Forest expansion of native woodlands across the montane subtropical deserts in South America: climate and grazing influences.

MARIANO MORALES; RICARDO VILLALBA

Vancouver, British Columbia, Canada.

Conferencia; First American Dendrochronology Conference.; 2008

Climatic fluctuations, interactions between living organisms, and natural and anthropogenic disturbances, are the major factors affecting forest structure and dynamics. Prosopis ferox woodlands form a widespread vegetation community in the high dry valleys of Northwestern Argentina and Southern Bolivia. Across its distribution range, P. ferox has been widely used as a local source of fuel, timber and food for livestock. Therefore, the current structure of the remnant woodlands reflects the long-term interactions between past climate and human activities in the region. In response to a regional urbanization process starting at the beginning of the 20th century, people migrated from rural areas to urban centers in the subtropical lowlands. This process was accompanied by a reduction in the intensity of land-use. Concurrent with the social processes, significant climatic changes were recorded in the region since the mid-19th century. In the context of the last 100 years, instrumental and dendrochronological records reveal an unprecedented increase in regional precipitation during the recent decades in Northwestern Argentina. Taken in account these environmental changes in the region, we were interested in evaluating how variations in rainfall and in the intensity of human-related activities have affected the extension and productivity of the Prosopis woodlands. To identify the vegetation responses to the recorded natural and anthropogenic changes, we determined the current structure and dynamics of the P. ferox woodlands in the Quebrada de Humahuaca. At different temporal and spatial scales, rainfall appears as the major factor affecting establishment and radial growth of Prosopis woodlands across the region. However, differences in tree-growth and survivorship are heavily affected by grazing at more local scales. The increase in precipitation in combination with a reduction in grazing pressure has favored the expansion of the native woodlands of Prosopis across the Humahuaca, with regional implications for the atmosphere-land interactions.