chapter  24
16 Pages

Climate–Fire Interactions and Savanna Ecosystems: A Dynamic Vegetation Modeling Study for the African Continent

ByAlmut Arneth, Veiko Lehsten, Kirsten Thonicke, Allan Spessa

Savannas are inherently “disturbed” ecosystems, but the regularly recurring disruptions play such a fundamental ecological role (Scholes and Archer, 1997) that “episodic events” rather than “disturbance” may the more apt terminology. From an atmospheric perspective, fire is the most significant of these episodic events. Fires shape community species composition; tree to grass ratio and nutrient redistribution; and biosphere-atmosphere exchange of trace gases, aerosols, momentum, and energy. Savannas’ estimated mean NPP of 7.2 ± 2.0 t C ha−1 year−1 amounts to nearly two thirds of tropical forest NPP (Grace et al., 2006); but remarkably little is known about savanna net carbon balance, especially for the African continent (Williams et al., 2007). In the absence of transient changes in the fire regime, such as could be introduced by climate change or fire-driven changes in land cover, savanna fires do not affect average annual net carbon uptake much, as the carbon released

CONTENTS

Introduction .........................................................................................................463 Dynamic Vegetation Models for Use in Savannas .........................................465 Tree to Grass Ratio as Affected by Mean Annual Precipitation,

Fire Return Interval, and Variable Rooting Depths ..............................466 Vegetation-Fire Interactions: Intensity and Fuel Accumulation .................. 469 Interactive Effects of Precipitation and NPP ................................................... 469 Dynamic Simulation of Wildfires ..................................................................... 473 Summary .............................................................................................................. 475 Acknowledgments .............................................................................................. 476 References ............................................................................................................. 476

is rapidly recaptured by regrowing vegetation. However, pyrogenic emissions include particles, ozone, methane, and other volatile hydrocarbons, all of which affect tropospheric chemistry and climate (Scholes and Andreae, 2000). Moreover, fires affect convective uplift via the interplay of surface albedo, Bowen ratio, and available soil moisture. In that way, fire intensity, timing, and total area burnt may also affect atmospheric circulation patterns, as was discussed for the Australian Monsoon (Lynch et al., 2007).