March 12, 2013
Long-Term Droughts, Rain Failures Revealed Through Tree-Ring Study
April Flowers for redOrbit.com - Your Universe Online
A new tree-ring study, led by the University of Arizona, reveals that long-term droughts in Southwestern North America often mean a failure of both summer and winter rains. This contradicts a long held belief that a dry winter rainy season is usually followed by a wet monsoon season, or vice versa.
"One of the big questions in drought studies is what prompts droughts to go on and on," said lead author Daniel Griffin, a doctoral candidate in the UA School of Geography and Development (SGD). "This gives us some indication that the monsoon and its failure are involved in drought persistence in the Southwest."
Covering most of Arizona, western New Mexico and parts of northern Mexico, the study reveals a 470-year-long history of summer precipitation. The findings of this study were recently published in the journal Geophysical Research Letters.
"This is the first time researchers have used tree rings to take a closer look at the monsoon in a large and important area of the American Southwest," said Griffin, who also is an EPA STAR Research Fellow at the UA Laboratory of Tree-Ring Research.
"Monsoon droughts of the past were more severe and persistent than any of the last 100 years," he said. "These major monsoon droughts coincided with decadal winter droughts."
Griffin points out that these droughts had major environmental and social effects. The late-16th-century megadrought, for example, caused landscape-scale vegetation changes, a 17th-century drought has been implicated in the Pueblo Revolt of 1680 and the 1882-1905 drought killed more than 50 percent of Arizona's cattle.
"The thing that's interesting about these droughts is that we've reconstructed the winter precipitation, but we've never known what the summers were like," Connie A. Woodhouse, UA associate head and associate professor of geography and development, said in a statement.
Prior large-scale, long-term tree-ring reconstructions of the region's precipitation history only focused on the winter rainy season because it has the strongest influence on annual tree growth.
"Now we see — wow — the summers were dry, too," Woodhouse said. "That has a big impact."
"In the Southwest, the winter precipitation is really important for water supply. This is the water that replenishes reservoirs and soil moisture," Woodhouse said. "But the monsoon mediates the demand for water in the summer."
Most tree-ring researchers, or dendrochronologists, have looked at the total width of trees' annual rings to reconstruct past climate until recently, with only a few teasing out the seasonal climate signal laid down in late summer. This signal is recorded in the narrow part of the growth ring, known as latewood (summerwood).
The research team measured latewood from tree-ring samples stored in the archives of the Tree-Ring Research lab, along with new samples taken in the field, to figure out the region's past history of monsoon precipitation. Annual growth rings from two different tree species, Douglas-fir (Pseudotsuga menziesii) and Ponderosa pine (Pinus ponderosa), were examined throughout the weather forecast region North American Monsoon Region 2 (NAM2).
Focusing on NAM2, which covers most of Arizona, western New Mexico and northern parts of the Mexican states of Sonora and Chihuahua, samples from 50 to 100 trees were taken at each of 53 different sites throughout southwestern North America.
Griffin said, "It was a massive undertaking — we employed about 15 undergraduates over a four-year period to measure almost 1 million tree rings."
Rain gauge records from 1950 — 2000 show dry seasons alternating with wet ones, so the results of the tree ring data surprised the team. The new multi-century record, going back to 1539, reveals that the wet/dry pattern of the latter part of the 20th century is not the norm, according to Griffin, not prior to the 20th century or in the present.
The team is considering avenues for continuing the research. One possible next step is to expand the current project to other areas of the Southwest and further into Mexico. The monsoon has a bigger influence on annual precipitation in those areas. Another possible step for the research would be to use tree-ring reconstructions of the Southwest´s fire histories to see how wildfires are related to summer precipitation.
Griffin said, "Before I moved to the Southwest, I didn´t realize how critically important the summer rains are to the ecosystems here. The summer monsoon rains have allowed humans to survive in the Southwest for at least 4,000 years."