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

VILLAGRA, P.E.; M. MORALES; E. CESCA; J.A. ALVAREZ; J.A. BONINSEGNA Y R. VILLALBA.

Akumal, México.

Workshop; The Assesment of Past, Present and future Climate Variability from Treeline Environments. Final Annual Science Meeting, IAI CRN03.; 2005

IAI

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). The Prosopis species are the dominant elements Prosopis flexuosa is 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 been traditionally used for timber, fuel and food production, therefore, they have played an important social and economic role for the local populations. The most significant environmental changes in Argentina arid lands have been: 1) The intense human use of natural resources leading to desertification processes, 2) The increase of precipitation during the second half of the XX century, and 3) of the intensification of the migration from rural to urban centers and intensive-cultivated valleys. We postulated that these processes have regulated the dynamics of Prosopis woodlands, but the influence of these forcings has been different between woodlands. We combined historical, dendrochronological, dasonomic and satelital 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 the processes of environmental change were also recorded. Prosopis flexuosa woodlands show a latitudinal decrease in the structural complexity, the growth rate, and the biomass, suggesting a large-scale climatic control on these forests. However, at local scale, the structure of Monte woodlands reflects the heavy logging in the early XX century, the subsequent transformation to pastoral use, and the different recover capacity. In the southern Monte, the dynamics are controlled by fires, which frequency has increased during the past decades, likely related to recent climatic changes. Traditionally, Prosopis woodlands were selected by humans for settlement. Most of the present urban and crop lands are located on areas originally covered by Prosopis due to the easier access of water and the microclimatic conditions generated by woodlands. The urbanization and irrigation processes leads to the reduction of Prosopis woodlands, and to the increase of the demand of forestal products from the remain 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. 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 the same interval. On the other hand, population and agronomic surveys indicate migration of the rural population to the oasis and valleys 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 increasing 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. of the woodlands in the arid lands of Argentina. On local- to regional-scales, the urbanization process and the concentration of population in valleys and oasis, have lead 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 woodlands determine the type of response to socio-economic processes as well as their future potential under the predicted scenarios of climatic change for the region. P. flexuosa, which appears to be an obligated phreatophytic species in the Monte, would not take advantage of increasing precipitation to expand its distribution across abandoned fields, whereas P. ferox, a facultative phreatophytic species, would expand with more abundant precipitations and less human intervention. Climatic, biological and economic factors strongly interact, and should be integrally considered in the study of the woodland dynamics in arid lands.