Grass‐to‐grass protection from grazing in a semi‐arid steppe. Facilitation, competition, and mass effect

Abstract

Plants of low palatability often serve as biotic refuges from grazing to palatable plants. Evidence for this facilitation comes from cases where the interacting species have different life form, which may minimize competition. Protected plants act as remnant seed sources that may maintain the palatable populations locally viable through mass effects. Here, we assess (1) the spatial association between a highly palatable Patagonian grass (Bromus pictus) and less preferred tussock grasses, (2) the role played by seed sources in maintaining the population in the face of heavy grazing by sheep, and (3) the facilitative and competitive components of the interaction. We quantified B. pictus density and its distance to nearest tussocks inside and outside a grazing exclosure. We also considered different distances from the exclosure, both leeward and windward, because strong westerly winds may be critical for dispersal. Additionally, we quantified several attributes of protected and unprotected B. pictus plants with and without grazing. Density of B. pictus was about 20 times greater inside the exclosure than outside. However, this difference was less pronounced in the leeward vicinity of the exclosure than in the windward one, which suggests a mass effect. B. pictus was significantly associated to less palatable tussocks, and the association became stronger under grazing and as distance from the exclosure edge increased. Protection under grazing was associated with a significant increase in plant biomass, height, tiller number, and panicle number, whereas protection in the absence of grazing, which could evidence competition, resulted in reductions of tiller number and panicle number, and an increase of height. These results suggest that in areas under grazing pressure on palatable grasses, other less palatable grasses may provide a protection from grazing that outweighs competitive effects. Such protection may generate small‐scale mass effects that maintain the population at relatively high density.