Abandonment in grazing systems:: Consequences for vegetation and soil

Many dehesa zones in marginal areas are suffering from the abandonment of traditional farming practices. Herbivore grazing affects grassland dynamics, changing the species colonisation and extinction relationships via consumption, mechanical disturbance, seed dispersal and altered soil fertility due to dung-borne nutrient input. This paper analyses changes to floristic composition, species richness and heterogeneity as well as soil and light resources in relation to grazing abandonment in two characteristic topography-related types of dehesa habitat (upper slope and lower slope grasslands). Vegetation surveys were undertaken using five replicates in each habitat type in grazed and abandoned grasslands in the Guadarrama range in Central Spain with distric cambisol soils. Additional measurements of light and soil parameters were also undertaken. The results show an effect of grazing on resource availability. In general there was a smaller proportion of fine matter (clay), organic matter, total nitrogen, assimilable potassium and readily available water in abandoned as opposed to grazed zones. We did not find any effect of grazing on phosphorous availability. Abandonment reduces light availability at ground level in lower slope grasslands (wet meadows) but does not produce a significant reduction in upper slope (driest) grasslands, although there is an increased spatial heterogeneity in the availability of light associated with the presence of scattered shrubs. While the total number of species in abandoned zones did not significantly differ from grazed zones, floristic composition changed dramatically with abandonment in both habitat types, with less than 50% of common species. Also, floristic heterogeneity at the small scale increased with abandonment. Designers of agri-environmental policies applied to Mediterranean grasslands should be aware of the risk of widespread grazing abandonment and promote the diversification of grazing regimes across the landscape to maximize species diversity and take advantage of the benefits of low-density grazing in other functions of the ecosystem such as their productivity and stability. (c) 2005 Elsevier B.V. All rights reserved.