Controls of forage selective defoliation by sheep in arid rangelands

Abstract

Selective grazing by domestic livestock is a major control of plant community structure anddynamics in drylands. However, grazing impact predictions supporting management decisions are fre-quently based on average biomass consumption, neglecting selectivity. We evaluated the relative impor-tance of grazing pressure, total and each species density, and plant dead biomass proportion as drivers ofselective defoliation by sheep in three dominant native grass species in Patagonian steppes. Species werePoa ligularis,Festuca pallescens,andPappostipa speciosa, whicha prioripresent different preference degree bysheep. The relevance of these drivers for differently preferred species has not been simultaneously studied.We recorded the defoliation frequency and degree of the three species (dependent variables) throughout112field surveys. Besides, we recorded grazing management and vegetation structure descriptors (inde-pendent variables).Poa ligulariswas highly defoliated (90% of plants), and grazing pressure was the lead-ing driver (asymptotic exponential relationship). ForF. pallescens, almost 70% of plants were defoliated,and defoliation non-linearly increased as grazing pressure rose and linearly decreased as both its dead bio-mass proportion andP. ligularisdensity increased. ForP. speciosa, defoliation was low (20% of plants) andlinearly decreased as both its dead biomass proportion and the density of the other two species increased.Grazing pressure played a negligible role in this species. These patterns confirmed thatP. ligularis,F. palles-cens,andP. speciosa,respectively, present high, intermediate, and low preference degree by sheep. In con-clusion, ourfindings suggest that (1) selective defoliation can be satisfactorily predicted as function ofgrazing pressure, species densities, and plant dead biomass proportion; (2) grazing pressure becomes amore relevant driver as species preference rises and its effect on defoliation is markedly non-linear; (3) thedead biomass proportion and the abundance of highly preferred species are the leading factors determin-ing less preferred species defoliation; and (4) grazing pressure management by itself is not enough toreduce the high defoliation of preferred species and increase the defoliation of non-preferred species. Thisknowledge is critical for developing effective management practices to control forage species defoliation inrangelands worldwide where species with different preference by herbivores coexist.