Fire and wind erosion in the northeastern portion of Patagonia

C.M. ROSTAGNO

Puerto Madryn, Chubut

Workshop; Multidisciplinary workshop on Southern South American Dust, Puerto Madryn, Argentina; 2007

CENPAT, FONCyT, National Science Foundation (USA)

  In the northeastern portion of Patagonia, the frequency of accidental fires has dramatically increased during the last 25 years. Many of these accidental fires have occurred at the end of the spring or during the summer time, under environmental conditions characterized by strong winds, high temperature and low humidity.  Under these extreme environmental conditions, fire can spread even in plant communities with low vegetation cover, consuming most of the above ground biomass and leaving an unprotected soil, highly susceptible to the erosion processes. Wind erosion has been recognized as a serious problem in the arid and semiarid regions where the relatively low vegetation cover permits a more direct impact of wind to the soil surface. Soil is an essential component of terrestrial ecosystems and its fertility can be severely affected by the removal of topsoil or selected removal of fine particles by wind. In some areas of Patagonia, wind erosion seems to have been an important geomorphologic process before the arrival of domestic livestock. However, its acceleration has been historically associated to sheep grazing, initiated around 1880. Fire has lately become an important disturbance factor, locally increasing the impact of wind erosion. In this presentation I analyze the impact on wind erosion acceleration of two accidental fires that burned 12.000 and 31.000 ha in the northeastern portion of the Chubut province. Average annual precipitation in the two sites is approximately 230 mm; mean annual temperature is 13,5° C. Soils are Xeric Haplocalcids, about 50 cm deep. The dominant vegetation is a shrub-grass steppe. In these rangelands, the small-scale landscape mosaic is comprised of two patch types: individual or aggregated shrubs, generally associated to mounds, and intershrub areas with bare soil or a low herbaceous cover. Fires occurred in January of 1994 and in January of 2001. Wind erosion was assessed along a one-year period in the area burned in 1994; a one-time assessment was practiced 8 months post-fire in the area burned in 2001. In both sites, vegetation was partially or totally eliminated by fire, leaving the mounds unprotected and exposed to the erosive agents. In these microenvironments, the mean layer of sediments removed 12 months after the 1994 fire was 27 mm, equivalent to 130 Mg ha-1. However, most sediment was removed within the first 10 days after the fire. An equally high erosion rate, 114 ton ha-1, was measured 8 month after the fire that occurred in 2001. Within the intermound areas, in which soil is somehow protected by a high stone cover, there was no sediment loss. On the contrary, part of the material removed from the eroded mounds was accumulated in the intermound areas. One of the consequences of soil erosion in the two burned areas was the changes in soil surface characteristics due to differential removal and sedimentation of the different soil fractions. The texture of the remaining soil became coarser as very coarse sand (1 -2 mm) and part of the coarse sand (0,5 - 1 mm) were not removed; gravels formed a desert pavement that covered part of the eroded mounds. Most very fine, fine and medium sand fractions (0,05 – 0,5 mm) were deposited within the burned area, forming small accumulation tongues that increased ecosystem heterogeneity. On the contrary, part of the very fine sand and more than 90% of clay and silt were removed outside the burned area as suspended material, originating frequent dust storms. A dust storm that occurred one week after the 1994 fire and lasted for 17 hs, highlights the high environmental impact of wildfires on the highly erodible soils of northeastern Patagonia.