91av

In the Pacific Northwest, no tree is as important as the Douglas fir (Pseudotsuga menziesii). The conifer is the dominant tree of the region’s rainforests. It was the basis of the traditional timber-based economy and continues to be an important source of timber in plantations around the world. But now, research suggests the iconic tree – famous for its prominence in – may face a future rife with disease and decline, fuelled by a warming climate.

The culprit is Swiss needle cast (SNC), a fungus that grows only on Douglas firs. SNC (Phaeocryptopus gaeumannii) can live on the tree benignly. However, its activity intensifies under certain environmental conditions, causing a yellowing and shedding of needles that slows growth. In severe cases, the tree is killed.

Because of limitations in our understanding of the historical, geographical and environmental pattern of infection, a team of scientists from the US Environmental Protection Agency and Oregon State University came together to reconstruct SNC outbreak conditions. In doing so, they found that the increasingly mild conditions expected due to climate change will drive an increase in SNC disease.

It is happening again

The team took tree core samples from mature Douglas firs in nine different forests in Oregon’s Coast and Cascade ranges, growing at various elevations and in a range of precipitation conditions. The growth rings in each core provided years of information, with sharp reductions in growth rate indicating SNC impacts.

By combining the tree chronologies with information on climatic fluctuations, the team illuminated ties between climatic events and outbreak intensity.

Periods with mild winters and increased summer rain led to some of the most extreme impacts from SNC. Similar climatic conditions are expected to become more frequent as a result of climate change. Traditionally, SNC outbreaks have been more common in the mild, coastal fog zone than in the high Cascade peaks, but warming of these inland elevations will invite major SNC outbreaks.

The owls are not what they seem

The effects of climate change are likely to extend beyond the trees, as Douglas firs are vital to many organisms. One example is the spotted owl (Strix occidentalis), says David Shaw, a forest biologist at Oregon State University and director of the . These threatened birds nest in cavities in bigger mature trees like Douglas firs. Slow growth from SNC may make such nesting opportunities scarce.

The owls also prey on red tree voles (Arborimus longicaudus), a rodent endemic to the region that feeds solely on conifer needles and is similarly threatened. “The red tree vole eats Douglas fir foliage and may be impacted by reduced needle quality and quantity,” says Shaw. “The bottom line is that lots of biota could be negatively influenced by widespread disease and decline of Douglas fir.”

If, as feared, SNC outbreaks worsen and Douglas fir growth declines, Shaw expects broad and fundamental changes to Pacific Northwest forests, including declining carbon fixation and impacts on forest harvests.

Regenerating the forest

The major effect of declining growth will be economic, says Mark Swanson, an ecologist at Washington State University.

“The timber industry certainly should have an interest in these findings, as it pertains to the productivity of a centrally important conifer species,” he says.

Given the increasing risk of outbreaks, it may be wise to actively encourage other species – like western hemlock (Tsuga heterophylla) or Sitka spruce (Picea sitchensis) – for commercial reforestation, Swanson suggests.

“At the very least, foresters should consider regenerating a mixed-species stand in the face of the challenge highlighted by this study.”