In a troubling new study just out in Nature Climate Change, a group of researchers says that a warming climate could trigger a “massive” dieoff of coniferous trees, such as junipers and piñon pines, in the U.S. Southwest sometime this century.

The study is based on both global and regional simulations – which show “consistent predictions of widespread mortality,” the paper says – and also an experiment on three large tree plots in New Mexico. The work was led by Nate McDowell of the Los Alamos National Laboratory who conducted the research along with 18 other authors from a diverse group of universities and federal agencies, including the U.S. Geological Survey.

“We have fairly consistent predictions of widespread loss of piñon pine and juniper in the Southwest, sometime around 2050,” said McDowell. The paper concludes that the consequences could be vast, citing “profound impacts on carbon storage, climate forcing, and ecosystem services.”

The study examined both an extreme warming scenario, which recent climate policies suggest we may be able to avert, and also a more modest scenario that would likely bring temperatures above 2 degrees Celsius above pre-industrial levels by the year 2100, but not necessarily by that much. The more extreme scenario was certainly worse for these trees, but even under the moderate scenario, the negative results were merely “delayed by approximately one decade,” the study found.

DRYER, HOTTER CLIMATE

The problem is that climate change is expected to not only increase the risk of drought, but will also drive heat up in general. And this could injure trees in two ways. It could simply dry them out, but also leading to “carbon starvation.” This could occur if, faced with dry conditions, tree leaves close their stomata to keep water in, but therefore cannot bring in more carbon dioxide and thus suffer from reduced or even fully halted photosynthesis.

In the field experiment, conducted over five years, the researchers found that depriving trees of 48 percent of usual rainfall led to 80 percent mortality of piñon pines more than 100 years old and a 25 percent loss for junipers.

The scientists then proceeded to use regional and global models to examine whether they would reproduce this phenomena on a much larger scale in a warming world. The result of regional models was that 72 percent of the U.S. Southwest’s needleleaf evergreen forests would “experience mortality by 2050, with nearly 100 percent mortality of Southwest USA forests by 2100.”

“Taken together, the validated regional predictions and the global simulations predict widespread conifer loss in coming decades under projected global warming,” the research concludes.

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