Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types

Shading and selection effect-mediated species mixing enhance the growth of native trees in dry tropical forests

Tropical dry forests remain vital to rural communities but are often degraded and require restoration. Biodiversity plays a crucial role in maintaining ecosystem functioning and resilience and in providing essential services in these ecosystems. In many cases, restoration involves planting monospecific plantations of robust exotic species; however, detailed ecological studies are required to understand how native species mixtures could become successful for restoration purposes. To address this knowledge gap, a tree diversity experiment (IDENT-Ethiopia) was conducted to examine the impact of species diversity and shading on the growth of nine native tree species in tropical dry forests. The experiment followed a block design with 270 plots, which included a gradient in native tree species richness (1-, 2-, and 4-species mixtures) and a second gradient based on the functional diversity of species traits, including plots of low, medium, and high functional diversity. A shading treatment (shaded and unshaded) was also replicated in triplicate. The stem volume growth of seedlings was measured 1 and 2 years after planting. The results revealed that seedling growth was significantly boosted by increased species richness and shading: stem volume growth increased by 50.9% in shaded environments and 30.5% in mixed plots. The study also demonstrated a positive diversity productivity relationship in 57% of all mixtures. Variance partitioning showed that this overyielding was a result of competitive dominance. In the shaded environment, these productive dominant species were Cordia africana, followed by Dodonaea angustifolia and Dovyalis abyssinica. Overall, the findings suggest that shading and species mixing are crucial factors for promoting seedling growth of native dryland species and ensuring the successful restoration of drylands.

Belowground energy fluxes determine tree diversity effects on above- and belowground food webs

Worldwide tree diversity loss raises concerns about functional and energetic declines across trophic levels. In this study, we coupled 160 above- and belowground food webs, quantifying energy fluxes to microorganisms and invertebrates in a tree-mycorrhiza diversity experiment, to test how tree diversity affects fluxes of energy above and below the ground. The experiment differentiates three mycorrhizal type treatments: only AM tree species (with arbuscular mycorrhizae), only EcM tree species (with ectomycorrhizae; one, two, and four tree species), or mixtures of both AM and EcM tree species (AM+EcM; two and four tree species). Our results indicate that most energy initially flowed through belowground communities, with soil microorganisms contributing 97.7% of total energy and belowground fauna accounting for 60.9% of energy to animals. Consequently, belowground fauna fueled surface (62.3% of predation) and aboveground (30.5% of predation) predators. Tree diversity increased ecosystem multifunctionality (indicated by total and averaged energy fluxes) by ∼30% and energy across most trophic levels in EcM tree communities, while it shifted food webs from fast (such as bacterial-dominated) to slow (such as fungal-dominated) channels in AM tree communities. Tree diversity primarily impacted energy fluxes through belowground communities and strengthened the coupling of above- and belowground food webs, with increasing importance of belowground prey for predators at the soil surface and above the ground. These findings highlight that tree diversity and mycorrhizal types drive above- and belowground ecosystem functioning via belowground energy fluxes.

Tree Architecture and Structural Complexity in Mountain Forests of the Annapurna Region, Himalaya

Mountain ranges comprise heterogeneous environments and high plant diversity, but little is known about the architecture and structural complexity of trees in mountain forests. We studied the relationship between tree architecture, environmental conditions, and tree structural complexity in forests of the Annapurna region in the Himalaya. We further asked whether and how tree structural complexity translates into forest stand structural complexity. The study covers 546 trees on 14 undisturbed study plots across wide ranges of elevation (1300 to 3400 m asl.) and annual precipitation (1180 to 3600 mm yr.-1). They were assessed by ground-based mobile laser scanning. We found that tree structural complexity, expressed as box-dimension (D b ), was lowest for the needle-leaved Pinus wallichiana and highest for the broad-leaved Daphniphyllum himalense. A high share of the variation in D b was explained by tree architecture. In multivariate models, tree height, crown radius, and crown length explained more than 60% of the observed variation in D b . Stem density of the plot accounted for 19% of the variation in D b , and there was no influence of tree diversity. Precipitation explained l3% of the observed variation in tree D b , but elevation and slope did not have significant influences. As expected, tree height decreased with increasing elevation, but small trees often had relatively high D b values. The standard deviation of tree-level D b within a plot explained 47% of the variation in stand-level structural complexity among plots, surpassing the maximum tree-level D b . This suggests that both the sole removal of small or large trees would reduce the stand-level complexity by 36%. We conclude that in the Himalayan forests, species identity and tree architecture play a significant role in determining tree structural complexity, while environmental factors have a smaller role. Furthermore, structural variation among the trees within a plot plays a crucial role for the structural complexity at the stand level.

Decoupling Responses of Phyllosphere and Rhizosphere Bacterial Communities to Spatiotemporal Environmental Changes

Understanding and predicting how plant-associated microbes respond to environmental changes is of key importance to understanding future plant performance. Yet, how aboveground and belowground plant-associated microbial communities, which may interactively affect plant performance, simultaneously respond to environmental changes, remains unknown. To fill this gap, we monitored temporal changes of phyllosphere and rhizosphere bacterial communities of three perennial species at 18 sites spanning a 1500 m elevational gradient. We showed distinct temporal trajectories of these community types, likely reflecting their differential responses to abiotic (e.g., air moisture) and biotic (e.g., plant specific leaf area) environmental factors. Further, using a transplantation experiment with the same plant species and their rhizosphere soils, we show that a portion of bacterial taxa from transplanted communities persisted in plants' rhizosphere 2 months after being transplanted to warmer sites. In contrast, phyllosphere communities were primarily harbored by taxa colonizing from local communities. Notably, the relative growth rate of transplanted plants at new versus original sites was positively correlated with the compositional dissimilarity between their phyllosphere and rhizosphere bacterial communities. Together, our results highlight the role of compartment-by-environment interactions in shaping the plant holobiome communities and emphasize the need to understand the impacts of such interactions on plant performance under global change.

Positive effects of species mixing on biodiversity of understory plant communities and soil health in forest plantations

Mixed-species plantations are increasingly recognized for their potential to maintain forest biodiversity and soil health; however, a comprehensive assessment of their global effectiveness is lacking. To fill this knowledge gap, we conducted a meta-analysis of 7,045 paired observations between mixed-species and monoculture plantations, derived from 311 studies across diverse forest ecosystems worldwide. Our results show that mixed-species plantations significantly increased understory plant biomass, cover, and species richness by 32.6%, 55.4%, and 32.2%, respectively, compared to monocultures. Furthermore, the Shannon and Pielou diversity indices increased by 28.2% and 8.6%, respectively, and the Simpson index increased by 9.6%. When understory shrub and herbaceous species were considered separately, species mixing had significantly positive effects on shrub diversity but had no effect on herbaceous diversity. Moreover, mixed-species plantations markedly improved soil physical and chemical properties compared to monocultures. These improvements include increases in soil nutrient content (9.6 to 17.8%) and nutrient availability (14.7 to 33.5%), soil microbial biomass (17.2 to 28.8%), and soil carbon sequestration (7.2 to 19.9%). These enhancements were particularly pronounced in plantations that included legumes. Our findings reveal that the benefits of species mixing are influenced by climatic conditions, geographic location, and stand age, with the most substantial effects observed in temperate regions and mature stands. This study underscores the critical role of mixed-species plantations in promoting sustainable forest management and mitigating the ecological limitations of monocultures.

Seasonal structural stability promoted by forest diversity and composition explains overyielding

The stability of forest productivity is a widely studied phenomenon often associated with tree species diversity. Yet, drivers of stability in forest structure and its consequences for forest productivity remain poorly understood. Using a large (10 ha) young tree diversity experiment, we evaluated how forest structure and multiple dimensions of diversity and composition are related to remotely sensed structural metrics and their stability through the growing season. We then examined whether structural stability (SS) across the growing season (April-October) could explain overyielding (i.e., the net biodiversity effect, NBE) in annual wood productivity. Using Uncrewed Aerial Vehicle-Light Detecting and Ranging (UAV-LiDAR), we surveyed experimental tree communities eight times at regular intervals from before bud break to after leaf senescence to derive metrics associated with canopy height heterogeneity, fractional plant cover, and forest structural complexity (based on fractal geometry). The inverse coefficients of variation for each of these three metrics through the season were used as measures of SS. These metrics were then coupled with annual tree inventories to evaluate their relationships with the NBE. Our findings indicate that wood volume and, to some extent, multiple dimensions of diversity and composition (i.e., taxonomic, phylogenetic, and functional) explain remotely sensed metrics of forest structure and their SS. Increases in wood volume as well as functional and phylogenetic diversity and variability (a measure of diversity independent of species richness) were linked to higher SS of forest complexity and canopy height heterogeneity. We further found that higher SS of forest complexity and fractional plant cover were associated with increased overyielding, which was mostly attributable to the complementarity effect. Structural equation models indicate that the stability of structural complexity explains more variation in NBE among plots than dimensions of diversity or variability, highlighting its value as an informative metric that likely integrates multiple drivers associated with overyielding. This study highlights the potential to integrate remote sensing and ecology to disentangle the role of forest SS in shaping ecological processes.

Nitrogen enrichment and vascular plant richness loss reduce bryophyte richness

Grasslands' high diversity is threatened by land-use changes, such as nitrogen fertilization, leading to productive but low-richness, fast-growing plant communities. Bryophytes are a key component of grassland diversity and react strongly to land use. However, it is unclear whether land-use effects are direct or mediated by changes in vascular plants. Increases in vascular plant cover are likely to decrease bryophyte abundance through light competition. Whether changes in vascular plant composition and richness also play a role remains unclear. We sampled bryophytes in a factorial grassland experiment manipulating nitrogen fertilization, fungicide, species richness, and functional composition of vascular plants crossed with moderate disturbances by weeding. Disturbance increased bryophyte richness and modulated treatment effects. In contrast to previous studies reporting indirect negative fertilization effects via increasing vascular plant productivity and reduced light levels, nitrogen fertilization directly reduced bryophyte cover and species richness, possibly because of toxic effects. Low vascular plant richness and dominance of fast-growing species reduced bryophyte richness. This might be because of decreased structural and resource niche heterogeneity in species-poor communities. Our results highlight novel mechanisms by which land-use intensification can affect bryophytes and suggest that a loss of vascular plant richness might have cascading effects on other taxonomic groups.

Satellite Observations Reveal a Positive Relationship Between Trait-Based Diversity and Drought Response in Temperate Forests

Climate extremes such as droughts are expected to increase in frequency and intensity with global change. Therefore, it is important to map and predict ecosystem responses to such extreme events to maintain ecosystem functions and services. Alongside abiotic factors, biotic factors such as the proportion of needle and broadleaf trees were found to affect forest drought responses, corroborating results from biodiversity-ecosystem functioning (BEF) experiments. Yet it remains unclear to what extent the behavior of non-experimental systems at large scales corresponds to the relationships discovered in BEF experiments. Using remote sensing, the trait-based functional diversity of forest ecosystems can be directly quantified. We investigated the relationship between remotely sensed functional richness and evenness and forest drought responses using data from temperate mixed forests in Switzerland, which experienced an extremely hot and dry summer in 2018. We used Sentinel-2 satellite data to assess aspects of functional diversity and quantified drought response in terms of resistance, recovery, and resilience from 2017 to 2020 in a scalable approach. We then analyzed the BEF relationship between functional diversity measures and drought response for different aggregation levels of richness and evenness of three physiological canopy traits (chlorophyll, carotenoid/chlorophyll ratio, and equivalent water thickness). Forest stands with greater trait richness were more resistant and resilient to the drought event, and the relationship of trait evenness with resistance or resilience was hump-shaped or negative, respectively. These results suggest forest functional diversity can support forests in such drought responses via a mixture of complementarity and dominance effects, the first indicated by positive richness effects and the second by negative evenness effects. Our results link ecosystem functioning and biodiversity at large scales and provide new insights into the BEF relationships in non-experimental forest ecosystems.

Temporal and habitat-specific variations in drivers of aboveground biomass dynamics in a Chinese subtropical forest

Understanding the mechanisms governing biodiversity-biomass relationships across temporal and spatial scales is essential for elucidating how abiotic and biotic factors influence ecosystem function in natural forests. However, the simultaneous contributions of multiple abiotic (e.g., topography) and biotic factors (e.g., structural diversity) to aboveground biomass dynamics (ΔAGB) over time and across habitat types remain inadequately understood. To address this gap, we evaluated changes in aboveground biomass across a decade and various habitats, disentangling the relative influences of topography and multidimensional diversity on ΔAGB through datasets from forest inventories conducted between 2007 and 2017, along with phylogenetic relatedness, functional traits, and environmental variables from a subtropical forest in China. Our findings indicate that aboveground biomass at community level experienced a significant decline followed by an increase over the decade, predominantly driven by changes in the low-valley habitat. In contrast, no statistically significant alterations were detected in the aboveground biomass of mid-hillside and high-ridge habitats. Furthermore, the determinants of ΔAGB exhibited temporal variation. During the 2007-2012 period, ΔAGB was primarily influenced by functional and structural diversity, accounting for 66.11% and 21.35% of relative importance, respectively. In the subsequent 2012-2017 period, phylogenetic and structural diversity emerged as key factors, explaining 48.46% and 36.43% of relative importance, respectively. Additionally, we observed that the drivers and effects impacting ΔAGB exhibited significant variability across different habitat types. In summary, our study underscores the significant spatiotemporal dependence of abiotic and biotic drivers on biomass dynamics within forest ecosystems, thereby enhancing our understanding of the complex biodiversity-ecosystem functioning relationships.

Canopy complexity drives positive effects of tree diversity on productivity in two tree diversity experiments

Forest canopy complexity (i.e., the three-dimensional structure of the canopy) is often associated with increased species diversity as well as high primary productivity across natural forests. However, canopy complexity, tree diversity, and productivity are often confounded in natural forests, and the mechanisms of these relationships remain unclear. Here, we used two large tree diversity experiments in North America to assess three hypotheses: (1) increasing tree diversity leads to increased canopy complexity, (2) canopy complexity is positively related to tree productivity, and (3) the relationship between tree diversity and tree productivity is indirect and driven by the positive effects of canopy complexity. We found that increasing tree diversity from monocultures to mixtures of 12 species increases canopy complexity and productivity by up to 71% and 73%, respectively. Moreover, structural equation modeling indicates that the effects of tree diversity on productivity are indirect and mediated primarily by changes in internal canopy complexity. Ultimately, we suggest that increasing canopy complexity can be a major mechanism by which tree diversity enhances productivity in young forests.

Mycorrhizal dominance influences tree species richness and richness-biomass relationship in China's forests

Mycorrhizal associations drive plant community diversity and ecosystem functions. Arbuscular mycorrhiza (AM) and ectomycorrhiza (EcM) are two widespread mycorrhizal types and are thought to differentially affect plant diversity and productivity by nutrient acquisition and plant-soil feedback. However, it remains unclear how the mixture of two mycorrhizal types influences tree diversity, forest biomass, and their relationship at large spatial scales. Here, we explored these issues using data from 1247 plots (600 m2 for each) across China's natural forests located mostly in temperate and subtropical regions. Both AM-dominated and EcM-dominated forests show relatively lower tree species richness and stand biomass, whereas forests with the mixture of mycorrhizal strategies sustain more tree species and higher biomass. Interestingly, the positive effect of tree diversity on biomass is stronger in forests with low (≤50%) than high AM tree proportion (>50%), reflecting a shift from the complementarity effect to functional redundancy with increasing AM trees. Our findings suggest that mycorrhizal dominance influences tree diversity and richness-biomass relationship in forest ecosystems.

Early overyielding in a mixed deciduous forest is driven by both above- and below-ground species-specific acclimatization

BACKGROUND AND AIMS

Mixed forest plantations are increasingly recognized for their role in mitigating the impacts of climate change and enhancing ecosystem resilience. Yet, there remains a significant gap in understanding the early-stage dynamics of species trait diversity and interspecies interactions, particularly in pure deciduous mixtures. This study aims to explore the timing and mechanisms by which trait diversity of deciduous species and competitive interactions influence yield, carbon allocation and space occupation in mixed forests, both above and below ground.

METHODS

A forest inventory was conducted in planted monocultures, two-species and four-species mixtures of European Acer, Tilia, Carpinus and Quercus, representing a spectrum from acquisitive to conservative tree species. Effects of competition were assessed with linear mixed-effects models at the level of biomass and space acquisition, including leaf, canopy, stem and fine root traits.

KEY RESULTS

Early above-ground growth effects were observed 6 years post-planting, with significant biomass accumulation after 8 years, strongly influenced by species composition. Mixtures, especially with acquisitive species, exhibited above-ground overyielding, 1.5-1.9 times higher than monocultures. Fine roots showed substantial overyielding in high-diversity stands. Biomass allocation was species specific and varied markedly by tree size and the level of diversity and between acquisitive Acer and the more conservative species. No root segregation was found.

CONCLUSIONS

Our findings underscore the crucial role of species trait diversity in enhancing productivity in mixed deciduous forest plantations. Allometric changes highlight the need to differentiate between (active) acclimatizations and (passive) tree size-related changes, but illustrate major consequences of competitive interactions for the functional relationship between leaves, stem and roots. This study points towards the significant contributions of both above- and below-ground components to overall productivity of planted mixed-species forests.

Sparse large trees in secondary and planted forests highlight the need to improve forest conservation and management

Large trees are essential for carbon storage and biodiversity conservation. While an increasing number of studies have focused on large trees in primary forests, little is known about them in secondary and planted forests. We surveyed 86,936 trees in secondary forests and 91,294 trees in planted forests in Zhejiang, China, to investigate the distribution patterns and determinants of large trees in these forests. We found a mean density of large trees (DBH ≥ 30 cm) of 15 ± 13 stems ha-1 in secondary forests and 11 ± 9 stems ha-1 in planted forests. Moreover, the mean density of trees with DBH ≥ 60 cm was 0.36 stems ha-1, indicating that large trees are particularly rare in secondary and planted forests. These large trees were primarily occurred in secondary forests that living in high-elevation area with less human exploitation and colder and wetter climates, and in planted forests with higher species richness and lower tree density. In addition, the density of large trees in these forests significantly increased with tree species richness and decreased with increasing tree density. These results indicate that the sparse large trees were the legacy of historical human activities in the studied area, but currently, the development of large trees is still limited by the improper forest structure characterized by low species diversity and high tree density. To better conserve large trees, there is an urgent need for enhanced conservation policies for secondary forests, such as establishing forest parks for forests with large trees, and implementing near-natural forest management practices for planted forests, which include planting mixed native tree species and maintaining moderate tree density.

Biodiversity and anthropogenic disturbances predominantly drive carbon sequestration rates across temporal scales in temperate forests

Addressing the escalating challenges of climate change necessitates a comprehensive understanding of the factors influencing carbon sequestration rates (CSRs) in forest ecosystems. Although the impact of various biotic factors, environmental, and anthropogenic factors on CSRs over different time scales is well recognized, their precise roles remain poorly defined. This study aims to clarify the mechanistic relationships between CSRs and these factors in large-scale natural temperate forests in northeastern China. We employed linear mixed-effects models and piecewise structural equation models were to analyze data from 310 vegetation plots, assessing the effects of biotic factors (including multidimensional diversity, structural diversity, and community-weighted mean (CWM) trait values) and abiotic factors (climate, topography, and anthropogenic disturbances) across different forest types and successional stages. Our analysis tested a series of hypotheses to identify the principal drivers of forest CSRs. The results indicate that while functional composition and standard environmental factors such as mean annual temperature and slope are significant, their influence is markedly less than that of biodiversity (encompassing multidimensional and structural diversity) and anthropogenic disturbance (as measured by the Human Modification Index). These findings support the dominance of the niche complementarity theory and the moderate disturbance hypothesis, with their importance increasing over time. Furthermore, this study advocates for forest management strategies that are specifically tailored to the unique characteristics of mixed and dense forests at different stages of succession. By elucidating the complex relationships between ecological variables and CSRs, our findings provide critical insights for the development of effective strategies aimed at optimizing forest carbon sequestration. This study underscores the necessity of integrating sustainable forest management with the conservation of ecological biodiversity.

Leaf nutrient traits exhibit greater environmental plasticity compared to resource utilization traits along an elevational gradient

Studying key leaf functional traits is crucial for understanding plant resource utilization strategies and growth. To explore the patterns and driving factors of key leaf functional traits in forests along elevational gradients under global change, we collected survey data from 697 forests across China from 2008 to 2020. This study examined the elevational patterns of Specific Leaf Area (SLA, m²/kg), Leaf Dry Matter Content (LDMC, g/g), Leaf Nitrogen (LN, mg/g), and Leaf Phosphorus (LP, mg/g), and their responses to climate, soil nutrients, and stand factors. The results showed distinct differences in these key leaf traits at different elevational gradients. Generally, as elevation increased, SLA decreased, while LDMC significantly increased (P < 0.001), and LN first increase and then decreased (P < 0.001). The direct influence of elevation on the spatial variation of key leaf traits was greater than its indirect effects (through environmental and stand factors). The elevational patterns of leaf traits related to resource utilization strategies (SLA and LDMC) were mainly influenced by climate (temperature and precipitation) and soil nutrient factors, showing opposite trends in response to environmental changes. The patterns of leaf nutrient traits (LN and LP) along elevational gradients were primarily influenced by climatic factors, with LN exhibiting greater environmental plasticity. Compared to other stand factors, forest age predominantly influenced the spatial variation of key leaf traits, especially SLA. These findings have significant theoretical implications for revealing how plants adapt to global change.

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

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

Mycorrhizal Types Regulate Tree Spatial Associations in Temperate Forests: Ectomycorrhizal Trees Might Favour Species Coexistence

In temperate mixed forests, dominant ectomycorrhizal (EM) tree species usually coexist with diverse arbuscular mycorrhizal (AM) understorey tree species. Here, we investigated the spatial associations between AM and EM trees in two > 20 ha temperate forest mega-plots to better understand the observed 'EM-dominant versus AM-diverse' coexistence. Overall, we found that positive spatial associations (e.g., facilitation) were mostly related to EM trees, while negative spatial associations (e.g., inhibition) were mainly related to AM trees. Because adult EM trees tended to facilitate surrounding AM and EM saplings and other EM adults in these two forests, facilitation hotspots that stabilize AM-EM tree coexistence should be centred around EM tree species rather than around AM tree species. Together, we propose a novel EM-stabilization mechanism, which emphasises how, despite some species-specific variation, EM tree species foster 'EM-dominant versus AM-diverse' coexistence in temperate mixed forests by facilitating other trees.

Increase in forest structural complexity along a precipitation gradient is mediated by partial harvests in temperate Patagonian forests

Increasing forest structural complexity is becoming a common goal in forestry worldwide. However, the lack of empirical quantification clouds its implementation. Here we quantified the long-term effects (> 30 y) of partial harvest on stand structural complexity and net primary productivity using the east-west precipitation gradient (318-2508 mm, mean annual precipitation-MAP) of western Patagonian as a study system. In this gradient, pairs of 1-ha plots on 20 sites (20 plots harvested and 20 plots unharvested) were installed. In each plot terrestrial laser scanning was used to quantify the stand structural complexity index (SSCI), and Sentinel satellite images to obtain the Enhanced Vegetation Index (EVI: proxy of net primary productivity). Generalized linear mixed-effect models were used to relate SSCI to MAP and EVI to SSCI, with harvesting as indicator variable, and site as random variable (two plots nested to same precipitation). Results showed that harvested plots on mesic-to-humid sites (but not on dry sites) had higher SSCI and EVI values compared to unharvested plots, likely due to a greater vertical canopy packing. These results show the influence of precipitation on SSCI, which resulted in a more diversified stand structure and higher EVI. Such insights support site-specific management aimed to increase forest structural complexity.

Mycorrhizal type and tree diversity affect foliar elemental pools and stoichiometry

Species-specific differences in nutrient acquisition strategies allow for complementary use of resources among plants in mixtures, which may be further shaped by mycorrhizal associations. However, empirical evidence of this potential role of mycorrhizae is scarce, particularly for tree communities. We investigated the impact of tree species richness and mycorrhizal types, arbuscular mycorrhizal fungi (AM) and ectomycorrhizal fungi (EM), on above- and belowground carbon (C), nitrogen (N), and phosphorus (P) dynamics. Soil and soil microbial biomass elemental dynamics showed weak responses to tree species richness and none to mycorrhizal type. However, foliar elemental concentrations, stoichiometry, and pools were significantly affected by both treatments. Tree species richness increased foliar C and P pools but not N pools. Additive partitioning analyses showed that net biodiversity effects of foliar C, N, P pools in EM tree communities were driven by selection effects, but in mixtures of both mycorrhizal types by complementarity effects. Furthermore, increased tree species richness reduced soil nitrate availability, over 2 yr. Our results indicate that positive effects of tree diversity on aboveground nutrient storage are mediated by complementary mycorrhizal strategies and highlight the importance of using mixtures composed of tree species with different types of mycorrhizae to achieve more multifunctional afforestation.

Microclimate modulation: An overlooked mechanism influencing the impact of plant diversity on ecosystem functioning

Changes in climate and biodiversity are widely recognized as primary global change drivers of ecosystem structure and functioning, also affecting ecosystem services provided to human populations. Increasing plant diversity not only enhances ecosystem functioning and stability but also mitigates climate change effects and buffers extreme weather conditions, yet the underlying mechanisms remain largely unclear. Recent studies have shown that plant diversity can mitigate climate change (e.g. reduce temperature fluctuations or drought through microclimatic effects) in different compartments of the focal ecosystem, which as such may contribute to the effect of plant diversity on ecosystem properties and functioning. However, these potential plant diversity-induced microclimate effects are not sufficiently understood. Here, we explored the consequences of climate modulation through microclimate modification by plant diversity for ecosystem functioning as a potential mechanism contributing to the widely documented biodiversity-ecosystem functioning (BEF) relationships, using a combination of theoretical and simulation approaches. We focused on a diverse set of response variables at various levels of integration ranging from ecosystem-level carbon exchange to soil enzyme activity, including population dynamics and the activity of specific organisms. Here, we demonstrated that a vegetation layer composed of many plant species has the potential to influence ecosystem functioning and stability through the modification of microclimatic conditions, thus mitigating the negative impacts of climate extremes on ecosystem functioning. Integrating microclimatic processes (e.g. temperature, humidity and light modulation) as a mechanism contributing to the BEF relationships is a promising avenue to improve our understanding of the effects of climate change on ecosystem functioning and to better predict future ecosystem structure, functioning and services. In addition, microclimate management and monitoring should be seen as a potential tool by practitioners to adapt ecosystems to climate change.