Modelling the effect of spatial scale and correlated fire disturbances on forest age distribution

Abstract

With the exponential model, Van Wagner (1978) gave us valuable insight in understanding stand age and forest age distribution in fire-disturbed landscapes. He showed that, under certain conditions, the probability distribution of the age of a stand subject to periodic renewal by fire is exponential. The extension of this model to the landscape-level results, also under certain conditions, in an exponential shape for the forest age distribution. Empirical studies have supported this hypothesis in some landscapes and not in others. The results are believed to depend on the size of the landscape in question, the patterns of fire disturbance, and changes in the disturbance regime over time and space. In this paper, we present additional insight into some of the fundamental factors that determine the forest age distribution. We analyzed some alternative spatial models of fire disturbance, and used a spatial simulation model (FLAP-X) to explore whether the forest age distribution has an exponential shape, and whether it would be stable or variable over time under different conditions. We use different spatial and temporal disturbance patterns, some of which represent correlation due to fire growth and episodes of high fire disturbance. We describe FLAP-X and give the results of computational tests based on hypothetical data. We found that, under characteristic boreal fire disturbance regimes, we should not expect to find forest age distribution stability even at very large spatial scales due to the spatial and temporal correlation of disturbances.