January 29, 2013
Dramatic Forest Death In Amazon Casts Shadow Over Future Of Climate Change
Alan McStravick for redOrbit.com - Your Universe Online
If a tree falls in the forest and no one is around, does it have a direct impact on climate change? A new study from the Lawrence Berkeley National Laboratory (Berkeley Lab) set out to answer this question.
Forests are critical to the equilibrium of Earth´s climate since they absorb large quantities of the greenhouse gas carbon dioxide directly from our atmosphere. The death of a tree, however, reverses the flow of carbon and releases it back into the atmosphere. The Berkeley Lab study sought to quantify tree mortality in the hopes that they might achieve a better understanding of the role forests play in our global climate system. Tropical old-growth forests like those in the Amazon River basin may play a significant part in carbon dioxide absorption, but the mortality patterns for these specific forests are not well understood.
For the study, lead researcher Jeffrey Chambers and his colleagues created a method for detecting forest mortality patterns and trends that combines satellite images, simulation modeling and fieldwork. The research team believes that their new method will be an invaluable resource for broadening our understanding of the role of forests in the process of carbon sequestration and its impact of climate change should there be a disturbance in this delicate balance.
Both NASA and the US Department of Energy´s Office of Science provided funding for this study. Collaborators on the team include Robinson Negron-Juarez of Tulane University along with researchers from Brazil´s National Institute for Amazon Research (INPA). For their study, the team selected a section of the Central Amazon near Manaus, Brazil that spans over a 1,000 square miles.
"One quarter of CO2 emissions are going to terrestrial ecosystems, but the details of those processes and how they will respond to a changing climate are inadequately understood, particularly for tropical forests," explained Chambers. "It's important we get a better understanding of the terrestrial sink because if it weakens, more of our emissions will end up in the atmosphere, increasing the rate of climate warming. To develop a better estimate of the contribution of forests, we need to have a better understanding of forest tree mortality."
The researchers were able to compare data using images collected from the Landsat Program, NASA´s fleet of low-Earth orbiting satellites which has been collecting images for over 20 years. What they determined was that between 9.1 and 16.9 percent of tree mortality has been missing from the more conventional plot-based method for analyzing forests. According to the researchers, this means that as many as a half million dead trees went unaccounted for each year in previous studies. This marked deficit in previous methods is important and must be taken into account in future forest carbon budgets, explains the team.
Chambers and his researchers published their paper online this week in the journal Proceedings of the National Academy of Sciences (PNAS).
"If these results hold for most tropical forests, then it would indicate that because we missed some of the mortality, then the contribution of these forests to the net sink might be less than previous studies have suggested," Chambers said. "An old-growth forest has a mosaic of patches all doing different things. So if you want to understand the average behavior of that system you need to sample at a much larger spatial scale over larger time intervals than was previously appreciated. You don't see this mosaic if you walk through the forest or study only one patch. You really need to look at the forest at the landscape scale."
ACCOUNTING FOR FOREST LOSS THROUGH STORMS
The global carbon cycle — a complex biogeochemical process in which carbon is exchanged between the atmosphere, the ocean, the biosphere and Earth´s crust — relies heavily on the participation of trees and other living organisms. With the elevated tree mortality discovered by the researchers, the depletion of the tree population means not only a weakening of the forests´ ability to absorb carbon, but also an increased release of carbon dioxide back into the atmosphere. The team argues that such a large-scale tree mortality in a tropical ecosystem could function as a positive feedback mechanism and accelerate global warming.
One factor that plays into the elevated tree mortality rates in this region are the fierce thunderstorms that sweep through the area, producing outbursts of violent winds of speeds as high as 170 miles per hour. At times, many acres of forest can be devastated by a single storm. Through the team´s data collection and field research, Chambers was able to provide a much more nuanced explanation of how storms affected the forest.
Satellite imagery taken both before and after the storm allowed the researchers to discern changes in the reflectivity of the forest. This reflectivity, they suspected, was due to damage to the canopy through tree loss. At this point, the field researchers were sent into the storm-ravaged areas to get an actual count of the number of trees that were felled by the storm. Through the combination of the field observations paired with satellite images, the researchers were able to draw a detailed mortality map for the entire landscape.
Chambers explains that the team discovered that tree mortality occurred in clusters, both in time and space. “It´s not [either] blowdown or no blowdown — it´s a gradient, with everything in between. Some areas have 80 percent trees down, some have 15 percent.”
One storm in particular was responsible for an extreme loss of tree life. Back in 2005, the squall line was wider than 150 miles and traveled 1,000 miles inland across the entire Amazon basin. Upon reviewing the satellite imagery, the team was able to estimate that the destruction of trees numbered into the hundreds of millions. The return of carbon dioxide to the atmosphere was equivalent to a significant fraction of the estimated mean annual carbon accumulation for the entire Amazon rainforest.
In addition to violent storm activity, drought is another major contributor to widespread tree mortality in the Amazon basin. Intense droughts for the region were reported in both 2005 and 2010.
It is important to understand the effects of these increasingly common storms and droughts on the forest ecosystems, as climatic warming is increasing not only the strength but also the frequency of these events. “We need to establish a baseline so we can say how these forests functioned before we changed the climate,” Chambers noted.
Though developed for the observation of tree mortality rates in the Amazon River basin region, the new method can also be used to assess tree mortality in other types of forests as well. In a 2007 issue of the journal Science, Chambers and his colleagues published a report detailing how the 2005 super Hurricane was responsible for killing or severely damaging upwards of 320 million trees. The amount of carbon contained in the these felled trees has been estimated as roughly equal to the net amount of carbon absorbed by all U.S. forests in a year.
The team cited not only Hurricane Katrina but also the recent Superstorm Sandy, pointing out the massive impact these weather events have to the terrestrial carbon cycle, forest tree mortality and carbon dioxide emissions from decomposition. Until now, however, these processes have been underrepresented or missed entirely in global climate models.
USING THE FINDINGS TO CURB CLIMATE CHANGE
The researchers believe that the first step towards addressing these issues is to gain an accurate knowledge of the extent of the damage. “A better understanding of tree mortality provides a path forward towards improving coupled earth system models,” says Chambers.
Chambers and his colleagues believe their findings will have a two-fold application. In addition to increasing the understanding of how a forest can affect the carbon cycle, they believe their new technique will also be instrumental in understanding how climate change will affect the forests. For decades, the atmospheric carbon dioxide concentrations have been on a steady upward trajectory. It´s only now we are starting to feel the effects of a warming climate, however. This is evidenced by the stronger and more frequent violent storms, brutal heat waves and the epidemic of glacial melt across the globe.
"But these climate change signals will start popping out of the noise faster and faster as the years go on," Chambers said. "So, what's going to happen to old-growth tropical forests? On one hand they are being fertilized by some unknown extent by the rising CO2 concentration, and on the other hand a warming climate will likely accelerate tree mortality. So which of these processes will win out in the long-term: growth or death? Our study provides the tools to continue to make these critical observations and answer this question as climate change processes fully kick in over the coming years."
Chambers' co-authors on the PNAS paper were Alan Di Vittorio of Berkeley Lab and Robinson Negron-Juarez, Daniel Marra, Joerg Tews, Dar Roberts, Gabriel Ribeiro, Susan Trumbore and Niro Higuchi of other institutions, including INPA, Brazil; Tulane University, USA; Noreca Consulting Inc, Canada; the University of California at Santa Barbara, USA; and the Max Planck Institute for Biogeochemistry, Germany.