Niche Complementarity Effects Research Articles

Tree species diversity drives the land surface phenology of seasonally dry tropical woodlands

Abstract Seasonal foliage display (leaf phenology) is a key determinant of ecosystem function. Variation in land surface phenology, observed via space‐borne remote sensing, can be explained at broad spatial scales by climate, but we lack understanding of how vegetation structure and floristic diversity mediates these relationships. This lack of understanding hampers our ability to predict changes in phenology and therefore ecosystem function, in light of rapid ongoing shifts in biodiversity and ecosystem structure due to land use and climate change. We combined a network of 619 vegetation monitoring sites across seasonally dry tropical deciduous woodlands in Zambia with land surface phenology metrics to investigate the role of tree species diversity, composition and vegetation structure on patterns of land surface phenology, including the phenomenon of pre‐rain green‐up. Tree species diversity was associated with earlier pre‐rain green‐up, a longer growing season, and greater cumulative foliage production. Independent of diversity, proportional abundance of Detarioideae species (subfamily of Fabaceae) was associated with a longer growing season by facilitating earlier pre‐rain green‐up. Woodland stands with larger trees green up earlier, suggesting access to deep groundwater reserves. Senescence metrics showed variation among sites but were not well‐explained by precipitation, temperature, structure or diversity. Synthesis: Tree diversity, composition and structure are co‐determinants of seasonal patterns of foliage display in seasonally dry tropical deciduous woodlands, as measured via land surface phenology, at regional scale. Our study identifies both a niche complementarity effect whereby diverse woodlands exhibit longer growing seasons, as well as a mass‐ratio effect whereby detarioid species drive earlier pre‐rain green‐up, providing insights into the mechanisms underlying the biodiversity ecosystem function relationship in this biome. We stress the importance of considering biotic controls on ecosystem functioning in the next generation of earth‐system models predicting the response of communities to global change.

Plant functional traits are dynamic predictors of ecosystem functioning in variable environments

Abstract A central goal at the interface of ecology and conservation is understanding how the relationship between biodiversity and ecosystem function (B–EF) will shift with changing climate. Despite recent theoretical advances, studies which examine temporal variation in the functional traits and mechanisms (mass ratio effects and niche complementarity effects) that underpin the B–EF relationship are lacking. Here, we use 13 years of data on plant species composition, plant traits, local‐scale abiotic variables, above‐ground net primary productivity (ANPP), and climate from the alpine tundra of Colorado (USA) to investigate temporal dynamics in the B–EF relationship. To assess how changing climatic conditions may alter the B–EF relationship, we built structural equation models (SEMs) for 11 traits across 13 years and evaluated the power of different trait SEMs to predict ANPP, as well as the relative contributions of mass ratio effects (community‐weighted mean trait values; CWM), niche complementarity effects (functional dispersion; FDis) and local abiotic variables. Additionally, we coupled linear mixed effects models with Multimodel inference methods to assess how inclusion of trait–climate interactions might improve our ability to predict ANPP through time. In every year, at least one SEM exhibited good fit, explaining between 19.6% and 57.2% of the variation in ANPP. However, the identity of the trait which best explained ANPP changed depending on winter precipitation, with leaf area, plant height and foliar nitrogen isotope content (δ15N) SEMs performing best in high, middle and low precipitation years, respectively. Regardless of trait identity, CWMs exerted a stronger influence on ANPP than FDis and total biotic effects were always greater than total abiotic effects. Multimodel inference reinforced the results of SEM analysis, with the inclusion of climate–trait interactions marginally improving our ability to predict ANPP through time. Synthesis. Our results suggest that temporal variation in climatic conditions influences which traits, mechanisms and abiotic variables were most responsible for driving the B–EF relationship. Importantly, our findings suggest that future research should consider temporal variability in the B–EF relationship, particularly how the predictive power of individual functional traits and abiotic variables may fluctuate as conditions shift due to climate change.

Functional trait composition of carabid beetle communities predicts prey suppression through both mass ratio and niche complementarity mechanisms.

Several components of predator functional diversity have been hypothesized to influence prey suppression through either niche complementarity or mass ratio effects. Nevertheless, most studies have used a functional group approach when assessing the role of these predators in ecosystem functioning. By adopting a trait-based approach, we evaluated the relative contributions of carabid diversity components in predicting prey suppression. Our results highlight the importance of both taxonomic and functional diversity components of carabids as key drivers of prey suppression. Prey suppression was best predicted by carabid densities, with the dominance of Poecilus cupreus potentially driving the positive effect of community total abundance through the mass ratio effect. Prey suppression increased with increasing the density of large carabids. In addition, carabid eye diameter and antennal length were key functional traits for predicting prey suppression. Furthermore, prey suppression increased with increasing carabid functional richness following the niche complementarity effect. In contrast to functional richness, functional evenness and functional divergence of carabid communities were weakly correlated with prey suppression. By identifying which diversity components of carabid communities contribute the most to increase prey suppression, our results can guide efforts aiming to predict the relationship between diversity of these predators and ecosystem functioning.

Water-limited environments affect the association between functional diversity and forest productivity.

The link between biodiversity and ecosystem function can depend on environmental conditions. This contingency can impede our ability to predict how biodiversity-ecosystem function (BEF) relationships will respond to future environmental change, causing a clear need to explore the processes underlying shifts in BEF relationships across large spatial scales and broad environmental gradients. We compiled a dataset on five functional traits (maximum height, wood density, specific leaf area [SLA], seed size, and xylem vulnerability to embolism [P50]), covering 78%-90% of the tree species in the National Forest Inventory from Italy, to test (i) how a water limitation gradient shapes the functional composition and diversity of forests, (ii) how functional composition and diversity of trees relate to forest annual increment via mass ratio and complementarity effects, and (iii) how the relationship between functional diversity and annual increment varies between Mediterranean and temperate climate regions. Functional composition varied with water limitation; tree communities tended to have more conservative traits in sites with higher levels of water limitation. The response of functional diversity differed among traits and climatic regions but among temperate forest plots, we found a consistent increase of functional diversity with water limitation. Tree diversity was positively associated with annual increment of Italian forests through a combination of mass ratio and niche complementarity effects, but the relative importance of these effects depended on the trait and range of climate considered. Specifically, niche complementarity effects were more strongly associated with annual increment in the Mediterranean compared to temperate forests. Synthesis: Overall, our results suggest that biodiversity mediates forest annual increment under water-limited conditions by promoting beneficial interactions between species and complementarity in resource use. Our work highlights the importance of conserving functional diversity for future forest management to maintain forest annual increment under the expected increase in intensity and frequency of drought.

Few functionally acquisitive big-sized trees restrict but soil nutrients promote soil organic carbon storage in temperate deciduous forests

Soil organic carbon (SOC) storage plays a crucial role in mitigating global climate change while maintaining forests and sustaining environmental conditions. Recent studies have shown that plant species diversity increases SOC storage in experimental grasslands and natural forests. Here, we aim to show how an integrating tree-size and trait-based ecological approach leads to mechanistic insights into SOC storage that may be overlooked in previous studies. We hypothesized that tree functional diversity effects on SOC storage would change with tree-sized-based species' functional strategies driven by soil properties in natural temperate deciduous forests. We used the forest inventory data, site-specific measured traits and soil properties from 99 temperate deciduous forest plots in northwestern Iran. We investigated the joint effects of topographic factors, soil chemical properties, and the functional attributes (multi-trait functional diversity indices and community-weighted trait mean indices) of big-sized trees (BsT; individual tree diameter ≥ 50 cm) and remaining-sized trees (RsT) on topsoil (0–30 cm) SOC storage. The multiple linear regression models showed that SOC storage increased with increasing soil available phosphorus (r = 0.36), soil exchangeable potassium (r = 0.27 to 0.31) and soil total nitrogen (r = 0.27). In contrast, SOC storage declined with increasing the community-weighted mean of specific leaf area (r = −0.38 to −0.40). The structural equation model showed that SOC storage was directly driven by the positive effect of soil nutrients (i.e. a latent variable of soil total nitrogen and available phosphorus; r = 0.42) and the negative effect of the community-weighted mean of specific leaf area (r = −0.31). The topographic factors as well as the other several functional attributes of BsT and RsT showed non-significant direct and indirect effects on SOC storage in both types of statistical modelling. Our study suggests that the mass ratio and soil fertility rather than the niche complementarity effects along with the big-sized trees effect regulates SOC storage through the acquisitive trait identity of BsT species as compared to RsT species in temperate deciduous forests. We argue that maintaining high levels of species diversity with the most efficient trait identity across tree sizes is important for biodiversity conservation and multiple forest functions such as SOC storage which should be a priority for plantation management plans or silvicultural practices in natural forests.

Interaction Effect of Stand Age and Diversity on Aboveground Wood Carbon Accumulation in Subtropical Mixed Forests of the Zhejiang Province (China)

Aboveground wood carbon (AWC) stocks in forest ecosystems are mediated by biotic and abiotic variables. Understanding the internal regulatory mechanisms of forests is important for future forest management and global climate change mitigation. However, how these factors affect AWC in subtropical mixed forests remains poorly understood. Using a database from the National Forest Inventory (NFI) from China, we observed the effects of climate variables (temperature and precipitation), stand structure indices (stand density and DBH coefficient of variation and diversity), stand diversity indices (taxonomic diversity, functional diversity, and phylogenetic diversity), and stand functional indices on coniferous mixed forests (CMF), coniferous–broadleaf mixed forests (CBMF), and broadleaf mixed forests (BMF). Meanwhile, we examined the AWC based on a linear mixed model and a structural equation model for each mixed forest. We found that both stand structure and stand diversity can affect the AWC through their indirect effects on the stand function, aligning with the niche complementarity effect. Stand age is an important factor affecting AWC because it interacts with stand structure and stand diversity. Our study highlights that AWC is dependent on the regulation of stand age and structure, which can be crucial for boosting high carbon stocks in subtropical forests.

Temporal and spatial niche complementarity in sunflower pollinator communities and pollination function

One of the most invoked mechanisms mediating the positive effect of pollinator diversity on plant reproduction is pollinator's niche complementarity (i.e. partitioning of resource use by different pollinator species). However, the influence of spatial and temporal pollinator's niche complementarity on crop pollination function is rarely tested. We investigated the influence of spatial and temporal niche complementary in explaining sunflower crop production by comparing pollination activity at the edge and centre of crop fields and over the day. We found weaker evidence for spatial niche complementarity than for temporal niche complementarity in pollinator visitation rates. Only the visitation rate of hoverflies slightly differed between the centre and the edge of the fields. Nevertheless, we observed no differences in seed weight between the edge and the centre of the fields, but interestingly, plants allowed to be pollinated only by small-sized pollinators experienced a decline in seed production with distance from the edge. Pollinators did show complementary peak activity periods throughout the day, with Bombus terrestris and honeybees preferring to forage early in the day and at cooler temperatures than B. lapidarius and solitary bees. Unexpectedly, only morning- and only afternoon-pollinated plants produced similar seed weights, but these were higher than in all-day exposed plants. These findings indicate that sunflower fields shelter a small number of complementary pollinator species groups, which become rapidly redundant as diversity increases. Overall, we show that temporal and spatial niche complementarity effects on yield can unfold in unexpected ways, which are hard to predict without testing for the specific mechanisms.

Environmental, structural, and taxonomic diversity factors drive aboveground carbon stocks in semi‐deciduous tropical rainforest strata in Cameroon

AbstractForest stratification plays a crucial role in the interception of light and plants' photosynthetic activities. However, there is still a lack of information on the contribution of tropical forest stratification to its functioning, despite the increasing number of studies. Here, we analysed from a perspective of the whole tree community (WTC) and forest strata (i.e., large trees, understory trees, and small stems), the relationship between abiotic, biotic factors and aboveground Carbon (AGC). The abiotic factors‐AGC relationships were positive for all strata and WTC. However, soil factors‐AGC relationship was stronger for small stems and understorey, while topography factor‐AGC relationship was stronger for large trees and WTC. Tree size inequality‐AGC relationship was positive and much stronger for WTC, large trees and small stems. In addition, a species diversity‐AGC relationship was found positive only for large trees and WTC. These results highlight the niche complementarity effect for driving positive relationships of species diversity and individual tree size variation with aboveground biomass at large tree strata and WTC. The lack of positive effect of species diversity on AGC for understorey and small stems strata might be attributable to the selection effect or resource complementarity among species.

Species α-diversity promotes but β-diversity restricts aboveground biomass in tropical forests, depending on stand structure and environmental factors

Forest plays a vital role in the global biogeochemical cycles through a high rate of carbon sequestration and harboring biodiversity. However, local species diversity is declining while also becoming increasingly homogenized across communities. Although effects of local biotic processes (e.g., species α-diversity and stand structural heterogeneity) and environmental factors on aboveground biomass (AGB) have been widely tested, there is a huge knowledge gap for the effect of regional biotic processes (i.e., taxonomic and functional β-diversity) in forests. Here, we hypothesized that regional and local environmental factors along with biotic processes jointly regulate AGB through species shifts in tropical forests. Using piecewise structural equation modeling (pSEM), we linked climatic water availability, soil fertility, stand structural heterogeneity (either tree DBH inequality, height inequality, or stand density), species α-diversity, taxonomic or functional β-diversity (and its two components; β-turnover and β-richness), and AGB across 189 inventory plots in tropical forests of Sri Lanka. Soil fertility and climatic water availability shaped local and regional biotic processes. Stand structural heterogeneity promoted species α-diversity but declined β-diversity (but increased β-taxonomic turnover). Species α-diversity and stand structural heterogeneity promoted AGB whereas taxonomic and functional β-diversity declined (but β-taxonomic turnover increased) AGB. The relationships of AGB with species α-diversity and β-diversity varied from significant to nonsignificant positive depending on the specific combinations of stand structural heterogeneity metrics used. This study shows that local biotic processes could increase AGB due to the local and regional niche complementarity effect whereas the regional biotic processes could restrict AGB due to the regional selection or functional redundancy effect under favorable environmental conditions. We argue that biotic homogenization, as well as drought conditions, may have strong divergent impacts on forest functions and that the impacts of tree diversity loss may greatly reduce carbon sequestration.

Functional composition of tall-statured trees underpins aboveground biomass in tropical forests

The influences of trait diversity (i.e., the niche complementarity effect) and functional composition (i.e., the mass ratio effect) on aboveground biomass (AGB) is a highly debated topic in forest ecology. Therefore, further studies are needed to explore these mechanisms in unstudied forest ecosystems to enhance our understanding, and to provide guidelines for specific forest management. Here, we hypothesized that functional composition would drive AGB better than trait diversity and stem size inequality in the (sub-) tropical forests of Nepal. Using data from 101 forest plots, we tested 25 structural equation models (SEMs) to link elevation, stem DBH inequality, trait diversity (i.e., trait richness, evenness, dispersion and divergence), functional composition [i.e., community-weighted of maximum height mean (CWM of Hmax), specific leaf area (CWM of SLA), leaf dry matter content (CWM of LDMC), and wood density (CWM of WD)] and AGB. The best-fitted SEMs indicated that CWM of Hmax promoted AGB while overruling the impacts of trait diversity indices on AGB. However, low trait diversity indices were linked with higher AGB while overruling the effects of CWM of SLA, LDMC and WD on AGB. In addition, AGB decreased with increasing elevation, whereas stem size inequality did not influence AGB. Our results suggest that divergent species’ functional strategies could shape AGB along an altitudinal gradient in tropical forests. We argue that forest management practices should include plant functional traits in the management plan for the co-benefits of biodiversity conservation and carbon sequestration that underpins human wellbeing.

Positive Effects of Legumes on Soil Organic Carbon Stocks Disappear at High Legume Proportions Across Natural Grasslands in the Pyrenees

Soil is the largest terrestrial carbon pool, making it crucial for climate change mitigation. Soil organic carbon (SOC) is suggested to depend on biodiversity components, but much evidence comes from diversity-function experiments. To disentangle the relationships of plant guild diversity with SOC storage (kg m−2) at broad spatial scales, we applied diversity-interaction models to a regional grassland database (n = 96) including wide environmental conditions and management regimes. The questions were: (1) Are the effects of plant guilds on SOC stocks in natural grasslands consistent with those found in experimental systems? (2) Are plant guild effects on SOC stocks independent of each other or do they show interactive—synergistic or antagonistic—effects? (3) Do environmental variables, including abiotic and management, modify guild effects on SOC stocks? Among our most novel results we found, legume effects on grassland SOC vary depending on legume proportion consistently across broad spatial scales. SOC increased with legume proportion up to 7–17%, then decreased. Additionally, these effects were strengthened when grasses and forbs were codominant. Grazing intensity modulated grass proportion effects on SOC, being maximum at relatively high intensities. Interpreting our results in terms of existing contrasted ecological theories, we confirmed at broad spatial scales and under wide-ranging environmental conditions the positive effects of plant guild diversity on SOC, and we showed how legumes exert a keystone effect on SOC in natural grasslands, probably related to their ability to fix inorganic N. Niche complementarity effects were illustrated when codominance of forbs and grasses at optimum legume proportions boosted SOC storage, whereas grass dominance increased SOC stocks at medium–high grazing intensities. These findings can facilitate the preparation of regional and local strategies to ameliorate the soil capacity to absorb carbon.

Tree species diversity enhances plant-soil interactions in a temperate forest in northeast China

The plant-soil interactions may drive the diversity and functioning of forests, but we do not fully understand how interrelationships between plant and soil compartments are underlined by multiple ecological mechanisms. Here, we hypothesize that positive plant-soil interactions enhance biodiversity and functioning in a temperate forest. To do so, we tested the relationships between plant diversity (i.e., tree and herb species richness) and functions (i.e., coarse woody productivity and litterfall productivity), and soil diversity (i.e. bacterial, fungal and nematode) and functions (i.e. soil nutrient and carbon stock), and their interrelationships in a temperate forest in northeast China. The positive relationship between diversity and functioning was predominant within plant and soil compartments, and hence, provide support to the niche complementarity effect. As such, the positive interrelationships between the diversity of soil and plant compartments provide support to the positive plant-soil interactions. Tree species diversity was positively related with herb species diversity and coarse-woody productivity. Importantly, tree species diversity had pronounced positive effect on soil biodiversity resulting in increased soil carbon stocks, indicating that tree species diversity effect matters for linking positive interrelationships between plant and soil compartments of a temperate forest. This study shows that tree diversity effect is the main regulating biotic mechanism for linking the positive connections between plant and soil compartments of a temperate forest, and hence, the niche complementarity effect can enhance forest functioning through positive interactions on resource supply. We argue that linking the multiple key functions and diversity indices of forests can enhance our knowledge on the main influential factors and underlying ecological mechanisms.

Structural diversity consistently mediates species richness effects on aboveground carbon along altitudinal gradients in northern Ethiopian grazing exclosures

Grazing exclosures have been promoted as an effective and low-cost land management strategy to recover vegetation and associated functions in degraded landscapes in the tropics. While grazing exclosures can be important reservoirs of biodiversity and carbon, their potential in playing a dual role of conservation of biodiversity and mitigation of climate change effects is not yet established. To address this gap, we assessed the effect of diversity on aboveground carbon (AGC) and the relative importance of the driving biotic (functional diversity, functional composition and structural diversity) and abiotic (climate, topography and soil) mechanisms. We used a dataset from 133 inventory plots across three altitudinal zones, i.e., highland, midland and lowland, in northern Ethiopia, which allowed local- (within altitudinal zone) and broad- (across altitudinal zones) environmental scale analysis of diversity-AGC relationships. We found that species richness-AGC relationship shifted from neutral in highlands to positive in mid- and lowlands as well as across the altitudinal zones. Structural diversity was consistently the strongest mediator of the positive effects of species richness on AGC within and across altitudinal zones, whereas functional composition linked species richness to AGC at the broad environmental scale only. Abiotic factors had direct and indirect effects via biotic factors on AGC, but their relative importance varied with altitudinal zones. Our results indicate that the effect of species diversity on AGC was altitude-dependent and operated more strongly through structural diversity (representing niche complementarity effect) than functional composition (representing selection effect). Our study suggests that maintaining high structural diversity and managing functionally important species while promoting favourable climatic and soil conditions can enhance carbon storage in grazing exclosures.

Stand structure determines aboveground biomass across temperate forest types and species mixture along a local-scale elevational gradient

Greater variation in stand structure (i.e., individual tree size variation and stand density) is thought to promote aboveground biomass (AGB) better than species richness due to the canopy packing by different sized trees in the forest community. Here, we hypothesized that the relationships amongst species richness, stand density, individual tree size variation and AGB are dependent on forest types but single-species dominated forest stands matter much for higher AGB through higher stand density and individual tree size variation. To do this, we analyzed the forest inventory data (i.e., tree diameter, tree height, and species names) from 365 forest plots (each 0.1 ha) which was taken at different elevational gradients of temperate forest types (i.e., dense pure conifer, dense mixed forests, sparse mixed forests, sparse pure broadleaved, sparse pure conifer, and dense pure broadleaved) as well as species mixture groups (single-species and multi-species stands) in the northern area (namely Gilgit-Baltistan) of Pakistan. We mainly used piecewise structural equation modeling for testing the fixed effects of elevation, species richness, individual tree size variation and stand density as well as the random effect of either forest types or species mixture on AGB, and then we used linear structural equation modeling to test the models across forest types and species mixtures. We found the consistent positive effect of stand density on AGB across whole data, forest types and species mixtures, albeit species richness also showed a positive effect in some of the forest types. Individual tree size variation had a greater positive effect on AGB in single-species dominated stands compared to multi-species dominated stands. Elevation declined AGB directly and indirectly via stand density. We conclude that variation in stand structures (stand density and individual tree size variation of single-species dominated stands) are key to higher AGB in most of the temperate forest types. The positive effects of stand density and species richness on AGB may provide support to the niche complementarity effect through stand packing, whereas the greater effect of individual tree size variation on AGB in single-species rather than multi-species dominated stands may also confirm the selection or competitive exclusion effect. Therefore, we argue that both of the niche complementarity and selection effects are acting simultaneously to determine the relationships amongst species richness, stand structure and AGB in the studied temperate forests.

Afforestation suppresses soil nitrogen availability and soil multifunctionality on a subtropical grassland

Microbes simultaneously drive multiple functions (multifunctionality) that support human well-being. However, the structure and function of microbial communities and their impact on soil multifunctionality following grassland afforestation remains unknown, thus hindering our ability to formulate conservation policies. We compared soil bacterial and fungal communities, soil abiotic properties, and soil nitrogen (N) function and multifunctionality in the afforested sites that were previously grassland, on a subtropical plateau in China. We also explored the degree to which the niche complementarity effect and the selection effect of microbes are linked to soil N function and multifunctionality. We found that afforestation of grassland significantly decreased pH, available N concentration and density, and soil multifunctionality. However, afforestation significantly increased C (carbon) limitation and shifted soil microbes from being limited by N to, instead, being co-limited by N and P (phosphorus). The significant decrease in available N was primarily driven by soil microbes. In shaping soil N availability, the effect of bacterial diversities was stronger than that of fungal diversities, and the effect of fungal functional diversities was stronger than that of bacterial functional diversities. The effect of functional diversities was greater than that of all the significant changes in the functions and, also, the significant changes in the N-related functions. These results further emphasized that functional niche complementarity dominated soil N availability. In addition, bacterial taxonomic diversities showed positive effects of niche complementarity on soil multifunctionality; ultimately, the losses in bacterial taxonomic diversities derived from the increases in C limitation and the shifts in NP limitation combined to impaired soil multifunctionality. Our results suggested that the optimization of soil microbial functional diversities might increase soil N availability, and that minimizing losses of soil microbial taxonomic diversities by optimizing soil abiotic environments might improve soil multifunctionality.

Stand Structural Diversity and Species with Leaf Nitrogen Conservation Drive Aboveground Carbon Storage in Tropical Old-Growth Forests

Tropical old-growth forests are essential for global carbon regulation. Although there is increasing evidence that species and functional diversity, stand structural diversity, functional compositions, and elevation play roles in ecosystem functioning, the relative strengths of these drivers and the underlying mechanisms (mass-ratio hypothesis or niche complementarity hypothesis) are not clear. Aboveground carbon storage, species diversity, stand structural diversity, community-weighted mean (CWM), and functional diversity (FDvar) of 12 leaf traits were analyzed using data from 56 old-growth forest communities in the Dawei Mountain area of Southwest China. Multiple regression models were used to test the relative importance of the predictor variables and the structural equation model was used to explore the direct and indirect influences on aboveground carbon storage. High structural diversity moderately enhanced aboveground carbon storage. CWM leaf nitrogen concentration in young leaves weakly affected aboveground carbon storage. Our final multiple regression model showed that aboveground carbon storage is mostly affected by diameter at breast height (DBH) diversity, followed by FDvar of dry matter concentration in mature leaves and CWM nitrogen concentration in young leaves. The structural equation model indicated that elevation negatively affects aboveground carbon storage via diameter at breast height (DBH) diversity. Our results suggest that niche complementarity effects moderately drive aboveground carbon storage in tropical old-growth forests, but do not fully support the importance of the mass-ratio hypothesis.

Forest strata-dependent effects of vegetation attributes and soil nutrients on decadal changes in aboveground net carbon stock in two temperate forests

Vegetation attributes and edaphic properties are pivotal determinants for forest ecosystem functioning. However, how these factors determine the aboveground net carbon stock change (net C change) across forest strata remains poorly understood, especially for temperate forests. Here, we used repeat forest inventories, functional trait and soil nutrient data from large permanent plots in two old-growth temperate forests in northeast China to evaluate the effects of vegetation attributes (diversity, trait composition and forest structure) and soil nutrients on the net C change of overstorey and understorey trees. Using structural equation modeling, we found that forest structure was the strongest driver of net C change across forest strata in both forest types, which was positively related to stand density in the overstorey, and basal area in the understorey stratum. Neither diversity nor trait composition had any significant effect on the net C change in the overstorey. However, in the understorey stratum, diversity was positively associated with the net C change through niche complementarity effects. Due to understorey vegetation attributes, overstorey stand density had negative direct and indirect effects on net C change in the understorey. Diversity and trait composition in the overstorey were not significantly related to net C change in the understorey, but overstorey diversity increased the diversity, trait composition and forest structure of understorey trees. The soil phosphorus concentration had positive effects on the net C change in the overstorey, but negative effects in the understorey stratum. In addition, soil phosphorus concentration increased overstorey diversity but had no effect on understorey vegetation attributes. Our results showed that vegetation attributes and soil nutrients had different effects on the net C change in the overstorey and understorey strata. Resource filtering by overstorey trees affected the ecosystem functioning of the understorey stratum as well as its responses to soil nutrients. By separately analyzing the overstorey and understorey trees in complex natural forests, this study provides a better insight into the ecological mechanisms that play key roles in the aboveground carbon sequestration of temperate forests.

Scale-dependent effects of neighborhood biodiversity on individual tree productivity in a coniferous and broad-leaved mixed forest in China.

The relationship between biodiversity and productivity has stimulated an increasing body of research over the past decades, and this topic still occupies a central place in ecology. While most studies have focused on biomass production in quadrats or plots, few have investigated the scale‐dependent relationship from an individual plant perspective. We present an analysis of the effects of biodiversity (species diversity and functional diversity) on individual tree growth with a data set of 16,060 growth records from a 30‐ha temperate forest plot using spatially explicit individual tree‐based methods. A significant relationship between species diversity and tree growth was found at the individual tree level in our study. The magnitude and direction of biodiversity effects varies with the spatial scale. We found positive effects of species diversity on tree growth at scales exceeding 9 m. Individual tree growth rates increased when there was a greater diversity of species in the neighborhood of the focal tree, which provides evidence of a niche complementarity effect. At small scales (3–5 m), species diversity had negative effects on tree growth, suggesting that competition is more prevalent than complementarity or facilitation in these close neighborhoods. The results also revealed many confounding factors which influence tree growth, such as elevation and available sun light. We conclude that the use of individual tree‐based methods may lead to a better understanding of the biodiversity‐productivity relationship in forest communities.

Functional traits influence biomass and productivity through multiple mechanisms in a temperate secondary forest

Niche complementarity, mass-ratio, and vegetation quantity effects have been identified as major drivers of the biodiversity-ecosystem functioning (BEF) relationships. However, their relative contribution to biomass and productivity is not yet clear in temperate secondary forests. Based on the observations from a 21.12-ha temperate secondary forest plot in northeastern China, we assessed how these mechanisms regulate forest biomass and productivity. The niche complementarity effect was quantified using a functional diversity metric that was calculated from six locally collected functional traits. The mass-ratio effect was described as functional trait composition using community-weighted mean trait values. Vegetation quantity effect was evaluated using vegetation biomass. We performed structural equation modeling to test the alternative mechanisms. Our results provide evidence for all three mechanisms. Functional diversity increased forest productivity, in line with the niche complementarity hypothesis. Acquisitive traits (e.g., greater specific leaf area and leaf nitrogen concentration) enhance productivity, while conservative traits (e.g., greater wood density) enhance the long-term accumulation of biomass, demonstrating the mass-ratio hypothesis. Furthermore, we observed a significant positive relationship between biomass and productivity, confirming the vegetation quantity hypothesis. We conclude that functional traits drive biomass and productivity through multiple mechanisms. Both niche complementarity and the mass-ratio effects play roles in this temperate secondary forest. In addition, we emphasize the importance of preserving sufficient biomass stock to ensure maximum productivity in secondary forests. Our study contributes to the identification of the mechanisms underlying BEF relationships and has practical significance for guiding temperate secondary forest management and conservation.

Environmental variation, functional diversity and identity predicting community biomass in an old-growth subtropical broad-leaved forest

The relationship between biodiversity and ecosystem functioning (BEF) has emerged as a central issue in ecosystem ecology. Two broad classes of hypotheses, the mass ratio and niche complementarity effects, are proposed to explain the positive relation between biodiversity and ecosystem functioning in multiple experimental and natural systems. Environmental variation is noted as a key regulator of forest community biomass in natural systems and the under-story tree layer is often overlooked, as the canopy tree species are more likely to contribute to total community biomass. We determined community biomass of over-story, mid-story and under-story tree layers and employed environmental factors, functional diversity metrics of leaf traits to explain the variation in community biomass in an old-growth forest. We found that topographic and edaphic factors were of vital importance in regulating the standing biomass of each tree layer. Although functional diversity displayed positive effects on standing biomass across all tree strata, the standing biomass was more affected by functional identity than diversity in over-story and mid-story tree layers. Across all strata, the most significant correlation between functional diversity and standing biomass in under-story tree layer may suggest strengthened interspecific complementary interactions. Our study confirms that environmental variation is a strong driver of forest community biomass, and the relative importance of the mass ratio and the niche complementarity hypothesis vary across tree strata in explaining diversity effects on standing biomass.