Findings from a study showing that the impacts of extreme drought conditions and high temperatures vary across a tropical rainforest’s landscape have implications for understanding global warming trends.
The findings of Carlos Silva GS and Jim Kellner, assistant professor of ecology and evolutionary biology, were published in the November issue of “Global Change Biology.”
Kellner and Silva, along with collaborators David Clark and Deborah Clark from the University of Missouri-St. Louis, were interested in how the forest responds to climate changes. They were especially curious about “the increasing levels of carbon dioxide in the atmosphere,” Silva said. For this study, they wanted to see how canopy height was affected by the 1997-8 El Nino — an anomalous series of weather conditions involving high temperatures and low precipitation brought about by warm ocean currents.
The study “really emphasizes the benefits of long-term field observation,” said Rosie Fisher, a climate expert and project scientist at the National Center for Atmospheric Research.
“Long-term data is rare and this data is very powerful,” she added. “It’s an example of responsiveness of a natural forest system to a natural drought.”
A field crew from the La Selva biological station forest reserve in Costa Rica spent 12 years, from 1999 to 2010, collecting data by individually measuring trees with poles and laser rangefinders from 18 different plots, Silva said. It was an arduous process for them, he added.
After receiving the data, “we were able to compare observed distribution of height every year to what the equilibrium would be,” Silva said. Equilibrium was defined as the expected heights of the canopy if El Nino had not occurred.
During the first three years of data collection, a higher proportion of trees were at a lower height than expected at equilibrium.
“We’re quite confident that the departures that we observed in 1999 are a legacy of that extreme heat and low rainfall in 1998,” Silva said.
On hill slopes, for example, the number of trees over 15 meters declined by about 10 percent.
“One of the most interesting findings is that the impact of what’s considered a relatively severe drought was transient, at least in terms of its apparent duration.” Kellner said.
Though the canopy height recovered after a few years, this does not mean that the forest recovered in terms of mass, Kellner added.
Kellner and Silva also discovered that impacts from El Nino-type weather were not uniform across landscapes. In low-lying areas where there was access to rain water reserves, trees showed no departure from equilibrium, Silva said.
“Trees die less when they have more water,” Fisher said. “The death was very concentrated in tall trees, the ones exposed to most sunlight.”
Their study focused on the effects of rainfall and temperature changes combined, two factors from which results are difficult to distinguish, Fisher said. She added that it is important to understand different effects of rainfall and temperature because they change independently.
“It was likely that soil moisture, not high heat directly, caused higher tree mortality during El Nino,” Silva wrote in an email to The Herald. He added that forests are more sensitive to drought than heat stress because the differences in elevation at La Selva are not large enough to have significant temperature differences.
In the future, the duo would like to see if their findings are specific to La Selva forest or if they apply to all rainforests.
“We need to be able to characterize how forests respond to drought much more generally and extensively,” Kellner said. “You hope you discovered something that is going to be general, but the more important thing you’re doing is getting an understanding.”
The tropical forest’s response to extreme weather conditions is important on a global scale because it influences atmospheric carbon dioxide concentrations, which can lead to global warming.
“Half of the anthropogenic carbon emissions are absorbed by the ocean and land. For the amount absorbed by land, half of that is in tropical forests,” Silva said.
“Think of forests as being gigantic reservoirs of carbon,” Kellner said.
When a tree dies, the carbon in its wood and leaves is “committed to the atmosphere,” he added.
The study “offers some information about where those carbon losses might be happening,” Silva said. “The question we’re trying to better understand is what fraction of tropical forests worldwide are likely to experience drought impacts and therefore carbon losses to the atmosphere in the future.”
Prior to coming to Brown, Silva and Kellner were at the University of Maryland-College Park — Silva as a graduate student and Kellner as assistant professor of geographical sciences. The duo spent two years compiling their data and preparing their results for publication.
“I got lucky. I was a new student of (Kellner’s) and when I arrived at the university, he said ‘Hey, I’ve got some data, would you want to get your feet wet?’” Silva said.
“The paper would not exist without (Silva’s) work and intellectual contribution,” Kellner said.
Kellner and Silva relocated to Brown this summer to continue their work and studies, respectively.
At Brown, Kellner is now working on obtaining remote sensing equipment. By using drones, or unmanned aerial vehicles, for example, “you can learn things about the structure, composition and physiological status of forest representation,” Kellner said.
The technology can perform multiple functions from the sky, like measuring canopy height or detecting how much nitrogen is in a leaf, Silva said.
Though remote sensing cannot substitute walking around in a forest to collect data, it has greatly improved the researchers’ ability to interpret certain data, Kellner said. He said he hopes to involve undergraduates and graduate students in remote sensing projects and obtain the necessary equipment by this summer.
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