Season- and herbivore-dependent competition and facilitation in a semi-arid rangeland

Empirical and theoretical evidence suggests that facilitation between plants, when it occurs, is more likely during periods of stress, while competition predominates under more moderate conditions.  Therefore, one might expect the relative importance of competition vs. facilitation to vary seasonally in ecosystems characterized by pronounced dry (stressful) and wet (benign) seasons. Herbivory also varies seasonally and can affect the net outcome of plant-plant interactions, but the interactive effects of seasonality and herbivory on the competition-facilitation balance are not known.  I experimentally manipulated neighboring plants and herbivory during wet and dry periods for two species of grass: Cynodon plectostachyus and Pennisetum stramineum, in the semi-arid Laikipia District of Kenya.   The experimental results indicate that Pennisetum was competitively dominant and consistently responded negatively to drought, grazing, and interspecific neighbor (competitor) presence. Cynodon showed more complex season-dependent responses: net facilitation by Pennisetum during the dry season, but net competition during the wet season.  Furthermore, Cynodon was facilitated only in the presence of herbivores.  These results illustrate how herbivory and seasonality can interact in complex ways to shift the competition-facilitation balance between species.  Additionally, because Cynodon and Pennisetum are key players in a local successional process, these results indicate that herbivory can affect the direction and pace of succession.