How Much Physiology Is Needed in Forest Gap Models for Simulating Long-Term Vegetation Response to Global Change? Introduction

A variety of types of models currently are being used to evaluate potential effects of elevated CO2 and climate change on terrestrial ecosystems and the feedbacks to the climate system (see reviews by Agren et al., 1991; Reynolds et al., 1996; Goudriaan et al., 1999). Such models are employed by scientists as research tools to synthesize scientific understanding, identify important uncertainties, and guide future experiments or field work. In addition, models of ecosystem responses to climate change increasingly will be utilized in comprehensive assessments of climate change designed to help decision-makers in making difficult policy decisions. A widely used class of ecological models is known as gap models (descendants of JABOWA, e.g., Botkin et al., 1972; Shugart, 1984; Bugmann et al., 1996). These models were originally conceived to study how vegetation structure and composition change over time under current climatic conditions, based on a consideration of plant population dynamics, i.e., establishment, growth and mortality of individual plants. These mathematical models assume horizontally homogeneous environmental and biotic conditions within the basic spatial unit, i.e., the patch; the death of a large individual creates a gap, thus the name ‘gap’ models. Typically, gap models are used to simulate vegetation dynamics in a relatively small area (i.e., 1–10 ha). Because they follow the fate of individual plants and species, they are not generally used to simulate vegetation dynamics for large regions, let alone continents or the entire globe. A review of gap models is presented in the accompanying paper by Bugmann (2001). With the emergence of global climate change as a major environmental issue, it was logical to utilize gap models to attempt to understand how climate change might affect species composition and vegetation structure in forests (Bugmann et al., 1996). However, the original gap models were based on very simple representa-

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