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.