Season affects fire behavior in annually burned humid savanna of West Africa

Background: Fire is important for the maintenance of African savanna ecosystems, particularly humid savanna. Despite the importance of fire behavior to our understanding of fire's ecological effects, few studies have documented fire behavior and its determinants in humid West African savannas and, in particular, whether fire behavior depends on season of the year. We analyzed fire behavior in the Guinean savanna of Lamto (Ivory Coast) during a 4-year field experiment. The fire regimes tested consisted of three different burning seasons: early-season fire, mid-season fire, and late-season fire. Nine 0.5 ha plots were burned annually to determine the rate of spread and fire intensity. Fuel characteristics and weather conditions were measured to assess their impact on fire behavior. Results: Understory grass height, total fresh fuel load, and moisture content had greater values in early-season fire than in mid-season and late-season fire. The rate of spread and intensity of both mid-season fire (0.14 +/- 0.03 m s(-1) and 3920 +/- 740 kW m(-1), respectively) and of late-season fire (0.12 +/- 0.02 m s(-1) and 3134 +/- 482 kW m(-1), respectively) were significantly greater than those of early-season fire (0.04 +/- 0.01 m s(-1) and 1416 +/- 252 kW m(-1), respectively). The best predictors of fire behavior were fuel moisture content and air humidity; these two explanatory variables were the sole significant predictors for fire intensity, rate of fire spread, and flame height. Conclusions: Given that there is no difference between intensity of mid-season and late-season fire, we suggest that the generally reported higher impact of late-season fire on trees in the West African humid savannas is due not to fire intensity per se, but rather to a more sensitive phenological stage of trees (e.g., leafless in mid-season), and to a longer time of exposure to lethal temperatures (> 60 degrees C) in the late dry season. These data provide important insights into fire behavior in the Guinean savanna-forest mosaic ecoregion, informing fire management.