Are semiarid shrubs resilient to drought and grazing? Differences and similarities among species and habitats in a long-term study

Abstract

We assessed long-term effects of grazing cessation and drought on the shrub community of a semiarid ecosystem with a long history of grazing, located in the Mediterranean-to-desert transitional zone in Israel. Effects of grazing and drought on the cover of dominant (Sarcopoterium spinosum) and subdominant (Thymelaea hirsuta, Noaea mucronata and Coridothymus capitatus) shrubs were monitored during 12 years in four topographic habitats. With the exception of the toxic shrub T. hirsuta, shrub cover increased by a few (at most five) percentage points soon after the establishment of fenced plots to prevent grazing, but the difference in cover between protected and grazed plots did not increase subsequently. Response of the woody vegetation cover to the drought pulse was more complex because it was affected by both species and habitat; it showed patterns of steady decrease, transient decrease, and transient increase. Recovery after the drought pulse was relatively slow, and total shrub cover did not return to its predrought level within 7 years. Varied responses to drought and grazing preclude consideration of shrubs as a single response group. The findings heighten concern for the stability of the ecosystem in light of the increasing frequency of dry seasons predicted by climate-change models.