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

1 A 60-year time-series of species recorded in visual surveys of the p lant communities of the Park Grass Experiment is analysed to detect ch anges through time in the frequency of species on seven plots with aci dified soil and on 35 non-acidified plots. 2 Of 14 species recor;ded o n the acidified plots, eight decreased with time, one (Agrostis capill aris) increased, four showed no trend and one (Chamerion angustifolium ) showed an outbreak with a peak in 1946. Of the 43 species recorded o n the nonacidified plots, six increased, five decreased, 10 showed out breaks and 22 showed no trend. 3 We used discriminant analysis to try to identify combinations of seven life-history and two habitat variabl es that would correctly classify species according to how their freque ncy changed on the non-acidified plots. Habitat variables (mean pH and mean hay yield of plots) were poor discriminators, but some life hist ory 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 sp ecies, both were more ruderal than average. We speculate that the rude ral species were all well equipped to spread across plots, but that on ly outcrossing species possessed sufficient genetic variation to be ab le to sustain a broad distribution in the heterogeneous environment re presented 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.