USA — A huge migration of trees has begun across the West, due to global warming, insect attack, diseases and fire.
In an enormous display of survival of the fittest, the forests of the future are taking a new shape. Many tree species are projected to decline or die out in regions where they have been present for centuries, while others move in and replace them.
In a new report, scientists outline the impact that a changing climate will have on which trees survive where. The studysuggests many species once able to survive and even thrive are losing their competitive footholds, and opportunistic newcomers will eventually push them out.
In some cases, once-common species such as lodgepole pine will be replaced by other trees, perhaps ponderosa pine or Douglas-fir, expanding their range.
Other areas may shift completely out of forest into grass savannah or sagebrush desert. In central California, researchers concluded more than half of the species now present would not be expected to persist in the climate conditions of the future.
“Some of these changes are already happening pretty fast, and in some huge areas,” said lead author Richard Waring, professor emeritus at Oregon State University. “In some cases, the mechanism of change is fire or insect attack. In others it’s simply drought.
“We can’t predict exactly which tree (species) will die or which one will take its place, but we can see the long-term trends and probabilities. The forests of our future are going to look quite different.”
Waring said tree species native to a local area or region are there because they can most effectively compete with other species, given the temperature, precipitation, drought, cold-tolerance and other factors. As those climatic conditions change, species that have been established for centuries or even millennia will lose their competitive edge, he said, and gradually decline or disappear.
The survey, based on remote sensing of large areas over a four-year period, compared 15 coniferous tree species found widely along the west coasts of Canada and the United States. The research explored impacts on 34 different “eco-regions,” ranging from the Columbia Plateau to the Sierra Nevada and the Snake River Plain to the Yukon Highlands.
It projected which species would be at highest risk of disturbance in a future that’s generally expected to warm by 5 to 9 degrees Fahrenheit by 2080, with perhaps somewhat more precipitation in the winterand spring and less during the summer.
Among the findings:
Some of the greatest shifts in tree species are expected to occur at the northern and southern extremes – British Columbia and Alberta in the north and California in the south.
Large declines are expected in lodgepole pine and Engelmann spruce, and more temperate species like Douglas-fir and western hemlock expanding to help fill the void.
Wilderness areas are among those at greatest risk for change. They will probably be the first to experience major species shifts.
Some of the wetter and milder parts of Western Oregon and Washington will face less overall species change than parts featuring a harsher, more limiting climate.
More than half of the evergreen species are experiencing a significant decrease in their competitiveness in six eco-regions. In conjunction with that, conditions have become more favorable for insect and disease outbreaks.
Warming will encourage growth at higher elevations and latitudes, and increased drought at lower elevations and latitudes. Fire frequency will continue to increase across the West, and any tree species lacking drought resistance will face special challenges.
“Ecosystems are always changing at the landscape level, but normally the rate of change is too slow for humans to notice,” said study co-author Steven Running, a professor at the University of Montana. “Now the rate of change is fast enough we can see it.”
“There’s not a lot we can do to really control these changes,” Waring said. “For instance, to keep old trees alive during drought or insect attacks they are no longer able to deal with, you might have to thin the forest, removing up to half the trees. These are very powerful forces at work.”
One of the best approaches to plan for an uncertain future, the researchers said, is to maintain “connective corridors” as much as possible, so trees can naturally migrate to new areas in a changing future and not be stopped by artificial boundaries.
Also collaborating on the research was Nicholas Coops at the University of British Columbia. Their work has been supported by NASA.
The study has been accepted for publication in two professional journals, Ecological Modeling and Remote Sensing of Environment.