Nonlinear rainfall effects on savanna fire activity across the African Humid Period

Fire is a key ecosystem process in tropical and subtropical savannas, with a varying role that depends on hydroclimate but also on feedbacks between fire and vegetation. In savannas, fire response to changes in rainfall depends on mean annual rainfall: in arid and semi-arid systems, burned area increases as rainfall increases fuel amount, whereas in mesic systems with high fuel moisture, burned area decreases as rainfall increases. The non-linear relationship between burned area and rainfall may be due in part to changes in the constraints on fire activity that shift the limiting factor for burning in savannas from fuel amount to fuel moisture with increasing rainfall and decreasing seasonality. Vegetation-fire feedbacks can also promote a shift from fire-prone savanna vegetation to forest taxa that suppress fire as rainfall increases. However, modern observations are, by definition, constrained to short-term dynamics, and the longer-term effects of precipitation changes on fire activity have not been evaluated. These longer-term impacts are especially relevant for evaluating biome transitions changing in response to variable hydroclimate and fire activity. The Late Pleistocene and early Holocene African Humid Period (AHP; ∼14.5–5.5 ka), when rainfall increased substantially across northern and equatorial Africa, provides an opportunity to examine long-term fire responses to increased precipitation at sites with different mean annual rainfall amounts. Here, we combine new polycyclic aromatic hydrocarbons (PAHs) records of paleofire activity at two East African lake basins (Lakes Victoria and Turkana) with previously published fire records (i.e., charcoal, levoglucosan) at five sites (GeoB7920-2, GeoB9508-5, Lakes Chala, Tanganyika and Bosumtwe) to examine responses of fire activity along a rainfall gradient (from <200 mm to 1500 mm). Our synthesis reveals fire dynamics that are consistent with modern ecosystem dynamics and shows that fire activity at each site followed predicted patterns across the AHP depending on initial mean rainfall, with increased fire activity at arid to semi-arid sites and decreased fire activity at mesic-to-humid sites. Results illustrate that fire responses to hydroclimate are nonlinear, such that the same direction of change in precipitation can elicit different fire responses depending on the total precipitation at a site. Accounting for heterogeneity in hydroclimate, even within biomes, may improve predictions of how fire activity will respond to future changes in rainfall regimes.