The effect of fire exclusion on the structure and tree mortality patterns of a caldén (Prosopis caldenia Burkart) woodland in semi-arid Central Argentina

We studied the effects of long-term fire exclusion on the structure and mortality of a caldén (Prosopis caldenia Burkart) woodland, a rare vegetation type remaining only in fragments in semi-arid Central Argentina. We tested the hypothesis that differences in caldén stand structure are consequence of differences in fire history by quantifying DBH and tree height in 30 plots set in caldén woodlands with fire occurring at least every 10 y, with fire excluded for 20-30 y and without fire for at least 60 y. 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). Our results showed that the fire exclusion increased stem basal area and density while decreasing mortality of larger size classes and reducing resprouting of stems of any size; the probable explanation for these changes is a shift in the prevalent thinning mechanisms from fire-induced to competition-induced mortality.