Espinal woodlands in Argentina are one of the most endangered ecosystems in the world because most of them have been transformed into agricultural fields and pastures, and only scattered fragments of the original Espinal vegetation remain. These woodlands were relatively open, with a low tree density; however, in recent decades, significant increases in densities of both native and non-native trees and shrubs have been observed but a clear explanation for these changes is lacking and their impact on Espinal ecosystem is unknown. We hypothesized that increases in woody plant density in Espinal woodlands are a consequence of exclusion of formerly frequent fires which had maintained low stand density by killing most trees. In addition, we expected that the increase in tree density caused by fire exclusion leads to decline in vascular plant diversity. To test our hypotheses we measured stem diameter (DBH) and height of both death and live trees, and surveyed understory and shrub vegetation in 60 plots set in Espinal woodlands with fire occurring at least every 10 years, with fire excluded for 20-30 years and without fire for at least 60 years.
Results/Conclusions
Stem height, density and basal area increased after 20-30 years of fire exclusion. The frequently burned woodland had high mortality in all but the largest (> 60 cm in DBH and > 8 m in height) size class, whereas the fire-excluded plots exhibited high mortality only in the smallest stems (< 15 cm in DBH and < 3 m in height) which indicates that fire exclusion leads to an increase in woody plants abundance due to a decrease in mortality of larger size trees. Plant diversity was higher in frequently burned plots and was negatively correlated with density of woody plants. Our results suggest that exclusion of frequent fires has a negative impact on the ecosystem of Argentine Espinal woodlands and that prescribed fires are needed to preserve structure and diversity of the last fragments of these rare woodlands.