You don’t need human beings to produce devastating droughts in the Southwest.
That’s the message that has emerged from a tree-ring study stretching back 1,400 years, presented at the American Association for the Advancement of Science.
The findings don’t invalidate the climate projections that predict more and longer droughts caused by the gradual warming of the planet due to the buildup of heat-trapping pollutants in the atmosphere. But they do indicate that the climate is surprisingly unstable even without those heat-trapping gases, for reasons that remain unclear.
Worse yet, recent studies also show that such extended periods with rainfall less than half of normal can have long-term effects on forests — and may account for the dismaying, but poorly understood, decline in aspen groves throughout the West.
The research on past dry periods provided a long-suspected, but previously tenuous link between “mega droughts” like the one that now grips the Southwest and wrenching social changes going back 1,000 years.
The researchers from Columbia University and the University of Arkansas laboriously matched the growth patterns preserved in the width of tree rings with measurements of rainfall dating back to 1895. They
established a strong link between the rainfall measurements and the growth ring patterns so they could then extend their reconstructions back to A.D. 800, even without rainfall data.
The researchers looked at the extended drought that caused the Dust Bowl of the 1930s and discovered it had left its mark in tree ring growth patterns all across the continent.
The researchers then hunted up other historical records to test against the growth rings taken from the drilled cores of thousands of trees, including beams used in ancient cliff dwellings.
They found the tree rings validated records left by Zebulon Pike in 1806 and Stephen Long in 1820 describing extremely dry conditions on the Great Plains and in the Rocky Mountains.
Going back further, the researchers discovered that the Great Pueblo Drought of 1276-1297 coincided with the abandonment of the Colorado Plateau by the ancient Pueblo Indians. The study showed this drought affected the entire Southwest. It may have therefore played a role in the destabilization of the interdependent civilizations of the Mogollon who lived in Rim Country, the Salado who lived along the Salt River, the Hohokam who lived in the Valley and the Sinagua who lived in Sedona and Flagstaff.
The study also documented mega droughts in the 8th and 16th centuries that connect to patterns of migration and abandonment.
As some researchers have documented the frequency and severity of droughts, other researchers have gained a greater understanding of the long-term effects of such hot, dry spells.
For instance, a drought that afflicted Colorado from 2000 to 2003 may account for a sharp decline in aspen forests, according to researchers at Carnegie Department of Global Ecology published in the online journal Global Change Biology.
Researchers have long noted that trees die not only during droughts, but after normal rainfall resumed. But they don’t know precisely why. Some speculated that the trees were starved for nutrients when the soil grew too dry for the roots to suck up and deliver to the leaves. Some suspected the trees’ system for pumping water from roots to leaves may have been damaged by the great thirst.
The Carnegie study supported that second theory, after permanent damage to the water transport systems of trees that survived.
The researchers reached their conclusion by carefully examining the oxygen isotopes they found in the sap, which flows through the tree’s veins and carries water and nutrients to the leaves. Depending on their history, the oxygen atoms in the water molecules have different isotopes — or ratios of neutrons to protons. The isotopes can reveal the difference between summer rains and winter rains — and between dry years and wet years.
The analysis found the aspen rely mostly on a wide, shallow, interlocking root system that sucks up water quickly from the surface, rather than going deep for groundwater. This makes the aspen very vulnerable to dry periods.
The researchers found dry summers damaged the water transport system in the trees, reducing its efficiency by a stunning 70 percent — even when rainfall returned to normal.
The findings could help account for the widespread decline of aspen in the West, although the change in the fire patterns likely also played a role.
The findings also raise the worrisome possibility that an increase in the number and length of droughts will lead to permanent changes in forest compositions, even if periods of normal rainfall intervene.
As it happens, Payson sits right on the edge of several major habitat types. Within a few miles of Payson lie the upper limits of the high desert chaparral, the heart of the pinyon pine zone and the lower reaches of the ponderosa pine forests — not to mention the increasingly rare, intact cottonwood willow habitats along streams like the East Verde River.
The latest research suggests the increase in droughts like the one currently gripping most of the Southwest could force long-term changes on all of those forests.