A comparative analysis of forest dynamics in the Swiss Alps and the Colorado Front Range

Abstract

Mountain forests in the European Alps and in the Rocky Mountains share many similarities but they are subject to strongly different climates and disturbance regimes. In this study, the factors that bring about the similar appearance of these systems are investigated based on a quantitative, dynamic model of the structure and composition of mountain forests with a specific focus on patterns of tree recruitment and growth. In a first step, the forest model F ORC LIM which had been used previously in different mountain ranges of the temperate zone but not in the Rocky Mountains, was evaluated and adapted with respect to its capability of portraying forest dynamics in the Rocky Mountains. This yielded several insights that point at necessary model improvements as well as the need for an increased understanding of the ecophysiological mechanisms controlling the biogeography of western tree species and better data on ecotypes of these species. A model version that is based on the tree species of the central Rocky Mountains proved capable of portraying the key features of Front Range forests along a climate gradient that extended from the lower to the upper treeline near Boulder, Colorado. In the second step, the model was used to quantitatively evaluate the limiting factors for tree establishment and tree growth at pairs of sites in the Swiss Alps and the Front Range. The results suggest that: (1) drought is important for shaping forest structure and composition at all elevations in the Front Range whereas it is generally of subordinate importance at high elevations in the European Alps; (2) large-scale disturbances such as fire are essential for predicting forest features in the Front Range but they are less important in the European Alps and (3) competition for light is lower in the Front Range than in the Alps. Intervals between successful establishment events in near-natural mountain forests of the two areas were found to be as large as 50–100 years. This very slow and highly stochastic nature of forest dynamics should be kept in mind when drawing conclusions on the future state and protective function of mountain forests.