biotic soil properties.     

The relationship between environmental complexity, species diversity and productivity in a natural reconstructed yeast community

Diversity, whether ecological or genetic, is widely believed to be beneficial to the functioning of ecosystems. Although many studies have investigated relationships between environmental complexity, species diversity and ecosystem function, few have examined these factors simultaneously. We propagated combinations of three naturally coexisting yeast species for 200 generations, in environments of increasing complexity represented by combinations of up to eight different carbon sources. We found that competitive ability was transitive, and not related to productivity, which was equal among the species. Species diversity had a positive effect on productivity and overyielding in mixtures was caused primarily by complementation. Environmental complexity and species diversity were positively correlated, though not significantly, and the sole case of coexistence of all three yeasts after 200 generations occurred on a single carbon source, melezitose. Environmental complexity also enhanced productivity, although this relationship failed, for unknown reasons, at the highest level of complexity. Our results suggest that maintaining species diversity contributes to ecosystem productivity, but that the mechanisms responsible for maintaining diversity may not be straightforward.     

Do temperate tree species diversity and identity influence soil microbial community function and composition?

Studies of biodiversity–ecosystem function in treed ecosystems have generally focused on aboveground functions. This study investigates intertrophic links between tree diversity and soil microbial community function and composition. We examined how microbial communities in surface mineral soil responded to experimental gradients of tree species richness (SR ), functional diversity (FD ), community‐weighted mean trait value (CWM ), and tree identity. The site was a 4‐year‐old common garden experiment near Montreal, Canada, consisting of deciduous and evergreen tree species mixtures. Microbial community composition, community‐level physiological profiles, and respiration were evaluated using phospholipid fatty acid (PLFA ) analysis and the MicroResp^? system, respectively. The relationship between tree species richness and glucose‐induced respiration (GIR ), basal respiration (BR ), metabolic quotient (qCO ₂) followed a positive but saturating shape. Microbial communities associated with species mixtures were more active (basal respiration [BR ]), with higher biomass (glucose‐induced respiration [GIR ]), and used a greater number of carbon sources than monocultures. Communities associated with deciduous tree species used a greater number of carbon sources than those associated with evergreen species, suggesting a greater soil carbon storage capacity. There were no differences in microbial composition (PLFA ) between monocultures and SR mixtures. The FD and the CWM of several functional traits affected both BR and GIR . In general, the CWM of traits had stronger effects than did FD , suggesting that certain traits of dominant species have more effect on ecosystem processes than does FD . Both the functions of GIR and BR were positively related to aboveground tree community productivity. Both tree diversity (SR ) and identity (species and functional identity—leaf habit) affected soil microbial community respiration, biomass, and composition. For the first time, we identified functional traits related to life‐history strategy, as well as root traits that influence another trophic level, soil microbial community function, via effects on BR and GIR .     

On the significance of belowground overyielding in temperate mixed forests: separating species identity and species diversity effects

Complementary soil exploration by the root systems of coexisting tree species has been hypothesised to result in a higher root biomass of mixed forests than of monocultures but the existing evidence for a belowground diversity effect in forests is scarce and not conclusive. In a species‐rich temperate broad‐leaved forest, we analysed the fine root biomass (roots ≤ 2 mm) and necromass in 100 plots differing in tree species diversity (one to three species) and species composition (all possible combinations of five species of the genera Acer, Carpinus, Fagus, Fraxinus and Tilia) which allowed us to separate possible species diversity and species identity effects on fine root biomass. We found no evidence of a positive diversity effect on standing fine root biomass and thus of overyielding in terms of root biomass. Root necromass decreased with increasing species diversity at marginal significance. Various lines of evidence indicate significant species identity effects on fine root biomass (10–20% higher fine root biomass in plots with presence of maple and beech than in plots with hornbeam; 100% higher fine root biomass in monospecific beech and ash plots than in hornbeam plots; differences significant). Ash fine roots tended to be over‐represented in the 2‐ and 3‐species mixed plots compared to monospecific ash plots pointing at apparent belowground competitive superiority of Fraxinus in this mixed forest. Our results indicate that belowground overyielding and spatial complementarity of root systems may be the exception rather than the rule in temperate mixed forests.