Data and code: Scale-dependent diversity-biomass relationships can be driven by tree mycorrhizal association and soil fertility

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 respond in contrasting ways (i.e., positively vs. negatively respectively) to increasing soil fertility, (ii) AM tree dominance contributes to higher tree diversity and EM tree dominance contributes to greater standing biomass and that as a result, (iii) mycorrhizal associations exert an overall negative effect on DBRs across spatial scales. To empirically test these hypotheses, we collected detailed tree distribution and soil information (nitrogen, phosphorus, organic matter, pH, etc.) from seven temperate and subtropical AM-EM mixed forest mega-plots (16–50 ha). Using 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 0.01–1 ha scales. We found first evidence overall supporting the above three 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 0.01–0.04 ha scales in nearly all forests and drove negative DBRs at 0.25–1 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.Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: Grant 31961133027Funding provided by: Key Research Program of Frontier Science, Chinese Academy of SciencesCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100018527Award Number: Grant ZDBS-LY-DQC019Funding provided by: K. C. Wong Education FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100012692Award Number: Funding provided by: China Postdoctoral Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100002858Award Number: 2021M703397Funding provided by: Special Research Assistant Project of Chinese Academy of Sciences*Crossref Funder Registry ID: Award Number: 2022000056Funding provided by: Institute of Applied Ecology, Chinese Academy of SciencesCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100015922Award Number: IAEMP202201Funding provided by: National Natural Science Foundation of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100001809Award Number: 31925027Funding provided by: Smithsonian InstitutionCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000014Award Number: Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: Funding provided by: National Zoological ParkCrossref Funder Registry ID: http://dx.doi.org/10.13039/100009196Award Number: Funding provided by: HSBC Climate Partnership*Crossref Funder Registry ID: Award Number: Funding provided by: International Center for Advanced Renewable Energy and Sustainability (I-CARES) at Washington University in St. Louis*Crossref Funder Registry ID: Award Number: Funding provided by: Tyson Research Center*Crossref Funder Registry ID: Award Number: Funding provided by: National Key Research and Development Program of ChinaCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100012166Award Number: 2022YFF1300501See the "Materials and Methods" section in the manuscript for details