TIME AND SPACE IN THE COMMUNITY STRUCTURE OF A SPECIES-RICH LIMESTONEGRASSLAND

The community structure of a species-rich grassland was investigated a t a small spatial scale (0.001 m(2)), to determine whether evidence su ggesting assembly rules could be found in temporal or spatial variatio n in either species richness or guild proportions. The community was a lvar limestone grassland on the island of Oland, Sweden. Three sites w ere sampled: two lightly grazed, the other recently ungrazed. Plots wi th and without fertilizer were compared. Evidence was sought for restr iction on the ability of species to co-occur within a limited spatial area. Restriction due to a limited number of niches available, 'niche limitation', could be manifest as lower variance in quadrat richness t han expected under a null model (i.e. RV(r), the ratio of observed: ex pected variance in richness, would be < 1.0). In several cases, RV(r) values were significantly < 1.0, even using a patch model to allow for possible spatial variation in the environment. Low RV(r) values were found only at the smallest square quadrat size, 10 cm(2). On Fertilize d plots in the years immediately after application of fertilizer, low RV(r) could not be demonstrated. Explanations of low RV(r) other than niche limitation are considered, such as environmental heterogeneity ( present and/or historical) and of individual plant modules.Assembly ru les based on guild membership were sought by looking at the variance a cross quadrats in the proportions of species from morphological guilds . An assembly rule would be seen as relatively constant proportion, es timated via RV(gp), the ratio of observed:expected Variances in guild proportions. Significant guild proportionality was found in some cases . There was no evidence of guild proportionality in the years after th e application of fertilizer. The significant effects in RV(r) were mor e numerous than expected on a random basis, though not observed in eve ry site in every year. Similar trends were seen in RV(gp). At the spac e/time scales examined, the species in a plant community may be constr ained by assembly rules only intermittently, e.g. when resources are m ore limiting (Wiens 1977). Under this concept, when competition is rel axed, such as following fertilizer application, there is a temporary m icrohabitat 'waterhole' in which more species can coexist, and the ass embly rules break down, at least temporarily whilst the species compos ition adjusts. There was some indication of a return to more determini stic community structure four years after fertilization commenced. Var iants of van der Maarel's Carousel model were tested. A Niche-limited Carousel Model (i.e. a model in which there is some limitation in the number of species that can occupy a microsite) would imply restricted variation in richness through time for a single quadrat (temporal RV(r )). Overall differences between years in species richness were demonst rated, and their effect removed; after this adjustment there was suppo rt for the Niche-limited Carousel Model. The extent of this limitation varied between sites. There were also consistent differences between quadrats in species richness. There was little evidence for constancy of guild proportions through time. The site that showed the strongest community structure in time and space, least year-to-year variation in mean species richness, and least response to fertilizer perturbation, is that on the shallowest soil. Possibly the thin soil results in gre ater resource limitation, supporting suggestions that assembly rules a re stronger when resources are more limited.