The effects of drought are most visible at the tops of our mountains.
Fires that destroyed much of the forest atop Mount Lemmon and the bugs that ate the top of Mount Graham were abetted by hot, dry conditions that scientists predict will recur in the coming years, making it difficult for forests to recover.
Catastrophic fire and insect outbreaks claimed up to 18 percent of the high- altitude forests in Arizona and New Mexico in the past 24 years, according to a report published this week by researchers at the University of Arizona and elsewhere.
A century of forest mismanagement is partially to blame, the report says, but a decade-long drought and rising temperatures are probable contributors, and the future looks bleak, the report says.
More than half of the Southwest's high-altitude forests could be gone by mid-century, under a worst-case scenario for continuing drought outlined by other UA researchers in a series of papers published this week in the Proceedings of the National Academy of Sciences. The eight papers sound warnings about the effects of drought on the region's ultimate sustainability.
The report does not predict that half our forests will disappear, said lead researcher Park Williams, a geographer at the University of California-Santa Barbara.
"We should not expect half the forest to be gone by 2050, but we should expect high levels of mortality," he said.
"We suggest it is time to accept that warming is going to continue, that trees in the Southwest are particularly sensitive to the warming and forests are going to change," he said.
Authors of the study, "Forest Responses to Increasing Aridity and Warmth in the Southwestern United States," include Tom Swetnam and Steven W. Leavitt of of the UA's Laboratory of Tree-Ring Research.
"The Southwest is really the microcosm of these multiple factors coming together to create the perfect firestorm - increases in fuels and invasive species and expansion of homebuilding out in wildland areas - on top of that an extraordinary drought and higher temperatures," Swetnam said.
The trees' defensive reactions to drought and high temperatures make them vulnerable to pests, Swetnam said. They close their stomata - pores underneath their leaves - more often, stopping the respiration that transforms carbon dioxide into needed chlorophyll. That makes these "carbon-starved" trees more vulnerable to beetle and other insect attacks and it could lead to massive die-offs even without the pests.
The study analyzed aerial and satellite data gathered by U.S. Forest Service researchers to determine the extent of the forest damage.
The measurements are "coarse" said Swetnam, the director of the tree-ring lab, but they are conservatively estimated. Swetnam believes the numbers are even higher than the report indicates.
They are drawn from U.S. Forest Service satellite surveys of fire damage since 1984 and the same agency's aerial surveys of beetle kill over a decade ending in 2008.
They indicate 7.6 percent, or 7,018 square miles, of tree mortality in all forest and woodland types due to bark beetles, and a 2.7 percent mortality, or 2,479 square miles, from fires. That broader measurement includes lower-elevation woodlands, characterized by piñon and juniper.
When limited to the high-altitude forests, characterized by ponderosa pine and Douglas fir, the percentage figure rises to 18 percent.
The study then analyzed precipitation and tree growth recorded in 1,097 tree-ring records (each representing 10 to 30 trees) in the U.S., concluding that that the Southwest region and the Colorado Rockies are the most likely to experience "widespread decreases in growth" as temperatures rise.
The Southwest region, for this paper, is Arizona, New Mexico and the southernmost portions of Utah and Colorado.
Drought and rising temperatures aren't the only things contributing to fire and insect death in Southwestern forests, the paper says. Grazing, fire suppression and invasive species all lend a hand. But the period of greatest forest loss coincided with the highest temperatures of the past 100 years, and the historical record shows a direct correlation between drought and tree mortality.
The paper calls for forest management that recognizes damaged forests may not be capable of returning to their former structures. Forests may convert to grasslands at lower elevations and change tree types higher up.
"Managing to resist undesired change" may be necessary for "highest-value resources," within or near forests, the paper says, but are "stopgap measures in the face of projections of rapid climate change."
Managers need to recognize that some of the changes of recent years are permanent and work with them, said Swetnam - introducing or allowing fire, for instance, to keep some previously continuous forest in its post-fire patchwork configuration.
Landscape-scale programs in Arizona's forests are good steps toward preparing for future drought, he said. They include the Coronado National Forest's Firescape Initiative and the 4 Forests Restoration Initiative, in the ponderosa pine forests of the Apache-Sitgreaves, Coconino, Kaibab, and Tonto national forests.
Drought will recur, says another of the papers from a team led by Connie Woodhouse of the UA's School of Geography and Development.
The current drought is worse than anything in the 150-year historical record, but it "pales" in relation to others reconstructed from paleontological records of temperature and stream flow, the paper says.
The "worst case" - a two-decade long drought in the mid-12th century - was "more extensive and much more persistent than any modern drought," the report says.
That drought occurred during a period of warm temperatures across much of the globe, but temperatures in the Southwest, though elevated, did not reach the level of the last two decades. With predictions of even warmer temperatures to come, the next drought may have no analog in the past, the study says.
This is not just a problem for the forests. The conifer-studded highlands are the primary watersheds for a rapidly growing region of the country that has yet to come to terms with its future water supply.
Contact reporter Tom Beal at email@example.com or 573-4158