Southwest forests are already in the early stages of a mega drought brought on by climate change that will result in massive tree die-offs and sweeping changes in Rim Country forests, according to an analysis published in the scientific journal Climate Change.
Severe drought will dominate much of this century, creating stresses on forests not seen for more than 1,000 years, according to the research that used tree ring samples from 13,000 trees, historical rainfall records and computer projections of future climate change.
The shifts will likely dramatically shrink the world’s largest ponderosa pine forest in northern Arizona, replacing pines with junipers at elevations like Payson and replacing junipers with chaparral and cactus at lower elevations.
The potentially rapid shift will come on the wings of bark beetle outbreaks and massive wildfires.
The researchers discovered that the combination of a light winter snowpack and hot, dry conditions in the summer and fall dramatically increase tree death, in both ponderosa and pinyon-juniper forests. Even the occasional wet winter can’t save the forests if they lose too much moisture to evaporation in the summer and fall, the researchers concluded.
Climate predictions suggest that those precise conditions will likely dominate in Arizona’s high country for the balance of this century.
Gila County currently remains in “severe drought,” with less than half its normal rainfall.
The National Oceanic and Atmospheric Administration predicts the drought will persist or intensify throughout the western U.S. through December.
This week, Roosevelt Lake is 46 percent full, Tonto Creek has gone dry before it reaches the lake and the Salt River carries about 70 percent of its normal flow. Last week, the Arizona Game and Fish Department warned that a bloom of golden alga, most likely brought on by warm water and low lake levels, has killed large numbers of fish in the lake, mostly shad on which the food chain depends.
Other studies suggest that the Southwest has already lurched into the era of mega drought and massive changes in the forests predicted in the Nature Climate Change study.
For instance, a study earlier this year, published in the journal Echohydrology, used satellite images to conclude pine beetles and drought have wiped out pinyon pine and juniper stands on some 2.5 million acres in the Colorado River basin. The die-off has affected 90 percent of the trees in many areas and will likely lead to big increases in erosion from wind and rain runoff.
The Nature Climate study used tree ring data going back nearly 1,000 years to document the impact of dry winters and hot summers on growth patterns across a wide area. The researchers from the University of Arizona, the U.S. Geological Survey and Los Alamos National Laboratory then applied the patterns recorded by tree growth and death in past centuries to the rainfall and temperature projections for coming decades.
The research demonstrates that trees throughout the Southwest grow best when they enjoy a wet winter with a deep snow pack and a moist, cool summer and fall due to active monsoons. Those damp summers prevent the heat from drying out plants and soils.
“When air is warmer, it can hold more water vapor, thus increasing the pace at which soil and plants dry out. The air literally sucks the moisture out of the soil and plants,” said A. Park Williams, the study’s lead author.
He noted that such conditions have already become common, which includes this past summer when humidity and temperatures produced some of the driest conditions on record in Rim Country forests.
Williams noted, “Satellite fire data from the past 30 years show that there has been a strong and exponential relationship between the regional tree-ring drought stress record and the area of Southwestern forests killed by wildfire each year. This suggests that if the drought intensifies, we can expect forests to not only grow more slowly, but also to die more quickly.”
University of Arizona Laboratory of Tree-Ring Research Director Thomas Swetnam said, “Among all the climate variables affecting trees that have ever been studied, this new drought index has the strongest correlation with combined tree growth, tree death from drought and insects and area burned by forest fires that I have ever seen.”
The strongest mega drought in the record dates back to the second half of the 1200s and probably played a role in the collapse and abandonment of ancient cultures throughout the Southwest.
Another prolonged, severe drought occurred in the late 1500s. Virtually every tree studied from that period showed years of narrow growth rings, together with a massive die-off across the region. The forests rebounded when normal rainfall resumed in the early 1600s.
“Following the 1500s’ mega drought, tree rings get wider and then there was a major boom in new trees. Nearly all trees we see in the Southwest today were established after the late-1500s drought, even though the species we evaluated can easily live longer than 400 years. If forest drought stress exceeds late 1500 levels, we expect that a lot of trees are going to be dying.”
Unfortunately, the team’s climate prediction models suggest that within the next 40 years the region will fall deep into mega drought conditions. The models predict that even the wettest, coolest years in the late 21st century will exceed mega drought levels. In that case, the drought conditions of the past decade will prove the new normal rather than a bad stretch.
Williams noted that while winters in the past decade haven’t been exceptionally dry, summer temperatures have soared. As a result, the stress on the trees in the past 13 years has exceeded mega drought levels about 30 percent of the time — conditions not matched for the previous 1,000 years.