High species diversity reduces fire susceptibility: Evidence from subtropical forests

Climate and human activities are widely recognized as major drivers of forest-fire susceptibility, but the role of plant species diversity in regulating it has received little attention. Here we address two questions: (1) are forests with higher tree species diversity less susceptible to forest-fire events? and (2) if so, what ecological mechanisms link species diversity to forest-fire susceptibility? To answer these questions, we conducted a field-based study in subtropical forests of Macheng, central China, integrating data on species diversity and fuel flammability with historical fire records over 2011–2021. We quantified species diversity using species richness, Shannon and Simpson indices, and fuel flammability using fuel load, equilibrium moisture content (EMC), and ignition points. Data were analyzed using generalized linear models (GLM), structural equation modeling (SEM), and XGBoost machine-learning methods. Our results show that high species diversity reduces fire susceptibility, leading to lower fire frequencies. SEM revealed that species diversity did not directly affect fire frequency but exerted an indirect effect through fuel flammability. Specifically, high diversity reduced fuel flammability ( β = −0.734, P < 0.001), which in turn increased fire frequency when fuel flammability was high ( β = 0.587, P < 0.001), yielding a significant negative indirect effect ( R 2 = 0.697; β = −0.431, P < 0.001) . XGBoost further identified fuel EMC, fuel load, and ignition points as the most influential predictors of fire frequency. Together, these results demonstrate that species-diverse forests lower forest-fire susceptibility primarily through changing fuel properties at the landscape scale and broader geographic replication is required to test generalisability at regional scale. Thus, promoting mixed-species forests represents a promising nature-based landscape-scale strategy for enhancing forest-fire resilience in subtropical systems.