Introduction

Models for simulating different aspects of vegetation dynamics have become increasingly popular during recent decades. Initially, mathematical modeling was only accessible to well-trained biomathematicians, but with the increasing availability of small and faster computers and with the development of modern software, it has been applied by more traditionally trained ecologists and foresters. Recently, many papers that present different models and applications within ecology have been published (e.g. Emanuel et al. 1984; van Tongeren & Prentice 1986; Running & Coughlan 1988; Tilman 1988; Costanza, Sklar & White 1990; Keane, Arno & Brown 1990).

Simulation models can help in the understanding and management of ecosystems. Such models are usually the only tool available for translating a collection of hypotheses for ecological processes into a testable representation of how the whole ecosystem functions. Simulation models can be used not only to evaluate hypotheses generated by field studies and ecological experiments, but also for situations where the more traditional ecological approach is less applicable, for example for studies that span several research generations, such as the study of processes involved in forest succession and gap-phase replacement of individual trees within a stand (Watt 1947). Ecological hypothesistesting by experiment and field studies for such long-term and largescale processes is almost inevitably incomplete and must be supplemented by simulation experiments.

Simulation models consist of a collection of hypotheses, most often in equation form. These hypotheses define how the major parts of the model change over time (Swartzman & Kaluzny 1987).