Drier conditions diminish plant coexistence via soil microbes in a tropical humid forest

Global environmental change can alter ecological mechanisms that maintain biodiversity. Interactions between plants and soil microbes mediate plant species coexistence, which can vary with abiotic factors such as light and soil moisture, and such context dependence is only beginning to be explored.

We assessed how variation in light and soil moisture alters soil microbial effects on the predicted coexistence of two common tree species (Litsea floribunda and Symplocos racemosa) native to the Western Ghats, India. We conducted a shade-house reciprocal transplant experiment with different soil origins in factorial combinations of high/low water and light, and predicted coexistence outcomes using metrics that decompose microbial effects into stabilization and fitness differences. For high-water, low-light conditions, we also evaluated whether soil microbes alter plant-plant interactions, using a structural framework that quantifies the feasibility domain of coexistence from species interaction coefficients.

Soil microbes were predicted to promote plant coexistence in high-water, low-light conditions, where stabilization exceeded the fitness differences due to microbes. Under low-water, high-light conditions, larger fitness differences predicted exclusion of S. racemosa. Microbes had weak effects on plant-plant interactions, but predicted coexistence improved slightly in soils with background microbes not shaped by either species, due to weaker estimated fitness differences in those soils.

Drier and brighter conditions may weaken the potential for microbially mediated plant coexistence in tropical forests. These findings hint at shifts in microbially mediated plant community dynamics in response to global change factors such as forest fragmentation and drought.