Smokejumpers, stand down! Savanna trees, such as these pines in Everglades National Park, may promote wildfires to prevent other trees from crowding in, according to a new study by UVM ecological modeler Brian Beckage. (Photo: Brian Beckage)
For a tree, wildfire spells doom. Or does it? A new study led by the University of Vermont's Brian Beckage suggests that, in some savannas, trees have evolved to actually promote fires by, for example, shedding lots of easy-to-burn needles and bark.
And this odd arboreal pyromania may help solve a mystery that ecologists have wrestled with for decades: how do savannas form and persist?
Savannas are grassy areas found around the world, especially common in Africa, but also in the southeastern U.S. They are not strictly grasslands however, but rather are grassy places sprinkled with trees. Often described as a transition zone between grassland and forest, savannas do not have the closed canopy of true forest.
The models that ecologists have used to understand savannas have been hard-pressed to show why these areas don't eventually give way to become full forests.
Beckage's study, which appears in the December 2009 issue of the journal The American Naturalist, developed a mathematical model of savannas that proposes an explanation. "The savanna tree," like the famed long-leaf pine or Florida's slash pine, "would initially invade a grassland," Beckage and his co-authors write, "but it would facilitate frequent fires, preventing the conversion to a forest and concurrently limiting its own density."
The result: widely scattered trees with an open canopy and persistent grassy groundcover. In other words, these fire-aiding savanna trees are self-serving ecosystem engineers.
A new model
Prior to this study, there were two general models of savannas: one views disturbances, like wildfires or hurricanes or droughts, as extrinsic and largely random occurrences in the environment. In these models, a savanna is not an ecosystem in equilibrium; it's a transitional state that occurs by chance when fires, for instance, sweep through. But then how have some savannas persisted for millennia despite changing climate and conditions?
A second basic approach explores how grass and trees in a savanna divvy-up limited available water: the trees plunge down to get the deeper ground water while the grass deftly captures rain at the surface. The result is a landscape split between grass and trees. These so-called partition models have quite a bit of explanatory power in dry places, like many African savannas, but what about wet savannas like in the southeastern U.S.?
Instead, Beckage, associate professor of plant biology, forged a third approach, a kind of disturbance model that developed differential equations to describe the three main components of a savanna: grasses, fire-tolerant savanna trees, and fire-intolerant forest trees.
"We came up with a model where fire is not an extrinsic factor in the environment, but, instead, the vegetation is driving the fire rather than just responding to it. It's a feedback loop," says Beckage, who co-wrote the study with William Platt, a biologist at Louisiana State University, and Louis Gross, director of the National Institute for Mathematical and Biological Synthesis in Tennessee.
"We were able to show that you get these stable savannas, sort of like you get in the older disturbance model," he says, except that in his new model the savanna approaches equilibrium in the same way that a full forest does: the trees promote conditions in the ecosystem that favor their own persistence.
"Savanna trees are contributing to fire probability," he says, "Basically, the trees that are competitively inferior can remove their competitors by increasing the frequency of fire. Regular, low-intensity fires stabilize the system in a savanna state."
Abrupt changes
The evolution of fire-facilitating, not just fire-resisting, traits in trees