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Trees in trouble: massive die-offs predicted with global warming

An experimental study of pinon pines at Biosphere 2 in Arizona shows that an increase in temperature makes the species more susceptible to die-off during drought. When temperatures were increased by 4 degrees Celsius (7.2 degrees Fahrenheit), the piñon pines died 28 percent faster than trees which experienced drought-conditions at current temperatures, according to a new study published in Proceedings of the National Academy of Sciences (PNAS).

The researchers transplanted mature piñon trees pinus edulis (also known as the Colorado Pinyon) to the Biosphere facility. As the researchers simulated drought-conditions on the trees, one set of trees experienced average temperatures while the other set received temperatures 4.3 degrees Celsius higher.

“All drought trees in the warmer treatment died before any of the drought trees in the ambient treatment,” the researchers write. “Our results imply that future warmer temperatures will not only increase background rates of tree mortality, but also result in more frequent widespread vegetation die-off events.”

Relating their findings to historic occurrences of drought across the Southwestern United States, the scientists predict tree die-offs in the region will occur five times more frequently than in the past. However the authors write, “this projection is conservative because it is based on the historical drought record and therefore does not include changes in drought frequency, which is predicted to increase concurrently with warming.”

With the likelihood of more droughts and continued stress from pests, like the bark beetle, the outlook for trees in America’s southwest, and possibly elsewhere, is troubling at best.

More tree die-offs is likely to increase the region’s carbon output as well. When trees die they release their stored carbon back into the atmosphere, at the same time massive declines in trees means less carbon storage. According to the authors, tree die-offs could also lead to losses in wildlife habitat, changes in the water cycle, increased opportunities for alien species, and a loss of ecosystem services.

The scientists suggest that similar research should be done on other tree species. In addition, they write, “the temperature sensitivity we document highlights the need to improve model predictions and could profoundly alter assessments of climate change impacts, which continue to reveal increasingly dangerous risks.”

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