Mycorrhizal feedbacks influence global forest structure and diversity

Abstract

One mechanism proposed to explain high species diversity in tropical systems is strong negative conspecific density dependence (CDD), which reduces recruitment of juveniles in proximity to conspecific adult plants. Although evidence shows that plant-specific soil pathogens can drive negative CDD, trees also form key mutualisms with mycorrhizal fungi, which may counteract these effects. Across 43 large-scale forest plots worldwide, we tested whether ectomycorrhizal tree species exhibit weaker negative CDD than arbuscular mycorrhizal tree species. We further tested for conmycorrhizal density dependence (CMDD) to test for benefit from shared mutualists. We found that the strength of CDD varies systematically with mycorrhizal type, with ectomycorrhizal tree species exhibiting higher sapling densities with increasing adult densities than arbuscular mycorrhizal tree species. Moreover, we found evidence of positive CMDD for tree species of both mycorrhizal types. Collectively, these findings indicate that mycorrhizal interactions likely play a foundational role in global forest diversity patterns and structure.

Fig. 1. Geographical extent of dataset captures known biogeographical patterns.

A map of the 43 ForestGEO network sites included in the study (A), collectively comprising over 3 million stems. Major expected relationships including the latitudinal gradient in tree species diversity (scaled to the maximum species richness across sites; gray, p < 0.001) and the gradient from AM dominated to EM-dominated sites (proportion of EM tree species in a site; green, p = 0.02) with increasing latitude (B) as well as the mycorrhizal bimodality of forests (C; proportion of AM per basal area of all tree species) are captured by our data. Conceptual figures depicting negative conspecific density dependence (CDD) in recruitment due to hypothesized higher densities of species-specific enemies around a conspecific adult tree (D) and positive conmycorrhizal density dependence (CMDD) in recruitment due to hypothesized shared mutualists with an adult tree of a conmycorrhizal heterospecific species (E).

Fig. 2. Mycorrhizal type mediates strength of conspecific density dependence.

Negative conspecific density dependence (CDD), estimated as the degree to which sapling densities decrease with increasing conspecific adult tree density (per-capita sapling density), is consistently stronger for arbuscular mycorrhizal (AM) compared to ectomycorrhizal (EM) tree species. The three panels show results from the integrated global model that incorporates random slopes and intercepts for each tree species-by-site, using raw sapling densities (A), sapling densities scaled to the maximum sapling density of each mycorrhizal type (B), and species-by-site estimates of change in per-capita sapling density with a standard increase in conspecific adult density (1 conspecific adult) extracted from this model (C, n = 2469, p < 0.001). Gray lines in (A) and (B) represent species-by-site curves; AM and EM tree species are shown in purple and green, respectively. Solid circles and horizontal lines in (C) represent means estimated from the model and standard errors, respectively.

Fig. 3. AM tree species benefit from shared mycorrhizal fungi.

AM tree species experience positive conmycorrhizal density dependence (CMDD), measured as conmycorrhizal per-capita sapling density, while EM plant species show weaker evidence for positive CMDD. Positive CMDD can be seen for both mycorrhizal types in the global model (A, B) and for AM tree species in the species-by-site estimates of change in per-capita sapling density with a standard increase in conmycorrhizal adult tree density (1 conmycorrhizal adult) extracted from this model (C, n = 2428). Gray lines in (A) and (B) represent species-by-site curves; AM and EM tree species are shown in purple and green, respectively. Solid circles and horizontal lines in (C) represent means estimated from the model and standard errors, respectively.