Spatial patterns of the germinable soil seed bank of coexisting perennial-grass species in grazed shrublands of the Patagonian Monte

Abstract

We compared the size and spatial pattern of the germinable soil seed bank (GSB) of the three dominant perennial-grass species in the arid Patagonian Monte of Argentina. These species differ in plant functional traits, seed morphology, and in their preference by grazers. During three consecutive years, we sampled the soil seed bank at 1 m intervals and the intercepted diameter of perennial-grass species and shrub patches along 50-m linear transects at three sites with different grazing pressure. Soil samples were incubated at field capacity during 12 weeks and emerged seedlings counted. The largest-seeded and most xerophytic Stipa speciosa did not form a GSB. The other two grass species with more mesophytic traits and smaller seeds than S. speciosa formed different GSB. Poa ligularis formed a smaller and more heterogeneous GSB than Stipa tenuis. Grazing had not effect on GSB size of P. ligularis but increased the proportion of seeds under dense shrub canopies. In contrast, grazing reduced the size and also increased the proportion of the GSB of S. tenuis under shrub canopies. We conclude that the size and the spatial pattern of the GSB of the dominant perennial-grass species in the arid Patagonian Monte were related not only to seed and plant traits but also to the spatial distribution of grass plants and their exposure to grazers.