Forests at the edge: Fire and the dynamics of tropical savanna-forest boundaries

Fire is a major determinant of tree density in tropical savannas.  Although counterintuitive, it is now becoming clear that rate of topkill (i.e. death of the aerial stem) and rate of sprout regrowth are more influential in determining tree density under frequent burning than is mortality rate per se.  It is not clear if this is also true when considering the effects of fire at savanna-forest boundaries, since fire is believed to generally cause high tree mortality rates in tropical lowland forests.  However, there is less information available regarding fire effects at savanna-forest boundaries, where forest trees have a long history of exposure to fire.  We studied the effects of fire on three savanna-forest boundaries in central Brazil.  We hypothesized that, relative to savanna species, forest species would exhibit higher rates of topkill and mortality, as well as slower rates of resprouting.  As predicted, forest species exhibited significantly greater rates of topkill, relative to savanna species, a trend accounted for by the lower investment in bark by forest species.  Contrary to predictions, mortality rates of forest species were not significantly greater than savanna species, nor was the rate of regrowth following topkill.  The allometry of resprouting differed significantly between savanna and forest species.  For small pre-burn stems, savanna species resprouted more vigorously than forest species, while for large pre-burn stems, forest species resprouted more vigorously.  This relationship is consistent with predictions from optimal allocation strategies under different fire regimes.