COMMUNITY STABILITY - A 60-YEAR RECORD OF TRENDS AND OUTBREAKS IN THE OCCURRENCE OF SPECIES IN THE PARK GRASS EXPERIMENT

1 A 60-year time-series of species recorded in visual surveys of the plant communities of the Park Grass Experiment is analysed to detect changes through time in the frequency of species on seven plots with acidified soil and on 35 non-acidified plots. 2 Of 14 species recor;ded on the acidified plots, eight decreased with time, one (Agrostis capillaris) increased, four showed no trend and one (Chamerion angustifolium) showed an outbreak with a peak in 1946. Of the 43 species recorded on the nonacidified plots, six increased, five decreased, 10 showed outbreaks and 22 showed no trend. 3 We used discriminant analysis to try to identify combinations of seven life-history and two habitat variables that would correctly classify species according to how their frequency changed on the non-acidified plots. Habitat variables (mean pH and mean hay yield of plots) were poor discriminators, but some life history variables (notably ruderalness, mating system and flowering time) were more successful. 4 Species which increased were more outcrossing whereas outbreak species were more selfing than the average for all species, both were more ruderal than average. We speculate that the ruderal species were all well equipped to spread across plots, but that only outcrossing species possessed sufficient genetic variation to be able to sustain a broad distribution in the heterogeneous environment represented by the PGE. 5 In view of the otherwise stable nature of the Park Grass communities we suggest that the existence of outbreaks in a significant number of species calls for a reevaluation of the concept of the stable plant community.