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

Abstract

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°C) in the late dry season. These data provide important insights into fire behavior in the Guinean savanna−forest mosaic ecoregion, informing fire management.

Varying relationships between fire radiative power and fire size at a global scale

“…This effect could be explained as follows: at the beginning of the fire season, when the fuel moisture content is still high, FRP is limited as energy is consumed by fuel moisture vaporisation (Alexander, 1982;Pyne et al, 1996), and consequently rate of spread and fire size also become limited. As the fuel becomes drier throughout the fire season (Sow et al, 2013;Sedano and Randerson, 2014;N'Dri et al, 2018), fires become more intense and potentially propagate further. However, the propagation of larger fires can hit some limits due to the fragmentation of the fuel matrix, from intrinsic anthropogenic fragmentation, roads or grazing fields.…”

“…This cut-off value corresponds to the maximum time difference between the burn date of neighbouring pixels belonging to the same fire patch. These global datasets have been thoroughly compared by the authors of the FRY database, locally compared using the US and Canadian forest service fire patch database (Chuvieco et al, 2016) and validated against Landsat fire polygons in the Brazilian Cerrado (Nogueira et al, 2017). The FRY database is organised in eight datasets (two survey times and four cutoff values) and provides for each individual fire patch a set of variables, called fire patch functional traits, including the geolocation of the patch centre, the fire patch size (later called FS, in hectares) and different indices on fire patch morphology.…”

Effect of fire regimes on the demographic parameters of the perennial tussock grasses of a humid savanna

“…Three separate blocks of 3.72 ha each were identified in the shrubby savanna zone. In each block, four plots were established in September 2013 to implement four fire treatments (N'Dri, Kone, Loukou, Barot, & Gignoux, ; N'Dri, Soro, et al., ): early fire (in November), mid‐season fire (in January), late fire (in March) and no fire (unburned plot). In each of these plots, a plot was delimited to monitor grass demography.…”

“…According to them, the late fire should rather stimulate tussock growth by reducing aboveground biomass, subsequently reducing competition and increasing light availability (Zimmermann et al, 2010). The stronger negative impact of the late fire, which does not have a higher intensity than the other fire regimes in Lamto savanna (N'Dri, Kone, Loukou, N'Dri, Soro, et al, 2018), is likely explained by the fact that grass tussocks have resumed their growth at the moment of late fire, so that late fire has a stronger negative impact on growth (and other demographic parameters, see above).…”

“…In West African humid savannas, three fire regimes are generally described (Bruzon, ; N'Dri, Soro, et al, ): (a) early fires occur at the beginning of the dry season when vegetation is not fully dry; (b) mid‐season fires occur in the middle of the dry season when grass biomass is dry and neither grasses nor trees are growing; and (c) late fires occur at the end of the dry season when trees and grasses have resumed their growth. Over one year, we analyzed the impact of these three fire regimes and the absence of fire on five demographic parameters (see above) of the nine dominant perennial grass species of the Lamto savanna (Abbadie, Gignoux, Le Roux, & Lepage, ) in Ivory Coast.…”