Effect of grazing and drought on seed bank in semi-arid patchy rangelands of northern Patagonia, Argentina

Abstract

Premise of research. The soil seed bank (SSB) plays a critical role in vegetation regeneration after stress and/or disturbance events. Here, we analyzed how drought and grazing influence the SSB of shrub patches and interpatches in a rangeland from northern Patagonia. Methodology. Soil samples were collected from shrub patches and interpatches on heavily grazed, mod- erately grazed, and ungrazed sites at the end of a multiyear drought and 2 yr after the end of the drought. Sampling was done immediately before seed dispersion of the dominant species in order to study the persistent seed bank. Seeds were separated from the mineral soil by elutriation and grouped into four life-forms: annual grasses, perennial grasses, forbs, and shrubs. Pivotal results. In all sampling conditions, the SSB was markedly dominated by forbs. Total seed density and seed density of every life-form were higher in shrub patches than in interpatches. Seed density of grasses and shrubs was not influenced by drought and grazing in both shrub patches and interpatches. In contrast, seed density of forbs was negatively influenced by drought in interpatches and positively influenced by grazing in shrub patches. Conclusions. The results suggest that in patchy rangelands of northern Patagonia the SSB of shrub patches is less severely depleted than that of interpatches during multiyear droughts, particularly for forbs under high grazing intensity. However, the growth of the SSB after drought is a slow process in both shrub patches and interpatches. Our results highlight the importance of grazing pressure alleviation during drought conditions to safeguard the SSB, which is the only source of seedling recruitment for most species in northern Patagonia rangelands.