The region's 60,000 square miles (about 155,000 square km) of pinyon-juniper woodlands became a lot less green starting in 2002, the team found. Moreover, the NDVI measurements for the site at LANL showed that the plot's greenness dropped at the same time and in a similar way.
During a previous multi-year drought in the 1950s, not as many trees died. To see how the two droughts differed, the researchers compared the four driest consecutive years of the earlier drought, 1953-1956, with those of the recent drought, 2000-2003.
"By every measure we looked at, the recent drought was hotter," Breshears said, adding that, if anything, the 1950s drought was drier.
The high heat combined with the extreme dryness put the trees under so much water stress that the attacks from bark beetles finished them off. Under such conditions, the trees cannot make enough pine sap to defend themselves against the insects.
"These trees are slow-growing trees, so we aren't going to have woodlands of this type back in this area for decades," Breshears said. He added that the lack of pinyon nuts will have negative effects on wildlife and on people who harvest the nuts for food and for sale.
Rich said, "The fate of the pinyon-juniper forest depends on what happens next, especially in terms of weather. If it's wetter, the trees may come back. If not we'll probably see shifts to species from drier ecosystems."
Having such a wealth of data and a range of expertise was crucial for figuring out what happened, said Cobb, leader of the Drought Impacts on Regional Ecosystems Network (DIREnet). "The NSF DIREnet project allowed us to bring all these researchers together."
Breshears said the team's next step is developing ways to predict how bad a drought must be to cause such large-scale die-offs.
Source:University of Arizona