The distribution of plant functional types on gradients of disturbance intensity and resource supply in an agricultural landscape
Abstract. In this study, plant functional types are understood as groups of plants with similar biological traits that display significant optima or maxima on a gradient plane of resource supply and disturbance intensity. The biological traits refer to expansion, vegetative regeneration, generative reproduction, dispersal and seed bank longevity.
129 relevés were made in an agricultural landscape in Southwest-Germany, covering a wide range of vegetation types on terrestrial soils (except forests and wetlands). For each site, soil data were also recorded. A simple model computed the mean daily soil moisture. The soil moisture, the balanced nitrogen supply and the available phosphorus were combined to the factor "resource supply". Also, the disturbance intensity was determined for each site. The intensity was aggregated from the frequency of disturbance, the disturbance depth below or above soil surface and the proportion of an area that is affected by a discrete disturbance event. Thirty plant groups of similar biology resulted from a cluster analysis, based on a compilation of 19 biological traits for a regional species pool. Logistic regression on a gradient plane of disturbance intensity and resource supply yielded response curves for 28 groups. The dependent variable was defined as the probability to find all members of a group in a relevé. Seventeen groups display a significant response curve on the gradient plane of disturbance intensity and resource supply. Plants with a potential for long-range dispersal are concentrated on sites with low or high disturbance intensities (e.g. fallow land; e.g. fields, lawns). On sites with medium disturbance intensity (e.g. meadows) and low to medium resource supply, small-range dispersal predominates. There are no distinct trends concerning seed bank longevity. The potential for vertical and lateral expansion increases with decreasing disturbance intensity. Only at medium disturbance intensities does vertical expansion correlate positively with resource supply. Rapid detachment of daughter individuals occurs more often on productive sites than on less productive sites. Diversity of groups with similar biological traits is highest on sites with medium disturbance intensities.