Climate change is causing Western red cedar and Western hemlock trees to die off on Bainbridge Island. It’s just too hot and dry.

Kirk Hanson, director of Forestry at Northwest Natural Resource Group, said that shade-tolerant and moisture-preferring trees are giving way to drought-tolerant trees from the south. A tree migration is taking place.

In 2012, the US Department of Agriculture changed the hardiness zones chart, which shifted all the zones up and added zones 11-13. With these new planting guidelines, property owners can replace dying trees with drought-tolerant species for future generations.

For those who want to plant trees, Hanson encourages them to research their soil type and plant “the right tree for the right location.” He suggests drought-tolerant species: Douglas fir, Madrone, Bigleaf maple, Oregon oak and Willamette Valley ponderosa pine, as well as Douglas fir and Western red cedar — trees grown from seed sources adapted to warmer winters and drier summers in southwestern Oregon.

There are other things that can be done to help tree growth.

Hanson said there are two pathways to achieve forest resiliency: let nature run its course through different natural disturbances, such as wildfires, disease, pests and wind storms, or use human intervention to thin forests.

The US Forest Service advocates reducing forest density by adopting climate-smart forestry practices and strategic forest management.

“This is where my organization is really trying to pioneer the science of ecologically based forestry by thinning forests in a way that emulates natural disturbance events and produces forest structure that is resilient and resembles natural forest composition,” Hanson said.

Conservation groups are reluctant to cut trees, but thinning is essential to nurture old-growth forests. “We have way more trees per acre than there were historically,” he said.

Hanson uses two key climate adaptation strategies: “thinning to reduce competition for limited soil moisture and replanting with climate-adapted species.”

The ecological thinning process takes decades. The first thinning removes 40% of the stand and targets trees in the understory that are showing signs of decline while retaining the most dominant, healthy and vigorous trees. The stand is left undisturbed for 15 years to allow the forest to firm up.

Additional thinnings continue every 15 years until a 50-90 trees per acre ratio is reached. At that stage, management objectives are reviewed. If the goal is old growth, the forest managers may decide to let natural disturbances such as disease, pests and wind shape the forest going forward. If the objective is timber production, the ratio may be 50 trees per acre, and the next thinning will reduce it to 25 legacy trees.

“These are the trees that will stay indefinitely as the future old growth—whether they’re standing or they end up falling down to be great big old growth logs in the forest,” Hanson said.

Trees are harvested with small-scale, low-impact commercial logging equipment and sold to pay for the thinning.

“From an ecological forestry standpoint, we use active management and commercial timber harvesting practices that emulate natural thinning and stimulate natural regeneration to create these multi-age, multi-cohort mixed canopy, complex forest structures,” Hanson said.

Historically, almost 90% of BI’s old-growth trees were clear-cut, and a lack of replanting laws at that time left trees to regenerate into a “hodgepodge of different species.” Now, these forests are incredibly dense with a wide range of species, he said.

“Not all the species are necessarily suited for every place on Bainbridge Island,” Hanson said. “They’re all competing for limited soil moisture and sunlight. A lot of the mortality that we’re seeing on Bainbridge and across the Northwest is a natural forest developmental process of self-thinning. Climate change is certainly exacerbating that, driving tree species to the furthest extent of their tolerance range.”

Hanson consults with many land trusts in the region, including BI’s Bloedel Reserve and the BI Land Trust, which was worried that climate change is causing the die-off of cedars and hemlocks on BI. After looking at their forests, he said, ”They have way too many trees per acre.”

At Bloedel, Kaslin Daniels, director of Horticulture and Design, said they are working with Hanson on a comprehensive forest management plan.

“The primary objective is to future-proof our seventy-plus acres of forest, ultimately increasing our resiliency to climate change and nurturing a rich spectrum of wildlife and their respective habitats,” Daniels said. “The Reserve’s plan will involve cultivating a diverse tapestry of tree species, varying in ages and sizes, interwoven within a complex framework of fallen logs, towering snags, canopy openings, and a mixture of both hardwood and conifer species that are adapted to future conditions.”

To manage forest density, Hanson usually looks for old-growth forests that average 25-100 trees per acre in the Olympics and the Cascades. But at Bloedel’s naturally regenerated Douglas fir stand, they average 250-300 trees per acre, which is eight to 15 times the carrying capacity for the site.

“The Reserve envisions a forest where the interplay of ecological forestry and heritage landscapes unite, resulting in a dynamic late seral forest ecosystem that thrives for generations to come,” Daniels said.

The record-breaking heat wave in summer of 2021 was especially damaging to forests.

“In parts of the Northwest, trees and forests were damaged by the heat as well. This was most noticeable for forests in the Coast Range, where trees are not adapted to such extreme high of temperatures and were in earlier stages of seasonal development,” the USDA reports on its website. “The heat made conditions worse for trees that were already suffering from the previous two years of drought, and trees displayed signs of scorched leaves. Some trees like western hemlock dropped their needles, leaving the trees bare. Many young saplings and seedlings died in the heat.”

That has made the work of Hanson and others even more important.

“There’s sort of a microclimate species migration occurring,” Hanson said. “I caution people to avoid always pointing fingers at climate change. What we have to keep in mind is that it’s only been about 100 years since we’ve radically changed forest composition in the Northwest since European settlement clear-cutting wiped out 95% of the original old-growth forests.”

Today, 95% of our forests are in a “disturbed condition” and in the process of re-establishing themselves on different soil types. “One of nature’s regenerative strategies is to throw a lot of seed from a lot of different species everywhere,” Hanson said. “Those trees will regenerate and they’ll grow, but it doesn’t mean that they’ll persist.”

Learn more at nnrg.org and find soil data at websoilsurvey.nrcs.usda.gov/.