If the temperature rises as predicted by climate scientists, needleleaf evergreen trees in the Southwest United States could suffer widespread premature deaths.
A University of Delaware researcher, Sara Rauscher, and other university colleagues, studied a combination of field results, a range of validated regional predictions and global simulation models of varying complexity. Rauscher has a background in climate science and global climate modeling. Her research focuses on regional processes that affect climate variability.
A combination of scenarios brought the researchers to the grim conclusion that the needleleaf forests were in for a terrible time, and they may disappear altogether by 2100.
The research team was lead by Nate McDowell, an ecologist at Los Alamos National Laboratory.
Sara Rauscher was a staff scientist at Los Alamos National Lab, and is now an assistant professor of geography working at the University of Delaware’s College of Earth, Ocean, and Environment.
“No matter how we investigated the problem, we got the same result. This consensus gives us confidence in this projection of forest mortality,” Rauscher said.
The research team looked at forests in the semi-arid Southwestern USA. The area is enclosed by Arizona, parts of New Mexico, California, Colorado, Utah and Texas, and other states.
The USDA Forest Association says this area contains 11 national forests across more than 20 million acres in Arizona and New Mexico, alone.
Loss Of Forests Exacerbates Problems
If the release of carbon into the atmosphere really is the problem many climate scientists say it is, the loss of these living forests creates a bigger problem. Those trees and the shrubs and bushes that form the understory vegetation, sequester carbon from the atmosphere. When those trees die, they stop capturing carbon, and that creates a negative feedback loop that can accelerate warming.
There have already been substantial tree deaths in this area, due to the recent droughts. Even drought-resistant species were affected.
During the 2002-2003 drought, Los Alamos scientists say they saw a high die-off rate in drought resistant trees such as pinon pine and juniper. The scientists heard similar reports from around the world. This event, reinforced by global reports, prompted their investigation into whether the losses were due to global changes in the climate.
“The rise in juniper mortality likelihood has alarming implications for conifers in general because juniper historically experienced far less mortality than other conifers in droughts,” the researchers said.
Field experiments in a pinon-juniper woodland in New Mexico were studied over a five-year period. Working with three one-mile square plots, they created drought conditions by preventing 50 percent of the natural rainfall from reaching the trees. Over that five-year period, 80 percent of the mature pinon pines in the three plots died. Other trees did not die, but experienced drought stress effects.
The Models
Over the same period, the researchers worked with multiple computational models, in an attempt to predict how the trees would respond to future drought, if the predicted global warming scenarios came true. There were empirical statistical models, and mechanistic models with algorithms that describe how plants function, and complex global models, including some that consider dynamic vegetation.
When researchers averaged the models, a devastating scenario played out, with the needleleaf evergreen forests suffering a 72 percent death rate by 2050, and almost complete destruction by 2100.
The global climate model experiment that uses eight different simulations, all with different sea surface temperatures was designed by Sara Rauscher and Xiaoyan Jiang, a post-doctoral scientist, at Los Alamos National Lab.
Rauscher says “We had to figure out how to make a single model behave in a way that would produce a range of future possibilities of how climate and vegetation will respond. To do this, we used sea surface temperature patterns that other models had predicted, since sea surface temperatures play an important role in shaping how precipitation may change in a warming world.”
The most interesting thing is that although each of the eight simulations produced different precipitation patterns, the climate model findings were always the same: there was widespread tree death.
Adaptation Not Taken Into Account
This unusual study combined observational data with the results of multiple worst-case greenhouse gas emission simulation models.
Surprisingly, the research team’s models did not take into account the possibility of adaptation of the vegetation to warmer, drier conditions. It also did not take into consideration whether trees in some areas would survive due to microclimate conditions, that would allow some species to survive or to ride out changes that might turn around at some time in the future.
The models also did not take into account the possibility of changes in the frequency and severity of wildfires, acceleration of insect populations, loss of topsoil, or failure of seedlings.
“This region of the U.S. has beautiful, old forests with historic trees like Ponderosa Pine that you don’t find in many other places. A treeless Southwest would be a major change not only to the landscape, but to the overall ecosystem,” Rauscher said. “There is always hope that if we reduce carbon emissions, if we continue to address climate change, then perhaps these dire projections won’t come to pass.”
The paper, “Multi-scale predictions of massive conifer mortality due to chronic temperature rise,” was co-authored by researchers at Los Alamos National Laboratory, Columbia University, University of New Mexico, University at Buffalo, INRA-Bordeaux Sciences, Duke University, U.S. Geological Survey, National Center for Atmospheric Research, University of Arizona, and Lawrence Berkeley National Laboratory.
