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Swiss needle cast (SNC), caused by a fungal pathogen,  Nothophaeocryptopus gaeumannii , is a major forest disease of Douglas-fir ( Pseudotsuga menziesii ) stands of the Pacific Northwest (PNW). There is mounting concern that the current SNC epidemic occurring in Oregon and Washington will continue to increase in severity, frequency and spatial extent with future warming.  Nothophaeocryptopus gaeumannii  occurs wherever its host is found, but very little is known about the history and spatial distribution of SNC and its effects on growth and physiological processes of mature and old-growth forests within the Douglas-fir region of the PNW. Our findings show that stem growth and physiological responses of infected Douglas-fir to climate and SNC were different between sites, growth periods and disease severity based on cellulosic stable carbon and oxygen isotope ratios and ring width data in tree rings. At a coastal Oregon site within the SNC impact zone, variations in stem growth and Δ 13 C were primarily influenced by disproportional reductions in stomatal conductance ( g  s ) and assimilation ( A ) caused by a loss of functioning stomates through early needle abscission and stomatal occlusion by pseudothecia of  N. gaeumannii . At the less severely infected inland sites on the west slopes of Oregon’s Cascade Range, stem growth correlated negatively with δ 18 O and positively with Δ 13 C, indicating that  g  s  decreased in response to high evaporative demand with a concomitant reduction in  A . Current- and previous-years summer vapor pressure deficit was the principal seasonal climatic variable affecting radial stem growth and the dual stable isotope ratios at all sites. Our results indicate that rising temperatures since the mid-1970s has strongly affected Douglas-fir growth in the PNW directly by a physiological response to higher evaporative demand during the annual summer drought and indirectly by a major SNC epidemic that is expanding regionally to higher latitudes and higher elevations.

Physiological responses of Douglas-fir to climate and forest disturbances as detected by cellulosic carbon and oxygen isotope ratios