Mountain Meadows Dwindling in the Pacific Northwest
Some high mountain meadows in the Pacific Northwest are declining rapidly due to climate change as reduced snowpacks, longer growing seasons, and other factors allow trees to invade ecosystems that once were carpeted with grasses, shrubs and wildflowers.
The process appears to have been going on for decades, but was highlighted in one recent analysis of Jefferson Park, a subalpine meadow complex in the central Oregon Cascade Range, in which tree occupation rose from 8 percent in 1950 to 35 percent in 2007. The changes in Jefferson Park are representative of a larger force that is affecting not only this beautiful meadow at the base of Mount Jefferson, scientists say, but many areas of the American West.
"The first awareness of declining meadows dates back to the 1970s, and we’ve seen meadow reduction at both high and low elevations," Zald said. "Between climate change, fire suppression and invasive species, these meadows and all of the plant, animal and insect life that depend on them are being threatened."
The meadow decline takes place over several decades, like the melting of glaciers. This also provides a way to gauge long-term climate change, Zald said, since the forces at work persist through seasonal, annual and longer patterns that are variably more wet, dry, hot or cold than average.
In this study, it appears that snowpack was a bigger factor than temperature in allowing mountain hemlock tree invasion of Jefferson Park, a 333-acre meadow which sits at the northern base of Mount Jefferson, a towering 10,497-foot volcano northwest of Bend, Ore. Seedlings that can be buried by snow many months every year need only a few more weeks or months of growing season to hugely increase their chance of survival.
The study also found surprising variability of tree invasion even within the meadow, based on minor dips, debris flows or bumps in the terrain that caused changes in snowpack and also left some soils wetter or drier in ways that facilitated tree seedling survival.
"The process of tree invasion is usually slow and uneven," Zald said. "But if you get all the conditions just right, some tree species can invade these meadows quite rapidly."