Fuel, fire and cattle in African highlands: Traditional management maintains a mosaic heathland landscape

Abstract

Shrubland ecosystems are often inherently flammable due to a canopy structure favourable for fire propagation. At the same time, the fuel bed is not spatially uniform, but a complex mix of shrubs and herbaceous vegetation that will change with time since fire. These patterns are further influenced by megaherbivores capable of consuming large quantities of biomass that otherwise would enter the fuel bed, but the net effects for temporal thresholds of flammability are poorly known. We quantified post-fire fuel succession and effects of free-ranging cattle in high-elevation Erica shrublands in Ethiopia where traditional fire management is still practised above the treeline at 3500 m, despite being challenged by authorities. We found a near-linear accumulation of canopy fuel at 2·6 Mg ha-1 year-1, but stands <5 years old did not burn due to spatial separation of individual Erica shrubs before canopy closure and lack of fine dead fuels. Inside cattle exclosures, Erica height growth was nearly three times faster and reached the assumed flammability threshold c. 3 years earlier than in browsed/grazed stands, where cattle also kept herbaceous vegetation between shrubs short, thus eliminating litter that could otherwise bridge the discontinuous fuel bed in early succession. Modelling of fire behaviour indicated progressively higher fire intensity and rate of spread for stands >5 years. But if stands escape fire for several decades, flammability again decreases as canopy fuels become increasingly separated from the ground. This may be the ultimate reason for sharp treelines on many tropical mountains where fire is confined mainly to higher elevations. Synthesis and applications. In shrublands where dominant plants can outgrow consumption by large herbivores, provision of good cattle habitat typically requires fire. We found that fire and cattle interact to maintain a relatively stable system, where fuel limitation in early succession creates fire breaks that prevent landscape-wide wildfires. The same negative feedback protects the Erica from degradation by too frequent fires. As shrublands can have widely different compositions, the response to variation in fire frequency and herbivore pressure is likely to differ, but for sustainable management fire and grazing have to be treated in consort. In shrublands where dominant plants can outgrow consumption by large herbivores, provision of good cattle habitat typically requires fire. We found that fire and cattle interact to maintain a relatively stable system, where fuel limitation in early succession creates fire breaks that prevent landscape-wide wildfires. The same negative feedback protects the Erica from degradation by too frequent fires. As shrublands can have widely different compositions, the response to variation in fire frequency and herbivore pressure is likely to differ, but for sustainable management fire and grazing have to be treated in consort.