RAINFALL EFFECTS ON FIRE ACTIVITY AND DISTURBANCE-MEDIATED VEGETATION STATES ARE NON-LINEAR ACROSS THE AFRICAN HUMID PERIOD

Tropical and subtropical savannas are largely maintained by feedbacks between fire, vegetation, and hydroclimate. Rainfall regimes in the tropics and subtropics are projected to change in the future, which will likely lead to shifts in fire activity. In modern savannas, fire response to changes in rainfall depends on baseline mean annual rainfall: in arid and semi-arid systems, burned area increases as rainfall increases, whereas burned area decreases as rainfall increases in mesic systems. This non-linearity may be due in part to changes in the constraints on fire activity (transitioning from fuel amount to fuel moisture limitation with increasing mean annual rainfall and decreasing seasonality) and in part to vegetation-fire feedbacks that transition savanna to forest that suppress fire as rainfall increases. The African Humid Period (AHP; 14-5 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 initial mean annual rainfall amounts. Here, we combine novel polycyclic aromatic hydrocarbons (PAHs) records of paleofire activity at two East African lake basins (Lakes Victoria and Turkana) with previously published fire (i.e., charcoal, levoglucosan) records at five sites (GeoB7920-2, GeoB9508-5, Lakes Chala, Tanganyika and Bosumtwi) to examine responses of fire activity along a rainfall gradient (from <200 mm to 1500 mm). Our synthesis is consistent with modern dynamics, and shows that changes in fire activity across the AHP differed across sites, with increased fire activity at arid to semi-arid sites and decreased fire activity at mesic-to-humid sites. Results emphasize that fire responses to hydroclimate are nonlinear, such that the same direction of change in precipitation can elicit different fire responses depending on the absolute precipitation at a site. Accounting for heterogeneity in hydroclimate, even within biomes, may improve predictions of how fire activity may respond to future changes in rainfall regimes.