Tree uprooting and timber harvesting cause soil disturbances in forests. Plants colonise disturbed patches in two ways: generative (with germination of seeds) and vegetative (with ramets). In this study, the relative importance of the two means of colonisation was assessed and the influences of seed availability and site conditions on colonisation were explored.
In a mixed mountain forest in southern Germany, topsoil was disturbed experimentally and colonisation with vascular plants was observed afterwards for 15 months. The compositions of the soil seed bank and the seed rain were investigated. Experiments were carried out in 12 forest stands with different canopy covers and forest management histories. In each stand between 10 and 18 soil disturbance plots were established.
Species richness on experimental plots was significantly higher 1 year after soil disturbance than before the disturbance. The most numerous species emerging after soil disturbance was Rubus idaeus, followed by species of the genus Carex. R. idaeus colonised disturbed plots both generatively and vegetatively, but the vegetative colonisation of this species was important only on clear-cuts with high light intensities.
The decisive factor for generative colonisation (considering all species) was the number of germinable seeds of R. idaeus in the soil seed bank, whereas the seed rain was unimportant. The factor with the highest explanatory power for vegetative colonisation was the cover of the adjacent vegetation.
The results indicate that soil disturbances in forests increase germination rates, which are responsible for small-scale increases in species richness. R. idaeus on clear-cuts has high cover values because (1) soil disturbances by vehicle traffic and timber hauling during the harvest enhance its germination and (2) the high light intensities on clear-cuts favour the spread of this species vegetatively. Since R. idaeus is a potential competitor for tree seedlings, these findings are important for planning forest management.