Another challenge to boosting Tacoma’s tree canopy by 2030 is choosing trees that can survive.
If planting for the future, you need to choose trees that can cope with hotter, drier summers, wetter winters with more extreme storms, less snowpack and smaller water reserves.
“Climate change is a really long process,” said Karen Ripley, an ecologist who heads the state Department of Natural Resources’ Forest Health program. “But we’ve seen weather in the last few years that’s forecast to become normal, decades from now.”
The change that weather could have on tree canopy is obvious if you drive down some local freeways, Ripley said. Trees have died lately, particularly in tough spots such as the median or roadside, killed by lack of water and heat stress.
And because trees are most susceptible to disease or drought in their first years, anything Tacoma plants from now on will need a lot of care — or be tolerant to drought in the first place, especially if city water restrictions come into effect as they did last summer.
Beyond forecasting hotter and drier conditions in the Puget Sound area, climate models predict more of the heavy winter rains and extreme storms that hit last winter and this fall, when rain records were broken throughout December.
Warmer winters will allow exotic species to invade, or expand beyond their present reach, and extreme weather will disturb the soil — ideal conditions for many of those invasives, Ripley said.
In fact, she said, plants already are adjusting to the new conditions: The most recent U.S. Department of Agriculture plant hardiness zoning shows warming climes creeping northward, along with the plants that go with them.
“We need resilient trees,” city planner Mike Carey said. “Trees that are salt-tolerant, pollution-tolerant, wind-tolerant, drought-tolerant.”
So what trees will thrive into the future?
“I don’t really have a good answer,” Carey said. “Not having a lot of data about climate change, we have to go on what we see now.”
Local research scientists have some suggestions.
Connie Harrington, an ecologist with the Pacific Northwest Research Station, has studied tree growth in changing environments for more than 30 years, focusing on climate change effects for the past eight.
In particular, she’s been researching a kind of natural selection in Douglas firs: how those grown in hot, dry areas (such as Southern Oregon) cope better with predicted conditions than varieties grown in cooler places (such as present-day Tacoma.)
Asked about Tacoma’s canopy goal, Harrington suggested planting more native Northwest trees.
“In particular, those with drought resistance,” she said. “But there’s a range in each species as to how drought-tolerant different varieties are — so you need to find seed sources … that are a better match between current conditions and future predictions.”
Carey, meanwhile, is relying on Tacoma-grown trees that have proved to thrive in current conditions and is aiming for as diverse a mix as possible.
Scientists and planners agree that trees will be critical to a city’s ability to cope with climate change.
“When you consider the abilities of trees to shade and cool … to absorb dust and pollution, to … reduce runoff in extreme weather events, it’s a win-win situation for using trees as a way to ameliorate the effects of climate change,” Ripley said.