Interacting effects of physical environment and anthropogenic disturbances on the structure of European larch (Larix decidua Mill.) forests

Most forested landscapes have been strongly influenced by humans, and hence prediction of response to future perturbations or climate change requires understanding of the interaction among human influences and the abiotic environment. Environmental and anthropogenic influences on forest structure in Larix decidua Mill. stands were investigated in two watersheds of the central Italian Alps (Valtellina, SO). We related three data sets (forest structure, anthropogenic influences, and topography) using ordination methods. Path models of correlative and causal relationships between these sets of variables were developed and used to differentiate the two watersheds with respect to levels of historical human influence. The two study areas (Musella and Ventina) were characterized by strong climatic and topographic gradients as well as a long history of human settlement, although historical intensity of agricultural activities was much greater for Musella. We hypothesized a weaker influence of abiotic variables on forest structure where the intensity of the primary human disturbance factors, forest thinning and cattle grazing, had been strongest. Stand structure types varied from sparse, larch-dominated forests at high elevations, to denser stands at lower elevations dominated by spruce. Correlations of environmental variables with dominant trends in stand structure were low with the exception of elevation. Anthropogenic variables were unimportant at Ventina, whereas the interactive effects of both anthropogenic and abiotic variables were needed to explain stand structure in Musella. The best-fit model indicated a negative effect of elevation and anthropogenic variables on overall tree size. Stands with greater tree density and canopy height layer diversity were located further from roads. Both watersheds were characterized by a strong dominance of larch stands, but their structure and spatial pattern differed greatly. Sparse larch forests were exclusively associated with moraines and upper elevations at Ventina, but were also common near low-elevation farms at Musella. Historical human influences were difficult to measure and may play a greater role in determining forest structure than was suggested. Our study emphasizes the importance of landscape context for interpreting the relative strengths of anthropogenic and abiotic influences on stand development pathways.