Drivers of herb-layer species diversity in two unmanaged temperate forests in northern Spain

The identification of the drivers of diversity in understorey plant assemblages is a major challenge in forest ecology. However, it is not yet fully understood whether the same factors consistently affect different facets of species organization, such as species richness, composition and turnover. Here, we compare the influence of fine-scale environmental variables and spatial organization on the herb-layer flora of two unmanaged and ecologically different forest stands in the Muniellos Biosphere Reserve, Northern Spain. The aims of our study are to identify the most important factors influencing different facets of herb-layer plant organization, and to test whether the effect of such drivers is consistent across facets and forest types. We used Generalized Linear Models, Redundancy Analysis and Multiple Regression on Distance Matrices to model, respectively, the response of species richness, species composition and species turnover to spatial distances as well as to several environmental factors, including forest structure, light conditions, soil and topographical features. We observed a substantial consistency among variables affecting the different facets of ground-layer species organization within stands, with a subset of topographical variables with a transversal effect across facets. Although potential solar radiation was the main variable influencing species richness in the two stands, the factors shaping species composition and turnover varied across forest types: in the beech stand, slope and canopy openness were the main determinants of herb-layer species diversity and turnover; in the oak stand, the main drivers of species composition were related to topography and spatial structure, while spatial distance was the main driver of species turnover. Our study shows that the ecological processes driving fine-scale variation of ground-layer plant richness and composition are similar to those driving species turnover. Although the ecological factors shaping different facets may be the same, we highlight that, at least in temperate forests, these factors are system-specific and vary according to forest types.