Though not as immediately devastating as wildfires, slowed growth rates nonetheless pose a grave threat to Western Oregon’s commercial timber operations, experts say.
Rising soil temperature and falling soil moisture from prolonged drought are reducing the growth rate of certain tree species by as much as 2% per year, eventually leading to stagnation and death, said Henry Lee, a research statistician at the U.S. Environmental Protection Agency.
“These declines are happening throughout western Oregon. If you think western Oregon is wet and trees are healthy, they’re not,” Lee said at Oregon State University’s Feb. 26-27 “Forest Health in Oregon” seminar in Corvallis.
While 2% may not sound like a lot, the compounding effect over time means that affected trees will stop growing entirely within 50 years — a trend that’s strongly associated with tree mortality, Lee said.
“This is just going to continue,” he said. “This is the path we’re on.”
Hemlock trees at lower elevations are the most profoundly affected by drought, with core samples of their heartwood indicating that growth rates have been slowing since the late 1970s, Lee said.
“Hemlocks are like the canary in the coal mine,” he said.
Declining growth rates have also been detected in true fir species since that time, though they’re somewhat more drought tolerant, Lee said.
“The tree ring records are pretty strongly correlated with soil moisture and soil temperature everywhere,” he said.
Douglas fir trees can withstand dry conditions more readily than hemlocks and true firs, but they’re unlikely to escape the impacts of repeated droughts forever, Lee said.
Growth rates have also slowed at lower elevation stands of this key commercial species since the 1980s, though trees in cooler high elevations aren’t yet experiencing this effect, he said.
In fact, over the short term, shorter winters will likely enhance the growth of high elevation Douglas fir stands as they emerge from dormancy sooner, he said.
Eventually, though, the beneficial impact of a longer growing season is negated by the lack of water as trees deplete soil moisture earlier in the year, Lee said.
A period of prolonged drought seen between 1917 and 1936 was the worst in the region in about 700 years, but the current drought period that began in the 1990s has exceeded that previous episode’s severity, he said. In fact, 2015 marks the worse single-year drought on record.
Aside from slowing growth, drought stress makes trees more susceptible to attacks from pests such as bark beetles and wood-boring insects, said Danny Norlander, forest health survey and monitoring specialist at the Oregon Department of Forestry.
“Drought seems to be persistent on the landscape and that’s concerning for a number of factors,” Norlander said.
A case study conducted by ODF found that trees survived repeated droughts between 2013 and 2016 but didn’t start dying until 2018, which is when the accumulated stress likely caught up to them, he said.
“Trees don’t really respond right away. There’s a lag period from when we have issues from drought, versus see issues from drought,” Norlander said. “We started picking up these impacts throughout the Coast Range.”
Trees along the Oregon Coast aren’t experiencing the same slowed growth that Henry Lee of EPA has found elsewhere, but they face another threat: Swiss needle cast, a fungal disease that causes Douglas firs to lose foliage.
The moist conditions that have shielded coastal forests from declining growth rates are also ideal for Swiss needle cast, which flourishes in the fog zone that stretches nearly 20 miles inland, Lee said.
The disease was historically never so severe in North America that it actually killed the infected host, but that’s changed in the last five years, he said.
Tillamook is already a “hot spot” for Swiss needle cast, which spreads more rapidly under elevated temperatures in the moist coastal fog zone, Lee said.
“The fungus thrives in warmer winters,” he said.
Thinning can help Douglas fir trees survive the disease by giving them more room to grow, which won’t help them retain older foliage but will allow them to develop trunks with larger diameters, said Gabi Ritokova, assistant director of OSU’s Swiss Needle Cast Cooperative.
Trees that have been bred to tolerate the fungus can be planted in areas with moderate levels of the disease, though not heavily infested areas, Ritokova said.
Hemlock and spruce trees, which aren’t affected by the fungus, will eventually overtake infected Douglas firs in mixed stands, which can be planted as a measure of “insurance,” she said. “At some point, you might do better with different species.”