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

PETER, G.; FUNK, F. A.; LUNA. M; TORRES ROBLE, S. S.

Rangelands: Ecology, Management and Conservation Benefits

Nova Science Publishers

Lugar: New Tork; Año: 2015; p. 87 - 110

In terrestrial ecosystems the vegetation and soil co-evoluted in response to somedisturbance. In arid and semi-arid lands around the world vegetation is distributed inpatches within a bare-soil matrix. Such vegetation and soil are more vulnerable todisturbance effects and highly susceptible to desertification, vegetation attributes andorganic fractions of the soil are the parameters widely used for evaluating changes inecosystems. In the North-east Patagonian Monte in Argentina, the vegetation is ashrubland steppe with patches dominated by shrubs, with grasses, forbs and cryptogamsunder their canopy. It was hypothesized that grazing increases patchiness across itsassociated trampling and selective foraging; and fires and posterior wind erosionhomogenize the distribution of soil particles and vegetation with consequence ondynamic 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, specificfrequency and richness of herbs and preferred grasses in ungrazed sites; 3) a randompattern of vegetation distribution in burnt areas; and 4) greater carbon soil stock inungrazed sites and lower in burnt ones. Aerial cover of all perennial species, distributionof soil particles sizes, and stocked organic carbon (SOC) of soil were measured at sixsites with different land-use histories: heavily grazed, medium grazed, ungrazed, longexclosure from grazing followed by grazing, burnt and then ungrazed, and burnt and thengrazed. Species were grouped into five functional types: shrubs, sub-shrubs, preferredgrasses, non-preferred grasses and forbs. The results showed significant differences in thecover of preferred and non-preferred grasses, forbs and total cover with previous grazingbut there was no evidence of shrub encroachment. Species frequency and richnessdecreased especially with increased grazing intensity. The pattern of spatial distributionchanged from aggregated in grazed sites to random in ungrazed and burnt sites for allplant functional types. Land use history affects mainly the distribution of particle sizes inthe soil and the bulk density. We did not find differences in total SOC, but SOC ininterpatches of highly grazed site is associated with mineral particles, whereas in patchesof medium grazed is greater in the coarse fraction. The content of carbon was notaugment by fire. It is concluded that moderate disturbance (fire followed by grazing ormedium grazing intensity) increased the plant diversity and the amount of rapidlyavailable carbon in the soil.