Influence of land use-climate interactions on the dynamics of Prosopis woodlands in the arid lands of western Argentina

PABLO E. VILLAGRA; MARIANO S. MORALES; ERICA CESCA; JUAN A. ALVAREZ; JOSE A. BONINSEGNA; RICARDO VILLALBA

Akumal, Mexico

Simposio; The assessment of past, present and future climate variability from treeline environments; 2005

Inter-American Institute for the Study of Global Change

The arid lands of the western Argentina include the Monte and Prepuna Phytogeographical Provinces which are characterized by water deficits during the major part of the year (mean annual rainfall ranges from 30 to 350 mm). Prosopis species are dominant with Prosopis flexuosa as the main tree species in the Monte desert growing in open woodlands with phreatic water supplement. Prosopis ferox grows in upper-elevation and intermontane valleys between 2400 and 3800 m in the Prepuna. Both species have traditionally been used for timber, fuel and food production and have therefore played an important social and economic role for the local populations.  The most significant environmental changes in Argentinian arid lands have been: 1) the intense human use of natural resources leading to desertification; 2) the increase of precipitation during the second half of the XXth. century; and, 3) the intensification of migration from rural to urban centers and intensively-cultivated valleys. We postulated that these processes have regulated the recent dynamics of Prosopis woodlands, but that the influence of these forcings differs between woodlands. We combined historical, dendrochronological, dasonomic and satellite data to evaluate the relative contribution of the factors controlling the dynamics of Prosopis woodlands in the arid lands of Argentina. The responses of different woodlands to these processes of environmental change were also recorded. Prosopis flexuosa woodlands show a southwards decrease in structural complexity, growth rate, and biomass, suggesting a large-scale climatic control of forest characteristics. However, at the local scale, the structure of Monte woodlands reflects the heavy logging in the early XXth. century, the subsequent transformation to pastoral use, and the differential recovery capacity. In the southern Monte, the dynamics are controlled by fires that have increased in frequency during recent decades, likely related to recent climatic changes. Traditionally, Prosopis woodlands were selected as preferred settlement sites by humans. Most of the present urban and crop lands are located on areas originally covered by Prosopis due to the easier access to water and the microclimatic conditions generated by woodlands. The urbanization and irrigation processes lead to the reduction of Prosopis woodlands, and to the increase of the demand of forest products from the remaining forests. Presently more than 20% of the original area of Prosopis woodlands in the Monte has been replaced. In the Prepuna, the radial growth of P. ferox is highly favored by abundant rainfall during the growing season. Therefore P. ferox establishment is also related to changes in precipitation. The abrupt increase in tree recruitment during the period 1973-1992 is associated with abundant precipitation during this interval. Over the same period population and agronomic surveys indicate migration of the rural population to oasis and valley sites and a reduction in the density of domestic animals during the 20th century. It is highly possible that the decrease in domestic animals and the consequent reduction of grazing pressure also favors P. ferox recruitment. Therefore a combination of increased precipitation with the reduction of grazing has lead to the expansion of P. ferox woodlands in the Prepuna. On large spatial scales, climatic conditions influence the establishment and growth rate of the Prosopis woodlands and, therefore, their economic potential. On local- to regional-scales, urbanization and the concentration of population in valleys and oases, has led to the degradation of the P. flexuosa woodlands in the Monte, but the expansion of P. ferox in the Prepuna. Different strategies of water utilization between species determine their response to socio-economic processes as well as their future potential under predicted scenarios of climatic change for the region. P. flexuosa, which appears to be an obligated phreatophytic species in the Monte, could not take advantage of increased precipitation to expand its distribution across abandoned fields, whereas P. ferox, a facultative phreatophytic species, can expand its range with more abundant precipitation and less human intervention. As these studies clearly indicate that climatic, biological and economic factors strongly interact, all of them should be integrally considered in the study of the woodland dynamics in arid lands