Warmer temperatures play big role in droughty tree die-offs

What does it take to kill a tree?

"That's a little bit of an embarrassing question," acknowledged David Bershears, an ecologist at the University of Arizona, during a chat in his office earlier this year.

Oh, sure, he knows about bulldozers, axes, fires, bugs, and diseases. But when it comes to understanding how plants respond to drought, "I don't think as a community we really know: What does it take to kill a tree?" he concedes.

Now, scientists are a lot closer to an answer. All other things being equal, temperature appears to play the dominant role, at least for piñon pines, according to a new study. It's authors, who include Dr. Breshears, estimate that as global warming tightens its grip in the US Southwest, large-scale loss of trees from drought is likely to occur five times more frequently than they have in the past century. You can find a description of the study here.

Why? The experiment conducted for this study indicates that in an extreme drought, a significant temperature increase -- in this case 4.3 degrees Celsius above current levels -- will kill off a tree far faster than a lack of water under cooler temps. (That temperature increase corresponds to atmospheric concentrations of carbon dioxide at just over twice preindustrial levels. If current emission trends continue, concentrations will overshoot that significantly.) So as temperatures warm, shorter droughts, which over the course of a century tend to occur more often than long ones, have the potential to trigger a level of damage long drought used to inflict.

And the five-fold rise in the frequency of large die-backs understates the effect -- for a couple of reasons.

"If you think about drought frequency, nobody expects it to stay the same as it was during the last 100 years," says Henry Adams during a phone chat. Mr. Adams is a graduate student Dr. Bershears is guiding toward his PhD in ecology and evolutionary biology. And he ran the experiment for the international research team and wrote up the results.

Nor do the results take into account other climate-related threats forests face. Bark beetles, for instance, are ravishing western forests. Global warming is a key driver, bug ecologists say, because the West is experiencing fewer winters with temperatures cold enough to kill off the larvae. More bugs emerge the following year to overpower the trees' natural defenses.

Following the trail of droughts 'n trees

For his part, Breshears traces his own work on the subject to the 1990s.

The challenge, he says, is that for a long time scientists have tried to see how various ecosystems might respond to long-term trends, such as rising CO2 levels or warming temperatures on average.

But organisms adapt to their environment not on the basis of how well they survive averages or gradual trends, but on the basis of how well they survive the climate extremes in their area.

Breshears says his work tends to focus these "threshold events." These don't just rock the ecological canoe. They can capsize it. They can force significant, fast changes in a region's status as habitat for wildlife, it water resources, its ability to act as a storage depot for atmospheric carbon that trees take up, as well as is ability to provide other so-called ecosystem services.

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