FOREST DYNAMICS MODELLING UNDER NATURAL FIRE CYCLES: A TOOL TO DEFINE NATURAL MOSAIC DIVERSITY FOR FOREST MANAGEMENT SYLVIE GAUTHIER Canadian Forest Service, Petawawa National Forestry Institute, PO. Box 2000, Chalk River, Ontario, KOJ 1JO, Canada and ALAIN LEDUC and YVES BERGERON Groupe de Recherche en Ecologie Forestidre ( GREF), Universit# du Qudbec ,~ Montreal, C.P. 8888, Succ. Centre-~lle, Montreal, Qudbec, H3C 3P8, Canada Abstract. In natural boreal forests, disturbances such as fire and variation in surficial deposits create a mosaic of forest stands with different species composition and age. At the landscape level, this variety of stands can be considered as the natural mosaic diversity. In this paper, we describe a model that can be used to estimate the natural diversity level of landscapes. We sampled 624 stands for tree species composition and surficial deposits in eight stand-age classes corresponding to eight fire episodes in the region of Lake Duparquet, Abitibi, Qu6bec at the southern fringe of the Boreal Forest. For six surficial deposit types, stand composition data were used to define equations for vegetation changes with time for a chronosequence of 230 years for four forest types. Using Van Wagner's (1978) model of age class distribution of stands, the proportion of each forest type for several lengths of fire cycle were defined. Finally, for real landscapes (ecological districts) of the ecological region of the "Basses-Terres d'Amos", the proportion of forest types were weighted by the proportion of each surficial deposit type using ecological map information. Examples of the possible uses of the model for management purposes, such as biodiversity conservation and comparisons of different landscapes in terms of diversity and sensitivity to fire regime changes, are discussed. 1. Introduction A current priority in forest management is the development of practices aimed at forest ecosystem sustainability. Factors other than timber production, such as recreation, wildlife, and biodiversity conservation must also be considered in the context of sustainable forestry. The last factor has gained much prominence in the last few years. Biodiversity can be measured at different levels: genetic, species, ecosystem, and landscape (Canadian Forest Service, 1993). However, a species- by-species approach for measuring biodiversity is difficult because: 1) there are numerous species of which many are unknown, 2) the task would be extremely time consuming, 3) the financial resources are not available, and 4) it will exhaust the general public's patience (Franklin, 1993). Hence, a large scale approach "is the only way to conserve the overwhelming mass of existing biodiversity" (Franklin, 1993; see also Noss, 1983, 1987; Mladenoff et al., 1993; Attiwill, 1994a). A land- scape approach can help define a framework to design and manage landscapes for Environmental Monitoring and Assessment 39: 417-434, 1996. (~) 1996 Kluwer Academic Publishers. Printed in the Netherlands.