Trees Face Dire Future From Global Warming

"It's not easy to kill a full-grown tree — especially one like the piñon pine. The hardy evergreen is adapted to life in the hot, parched American Southwest, so it takes more than a little dry spell to affect it. In fact, it requires a once-in-a-century event like the extended drought of the 1950s, which scientists now believe led to widespread tree mortality in the Four Corners area of Utah, Colorado, New Mexico and Arizona.

So, when another drought hit the area around 2002, researchers were surprised to see up to 10% of the piñon pines die off, even though that dry spell was much milder than the one before. The difference in 2002 was the five decades of global warming that had transpired since the drought in the 1950s. That led terrestrial ecologists at the University of Arizona (UA) to pose the question, With temperatures set to rise sharply over the coming century if climate change goes unchecked, what impact will it have on the piñon pine?

Unsurprisingly, the outcome doesn't look good. In a new study published April 13 in the Proceedings of the National Academy of Sciences (PNAS), scientists at UA found that water-deprived piñon pines raised in temperatures about 7° Fahrenheit (4° Celsius) above current averages died 28% faster than pines raised in today's climate. It's the first study to isolate the specific impact of temperature on tree mortality during drought — and it indicates that in a warmer world trees are likely to be significantly more vulnerable to the threat of drought than they are today. "This raises some fundamental questions about how climate change is going to affect forests," says David Breshears, a professor at UA's School of Natural Resources and a co-author of the PNAS paper. "The potential for lots of forest die-off is really there."

The PNAS study, led by Henry Adams, a doctoral student at UA's ecology and evolutionary biology department, also confirms that hotter temperatures actually suffocate trees in dry times. Piñon pines respond to drought by closing the pores in their needle-like leaves to stop water loss. That keeps them from going thirsty, but it also prevents them from breathing in the carbon dioxide they need to live — and eventually, the drought-stressed trees simply suffocate."

ORIGINAL SOURCE: TIME