GRAZING EFFECTS ON PATCHY DRYLAND VEGETATION IN NORTHERN PATAGONIA

In this study the spatial patterns and dynamics of vegetation patches along a grazing gradient in the steppe of Larrea divaricata and Stipa spp. in NE Patagonia (Argentina) are described. A general effect of gr azing is the reduction of total plant cover resulting from the decreas e in cover of perennial grasses (Stipa speciosa, Poa ligularis, Stipa tenuis) and some tall shrubs (Chuquiraga hystrix, Bougainvillea spinos a, Lycium chilense). Dwarf shrubs (Nassauvia fueguiana and Junellia se riphioides) increase their cover under medium and/or high grazing pres sures. Plant species are spatially grouped into patches which alternat e with areas of bare soil. Eleven types of vegetation patch differing in the dominant plant functional type or species, floristic richness a nd size were identified with different relative frequency along the gr azing gradient. Based on these results, it is postulated that grazing forces the replacement of large patches dominated by tall shrubs with high species richness, by Larrea divaricata patches or small dwarf shr ub patches with low species richness and the extinction of grass patch es. This results from: (1) disruption of local balances of species del etions and additions; (2) fragmentation of large patches; and (3) form ation of new vegetation patches. These changes lead to differing plant spatial organization and heterogeneity along the grazing gradient whi ch may be described by characteristic arrays of vegetation patches. (C ) 1997 Academic Press Limited.