Fire-Mediated Forest Encroachment in Response to Climatic and Land-Use Change in Subtropical Andean Treelines

Abstract

We used dendroecological techniques to analyze the effects of rainfall and grazing on fire regime and its implications for tree regeneration in subtropical mountains of northwestern Argentina during the 20th century, a period characterized by increasing rainfall and decreasing land-use intensity. We dated fire scars and establishment of Alnus acuminata (the dominant tree species) in six watersheds along a 600km latitudinal range. We correlated fire frequency with rainfall records and performed Superposed Epoch Analyses to assess the relationship between rainfall and fire events during the century, and in two sub-periods: 1930-1965 (low rainfall, high grazing) and 1966-2001 (high rainfall, low grazing). We performed permutation analyses to assess the association between fire events and tree establishment, and to describe the spatial distribution of fires and forests in relation to hillslope aspect. Rainfall was associated with regional fires at interannual and decadal scales: fire probability increased after growing seasons with above-average rainfall and through the century, in concurrence with rainfall increase. The climatic control of fire was stronger under lower land-use intensity. Tree establishment was temporally associated with fire events, which occurred mainly in north facing slopes, where grassland cover is more extensive and forest colonization more likely. These results suggest that fire is limited by the availability of fine fuels, which is enhanced by high rainfall and reduced grazing; and tree establishment is limited by the competition with grasses. Consequently, increasing rainfall and decreasing grazing favored higher fire frequency, thus promoting forest encroachment during 20th century.