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Regional patterns of increasing Swiss needle cast impacts on Douglas-fir growth with warming temperatures.
Lee, EHenry, P. Beedlow, R. Waschmann, D. Tingey, S. Cline, Mike Bollman, C. Wickham, AND C. Carlile. Regional patterns of increasing Swiss needle cast impacts on Douglas-fir growth with warming temperatures. Ecology and Evolution. Wiley-Blackwell Publishing, Hoboken, NJ, 7(24):11167-11196, (2017).
By the end of the 21st century, climate models predict hotter, drier summers and warmer, wetter winters in the Pacific Northwest (PNW), resulting in decreased snowpack, earlier snowmelt, and increased summer water balance deficit. These changes are already affecting sensitive forested ecosystems, raising concerns that forests are becoming increasingly susceptible to tree pathogens, phytophagous insects, and fires. Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirb.) Franco) is a dominant PNW tree species that is infected by the foliar fungus Phaecryptopus gaeumannii (Rhode) Petrak which causes Swiss needle cast (SNC) disease. The fungus is present wherever Douglas-fir is found but very little is known about the geographic extent of SNC impacts. WED scientists modeled Douglas-fir growth response to climate and SNC using tree-ring width data collected at nine mature forest stands in western Oregon, USA. WED scientists were first to show that: 1) SNC impacts occur wherever Douglas-fir is found and are not limited to the coastal fog zone as previously believed; 2) SNC impacts on Douglas-fir growth have been underreported and incorrectly attributed to temperature and water stress regionally; and 3) fungal dynamics are climate mediated and SNC is enhanced by warmer winters and cooler, wetter summers. Our work is important for filling in the gaps of knowledge in understanding the complex interactions of temperature, water, and biotic disturbance agents on conifer forests in the PNW under climate change scenarios. Because the greatest warming due to climate change is predicted to occur in the winter and summer, SNC is expected to intensify in frequency and magnitude at higher elevations and/or higher latitudes along the coast and inland where current winter temperatures are a primary limiting factor to fungal growth. SNC in combination with climate stress are predicted to decrease forest health and condition. This paper contributes to ACE CIVA 2.4.
The fungal pathogen, Phaeocryptopus gaeumannii, occurs wherever Douglas-fir is found but disease damage is believed to be limited in the Pacific Northwest (PNW) to the Oregon Coast Range and is of no concern outside the coastal fog zone . However, knowledge remains limited on the history and spatial distribution of Swiss Needle Cast (SNC) impacts in the PNW. We reconstructed the history of SNC impacts on mature Douglas-fir trees based on tree ringwidth chronologies from the west slope of the Coast Range to the west slope of the Oregon Cascade Mountains. Our findings show that SNC impacts on growth occur wherever Douglas-fir is found in western Oregon and is not limited to the coastal fog zone. The spatiotemporal patterns of growth impact from SNC disease were synchronous across the region, displayed periodicities of 25-30 years, and strongly correlated with winter and summer temperatures and summer precipitation. While winter and summer temperature and summer precipitation influenced pathogen dynamics at all sites, the primary climatic factor of these three limiting factors varied spatially by location, topography, and elevation. SNC impacts were least severe in the first half of the 20th century when climatic conditions during the warm phase of the Pacific Decadal Oscillation (PDO, 1924-1945) were not conducive to pathogen development. At low- to mid-elevations, SNC impacts were most severe in 1984-1986 following several decades of warmer winters and cooler, wetter summers including a high summer precipitation anomaly in 1983. At high elevations on the west slope of the Cascade Mountains, SNC impacts peaked several years later and were the greatest in the 1990s and 2000s, a period of warmer winter temperatures. Climate change is predicted to result in warmer winters and will likely continue to increase SNC severity at higher elevations, north along the coast from northern Oregon to British Columbia, and inland where low winter temperatures currently limit growth of the pathogen. Our findings indicate that SNC may become a significant forest health problem in areas of the PNW beyond the coastal fog zone.