Overstory structure and topographic gradients determining diversity and abundance of understory shrub species in temperate forests in central Pyrenees (NE Spain)

Understanding the factors that determine the distributions of understory plants is important because they represent the largest component of temperate forest diversity. In the present study, we investigate the influence of topographic gradients and overstory structure on composition and abundance of understory shrub species in Massís de l’Orri (central Pyrenees, Lleida, Spain). We established 329 plots in two opposing hills in the massif at elevations ranging from 1500 to 2200 m, where we measured understory composition, overstory structure and physiographic features. Overall richness and diversity did not vary with aspect and showed only a small decrease with elevation. However, understory composition showed drastic changes related to elevation and aspect. Most species had differences between slopes, some of them showing higher cover in south-facing plots and the others in north-facing plots. Elevation also influenced percent cover of many understory shrub species, which showed either a decrease (Arctostaphylos uva-ursi, Rosa sp.), a maximum at intermediate values (Cytisus purgans) or an increase (Rhododendron ferrugineum) with elevation. The effect of the two variables was not independent, and percent cover of the different species depended on the overall effect of aspect and elevation. The species with highest occurrence in plots, Vaccinium myrtillus, increased with elevation in south-facing slopes but showed similar values in north-facing slopes. Compared to topographic variables, the effect of overstory variables on understory was considerably smaller. Understory species richness and diversity were inversely but weakly correlated with overstory cover but not with basal area. These overstory variables only affected negatively the presence of C. purgans and Juniperus communis, two shade-intolerant species that decrease their abundance when light decreases. Overall, the results obtained suggest that topographic variables have a stronger influence on understory composition than overstory structure in the study area, which is located in the southern distribution limit of temperate forests in Europe.