Efectos del pastoreo sobre el banco de semillas germinable y la vegetación establecida en pastizales de montaña del centro de Argentina

Abstract

We studied the effect of grazing on the standing vegetation and the germinable soil seed bank in natural mountain grasslands at Pampa de Achala, Córdoba Province, Argentine. We analyzed the germinable seed bank at two depths (0-5 and 5-10 cm) in five replicates from sites grazed at a moderate to intense grazing intensity, and in five replicates from sites excluded from herbivores during the last 10 years. We recorded species frequency on the standing vegetation and species abundance in the germinable seed bank. Grazing did not produce significant changes in the number of species of the standing vegetation, but produced an increase in species diversity due to a decrease in the dominance of perennial grasses such as Deyeuxia hieronymi and Festuca tucumanica. We registered only three alien species in the grasslands under study. Grazing did not produce significant changes in the seed bank richness and diversity. Average seed bank density significantly did not change after grazing, however, the density of the annual grass Muhlenbergia peruviana increased, while that of the tussock grass Deyeuxia hieronymi decreased in grazed plots. Seeds present in litter at the excluded sites comprised 20 % of the total number of species and 43 % of the total seed density. Ordination analysis, taking into account floristic composition of standing vegetation and the germinable seed bank, showed higher segregation between grazed-ungrazed samples than samples from vegetation and from the seed bank. At both grazed and protected sites, there were dominance of species with transient seed banks, followed by species with short and long persistent seed banks, respectively. Grazing, or its absence, did not modify plant species abundance of most species at the germinable seed bank, moreover, the two species that changed their seed density showed two opposite plant strategies, as observed previously: one dominates primary productivity in absence of grazing, the another could play an important role in gap restoration at a micro patch scale in heavy grazed sites.