Aim We explored the relationships among rainfall, fire and tree population dynamics during the twentieth century. Relationships were analysed on seasonal, interannual and multiannual time scales. Location The study was conducted at ecotones between Andean grasslands and montane forests in north-western Argentina. In this area of subtropical monsoonal climate (wet summers, dry winters), annual rainfall has increased and forests have expanded into grasslands and shrublands during the 20th century. Methods We used annual tree-rings of alder (Alnus acuminata H. B. K.), the dominant tree species, to date fire scars on 265 trees and determine establishment dates of 455 trees dispersed over areas > 100 ha at five sites. We also sampled tree establishment and resprouting in 21 plots of 0.1-5 ha and determined mortality patterns in one recently burnt stand. Results Fire occurrence during the winter dry season tends to lag by 1 year after years of above-average moisture availability. This pattern is probably due to enhanced production of fine fuels during the growing season of the preceding years. Over 5-year periods, higher fire frequency is associated with greater variability in rainfall. On a 5-year time scale, tree establishment is also associated with higher rainfall variability. Tree age structures suggest that a decades-long trend towards increased rainfall may also promote forest expansion. By the age of 10-20 years, Alnus acuminata attains a large enough size to survive the low-intensity fires typical of these open woodland sites. Recovery of A. acuminata after fire is also assured by vigorous resprouting and seedling establishment after fire removes or reduces the herbaceous understorey. During relatively wet periods or at relatively wet sites, fire appears to favour forest expansion by reducing competition between tree juveniles and non-forest vegetation. Main conclusions These results demonstrate the importance of variability in interannual rainfall as an influence on the dynamics that control ecotones between high Andean grasslands and subtropical montane forest.
