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Tree-ring history of Swiss needle cast impact on Douglas-fir growth in western Oregon: Correlations with climatic variables
Citation:
Lee, E., P. Beedlow, R. Waschmann, S. Cline, M. Bollman, C. Wickham, AND N. Testa. Tree-ring history of Swiss needle cast impact on Douglas-fir growth in western Oregon: Correlations with climatic variables. Journal of Plant Science and Phytopathology. Heighten Science Publications Inc. (HSPI), East Windsor, CT, , 76-87, (2021). https://doi.org/10.29328/journal.jpsp.1001065
Impact/Purpose:
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 Nothophaecryptopus 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 history and geographic extent of SNC impacts. PESD scientists modeled Douglas-fir growth response to climate and SNC using tree-ring width data collected at mature forest stands and from ancient Douglas-fir logs (~53K radioactive years before present) in western Oregon, USA. PESD scientists were first to show that the history of SNC impacts on Douglas-fir growth extends as far back in time as ~53K years BP which is the earliest known existence of Douglas-fir in the Pacific Northwest. The ancient Douglas-fir logs represent only the ninth find of ancient wood in the whole world. SNC impacts on Douglas-fir growth are climate mediated, enhanced by warmer winters and cooler, wetter summers, and display sinusoidal patterns which are synchronous across western Oregon. 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.
Description:
The fungal pathogen, Nothohaeocryptopus gaeumannii, occurs wherever Douglas-fir is found but disease damage is believed to be limited to the Coast Range and is of no concern outside the coastal fog zone (Shaw et al., 2011). However, knowledge remains limited on the history and spatial distribution of Swiss Needle Cast (SNC) impacts in the Pacific Northwest (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 high Cascades of Oregon. 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, strongly correlated with winter and summer temperatures and summer precipitation, and matched the patterns of enriched cellulosic stable carbon isotope indicative of physiological stress. 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. In the 20th century, SNC impacts at low- to mid-elevations were least severe during the warm phase of the Pacific Decadal Oscillation (PDO, 1924-1945) and most severe in 1984-1986, following the cool phase of the PDO (1945-1977). At high elevations on the west slope of the Cascade Mountains, SNC impacts were the greatest in the 1990s and 2000s, a period of warmer winter temperatures associated with climate change. Warmer winters 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. Surprisingly, tree-ring records of ancient Douglas-fir logs dated ~53K radioactive years B.P. from Eddyville, OR displayed 7.5- and 20-year periodicities of low growth, similar to those found in modern day coastal Douglas-fir tree-ring records which we interpret as being due to cyclic fluctuations in SNC severity. Our findings indicate that SNC has persisted for as long as its host, and as a result of changing climate, may become a significant forest health problem in areas of the PNW beyond the coastal fog zone.
URLs/Downloads:
DOI: Tree-ring history of Swiss needle cast impact on Douglas-fir growth in western Oregon: Correlations with climatic variables