Mechanisms regulating understory plant diversity in three forest types: a simulation study

Understory plant communities play a crucial role in forest ecosystems, significantly contributing to nutrient cycling, providing habitats, and supporting forest restoration. While light penetration and soil quality are known drivers of these communities, the mechanistic role of the canopy structure remains understudied. This paper examines how different canopy characteristics, specifically canopy percentage and the canopy effect intensity, affect plant diversity in the understory layer through a lattice simulation model. Based on the lattice Lotka-Volterra competition model with microhabitat locality (Tubay and Yoshimura 2018), we simulated understory population dynamics across three forest types: evergreen, deciduous, and mixed. Our results reveal that the canopy structure acts as a critical biological filter for the understory plant species, identifying a tipping point in evergreen forests where species diversity collapses once canopy cover exceeds 80%. In contrast, we show that seasonal leaf shedding in deciduous forests acts as a temporal buffer, preventing competitive exclusion and maintaining high diversity even under high canopy cover and high canopy effect. These findings demonstrate that temporal variations in canopy cover are as vital as spatial heterogeneity in maintaining forest biodiversity. By providing novel simulation-based evidence for these mechanisms, this study offers a mechanistic framework that can help inform forest management and conservation strategies, particularly regarding the potential impact of canopy density thresholds on understory biodiversity.