Understory floristic heterogeneity within a Mediterranean oak forest: how much is explained by edaphic, spatial and overstory components?

Abstract

The understory floristic heterogeneity within a well-preserved Mediterranean forest was studied, in the Natural Park of Sudoeste Alentejano e Costa Vicentina (SW Portugal), to investigate if floristic trends were related to the soil chemical parameters, to the spatial component and/or to the overstory sinusia. Three bottom-top hillside and parallel to each other transects were traced, for sequential sampling of floristic, vegetational, edaphic and spatial variables. Multivariate analyses of woody species composition (by DCA), of soil variables (by PCA), and of the edaphic-community relationships (by CCA and pCCA) were performed. Several and particular steps of analysis were performed, for partition of total variation of the species data in: (a) non-spatially structured variation; (b) spatially structured edaphic variation; (c) spatial variation not shared by the soil variables; and (d) unexplained variation. A strong bottom-top gradient of edaphic and understory floristic variation was identified. All the spatial components were important to predict the understory floristic variation and, when those were excluded, soil NH4+ and NO3- were the best predictors of that variation. We concluded that both the edaphic and spatial parameters, together with the overstory sinusia structure and composition, played a significant role in explaining the understory variation.