Climatic and Anthropogenic Influences on the Dynamics of Prosopis ferox Forests in the Quebrada de Humahuaca, Jujuy, Argentina

MARIANO MORALES; RICARDO VILLALBA

La Paz, Bolivia

Simposio; Linking mountain diversity with fire, grazing and erosion; 2003

Global Mountain Biodiversity Assesment

The recent increase of Prosopis ferox, both in population density and distribution range in the Quebrada de Huamahuaca, Jujuy, Argentina, appears to be related to major changes in land use and regional precipitation. P. ferox grows between 2600 and 3800 m elevation in the upperelevation intermontane valleys in northwestem Argentina and southem Bolivia. Across its range of distribution, P. ferox has been largely used as a local fuel source, as construction material, and as fodder for livestock. To determine the factors affecting the recent changes in the population structure of P. ferox, we compared the age structure of the woodlands with human demographic and land use changes, and with regional variations in precipitation. Based on age structures from 12 stands, four periods of recruitment were identified. Reduced recruitment was recorded until 1960 followed by a gradual increase between 1960 and 1970. An abrupt increase was observed around 1974, which persisted to 1990. Finally, low recruitment was recorded during the past 10 years. These regional pattems of recruitment are consistent with the recent expansion of the woodlands. Variations in recruitment reflect regional variations in precipitation since the middle of the 20th century. Furthermore, demographic and agrarian censuses show significant changes in the relationship between rural and urban populations, and a substantial reduction of livestock density between 1974 and 1988. Reduced grazing by livestock during this interval might have also favored the recruitment of P. ferox trees. Similar or lower grazing pressure during the 1990s suggests a greater influence of precipitation than livestock on the reduced tree establishment during this decade