Changes in vegetation and in the organic fraction soil related with grazing and fire histories in the Patagonian Monte, Argentina

Abstract

In terrestrial ecosystems the vegetation and soil co-evoluted in response to some disturbance. In arid and semi-arid lands around the world vegetation is distributed in patches within a bare-soil matrix. Such vegetation and soil are more vulnerable to disturbance effects and highly susceptible to desertification, vegetation attributes and organic fractions of the soil are the parameters widely used for evaluating changes in ecosystems. In the North-east Patagonian Monte in Argentina, the vegetation is a shrubland steppe with patches dominated by shrubs, with grasses, forbs and cryptogams under their canopy. It was hypothesized that grazing increases patchiness across its associated trampling and selective foraging; and fires and posterior wind erosion homogenize the distribution of soil particles and vegetation with consequence on dynamic of soil organic carbon. It was predicted that there would be: 1) greater cover, specific frequency and richness of shrubs in grazed sites; 2) greater cover, specific frequency and richness of herbs and preferred grasses in ungrazed sites; 3) a random pattern of vegetation distribution in burnt areas; and 4) greater carbon soil stock in ungrazed sites and lower in burnt ones. Aerial cover of all perennial species, distribution of soil particles sizes, and stocked organic carbon (SOC) of soil were measured at six sites with different land-use histories: heavily grazed, medium grazed, ungrazed, long exclosure from grazing followed by grazing, burnt and then ungrazed, and burnt and then grazed. Species were grouped into five functional types: shrubs, sub-shrubs, preferred grasses, non-preferred grasses and forbs. The results showed significant differences in the cover of preferred and non-preferred grasses, forbs and total cover with previous grazing but there was no evidence of shrub encroachment. Species frequency and richness decreased especially with increased grazing intensity. The pattern of spatial distribution changed from aggregated in grazed sites to random in ungrazed and burnt sites for all plant functional types. Land use history affects mainly the distribution of particle sizes in the soil and the bulk density. We did not find differences in total SOC, but SOC in interpatches of highly grazed site is associated with mineral particles, whereas in patches of medium grazed is greater in the coarse fraction. The content of carbon was not augment by fire. It is concluded that moderate disturbance (fire followed by grazing or medium grazing intensity) increased the plant diversity and the amount of rapidly available carbon in the soil.