Responses of tropical tree seedlings in the forest-savanna boundary to combined effects of grass competition and fire

Co-occurring tree functional types (TFTs) within forest-savanna
transitions may differ in seedling responses to grass competition and fire
in savannas. We performed a common garden experiment in the Guinea savanna
of Ghana to test hypotheses related to competition effects on growth,
allocation to root storage reserves and subsequent survival responses to
dry season fire for savanna-transitional TFT (i.e., species occurring both
in forest and savanna) and forest TFT. The experiment included factorial
combinations of TFT, comprising four species each of forest vs
savanna-transitional trees, wet season grass competition vs no-grass
competition, and dry season fire vs no-fire. Partly consistent
with prediction, we found that grass competition suppressed tree seedling
growth and caused a 17% reduction in root non-structural carbohydrates
concentration [NSC] but had no effect on direct survival regardless of TFT
at the end of the wet season. Post-fire survival averaged 6% for forest
versus 91% for savanna-transitional TFTs across competition treatments. In
contrast to our prediction that grass competition
decreases post-fire seedling survival, wefound that a history of
grass competition did not result in lower post-fire survival regardless of
TFT, although plant mass, root mass fraction and root [NSC] at the end of
the dry season were lower for tree seedlings with a history of
competition. Our results demonstrate that grass competition suppresses
tree seedling growth and root storage reserves irrespective of TFT, and
that competition alone (without fire) may not preclude the establishment
of forest seedlings in savannas close to forests.