Is understory plant species diversity driven by resource quantity or resource heterogeneity?

What maintains plant species diversity has been the subject of much debate with no general consensus. In forest ecosystems in which understory plants account for the majority of floristic diversity, a crucial question is whether understory plant diversity is driven by resource quantity or resource heterogeneity. This study sought to reconcile the two hypotheses in relation to their effects on understory plant diversity in forest ecosystems. A database of studies that investigated the effects of resources on understory plant diversity was compiled and analyzed using log-linear models. Whether resource quantity or resource heterogeneity is the determinant of understory plant diversity in individual studies was dependent on stand successional stage(s), presence or absence of intermediate disturbance, and forest biome within which the studies were conducted. Resource quantity was found to govern species diversity in both young and mature stands, whereas resource heterogeneity dominated in old-growth stands. Resource quantity remained the important driver in both disturbed and undisturbed forests, but resource heterogeneity played an important role in disturbed forests. We argue that neither resource quantity nor heterogeneity alone structures species diversity in forest ecosystems, but rather their influences on understory plant diversity vary with stand development and disturbances in forest ecosystems.