Tree mycorrhizal strategies regulate diversity–productivity relationships across forest strata along soil fertility

The symbiotic relationships between trees and different mycorrhizal fungi affect the biodiversity-ecosystem functioning relationships within forest communities, particularly across different strata from the understorey to the overstorey. However, current research on this relationship has not yet reached a definitive conclusion. In this study, we used regression analyses and structural equation modelling based on a 9-ha mixed broadleaved-Korean pine ( Pinus koraiensis ) forest plot to explore the effects of different types of mycorrhizal fungi (arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi) on the relationship between tree species diversity and productivity. The study found that the dominance and productivity of AM species increased in the understorey with increasing soil nutrients, whereas the productivity of EM species declined despite increasing diversity. In the overstorey, the productivity of AM species continued to increase with increasing soil nutrients, while that of EM species decreased due to increased competition. As for diversity-productivity relationships (DPRs), in the understorey, both AM and EM tree species demonstrated a positive DPR, suggesting the presence of the niche complementarity effect. In the overstorey, AM species continue to exhibit positive DPR due to their competitive advantage in nutrient-rich environments. In contrast, EM species exhibit negative DPR due to increased competition or less efficient resource use. Additionally, slope influenced forest productivity indirectly by altering the accumulation and distribution of soil nutrients, thereby affecting species distribution and growth conditions. This indirect effect highlights the potential negative impact of slope on soil nutrients within forest ecosystems, as well as its influence on the relationships between biodiversity and ecosystem functions. The study reveals how AM and EM trees influence forest productivity through distinct adaptability and competitive strategies across different forest strata, with a particular emphasis on the strata-dependent effects of mycorrhizal association on DPRs. This finding offers a new perspective on how mycorrhizal types modulate the complex relationships between biodiversity and ecosystem functions across various strata in temperate forests. • Tree mycorrhizal associations drive biodiversity-productivity relationships. • Arbuscular mycorrhizal (AM) trees adapt well to high fertility soils. • Ectomycorrhizal (EM) trees excel in low fertility conditions. • Species richness boosts understory productivity. • Overstory AM tree productivity rises with species richness; EM tree declines.