The inexorable rise of average temperatures will batter forests in the Southwest by increasing the number of megafires and plagues like bark beetle outbreaks, according to the alarming results of recent studies.
Climate scientists have documented a rise in average global temperatures of about one degree in the past 50 years — and predict accelerating changes in coming decades. Perhaps even more important, the coldest winter nights globally are about 7 degrees warmer than half a century ago, according to a review of 500 studies by researchers from Dartmouth published recently in the journal Ecological Monographs and reported on the Science Daily Web site. (www.sciencedaily.com).
That warming effect will create complicated effects on North American forests — especially in the Southwest.
The mounting piles of evidence underscore the urgency of thinning unhealthy, badly overgrown forests throughout central Arizona. The U.S. Forest Service has embraced the Four Forest Restoration Initiative (4FRI). However, that effort remains mired in difficulties finding a contractor and getting thinning projects started.
The region may be rapidly running out of time, according to the latest studies.
For instance, the Dartmouth-led study concluded the warming trend helps account for widespread outbreaks of pine beetle epidemics and a major increase in the size of wildfires. On the other hand, the shift has also caused many forests to increase their growth rates.
Climate scientists mostly agree on the observed rise in average temperatures — and the odds that such a warming trend will continue. Studies show that the climate has long
undergone poorly understood warming trends — often lasting for centuries at a time. However, the buildup of heat-trapping gasses in the atmosphere as a result of human activities has contributed to the current warming trend and increased its intensity.
But how will that affect places like Rim Country, which sits on the ecological boundary between different habitat types? For instance, Payson sits right on the boundary between pinyon juniper and oak woodlands and the start of the world’s largest ponderosa pine forest.
A growing body of evidence suggests that in coming decades Rim Country will likely see major changes.
For instance, it looks like just the rise in temperature will drive fundamental changes even if the added energy in the atmosphere brings more rainfall in certain areas.
Rim Country relies on complicated wet and dry cycles driven largely by sea-surface temperatures in the Pacific. One study published in the Journal of Geophysical Research predicted a delay in the summer monsoon rains that deliver about half of Rim Country’s 22 inches of rain a year. Researchers from NASA and other institutions predicted that the peak of the monsoon will shift from July and August to September and October. Such a shift could dramatically increase the region’s fire danger, since the worst fires occur in the months just before the arrival of the monsoons.
But no matter what happens to rainfall patterns, the predicted warming could cause widespread tree death in the Southwest, especially in lower-elevation forested areas like Payson, recent studies say.
For instance, drought-adapted trees like ponderosa pines have in many areas completely failed to come back into areas burned in these warmer conditions, according to a study by researchers from Oregon State University published in the Forest Ecology and Management journal. The researchers noted that ponderosa pine forests have not recovered in openings on hotter, drier south-facing slopes created by fires a century ago, even before the temperature rise and increase in the size of wildfires became evident.
Unfortunately, most studies agree that shifts in the climate will produce more widespread tree death in the Southwest.
For instance, the forests of the Southwest may prove more vulnerable to even modest temperature increases than any forests in the country, according to a study by researchers from the University of California at Santa Cruz published in the Proceedings of the National Academy of Sciences. These forests are especially vulnerable to temperature increases in the spring and summer, the very months in which average temperatures have been rising the fastest, according to the study.
Some 18 percent of the forests in the Southwest have already seen massive tree deaths in just the past 20 years.
The researchers used tree ring data to correlate tree growth and death with past changes in temperature and rainfall. They then used those results to estimate how predicted changes in the climate will affect existing forests.
The findings suggested forests can adapt to changes up to a point, stubbornly hanging on despite changes in temperature and rainfall. However, at a certain point different tree species reach a threshold and change accelerates.
A growing number of studies and projects indicate that the forests of the Southwest may have already nearly reached that threshold, making projects like 4FRI critical.