Regeneration of willows and herbaceous vegetation on river banks after grazing by small ruminants

Willow bushes (Salix) in a biosphere reserve cause build up of water when the river is in flood. In order to avoid flood disaster as well as for nature conservation reasons we studied whether small ruminants might suppress the willows without causing long-lasting damage both to the reeds and the vegetation of the banks. Comparing the effects of single (one time) and repeated grazing in a two-year trial, 109 sheep and 10 goats grazed up to 13 days on 8000 m2 plots with direct access to the river. A single grazing resulted in extensive reduction of the herbaceous vegetation, but its complete regeneration within three months. This is due to a variety of strategies for survival in a dynamic bank habitat in which grazing imitates natural disturbance by changing water levels; 77.3% of the 128 species persisted under a single grazing, 10.2% were new, 12.5% were not found afterwards. Heavily reducing the biomass stimulated less competitive herbaceous species. Death of willow stems occurred only where the bark had been completely removed. The proportion that died rose from 21.7% after first grazing to 36.3% after first penning and up to 44.1% after second penning. Early grazing in the season is much more efficient (58.6%) than late (23.1%) in controlling willow growth. Insufficient damage to the bark of old multi-branched willows also reduced the proportion that died. Sustainable bark reduction of willow growth depends on damaging them severely by single grazing and then penning once early in the year, and to avoid their full regeneration by repeating this treatment every second year. Over the long-term a cyclic grazing treatment will weaken the willows and enable the herbaceous life forms to take and over reinvigorate sufficiently, suggesting it as an option for flood protection in the biosphere reserve.