Response of the clonal plant Apium repens (Jacq.) Lag. to extensive grazing

The target of this paper is the correlation of disturbance characteristics of a traditionally used extensive summer pasture in the Alpine foothills (Southern-Bavaria, Germany) and corresponding seasonal population dynamics of the clonal plant Apium repens (FFH Annex II species). The basic hypothesis is that an extensive cattle grazing of its habitats (flood meadows) provides an optimum disturbance regime for long-term persistence of the species. Spatial and temporal scales of the investigation were adapted to the dimensions of the most prominent disturbance impacts (rotation grazing and hoof trampling by cattle) and response rates of Apium which are mainly controlled by stolon propagation. A total of 603 microplots, each measuring 10 cm×10 cm, were arranged within three transects and surveyed for two seasons. Statistical analyses include spatial second order statistics and mixed linear models. Random Apium extinctions during grazing periods and fast re-invasions of gaps during subsequent resting periods result in strong population fluctuations. The probability of re-invasion depends on the distance to next Apium plant which must not exceed much more than 1 dm. The morphological plasticity of stolon length indicates presence of “foraging behaviour” which facilitates the preoccupation of favourable gaps. Disturbances favour the coexistence of A. repens in the pasture due to a combined effect of reduced shoot competition (destruction of tall competitors) and more efficient “foraging” strategies. Attempts to explain Apium coexistence in these flood meadows must take into account both the fine-tuned balance between different disturbance parameters and corresponding functional plants traits.