Last summer I received a voice message asking, “What’s happening with the trees on Grant Butte?” I was pretty sure I knew the answer, but I headed out to west Gresham to check out the forest on the butte. The best view was from the neighborhood to the east of the butte, where the dead trees and dying tops were visible among the summer green of the healthier trees. Most of the dead and dying trees were red alders, but the bigleaf maple and black cottonwoods also had dying tops.
Planting native trees at your home
If you are thinking about what type of tree to plant, OSU Extension Service offers good advice on native drought-resistant trees. To take care of yard trees, consider thinning the vegetation to reduce competition for water. Mulching out to the edge of the tree crown helps retain soil moisture, but don’t leave mulch on tree trunks, since that can promote disease. Protect the soil around the trees from compaction. Avoid driving or parking under trees as roots do not grow well in compacted soil. Irrigation is an option. Deep watering twice a month during periods of extended dry weather benefits most species.
The calls and conversations around dead and declining trees continue today. We see declining red alder in the East Buttes forests and along Johnson Creek, dying Douglas fir in valley-floor forests and browning foliage in bigleaf maples at Chehalem Ridge.
The simple reason is summer drought. Yet as with all things in nature, it is more complex than that. Drought is often the triggering event but the tree’s response is complicated by growing conditions, insects, disease and other factors.
In the northern Willamette Valley, some species of trees expanded into new areas when the periodic fires used by Indigenous communities ended with colonization. Douglas fir, red alder, grand fir and other species took root in challenging places such as thin soils or hot, south-facing slopes.
Then the climate started to shift. Although our trees are adapted to a periodic summer drought, our more recent summers have been hotter and drier than typical. Last summer we had the greatest number of days above 90 degrees — 31 in the Portland area. And particularly for trees growing on marginal sites, these hotter droughts are very stressful. Insects and diseases take advantage of drought-weakened trees, contributing to further decline and death.
What does an increase in tree mortality mean for our natural areas? More dead wood habitat for insects, woodpeckers and other wildlife! Dead wood is a part of a functioning forest and, fortunately, the region is not yet seeing the extremely high tree mortality that is occurring in California and other places. Where dead or dying trees threaten a structure or public safety, Metro cuts them and removes them if necessary.
The hotter droughts and subsequent increase in tree decline and death are just one of the impacts of the changing climate. It does have my colleagues and me thinking differently about our restoration and land stewardship. Metro incorporates recommendations based on climate science to promote resilient forests. We use a climate resiliency lens when developing our planting lists, including understanding the specific growing conditions of each site.
For example, in a low-elevation valley floor forest, I might favor ponderosa pine in my planting mix even if Douglas fir is present. Ponderosa is native to the Willamette Valley. More drought tolerant than Douglas fir, it may better tolerate the trend toward hotter temperatures and lower recipitation rates. Thinning also helps trees by reducing the competition for moisture. Over the past decade we’ve thinned many of our forests, reducing the number of trees but keeping the species diverse.
Due to the tough conditions last summer, we are likely to see more tree deaths this year. But on a bright note, our Oregon white oaks, Pacific madrones and ponderosa pines are drought tolerant, and so far are doing okay with the warmer summers.