Trees in regions where fire is common, such as savannas and the forests of
western North America, tend to have thicker bark, while trees in tropical
rainforests have thinner bark, researchers at Princeton University and
collaborating institutions reported Jan. 9 in the journal Ecology
Letters. Bark protects the inside of the trunk from overheating and is one
of a handful of adaptations that trees use to survive fire.
"We found large-scale evidence that bark thickness is a fire-tolerance trait,
and we showed this is the case not just in a particular biome such as a savanna,
but across different types of forests, across regions and across continents,"
said first author Adam Pellegrini, a NOAA Climate and Global Change Postdoctoral
Fellow at Stanford University who led the study while a graduate student in
Princeton's Department of Ecology and Evolutionary Biology.
The research suggests that the link between bark thickness and fire
resistance should be included in global climate
models, Pellegrini said. "Trees from regions that burn frequently could
still become vulnerable if the risk of fire increases," he said. "The open
question is whether the bark is thick enough to help trees survive."
Pellegrini and his colleagues looked at 572 tree species in regions across the
globe. They compared bark thickness from trees in areas that experience frequent
wildfires—and where rain falls only seasonally—to trees in regions where fires
are rare, such as tropical rainforests. They found that in areas where fires are
frequent, most trees, no matter the species, have thicker bark than closely
related tree species growing in low-fire areas.
The study suggests that tropical rainforests—which are mostly composed of
thin-barked trees—may have a more difficult time recovering from fire, whereas
savannas and seasonal forests with thickly barked trees should be able to better
withstand fire. A savanna was defined as land with continuous grass cover that
is 20 to 80 percent trees, while a forest was defined as having complete tree
coverage and little to no grass.
Periodic fires are necessary for the health of some types of savannas and
forests. Fires burn off excess plant matter such as dead wood and grass—as well
as competing fire-sensitive species—and rejuvenate the soil so that the
dominant, fire-resistant plant species can flourish. However, fires also can be
detrimental to the environment by releasing stored carbon back into the
atmosphere, and causing the decades-long loss of a valuable carbon-storage
The researchers also addressed the question of where thick-barked trees come
from: Did they evolve to have thick bark in response to living in a fire-prone
region, or do thick-barked trees come from plant families with species that all
tended to develop thick bark irrespective of fire activity?
To find out, the researchers compared related tree species that live in
fire-prone areas against those that are found in non-fire-prone regions. The
researchers found that the bark thickness of closely related species is linked
to whether the species lived in a fire-prone or non-fire-prone region, which
provided further evidence that bark thickness is an evolutionary adaptation to
Tim Coulson, a professor of zoology at Oxford University, said that the study
illustrates how climate change could create conditions that already-endangered
ecosystems cannot withstand.
"As periods of drought begin to be seen more frequently in tropical
forests—the lungs of our planet—the risk that these ecosystems will burn
increases," said Coulson, who is familiar with the study but had no role in
"Because the species found there are not well-adapted to cope with fire, the
consequences could be devastating," he said. "[This] work highlights that the
changes we are making to our climate can put ecosystems at risk to factors, such
as fire, that they are poorly equipped to deal with."
The paper, "Convergence of bark investment according to fire and climate structures
ecosystem vulnerability to future change," was published Jan. 11 by Ecology