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? (See the top 10 green stories of 2008.)
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. (See pictures of activists defending backcountry forests from logging.)
The higher levels of atmospheric CO2 that would likely be seen in a warmer future won't make much of a difference either if the pine needles' pores are closed to prevent water loss, CO2 simply won't get in. Even more worrisome, the PNAS study doesn't take into account possible changes in precipitation patterns in a warmer future, which many climate models say could be drier, exacerbating the impacts of higher temperatures. "We can envision the landscape getting hammered over and over again," says Breshears.
The study took advantage of the university's unique Biosphere 2 research facility. The 7.2 millioncubic-foot dome famous for an experiment in the early 1990s when eight people lived inside it for two years allows scientists to recreate almost any climate on Earth. Adams and his collaborators kept two groups of piñon trees inside Biosphere 2 in nearly identical conditions. One key difference: for the experimental group, researchers ramped up the temperature 7° Fahrenheit (4° Celsius), the rough midpoint of the Intergovernmental Panel on Climate Change's business-as-usual predictions for warming in this century. "We thought temperature might play a big role, but that was speculation until we could conduct an experiment," says Adams. "The great thing about Biosphere 2 is that it allowed us to test this out."
Adams' paper is the latest in a number of recent studies that paint a grim fate for the world's forests if warming isn't slowed. A major Science study published in January found widespread increase in tree mortality rates in the western U.S., thanks in part to regional warming trends and growing water scarcity. Another study published last month, also in Science, found that even the seemingly limitless Amazon rainforest could be highly vulnerable to drought. And since living trees suck up CO2 from the atmosphere, massive tree mortality due to warming could produce a feedback effect, further intensifying climate change. In the end, we might need a bigger Biosphere 2, because we're on track to screw up Biosphere 1 otherwise known as the Earth.