Christmas Trees At Risk: New Study Shows Evergreens Could Disappear From US Southwest By 2100

Is your Christmas tree at risk? A new study shows that current rates of global warming and impending droughts could wipe out needle-leaf evergreen trees (NET) within the Southwest United States by the year 2100.

In the American Southwest, that semi-arid region comprising Arizona and parts of New Mexico, California, Colorado, Utah and Texas, 11 national forests capture and store significant amounts of atmospheric carbon. Therefore, researchers believe the loss of broad-scale forest cover across the Southwest could contribute additional carbon to the atmosphere and accelerate climate change, according to a news release.

Recent droughts across the Southwest have caused widespread tree loss, even among drought-resistant species. Water-thrifty conifers such as piñon pine and juniper experienced a high rate of mortality between 2002 and 2003. This, along with similar reports from around the world, prompted researchers to investigate if climate change was to blame.

For their study, researchers from the University of Delaware (UD) monitored trees in a piñon-juniper woodland in New Mexico over a five-year period. To mimic drought conditions, researchers restricted nearly 50 percent of the rainfall from three one-mile square plots and applied their findings to global simulation models. Ultimately, 80 percent of the mature piñon pines in the three plots died, while others suffered from other side effects related to drought stress.

Compared to deciduous trees that shed their broad leaves annually when the weather gets cooler, evergreen trees have needles that they shed periodically so they are never completely bare. When fewer nutrients are available -- such as water -- evergreens generally have an advantage because they have adapted to growing in warmer areas with poor soil.

Based on the results of their field experiments, researchers were interested to see how the trees would respond to drought in the future under predicted global warming scenarios. Using a series of computational models, they found 72 percent of the region's evergreen forests will die by 2050. Furthermore, nearly 100 percent of evergreens in the Southwest U.S. will die out by 2100.

"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," Sara Rauscher, one of the study researchers and an assistant professor of geography in UD's College of Earth, Ocean, and Environment, explained in the university's release.

Despite some variation among the computational models and predicted precipitation patterns, the results were always the same: widespread tree death.

"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 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."

Their study was recently published in the journal Nature Climate Change.

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