Effects of contagious disturbance on forest temporal dynamics

We investigated the possible impacts of two types of stand replacing disturbances on the boreal forest temporal dynamics by using a landscape model: a type I disturbance, characterized solely by the disturbance frequency, and a type II disturbance, characterized by both the disturbance frequency and the presence of contagion. For the type II disturbances, we examine a major factor which influences the probability of contagion for a given spatial cell: the distance from the disturbance center. Two mechanisms of disturbance initiation and two kinds of forest renewal process are examined for their effects on the average biomass of the forest. We find that if the disturbances are randomly initiated, then there are no significant differences between temporal forest dynamics generated by disturbances involving a contagious process (type II) and those that do not (type I). When the disturbance initiation is related to the age of the forest, however, the average biomass is significantly higher with type II disturbances than with type I disturbances. These differences in average biomass arise because older stands are disturbed more frequently in type I disturbances than in type II; hence for a given disturbance frequency, type I disturbances result in younger forests than do type II. Our results therefore suggest that more attention should be paid to spatial dynamics affected by contagious disturbance processes when simulating biomass dynamics of forests, such as the boreal forest, which are strongly influenced by disturbance regimes.