"This is pervasive across a wide variety of forest sites and ages of trees," said Phillip van Mantgem of the US Geological Survey. "At the same time, the rate of re-establishment didn’t change. If trends continue, forests will become sparser and the average tree age will decrease by half."
Previous studies of this type have focused on tropical forests, where both mortality and recruitment (re-establishment) rates have tended to increase over the past few decades.
In this study, the period for tree mortality to double ranged from 17 years in the Pacific Northwest to 29 years in the interior. The trend was visible across elevations of 130 to 3353 m, for different sizes of tree, and for trees from the Pinus, Abies, and Tsuga families.
As a result, the coniferous forests in the region may store less carbon in future, possibly even becoming net carbon sources. According to van Mantgem, they may also become less suitable habitat for species that depend on old-growth forest.
Together with colleagues from the US Geological Survey, USDA Forest Service, University of Washington, Northern Arizona University, Oregon State University, University of Colorado, Boulder, Pennsylvania State University, all in the US, and the University of British Columbia, Canada, van Mantgem examined a number of possible causes for the increased death rate. Having ruled out both air pollution, which is low in the Pacific Northwest where relatively clean air blows off the ocean, and internal forest dynamics, the researchers found that climate change is the most likely culprit.
The increased mortality was correlated with increased temperature, according to Nathan Stephenson of the US Geological Survey. Rising temperatures are causing more precipitation to fall as rain rather than snow, giving a smaller winter snowpack. Combined with the fact that the snow is melting earlier, this is leading to a longer summer drought in the western US and increasing water stress on plants. In addition, warmer temperatures can be beneficial to pathogens and insects.
Stephenson says that if it is warmer temperatures causing the problem and temperatures continue to rise, it’s very likely that the mortality increase will rise, too.
"We suggest the increased tree mortality may be due to temperature-related stress that may increase susceptibility to more abrupt causes such as bark beetle," added Thomas Veblen of the University of Colorado, Boulder, who also highlighted that Colorado has seen a steep increase in droughts since the 1990s.
Veblen believes the notion of avoided carbon release should be on the table in climate negotiations. "The consequence of cutting down old forest is a very large emission of carbon dioxide to the atmosphere," he said. "There is no way you can make up for that, even by planting a large stand of new trees." Veblen reckons you can think of trees like dollars. "Young trees have a high rate of carbon sequestration but not much already stored," he explained. "That’s like having a high cash flow but low capital. Old forests have lots of capital but modest cash flow. If you allow people to cut down those forests you basically lose all that capital."
Jerry Franklin of the University of Washington stresses the importance of having long-term data sets for detecting changes in ecological phenomena as this study did. "It’s difficult to obtain funding to maintain these plots," he said. "It’s not particularly fashionable and many are down to individuals. We need much more investment in this kind of monitoring."
The researchers reported their work in Science.