What happens below the canopy? Direct and indirect influences of the dominant species on forest vertical layers

Temperate forests are one of the most important ecosystems in the world, and thus disentangling the factors that drive diversity within these ecosystems is of major concern. However, due to the complex interactions among forests layers, topography and soil factors, discovering the drivers of diversity is often complicated. In this study, we tested three a priori hypotheses about the effect of the dominant competitor (Pinus koraiensis) on the different forest layers in a 25 ha full mapped plot of temperate forest in the Changbai Mountain of northeastern China. Structural equation modelling (SEM) was used to study the direct and indirect interactions between four vertical forest layers (dominant competitor, canopy composition, sub-canopy diversity and shrub diversity), topographic factors, edaphic factors to discover sub-canopy and shrub diversity drivers. Our results suggest that the dominant competitor (Pinus koraiensis) is a key factor explaining canopy variation, and sub-canopy diversity and shrub diversity, and that this competitor can act directly (through shading) and indirectly (through the modification of the soil). Topographic heterogeneity also had significant effects on the soil variation and the diversity of the sub-canopy and shrub layers. Finally our results indicate that the influence of canopy composition on the diversity of the rest of forest layers is indirect and positive, suggesting that the dominant competitor is the main factor limiting diversity. In conclusion, we have found strong evidence that the dominant species of the canopy can influence, both directly and indirectly, the diversity of the different vertical forest layers. Patterns of diversity in forests are driven by a multiplicity of factors that are inherently related.