Introduction

This book is motivated by studies of global climate change and its impact on the environment, a topic which is increasingly being discussed among scientists as well as by the general public. One major point of concern is the possible shift of vegetation zones to higher altitudes and greater latitudes. In particular, attention is being given to northern and mountain timberlines, which are readily visible and for which past records are available. Undoubtedly, the upper timberline is the most conspicuous vegetation limit in high-mountain areas of all continents, with the sole exception for the Antarctic. Actually, impressive maps and graphs projecting the future positions of vegetation zones and altitudinal belts have already been published.

These predictions, however, are based on very simple assumptions that cannot encompass the ecological complexity and great heterogeneity of such boundaries. The expected shift of timberline is estimated by extrapolating the rough coincidence of the existing timberlines and present thermal conditions, usually described in terms of temperature sum, number of growingdegree days, or mean temperature of the warmest month (Section 4.3.1). However, timberline is not an organism that will individually respond to any change of the environment, nor is it a line that will advance or retreat parallel to an altitudinal shift of any isotherm considered to be essential to tree growth. Timberline is a biological boundary, a more or less wide ecotone that has to be understood as a space- and time-related phenomenon that will not respond linearly to changing temperatures or other environmental factors (Chapter 5).