New technique focuses on late-summer tree ring growth to pinpoint seasonal climate patterns
By Summit Voice
SUMMIT COUNTY — A detailed new study of tree rings in the Southwest shows that patterns of alternating wet and dry seasons observed since the 1950s is not the norm. Rather, the analysis suggests that, during historic decadal droughts, both winter and summer precipitation was below the long-term average.
The new 470-year-long history (1539 to 2008) of summer precipitation in the Southwest covers most of Arizona, western New Mexico and parts of northern Mexico. It was established by looking at rings from Douglas firs and ponderosa pines, including tree-ring samples stored in the archives of the University of Arizona Laboratory of Tree-Ring Research.
Until recently, most tree-ring researcher have looked at the total width of trees’ annual rings to reconstruct past climate. Few teased out the seasonal climate signal recorded in the narrow part of the growth ring laid down in late summer known as latewood.
“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. “This gives us some indication that the monsoon and its failure is involved in drought persistence in the Southwest … It was a massive undertaking … we employed about 15 undergraduates over a four-year period to measure almost 1 million tree rings.”
“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.”
Those droughts had major environmental and social effects, Griffin said, pointing out that the late-16th-century megadrought 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.
Co-author Connie A. Woodhouse, UA associate head and associate professor of geography and development, said, “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.”
Because winter precipitation has the strongest influence on annual tree growth, previous large-scale, long-term tree-ring reconstructions of the region’s precipitation history had focused only on the winter rainy season.
“Now we see – wow – the summers were dry, too,” she 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.”
The report was scheduled for publication March 11 in Geophysical Research Letters, a journal of the American Geophysical Union. Additional UA co-authors are David M. Meko, Holly L. Faulstich, Carlos Carrillo, Ramzi Touchan, Christopher L. Castro and Steven W. Leavitt. Co-author David W. Stahle is from the University of Arkansas in Fayetteville.
The National Science Foundation, the National Oceanic and Atmospheric Administration and the U.S. Environmental Protection Agency supported the research.