Managing the human component of fire regimes: lessons from Africa

Abstract

Human impacts on fire regimes accumulated slowly with the evolution of modern humans able to ignite fires and manipulate landscapes. Today, myriad voices aim to influence fire in grassy ecosystems to different ends, and this is complicated by a colonial past focused on suppressing fire and preventing human ignitions. Here, I review available evidence on the impacts of people on various fire characteristics such as the number and size of fires, fire intensity, fire frequency and seasonality of fire in African grassy ecosystems, with the intention of focusing the debate and identifying areas of uncertainty. Humans alter seasonal patterns of fire in grassy systems but tend to decrease total fire emissions: livestock have replaced fire as the dominant consumer in many parts of Africa, and fragmented landscapes reduce area burned. Humans alter the season and time of day when fires occur, with important implications for fire intensity, tree-grass dynamics and greenhouse gas (GHG) emissions. Late season fires are more common when fire is banned or illegal: these later fires are far more intense but emit fewer GHGs. The types of fires which preserve human livelihoods and biodiversity are not always aligned with the goal of reducing GHG concentrations. Current fire management challenges therefore involve balancing the needs of a large rural population against national and global perspectives on the desirability of different types of fire, but this cannot happen unless the interests of all parties are equally represented. In the future, Africa is expected to urbanize and land use to intensify, which will imply different trajectories for the continent's fire regimes.This article is part of the themed issue 'The interaction of fire and mankind.

Keywords: fire management; fire return period; fire season; ignition.

Figure 1.

(a) The direct and indirect pathways by which humans impact fire and the aspects of fire that are impacted. The relative importance of these pathways depends on the socio-ecological context. In Africa, the impacts on fuels and timing of ignitions currently outweigh the impacts on ignition number or climate. (b) The estimated timing of these impacts in four different parts of the globe. Impacts on fire in Africa accumulated slowly over time, but more recently and abruptly in other continents. Impacts on fuels can be positive (extermination of herbivores) or negative (livestock grazing, cultivation, fragmentation). Climatic impacts have occurred the most recently, and uniformly throughout the globe.

Figure 2.

Showing the extent of the human-derived pyrome: areas of the world where fire characteristics are largely controlled by human impacts. Diverse biomes and environments converge on a homogeneous fire regime under high human impacts. Adapted from [10].

Figure 3.

Fire frequency in different parts of Africa (the number of times a fire was recorded in a 500×500 m MODIS pixel over 10 years from 2000 to 2010). Clearly National Parks (the least inhabited parts of the continent) burn more extensively, but not more frequently, than non-protected and inhabited regions. This is clear demonstration of the importance of indirect impacts of humans on fuels in grassy systems: by replacing fire with livestock and fragmenting landscapes they have greatly reduced the total area burned in Africa.

Figure 4.

(a) Relationship between number of people and the number of fires. (b) Relationship between the number of people and the size of fires. Points represent the value per 50 km grid cell. Solid lines represent the median values in (a), and the 95th quantiles in (b). All data are for Africa south of the Equator and were derived by identifying individual fire events using the methods described by Archibald & Roy [28], i.e. they represent individual ignition events, not active fire hotspots.

Figure 5.

(a) Rainfall strongly controls fire return intervals by mediating regrowth rates of the grassy fuels. In Africa, fire return intervals are minimized between 900 and 1700 mm MAR above which the moisture (flammability), rather than biomass of fuel constrains return times [13]. (b) High human densities slightly increase the return interval of fire at a location in space despite their frequent ignitions, i.e. the fuels drive the fire regime and humans impact fire via their impacts on fuels. The methods to produce this figure are described by Archibald et al. [45].

Figure 6.

Current seasonal patterns of ignitions in Southern Hemisphere Africa (grey bars) in relation to the maximum fire radiative power values recorded over the season in the region (black lines). Dashed grey lines represent the seasonal pattern of lightning strikes. The late dry season is characterized by the potential for high-energy fires. Currently nearly half (41%) of the fires occur before the late dry season. When lightning was the major ignition source there was still the potential for high-energy fires in the late dry season. Patterns for Northern Hemisphere Africa are similar, but inverted, with peak fires in December [50].

Figure 7.

Seasonal patterns of burning in the Kruger National Park when management aimed to exclude all fires except lighting (a) and when they proactively burned early season fires (b). The total area burned was insensitive to these interventions, but the proactive management period resulted in three times as many fires (171 versus 52 per year), and a higher proportion of early season fires (8% and 22% of the area was burned before July in the lightning and proactive management periods, respectively).

Figure 8.

Results of a Southern African Fire Network meeting in Tanzania. Participants were asked to indicate places in Africa where fire should be managed to increase woody cover (dark blue) or to decrease woody cover (light pink). Although the spatial distribution reflects the knowledge of the participants, clearly there are contrasting fire management objectives on the continent.