A burning regime refers to the timing, intensity, efficiency, and frequency of fires. In West Africa, the timing of burning is extremely important in terms of a fire‟s effects on land cover change and greenhouse gas emissions. Dry season burnings are commonly divided into three periods: early, middle and late dry seasons (Monnier 1968; César 1990a; Bassett and Koli 1998; Mbow et al. 2000; Koné et al. 2008). Fires set in the early dry season are less destructive to vegetation than middle and late dry season fires. This timing is also important for the amount of fuel burned and gases and particulate matter emitted. Despite these important distinctions on the varying intensity and efficiency of burning within the dry season, the climate change literature rarely considers the temporal dimension of burning. For example, Nguyen et al. (1993: 209-210) write, “In tropical Africa, savannas are burned almost continuously, from December to March in the Northern Hemisphere”. Their field experiments were conducted in January 1989 and January 1991 during the middle dry season burning in the Guinean savanna at the Lamto research station in Côte d‟Ivoire. In their important study, the “dry season” is typically
viewed as homogeneous in both space and time in West Africa. The temporal dimension has two components. First, fires are assumed to be destructive, middle and late dry season fires. Second, there is no discussion of duration and location of the “dry season” which does vary with latitude in West Africa. The north-south movement of the Inter-Tropical Convergence Zone (ITCZ) results in the dry season starting early and lasting longer in the sudanian savanna in contrast to the shorter dry season in the southern Guinean savanna (Fig. 1.1).
Fig. 1.1Savanna vegetations in Lamto (11/12/2008) and Katiali (11/20/2008) in the same period. The vegetation dries (B) and burns (C) earlier in Katiali than in Lamto (A).
Fire studies that report differences in the timing of burning do not link the temporal and spatial distributions of biomass burning. For instance, Cros et al. (2000: 29, 348) argue, “Generally, the fire period (corresponding to the dry season) begins in early November, peaks in January, and ends in February/March.” Mbow et al. (2000: 572) underscored the existence of early fires and late fires in Sénégal. Abbadie et al. (2006: 52) informed the literature about the impacts of the timing of burning. They state, “The early fire regime has almost no effect compared to complete fire exclusion. Because fires in Lamto occur regularly during the dry season, as middle fires, they may not be sufficient to prevent tree invasion; a late fire from time to time might be needed to reduce tree density”. The early dry season fires in the pre-forest savanna landscapes at Lamto station in Côte d‟Ivoire do not correspond to the early dry season fires of the sudanian savanna, which occur in late October-November (Fig. 1.1). It is important to make these distinctions in our analyses of biomass burning and its effects on land cover change, and gas and particulate matter emissions.
The timing of burning is crucial to understanding burning regimes. Bush fires that are set in the early dry season are less intense than those of the middle and late dry
seasons. Because of the relatively high water content, air humidity, and low wind speed, early dry season fires produce unburned patches throughout the landscape. Many authors underscore that more bush fires are occurring in the early dry season, leading to less intense burnings. Thus, they argue that there is a change in burning intensity (Bassett and Koli 1998; Eva and Lambin 2000; Jenkins and Ryu 2003; Laris 2002).
Combustion efficiency in savanna landscapes also remains unclear. The combustion efficiency is different depending on the vegetation type, combustion stage, and burning period. Ward and Hardy (1991) conducted an experiment and said that the combustion efficiency is 50-80% for smoldering combustion and 80-95% for flaming combustion. For different vegetation types, Jain et al. (2006: 4) used 0.75 for savanna combustion completeness, tropical evergreen 0.50; tropical deciduous 0.50; temperate evergreen 0.50; temperate deciduous 0.50; grassland and pastureland 0.83, shrubland 0.75, cropland 0.86. Savanna is a heterogeneous landscape mainly composed of woodland and open forest, gallery forest, bush savanna, grass savanna, cropland and fallow fields that have specific combustion efficiency coefficients.
Abbadie et al. (2006: 53) suggest a method to estimate burning intensity. According to them, “the easiest way to measure fire severity is through temperature measurements. (…) This can be done with a thermocouple or more easily, with thermal paints and pencils (paints that show a color change at a fixed temperature). Thermocouples measure the instantaneous temperature, while thermosensitive paints include a time effect in their response and theoretically measure the maximal temperature over the time they have been exposed to.” They also state, “Spatial and temporal distribution of fire severity are important, due to many causes acting at different scales. Such variations have possible effects on vegetation dynamics” (Abbadie et al. 2006: 55). Other studies link burning intensity to the burning period and to biomass characteristics. Burning intensity also varies according to the time of the day. Solar radiation and exposure to the sun affect burning intensity. Thus, fires set in the mornings and in the evenings are less intense than mid-day burns. Many authors underscore that more and more bush fires are occurring in the morning and evening by farmers and herders, especially early in the dry season leading to less intense fires. They suggest that past fires
were more destructive than current savanna fires (Bassett and Koli Bi 1998; Eva and Lambin 2000; Jenkins and Ryu 2003).