Scale‐dependent diversity–biomass relationships can be driven by tree mycorrhizal association and soil fertility

Abstract

Diversity–biomass relationships (DBRs) often vary with spatial scale in terrestrial ecosystems, but the mechanisms driving these scale‐dependent patterns remain unclear, especially for highly heterogeneous forest ecosystems. This study explores how mutualistic associations between trees and different mycorrhizal fungi, i.e., arbuscular mycorrhizal (AM) vs. ectomycorrhizal (EM) association, modulate scale‐dependent DBRs. We hypothesized that in soil‐heterogeneous forests with a mixture of AM and EM tree species, (i) AM and EM tree species would respond in contrasting ways (i.e., positively vs. negatively, respectively) to increasing soil fertility, (ii) AM tree dominance would contribute to higher tree diversity and EM tree dominance to greater standing biomass, and that as a result (iii) mycorrhizal associations would exert an overall negative effect on DBRs across spatial scales. To empirically test these hypotheses, we collected detailed tree distribution and soil information (e.g., nitrogen, phosphorus, organic matter, pH) from seven temperate and subtropical AM–EM mixed forest megaplots (16–50 ha). Using a spatial codispersion null model and structural equation modeling, we identified the relationships among AM or EM tree dominance, soil fertility, tree species diversity, and biomass and, thus, DBRs across 0.01‐ to 1‐ha scales. We found the first evidence overall supporting the three aforementioned hypotheses in these AM–EM mixed forests: (i) In most forests, with increasing soil fertility, tree communities changed from EM‐dominated to AM‐dominated; (ii) increasing AM tree dominance had an overall positive effect on tree diversity and a negative effect on biomass, even after controlling for soil fertility and number of trees. Together, (iii) the changes in mycorrhizal dominance along soil fertility gradients weakened the positive DBR observed at 0.01‐ to 0.04‐ha scales in nearly all forests and drove negative DBRs at 0.25‐ to 1‐ha scales in four out of seven forests. Hence, this study highlights a soil‐related mycorrhizal dominance mechanism that could partly explain why, in many natural forests, biodiversity–ecosystem functioning (BEF) relationships shift from positive to negative with increasing spatial scale.

Long‐term stability of sapling dynamics is regulated by soil phosphorus availability in subtropical forest

“…In this study, we have further found that the majority of tree species were more abundant in soil with higher levels of inorganic P, and quadrats with higher organic P sustained many fewer sapling species (Figure S16). Hence, the distinct effects of inorganic and organic P on community stability could be explained by the soil-related mycorrhizal dominance mechanism (Mao et al, 2023) stability (Figures 4b and 6). We found that average population stability had a significantly positive relationship with species diversity, which is opposite to the theoretical prediction by Lehman and Tilman (2000).…”

“…Meanwhile, many previous studies have reported that AM tree dominance would contribute to higher tree diversity, and ECMdominated plant communities tend to be less diverse than AMdominated communities across local and geographic scales (Gerz et al, 2016;Mao et al, 2023). Four mechanisms, including access to organic nutrients, accumulation of organic material and allelopathic compounds, and positive plant soil feedback, act synergistically in ECM-dominated plant communities to maintain community monodominance over multiple generations (Johnson et al, 2023;Tedersoo et al, 2020).…”

“…Most trees in subtropical forests associate with mycorrhizal fungi, and these plant‐fungal symbioses play critical roles in vegetation dynamics and nutrient cycling, leading to plot level differences in tree dominance associating with differences in soil inorganic and organic P, that is, plots with higher inorganic P are dominated by arbuscular mycorrhizal (AM) tree species, and plots with more organic P are dominated by ectomycorrhizal (ECM) host trees (Johnson et al., 2023; Phillips et al., 2013; Rosling et al., 2015). Meanwhile, many previous studies have reported that AM tree dominance would contribute to higher tree diversity, and ECM‐dominated plant communities tend to be less diverse than AM‐dominated communities across local and geographic scales (Gerz et al., 2016; Mao et al., 2023). Four mechanisms, including access to organic nutrients, accumulation of organic material and allelopathic compounds, and positive plant soil feedback, act synergistically in ECM‐dominated plant communities to maintain community monodominance over multiple generations (Johnson et al., 2023; Tedersoo et al., 2020).…”

Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types

“…While EM tree species generally benefit from greater nitrogen mobilization from organic matter and enhanced organic and inorganic resource uptake (32), trees associated with AM fungi mainly benefit from greater uptake of less mobile nutrients such as phosphorus (26)(27)(28)(29). Mycorrhizal associations with AM or EM fungi are known to influence tree productivity differently (33)(34)(35). For instance, Deng et al (36) found that differences in nutrient acquisition strategies affect the direction of BPRs, with positive effects of tree species richness on AM tree productivity but negative effects for EM trees.…”

Tree diversity increases productivity through enhancing structural complexity across mycorrhizal types

“…While EM tree species generally benefit from greater nitrogen mobilisation from organic matter and enhanced organic and inorganic resource uptake (32), trees associated with AM fungi mainly benefit from a greater uptake of less mobile nutrients such as phosphorus (26,29). Mycorrhizal associations with AM or EM fungi are known to influence tree productivity differently (33)(34)(35). For instance, Deng et al (2023) found that differences in nutrient acquisition strategies affect the direction of BPRs, with positive effects of tree species richness on AM tree productivity, but negative effects for EM trees (36).…”