SPATIAL PATTERNING IN YOUNG STANDS OF PINUS-SYLVESTRIS IN RELATION TOMORTALITY CAUSED BY THE SNOW BLIGHT PATHOGEN PHACIDIUM INFESTANS

In two young (5-10 year old) populations of Pinus sylvestris, severe i nfection by the pathogen Phacidium infestans led to significant mortal ity. 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 sit uations changes in the degree of aggregation were more obvious in anal yses 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 greate r aggregation (site 2: one plot). In one of the former areas, the dist ribution of surviving P. sylvestris individuals was still significantl y aggregated after four years, while in the other, the pattern approac hed spatial randomness. Increasing aggregation on the second site prob ably has resulted from a complicating biotic interaction between the p athogen and other plant species.