Early-season effects of fire on soil chemistry and undergrowth diversity in teak and cashew plantations in a tropical savanna woodland

Background/Question/Methods: Fire has evolved inseparable from savanna vegetation of Africa, but its effects on soil chemistry and undergrowth diversity in managed plantations is still subject to intense debate. This study assessed early-season effects of fire on soil chemistry and undergrowth diversity in teak and cashew plantations in savanna woodland of Nigeria. Data were collected on soil pH, organic carbon/matter, N, P, Na, K, Ca, Mg from top and sub soils in fifteen delineated subplots in the burned and unburned zones of Cashew and Teak plantations after annual wildfire had occurred. Species diversity (H’) and similarity in the burned and unburned zones of the plantations were assessed using composition and abundances of undergrowth flora two months after the fire event. Canonical correspondence analysis was used to relate environmental variables (plantation, soil fertility and fire) to species composition.

Results/Conclusions: Burn incidence significantly improved OC, OM and Ca while reducing total N and Mg in the plantations. Soil nutrients in the burned zones varied significantly with plantation type but not with soil depth (except Na). Twenty-eight species occurred in the plantations and were distributed among Rubiaceae (10), Poaceae (5), Asteraceae (5), Malvaceae (3), Euphorbiaceae (1), Lamiaceae (1), Portulacaceae (1), Rubiaceae (1) and Solanaceae (1). Burned Cashew plantation had higher species abundance (4123 plants) but lower diversity (2.810) compared with the unburned plantation. The burned Teak plantation had significantly higher species abundance (3942 plants), richness (23 species) and diversity (2.785) than the unburned Teak. Burned and unburned zones of Cashew and Teak plantations had 90.2% and 86.4% of the species similar. CCA ordination revealed fire event, soil OC and OM in the burned Cashew plantation restricted Tephrosia pedicellata, Desmodium. tortuosum, Daniellia oliveri, Senna obtusifolia and Zornia latifolia to the site. Presence of Euphorbia heterophylla, Eriosema psoraleoides and Crotalaria retusa in the burned Teak plantation is linked to the direct influence of fire on soil Na and Ca rather than on the species. The study concluded that burning utterly influenced soil fertility but differently affected the diversity of undergrowth flora in the plantations.