Perennial grass seed bank in litter patches. Co-occurrence with forb and annual grass seed banks constraints the reestablishment in disturbed Patagonian Monte rangelands

Abstract

We assessed the effect of grazing and plant patchiness on the perennial grass seed bank in litter patches in relation to co-occurring shrub, annual grass, and forb seed banks in disturbed rangelands of the Patagonian Monte. We hypothesized that litter patches in disturbed rangelands have a perennial grass seed bank but co-occurring seed banks of annual lifeforms reduce the opportunities for perennial grass reestablishment at these patches. We evaluated the effect of microsite (bare soil vs. vegetated) and grazing (grazed vs, non-grazed) on the (1) size (seeds m⁻²) of the four seed banks, (2) frequency of microsites with the respective seeds, and (3) frequency of co-occurrence of the perennial grass with shrub, annual grass and forb seed banks in litter patches at 5 dates (2009–2011). We found the smallest perennial grass and annual grass seed banks in litter patches under grazing and in bare soil microsites. The shrub and forb seed banks in litter patches were smaller at vegetated than at bare soil microsites under grazing and the reverse occurred in non-grazed conditions. The sizes of all seed banks were positively related to the frequency of litter patches with the respective seeds. In these litter patches, the size of perennial grass and shrub seed banks did not vary with grazing, and/or microsite while the size of annual grass and forb seed banks decreased with grazing and increased in vegetated microsites. The perennial grass seed bank in litter patches co-occurred mostly with forb and annual grass seed banks (several times larger). This is probably a constraint for the reestablishment of perennial grasses. However, although scarce, litter patches with exclusive perennial grass seed bank or co-occurring perennial grass and shrub seed banks could be an opportunity for perennial grass reestablishment.