Moto Kichakani:

Fire in the Bush

By Eric LaMalfa
 

Above, plots in the Kenya Long-term Exclosure Experiment (KLEE) at Mpala Research Centre undergo controlled burning to test interactions between fire, herbivores, and grass and tree cover.

After crunching the numbers and following trends for the last four years, I have concluded that Acacia drepanolobium trees live in a world much like a casino. Life is a gamble, and by playing the long game the house wins. When you live in an environment famous for recurring drought, fire, and one of the world’s diverse herbivore communities, you make unexpected allies, invest in yourself, prepare for the worst, and celebrate rare bonanzas.

My interest in the Whistling Thorn Tree (Acacia drepanolobium) began over a decade ago when I was invited to work as an intern for the Pastoral Risk Management program (PARIMA). I traveled to Yabello, Ethiopia near the southern boundary shared with Kenya, where I participated in workshops and meetings with Boran pastoralists, community elders, government officials, and non-government organizations like CARE and Save the Children. The goal of PARIMA was to reintroduce fire to the rangelands for the purpose of reducing tree cover and increasing forage security for the community. This formative life experience sculpted my first intimate perception of natural resource and food security issues in East Africa.

Above left, Eric LaMalfa discusses the traditional use of fire with participants of a PARIMA fire workshop, near Negele, Ethiopia, in 2007. Above right, symbiotic ants (Chrematogaster mimosae) residing in the swollen thorns of Acacia drepanolobium trees provide defense against herbivores.

Fire was once widely used by pastoralists to reduce tree cover and increase grass forage for cattle. During the last century, both human and cattle populations have been increasing while the use of fire and the abundance of wildlife are generally declining. These changes in savanna land use are altering tree cover and have often been associated with loss of ecosystem services, including forage for livestock. Many of the trees that occur in east African savannas are Photo by Victor Kasii.highly adapted to protect themselves against the wildlife that forage on woody tree tissues and fires that damage or kill trees. For instance, long spines and symbiotic ants that defend their host trees discourage browsing animals, and thick bark insulates trees from heat damage during fire. Yet shorter trees remain vulnerable to fire because flames can “top-kill” their upper-most branch tissues. Even after being severely damaged by fire, most trees can resprout from surviving belowground tissues. However, resprouting trees get caught in a “fire trap” cycle: they repeatedly resprout following repeated fires, and are “trapped” as short trees. Trees can escape the cycle when there is a long enough fire-free period for them to grow taller than the flames. 
 

Above left and below right, LaMalfa and his research team measure the growth of saplings within the Kenya Long-term Exclosure Experiment (KLEE). Below left, an electric fence marks a KLEE plot border.

Photo by Victor Kasii. Photo by Victor Kasii.

Tenacity mixed with rare  opportunity led me back to East Africa and the black cotton soils where A. drepanolobium thrives. In 2013, I began my research at Mpala with a group of community ecologists interested in understanding how the combination of wildlife, livestock, and fire affect ecosystem function. To investigate how herbivores influence trees’ ability to escape the fire trap, we used a series of prescribed fires within the Kenya Long-Term Exclosure Experiment (KLEE), to see how post-fire tree size was influenced by different combinations of cattle, meso-wildlife (such as zebras, impala, and gazelles), and megaherbivores (elephants and giraffes).
 
Surprisingly, we found that instead of increasing height of resprouting trees, cattle slightly reduced their height, perhaps by inadvertently trampling the small resprouting trees. But more importantly, fire reduced the abundance of ants that defend resprouting trees against browsing, and this in turn increased trees’ vulnerability to wildlife herbivory. We also found that elephants preferentially browsed the largest resprouting trees after fire - that is, those trees that would have had the greatest potential to escape the fire trap. Our results suggest that, over time, the combination of fire and wildlife – especially elephants – will reduce woody cover.
 
The Boran elders I worked with a decade ago reintroduced fire in order to reduce tree cover and thereby increase forage for cattle. Our research has shown that the reduction in tree cover following fire also depends on which herbivores show up after the flames go out.  In parts of East Africa where wildlife have largely been replaced by livestock, fire may not be as effective as it once was in reducing woody cover. Although fire may still be the most cost-effective tool for managing vegetation across large landscapes, it will be most effective where wildlife, and particularly elephants, have access to browse on resprouting trees. And wildlife can provide a service to pastoralists: by limiting post-fire tree growth, they help to maintain grass productivity for livestock.



Above, an experimental fire sweeps across a burn plot within the KLEE experiment, leaving smoke in its wake.
 
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