Loss of summer rains lead to long droughts in southwest U.S.

is that we’ve reconstructed the winter precipitation, but we’ve never known what the summers were like,” said co-author Connie A. Woodhouse, also of the University of Arizona, Tuscon.

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,” 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,” she said. “But the monsoon mediates the demand for water in the summer.”

Until recently, most tree-ring researchers, known as dendrochronologists, 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.

To figure out the region’s past history of monsoon precipitation, the scientists needed to measure latewood from tree-ring samples stored in the archives of the University of Arizona Laboratory of Tree-Ring Research and go into the field to take additional samples of tree rings.

The team looked at annual growth rings from two different species, Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) throughout the weather forecast region called North American Monsoon Region 2, or NAM2.

In all, the researchers used samples from 50 to 100 trees at each of 53 different sites throughout southwestern North America. The team’s climate analyses focused on NAM2, which covers most of Arizona, western New Mexico and northern parts of the Mexican states of Sonora and Chihuahua.

Griffin said, “It was a massive undertaking — we employed about 15 undergraduates over a four-year period to measure almost 1 million tree rings.”

One possible next step, Woodhouse said, is to expand the current project to other areas of the Southwest and into Mexico, where the monsoon has a bigger influence on annual precipitation.

Another would be using tree-ring reconstructions of the Southwest’s fire histories to see how wildfires are related to summer precipitation.

“Before I moved to the Southwest, I didn’t realize how critically important the summer rains are to the ecosystems here,” Griffin said. “The summer monsoon rains have allowed humans to survive in the Southwest for at least 4,000 years.”

The National Science Foundation (NSF), the National Oceanic and Atmospheric Administration (NOAA), and the U.S. Environmental Protection Agency (EPA)supported the research.

— Read more in Daniel Griffin et al., “North American monsoon precipitation reconstructed from tree-ring latewood,” Geophysical Research Letters (11 MAR 2013) (DOI: 10.1002/grl.50184)