Spatial Patterning in Young Stands of Pinus sylvestris in Relation to Mortality Caused by the Snow Blight Pathogen Phacidium infestans

Abstract

In two young (5-10 year old) populations of Pinus sylvestris, severe infection by the pathogen Phacidium infestans led to significant mortality. Previous studies of this interaction have shown some evidence of density-dependence. Analyses of the spatial distribution of surviving individuals was performed using two contrasting methods based on: (1) a dispersion index; and (2) on nearest neighbour distances. In all situations changes in the degree of aggregation were more obvious in analyses based on the frequency of individuals in small areas (dispersion index) than in those based on the distance between nearest neighbours. These methods detected trends both away from aggregated towards more random patterns of distribution (site 1: two plots) and towards greater aggregation (site 2: one plot). In one of the former areas, the distribution of surviving P. sylvestris individuals was still significantly aggregated after four years, while in the other, the pattern approached spatial randomness. Increasing aggregation on the second site probably has resulted from a complicating biotic interaction between the pathogen and other plant species.