The widespread die off of the Southwest's trademark pinyon-juniper woodlands is linked to shrinking water stores in the soil, according to a new study. And the loss of those trees could cause extensive soil erosion and reduce Colorado River water supplies, its authors suggest.

The analysis found that a combination of drought and pine beetle outbreaks has killed more than 2.5 million acres of pinyon pine and juniper trees in the Southwest over the past 15 years. The study was published last week in the journal Ecohydrology by scientists from the College of Forestry at Oregon State University and the Conservation Biology Institute in Oregon.

As the trees die, they leave the soil more vulnerable to erosion, said Wendy Peterman, an OSU doctoral student and soil scientist with the Conservation Biology Institute.

"With the scale at which these trees have died, we're looking at some pretty serious erosion issues," she said.

Some of that eroded soil can be carried by the wind onto mountaintops, where it can blanket snowpacks, causing them to absorb heat from the sun and melt more quickly. That, in turn, affects flows in the already beleaguered Colorado River Basin.

Pinyon pine and juniper trees, which are the dominant tree species in much of the Southwest, can tolerate a year or two of drought, so the mass die off signals a significant change in the ecosystem, Peterman said.

"Pinyon pine and juniper are naturally drought-resistant, so when these tree species die from lack of water, it means something pretty serious is happening," she said.

Pinyon pine and juniper trees are found throughout Arizona, New Mexico, southern Colorado and Utah. In some places, up to 90 percent of the trees have died, many of them during a major drought in 2003 and 2004.

How much water is available in the soil seems to be a major determinant of whether a drought-stressed tree will live: Most of the mortality occurred in shallow soils with less than 4 inches of available water in the top 5 feet of soil, the study found.

"In some soils, water may drain very quickly, whereas in other soils, they may have a greater ability to store water," Peterman said.

More mature trees are the most vulnerable to changes in soil moisture, she added.

The die off could change the landscape of the Southwest. If dry conditions continue -- as climate experts predict -- what are now pinyon-juniper forests eventually could become grasslands, she said. That is a reversal of an expansion trend over the past century, when pinyon pine and juniper trees may have taken advantage of wetter conditions to spread into new areas.

And the die off could turn pinyon-juniper woodlands into a carbon source instead of a carbon sink, since dead trees release carbon dioxide into the atmosphere, Peterman added.

It is not entirely clear whether the mass die off is related to climate change, but it does align with climate change projections. The 2007 report of the Intergovernmental Panel on Climate Change suggested that while most of the United States would become warmer and wetter, the Southwest would turn warmer and drier. And the Southwest is warming faster than any other region in the United States, according to a study issued last month by Climate Central, a nonprofit research and journalism organization (Greenwire, June 12).

The trees, which do not grow very large compared with trees at higher elevations, such as ponderosa pines and Douglas firs, have little commercial value, although they occasionally are used for firewood, and some residents sell pine nuts collected from pinyon pines.

They are vital to the ecosystem, however. The trees keep soil in place, preventing erosion, and provide food, shelter and nesting sites for birds, small mammals and reptiles and other wildlife. They also store carbon in their biomass and in the soils beneath their canopies.

Click here to read the study.

Reese writes from Santa Fe, N.M.