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20 January 2016, Mongabay news - Researchers published a paper last month in the journal Nature Climate Change predicting widespread death of needleleaf evergreen trees in the Southwest United States by the year 2100 due to global warming.

Photo by Tamara Beeson.

Trees have already suffered mightily as historic drought conditions exacerbated by climate change have persisted across large portions of the Southwest, causing substantial tree death even among drought-resistant species.

The U.S. Forest Service said last year that as many as 12 million trees had been killed by the drought in California alone.

Subsequent research found that nearly 60 million large trees in the Golden State have suffered severe canopy water loss and are at risk as long as the drought continues — and that the damage was so bad already that they would still be at risk even after a respite if drought conditions were to recur in the future.

There have been other die-offs in recent history, and it was one of these that prompted the present study. Between 2002 and 2003, scientists in Los Alamos, New Mexico, noticed a high rate of death among drought-resistant species like piñon pine and juniper.

What's more, the authors of the Nature Climate Change paper say, there were similar reports from around the world, which made them wonder whether global climate change could be the culprit.

"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 authors of the paper wrote.

The research team that conducted the study performed field tests and studied a range of validated regional predictions and global climate models in order to reach their dire conclusion.

"No matter how we investigated the problem, we got the same result," the University of Delaware's Sara Rauscher, who began working on the study while she was a staff scientist at Los Alamos National Lab, said in a statement. "This consensus gives us confidence in this projection of forest mortality."

A five-year field experiment led by the paper's lead author, Nate McDowell, an ecologist at Los Alamos National Laboratory, mimicked drought conditions in a piñon-juniper woodland in New Mexico by preventing nearly half of the rainfall from reaching the ground in three one-mile square plots. Some 80 percent of the mature piñon pines in the three plots died.

At the same time, the researchers used a number of computational models of varying complexity to determine how trees would respond to future droughts predicted under several different global warming scenarios.

They say that averaging the results of all of these models together suggests that 72 percent of the region's needleleaf evergreen forests will die by 2050, and nearly 100 percent will die by 2100.

"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," Rauscher said. "A treeless Southwest would be a major change not only to the landscape, but to the overall ecosystem."

As grim as that sounds, Rauscher also says that all is not necessarily lost. There's always the chance that some species will adapt to warmer temperatures and different precipitation patterns, or that some areas will be impacted by rising global temperatures less than others and will provide a refuge where vulnerable species can essentially wait it out.

There's also the possibility that humans will get their act together and do something to save the forests of the American Southwest, Rauscher added: "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."

Source: http://news.mongabay.com/2016/01/widespread-death-of-evergreens-predicted-in-american-southwest/?n3wsletter