Report: trees are dying from no obvious cause in the Rocky Mountains

A report issued by the Union of Concern Scientists concluded that in some areas of the Rocky Mountains trees are dying from no obvious cause:

Besides increases in tree-killing insects and wildfires, scientists have found a rise in “background mortality”—the rate at which trees die from no obvious cause. For example, tree mortality in relatively undisturbed old-growth forests across the West has doubled in recent decades, with no compensating increase in the number of tree seedlings. And tree mortality has been highest in recent years.

But, the report goes on to suggest that hotter and drier conditions across the West are driving these changes. There are other factors impacting the health of trees the document says, including, tree-killing insects and more wildfires.

Notable are the effects on three “iconic tree species”, as you can see in this excerpt:


Whitebark pines (Pinus albicaulis)—a high-elevation species with unique ecological importance in the Northern Rockies— have faced both blister rust and epidemic-level infestations of mountain pine beetles, part of the recent West-wide outbreak. Earlier outbreaks of mountain pine beetles at high elevations were shorter and less severe, because winter temperatures were typically cold enough to kill the beetles. However, the sustained higher temperatures of recent winters have allowed the beetles to overwinter and thrive. Today whitebark pines are in catastrophic decline throughout their range in western North America. Mortality in some areas has been 90 percent to 100 percent. This die-off has led the U.S. Fish and Wildlife Service to determine that they are in such risk of extinction that they qualify for listing under the Endangered Species Act.

Quaking aspens (Populus tremuloides), an emblematic species of the Rocky Mountains, have seen abrupt and extensive die-off across large areas of their range, in response to extreme heat and dryness at the beginning of this century. From 2000 to 2010, some 1.3 million acres in the Southern Rockies saw significant aspen decline, and regeneration of new aspens has been much lower than normal.

Piñon pines (Pinus edulis) are a foundation species of the forests that flank the Southern Rockies and many other areas in the Southwest. In 2002–2003, these areas suffered a mass die-off of piñon pines triggered by severe drought and exceptional heat. Sites in Mesa Verde National Park in Colorado, near Los Alamos in northern New Mexico, and near Flagstaff, AZ, lost some 90 percent of their piñon pines. One team of scientists described the mass piñon pine die-off as “one of the most extensively documented examples of a sudden ecosystem crash in response to climate change.”  ”