Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types

Abstract

Tree species diversity and mycorrhizal associations play a central role for forest productivity, but factors driving positive biodiversity-productivity relationships remain poorly understood. In a biodiversity experiment manipulating tree diversity and mycorrhizal associations, we examined the roles of above- and belowground processes in modulating wood productivity in young temperate tree communities and potential underlying mechanisms. We found that tree species richness, but not mycorrhizal associations, increased forest productivity by enhancing aboveground structural complexity within communities. Structurally complex communities were almost twice as productive as structurally simple stands, particularly when light interception was high. We further demonstrate that overyielding was largely explained by positive net biodiversity effects on structural complexity with functional variation in shade tolerance and taxonomic diversity being key drivers of structural complexity in mixtures. Consideration of stand structural complexity appears to be a crucial element in predicting carbon sequestration in the early successional stages of mixed-species forests.

Tree diversity and mycorrhizal type co‐determine multitrophic ecosystem functions

“…where f is a cylindrical form factor of 0.5 (an average value for young broadleaved trees) to account for the deviation of the tree volume from the theoretical volume of a cylinder (Fichtner et al, 2018;Pretzsch, 2009;Ray, Delory, et al, 2023).…”

“…Furthermore, we quantified above-ground vertical structural complexity using the effective number of layers (ENL) determined by terrestrial laser scanning in the core area of each plot in September 2021 (Ray, Delory, et al, 2023;. ENL provides a holistic and reproducible description of the three-dimensional (3D) vertical stand structure, independent of human estimation (possible observer bias) and classic dendrometric parameters, for example, basal area, density, and height (Ehbrecht et al, 2016;Knuff et al, 2020).…”

“…In contrast, in AM plots, we found ENL only positively correlated with the mean of total stand volume but not with its standard deviation (Table S12); so, the larger numbers more likely indicate dense and thick-layered canopies (Figures S12.1 and S13A). The denser canopies in polycultures of AM-species in particular (Ray, Delory, et al, 2023) decreased the predation rates on P. avium and S. aucuparia (Figure S11; Nell & Mooney, 2019;Muiruri et al, 2016;Yang et al, 2018), likely resulting in the significantly negative selection effects (Figure S6D). The opposing responses of AM-species and EcMspecies to ENL may explain (1) that the significant correlations did not ture to better understand the drivers of multitrophic communities (Schuldt et al, 2019) and ecosystem functions (Aikens et al, 2013;Muiruri et al, 2016;Williams et al, 2017).…”

“…For example, EcM-species supposedly favour a more conservative nutrient-use strategy than AM-species (Averill et al, 2019) and tend to produce less biomass during the sapling stage (Dietrich et al, 2023). This likely allows for height variation, complementing each other through the partitioning of canopy space (Ray, Delory, et al, 2023;Williams et al, 2017).…”

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

“…Plant diversity was found to foster canopy thickness and complexity (Peng et al, 2017;Perles-Garcia et al, 2021;Williams et al, 2017), potentially also enhancing the buffering capacity of vegetation cover on microclimatic processes in the understorey. Results from experiments manipulating plant diversity in the field (BEF experiments) indicate that microclimatic conditions are even more stable with increasing plant species richness (Huang et al, 2023;Schnabel et al, 2023;Zhang, 2022) and that microclimate interacts with stand complexity in promoting forest productivity (Ray et al, 2023). Similar effects by more diverse plant communities on microclimatic conditions via thicker canopies were also observed in grasslands (Huang et al, 2023), yet empirical evidence in grassland ecosystems remains limited, particularly F I G U R E 2 Climate dependencies of various processes across different levels of organization, here using temperature dependency as an example: (1) Net ecosystem CO 2 exchange (NEE, Niu et al, 2012), (2) soil respiration (Jones et al, 2006), (3) tree seedlings survival (Dingman et al, 2013), (4) insect activity (Colinet et al, 2015) and (5) Rubisco activity (Galmés et al, 2019).…”