Fire and water: the role of grass competition on juvenile tree growth and survival rates in a mesic savanna

Trees and grasses compete for space in savanna landscapes where fire also suppresses trees, maintaining a lower tree/grass ratio than precipitation levels can support. While the effects of competition and fire have been considered largely in isolation, theory suggests that competition with grasses for water may limit the ability of juvenile trees to grow rapidly enough to escape flames holding them in a firetrap. This study examined the growth and survival rates of trees at different life stages on plots subjected to four different grass removal treatments with those on plots subjected to three different fire treatments in two mesic savannas in Mali. Two hypotheses were tested: (i) grass removal treatments will not significantly increase tree height growth rates; and (ii), burning earlier in the dry season will result in faster height growth rates and lower death rates than burning later. Trees were measured over 2 years. To compare how treatments affected tree growth, data were analyzed using a one-way analysis of variance with treatment type as a fixed factor. We find that grass removal resulted in increased juvenile tree growth varying from 147 to 205% depending on treatment. Grass treatments of hoeing, clipping and herbicide resulted in fewer tree deaths. Both early and late fires resulted in increased tree growth rates, although late fires resulted in more deaths. Mid-season fires did not significantly affect growth rates. Findings suggests that competition with grasses slows juvenile tree height growth and that a change in competition intensity is sufficient to cause a shift in savanna state to a more tree dominated one with implications for savanna management and carbon sequestration schemes.