Sheep trampling on surface-lying seeds improves seedling recruitment in open sand ecosystems

Low-intensity grazing with domestic ungulates is an important tool in restoration to enhance habitat openness and foster light-demanding plant species. Ungulates affect plant regeneration in different ways; one important factor is trampling. We hypothesized that in dry open sand ecosystems sheep trampling is beneficial for seedling recruitment. Therefore, we set up a standardized laboratory experiment, where we simulated trampling on surface-lying seeds and measured the number of seedlings, mean germination time, synchrony of germination, and aboveground phytomass of seedlings. We studied seeds of 16 herbaceous species occurring in European open inland sand ecosystems, which depend on continuous disturbance dynamics. The species set comprised four plant families (Asteraceae, Caryophyllaceae, Fabaceae, and Poaceae) and two species groups (target and ruderal nontarget species). Across species, trampling had profound effects on seedling emergence, that is it induced a 3.5-fold increase in the number of emerging seedlings. In addition, germination synchrony and aboveground phytomass increased after trampling, whereas mean germination time decreased. Interestingly, ruderal species seemed to be more susceptible toward trampling than target species. We conclude that sheep trampling strongly promotes seedling emergence in grazed open sand ecosystems by pressing seeds into the uppermost soil layer and improving microclimatic conditions.