Swiss needle cast, a foliage disease specific to Douglas-fir resulting in needle loss and reduced growth, is perhaps the most well-studied foliage disease globally. Knowledge about this disease, caused by the native pathogenic fungus Nothophaeocryptopus gaeumannii, is used to study other foliage diseases worldwide.

This is due to the influence of the Swiss Needle Cast Cooperative at Oregon State University. “Since 1996, the cooperative has been focused on research and management of Douglas-fir in zones where Swiss needle cast occurs,” says professor and forest health specialist Dave Shaw, the current director of the cooperative.

Now, a quarter of a century later, the cooperative has published an article in the Journal of Forestry, summarizing their current understanding of Swiss needle cast based on twenty-five years of research.

Swiss needle cast, considered one of the top threats to Douglas-fir plantation health and productivity in western Oregon, Washington and SW British Columbia, was first identified on Douglas-fir growing in Switzerland in the early 20th century. Forest pathologists in North America found the fungus was common in native Douglas-fir stands but was not causing problems.

The disease emerged in Christmas tree plantations in Washington and Oregon in the 1970s, and by the 1990s, it had intensified in coastal Oregon and Washington Douglas-fir plantations. In January 1997, in response to the disease epidemic, the Swiss Needle Cast Cooperative was formed by private forest landowners, federal and state agencies, and the Oregon State University College of Forestry to conduct research and address management practices.

“The more we learn, the better we will be able to sustain the productivity of our forests,” Shaw says. “However, this is a native disease that has a role here in the PNW, and we have some ability to manage forests effectively in the presence of the disease.”

Swiss needle cast symptoms include chlorotic, yellowish foliage, low needle retention, thin crowns and reduced tree growth. The fungus occurs wherever its only host, Douglas-fir, is grown. Disease, however, is only expressed when the fungus causes significant defoliation of two and three-year-old needles.

This, says Shaw, is an essential point for managers. The fungus may be present and yet have no effect on Douglas-fir productivity.

The fungus lives inside the needles of Douglas-fir. It only impacts needle function when fungal fruiting bodies called pseudothecia emerge into and plug the stomates, or air pores, on the underside of the needle, blocking gas exchange. When too many stomates on a needle get plugged, the needle dies and is cast, or dropped, from the branchlet.

Even after twenty-five years of research, Shaw says the science can still be surprising.

“We recently found that Swiss needle cast is distinctly a young tree disease, and older stands, except in extreme examples, are not as affected. There is something unique about young stands that makes the disease more prevalent,” Shaw says. This work was based on collaborations between graduate student Sky Lan, Shaw, and scientists from the Environmental Protection Agency who climbed mature and old-growth trees over two hundred feet tall to get samples and measure microclimate.

While Shaw and others do not yet know why this disease affects younger trees more often, they hypothesize the difference may be due to temperature dynamics within the canopy.

According to Shaw and Gabriela Ritokova, the associate director of the cooperative and a forest pathologist with the Oregon Department of Forestry, the management of Swiss needle cast is nuanced and site-specific. A “Guide to the Silviculture of Swiss Needle” is available on the Swiss Needle Cast Cooperative website.

Shaw explains that future challenges include understanding how climate change will influence the disease.

“Epidemiology-wise, winter temperature, and late spring-summer leaf wetness control disease,” Shaw says. “As we continue to increase winter warming, we may see the disease spread from the core areas now impacted to higher elevations and the western Cascades.“

A version of this story appeared in the Spring 2022 issue of Focus on Forestry, the alumni magazine of the Oregon State University College of Forestry.

The amount of carbon stored in soils is about three times that in living plants and double that in the atmosphere.

But, we often overlook soils’ powerful ability to store carbon.

“Concern about rising atmospheric carbon dioxide concentrations has heightened interest in the role that forests play in carbon sequestration, storage and cycling,” says Jeff Hatten, associate professor of soil science and head of the forest engineering, resources and management department at the Oregon State University College of Forestry. “Living trees sequester and store carbon, but we give less recognition to soils’ role.”

But what happens to the soil’s carbon levels after forest harvests?

According to published research by OSU and Weyerhaeuser Company, conventional timber harvesting has no effect on carbon levels in the mineral soils of the western Pacific Northwest for at least three-and-a-half years after harvest.

Historic in its scope, a collaborative and long-term effort between Hatten and Scott Holub of Weyerhaeuser monitored nine managed Douglas-fir forest stands in Oregon and Washington before and after traditional timber harvest and replanting, analyzing more than 50,000 soil samples from 2700 sample points.

“Our original hypothesis that timber harvesting would decrease soil carbon in the short term was disproven,” Hatten says. “Even where you have the highest soil temperatures and the highest soil moistures – the strongest environment for decomposition that releases carbon dioxide into the atmosphere – harvesting doesn’t seem to have an impact in the areas we studied.”

Across all the sites combined, after harvest, the scientists found little change (+2%) in mineral soil carbon content and a 184% hike in forest floor carbon, the result of harvest residue.

“We should not overlook the importance of the 184% increase in forest floor carbon,” says Katherine McCool, a master’s student in Hatten’s lab who is researching forest soil and watershed processes. “We can expect this increase because the branches, needles and bark that fall to the ground as harvest residue, in addition to the dead roots and stumps from harvested trees, will provide much of the future carbon that will infiltrate into the soil.”

The study, the most extensive sampling ever conducted to determine if harvesting has an impact on soil carbon, is essential because of soil’s ability to store carbon to mitigate and prevent increased greenhouse
gases and maintain a stable carbon balance in the soil for sustainable forest management.

“Soil carbon also is useful in establishing a suitable microbiome for plant and fungal growth,” says McCool. “Without ample soil carbon, new seedlings in post-disturbance areas will not be able to grow.”

Hatten had plans to resample these research sites in coming years and decades to look at the longer-term impacts. Then the 2020 Holiday Farm fire occurred, tearing through some of the research sites near Eugene, Oregon, and dramatically altering the forest landscape. The research continues, but now Hatten and McCool are also studying the role and effect of fire on soil carbon dynamics, examining one of the nine research sites which burned and assessing how soil changes after a severe wildfire in a harvested stand.

“The existence of this preestablished soil carbon site provides an excellent opportunity to study the difference between pre- and post-fire soil carbon, which is often hard to come by due to the spontaneity of wildfire,” says McCool.

Hatten and McCool found a 97% decrease in forest floor carbon within the research site, which equates to 14-times less forest floor carbon than in the pre-harvest stand. The fire’s impact on the mineral soil carbon is still undetermined, but given that 90% of the soil’s carbon lies below the forest floor, the results will have big ramifications for the total carbon budget of the site.

“These initial results show that disturbances like fire can reduce carbon sequestration progress,” says McCool. “Therefore, management activities need to include a focus on fire-safe landscapes if we want to prioritize carbon containment.”

A version of this story appeared in the Spring 2022 issue of Focus on Forestry, the alumni magazine of the Oregon State University College of Forestry.