Changes in forest structure and in the relative importance of climatic stress as a result of suppression of avalanche disturbances

Natural disturbances have been among the most important driving factors in many ecosystems. Anthropogenic suppression of various disturbances has led to documented changes in ecosystem structure and function. Avalanche disturbances are one of the most important processes in many subalpine ecosystems world-wide and avalanche tracks provide unique habitat for various animal and plant species. Over the past decades the natural avalanche regime in the Alps has been disrupted by snow-supporting structures and other measures intended to prevent the occurrence of avalanches. We hypothesized that suppression of avalanche disturbances changes stand structure and composition and increases the relative importance of climatic stress (the degree to which climatic conditions limit the growth of vegetation). We analyzed stand structure and tree growth at high and low elevations in pairs of active avalanche tracks and tracks from which avalanches have been excluded in the Swiss Alps. Data on density and size of all tree species were collected in the field to analyze stand structure and increment core samples were collected to analyze tree growth. In tracks from which avalanches have been excluded, dbh (diameter at breast height), tree height, annual tree-ring widths, correlation of ring widths between trees, and correlation of ring-width indices with growing-season temperature were all greater than in active tracks, indicating an acceleration of ecosystem development and an increase in the relative importance of climatic stress. Within tracks from which avalanches have been excluded, increases in tree size and ring width were more pronounced at lower elevations while correlation of ring-width indices with growing-season temperature was more pronounced at higher elevations. The anthropogenic alteration of the natural avalanche regime is beginning to cause changes in the structure and function of some subalpine forests. The continued suppression of avalanches, while valuable for social reasons, is likely to eventually lead to the decline of certain ecological communities and may alter subsequent forest dynamics. In addition to changing forest structure, the suppression of avalanche disturbances is also changing the relative importance of the factors that are controlling ecosystem development by increasing the role of climatic stress in ecosystem development. We suggest that, in general, the suppression of disturbances predictably changes the relative importance of stress and competition along existing environmental gradients. Such changes represent a major alteration of ecosystem structure and function. # 2005 Elsevier B.V. All rights reserved.