Microbial drivers of woody plant encroachment in fire-maintained grasslands and savannas

Woody plant encroachment into grasslands and savannas is a global phenomenon with wide-ranging consequences for people and nature, but we lack a comprehensive understanding of its drivers. Various factors can contribute to woody encroachment across ecosystems, but a notable commonality is that transitions from herbaceous to woody vegetation are spatially and temporally abrupt, suggesting that positive feedbacks play a role. Positive feedbacks are well-studied in the context of vegetation--fire dynamics, but growing evidence points to the potential importance of microbially mediated plant--soil feedbacks in woody plant encroachment. For example, ectomycorrhizal association, often associated with positive feedback, is especially common among woody plants known to encroach grass-dominated systems, while herbaceous plants tend to accumulate self-limiting microbial communities. To fill this gap, we developed a novel patch occupancy modeling framework for predicting microbial impacts on vegetation dynamics in fire-maintained grasslands, and parameterized this model with empirically derived estimates of plant--microbe interactions from global meta-analyses. We find that empirically measured microbial feedbacks can sharply constrain the fire regimes under which herbaceous communities are maintained. We also show that woody-favoring soil communities increase the duration of fire-based management necessary to recover grassland after encroachment and narrows the conditions under which such recovery is possible. In all, our model points to the overlooked yet consequential role of plant--soil feedbacks in driving changing vegetation patterns in fire-prone grass-dominated systems and the urgent need for empirical data testing this process.