Regional patterns of increasing Swiss needle cast impacts on Douglas‐fir growth with warming temperatures

Abstract

The fungal pathogen, Phaeocryptopus gaeumannii, causing Swiss needle cast (SNC) occurs wherever Douglas‐fir is found but disease damage is believed to be limited in the U.S. Pacific Northwest (PNW) to the Coast Range of Oregon and Washington (Hansen et al., Plant Disease, 2000, 84, 773; Rosso & Hansen, Phytopathology, 2003, 93, 790; Shaw, et al., Journal of Forestry, 2011, 109, 109). However, knowledge remains limited on the history and spatial distribution of SNC impacts in the PNW. We reconstructed the history of SNC impacts on mature Douglas‐fir trees based on tree‐ring width chronologies from western Oregon. Our findings show that SNC impacts on growth occur wherever Douglas‐fir is found 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 12–40 years, and strongly correlated with winter and summer temperatures and summer precipitation. The primary climatic factor limiting pathogen dynamics 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 (1924–1945) were less 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 Range, SNC impacts peaked several years later and were the greatest in the 1990s, 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.

Environmental variables associated withNothophaeocryptopus gaeumanniipopulation structure and Swiss needle cast severity in Western Oregon and Washington

“…Previous studies have identified significant associations between environmental variables, colonization by N. gaeumannii, and SNC symptoms (Coop & Stone, 2010;Hansen et al, 2000;Lee et al, 2013Lee et al, , 2017Manter et al, 2005;Rosso & Hansen, 2003;Shaw, Woolley, & Kanaskie, 2014;Stone, Coop, et al, 2008;Stone et al, 2007;Watt et al, 2010;Zhao et al, 2012Zhao et al, , 2011 Given that the occlusion of stomata causes the foliage loss associated with SNC, we expected the correlation between AFR and CI to be stronger, as reported in previous studies (Manter et al, 2003;Watt et al, 2010). In our dataset, there was considerable variation in AFR for a given value of CI, suggesting genetic variation in host tolerance (i.e., some host genotypes can tolerate higher levels of infection before needle abscission occurs) and/or that AFR may be affected by environmental factors in combination with SNC severity.…”

“…Included as vectors on the joint plot were SNC severity (AFR and CI), the relative proportion of Lineage 2, latitude, longitude, elevation, and several environmental variables related to SNC severity and N. gaeumannii abundance (Lee et al, 2017;Manter et al, 2005;Rosso & Hansen, 2003;Stone, Coop, et al, 2008;Stone et al, 2007;Watt et al, 2010;Zhao et al, 2011). Included as vectors on the joint plot were SNC severity (AFR and CI), the relative proportion of Lineage 2, latitude, longitude, elevation, and several environmental variables related to SNC severity and N. gaeumannii abundance (Lee et al, 2017;Manter et al, 2005;Rosso & Hansen, 2003;Stone, Coop, et al, 2008;Stone et al, 2007;Watt et al, 2010;Zhao et al, 2011).…”

“…Leaf wetness and free moisture (as precipitation, fog, or dew) during the spring and early summer are necessary for spore dispersal, adhesion, and germination on the needle surface (Capitano, 1999;Manter et al, 2005), and thus are highly influential in predictive spatial models of SNC severity when severity estimates are based on the abundance of N. gaeumannii. Temperatures above 30°C inhibit the growth of N. gaeumannii, and thus, warmer average summer temperatures also are associated with less severe SNC symptoms (Capitano, 1999;Lee et al, 2017;Rosso & Hansen, 2003;Zhao et al, 2011). Predictive models based on combinations of these factors explain much of the variability (R 2 = 57%-78%) in SNC severity in western Oregon and Washington forests (Lee et al, 2017;Manter et al, 2005;Rosso & Hansen, 2003;Stone, Coop, et al, 2008).…”

“…Temperatures above 30°C inhibit the growth of N. gaeumannii, and thus, warmer average summer temperatures also are associated with less severe SNC symptoms (Capitano, 1999;Lee et al, 2017;Rosso & Hansen, 2003;Zhao et al, 2011). Predictive models based on combinations of these factors explain much of the variability (R 2 = 57%-78%) in SNC severity in western Oregon and Washington forests (Lee et al, 2017;Manter et al, 2005;Rosso & Hansen, 2003;Stone, Coop, et al, 2008). A trend of increasing mean winter temperatures and spring precipitation in the Pacific Northwest in recent decades has resulted in conditions conducive to the intensification and expansion of SNC (Abatzoglou, Rupp, & Mote, 2014;Lee et al, 2017;Stone, Coop, et al, 2008).…”

“…Nakash., U. Braun & Crous. However, the loss of vigor associated with SNC may diminish the ability of Douglas-fir to compete with other conifer species (e.g., Tsuga heterophylla), thereby altering the ecology and successional trajectory of Douglas-fir in mixed natural stands (Lee et al, 2017;Stone, Coop, & Manter, 2008;Zhao, Maguire, Mainwaring, & Kanaskie, 2014). Impaired carbon assimilation and reduced photosynthetic capacity due to foliage loss result in height and diameter growth reductions of 20%-50% relative to healthy trees (Maguire, Kanaskie, Voelker, Johnson, & Johnson, 2002;Manter et al, 2003).…”