Cartografía de un nuevo Modelo de Estados y Transiciones de la Estepa Magallánica Seca en la Patagonia argentina

Abstract

We analyzed 103 vegetation relevés in the Dry Magellan Steppe, a 12000 km2 semiarid biozone (170-270 mm of annual rainfall) dominated by grass steppes in the SE of Patagonia. The objective was to map rangeland states within the State and Transition Model (STM). Cluster analysis showed 10 communities, some of them not included in previous STM models of the region, so that a new model was designed. In addition to already described transitions such as those driven by erosion that exposes argilic soil horizons covered by dwarf Shrub Steppes, the new model includes transitions driven by eroded soil accumulation that benefit Pappostipa chrysophylla, an invasive psammophyle grass species. Three states previously differentiated by short grass cover were combined in order to reflect only irreversible transitions. Maps were obtained by drawing polygons around each relevé, grouping them in accordance to the classes defined by cluster analysis and extracting spectral firms of these areas in a Landsat 8 image. These classes were grouped into five states. We estimated 61% precision of the map. Tussock grasslands in good conservation state occupied 39% and were dominant in the south of the area. Degraded dwarf shrub steppes with P. chrysophylla (18%) and dwarf shrub steppes (15%) were next in importance. Fence contrasts in the map show some of the hypothesized transitions, such as those that lead to dwarf shrub or dwarf shrubs with P. chrysophylla in highly grazed wintering areas in the north. The map and new STM will help to interpret the dynamics of the Magellan Steppe, one of the most diverse and productive biozones of Patagonia that shows clear evidence of past degradation. Some of these rangelands should probably be protected in the future, as conservation areas are almost nonexistent. In this sense, all of the region should be conserved in productive units by managing grazing with moderate stocking rates and rests in order to ensure conservation of biodiversity, productivity and other ecosystem services.