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

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. N. 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, Δ13C and δ18O were primarily influenced by disproportional reductions in 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 δ18O and positively with Δ13C, indicating stomatal conductance (gs) decreased in response to high evaporative demand rather than a reduction in A. Current- and previous-years summer vapor pressure deficit (VPD) 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.

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 Nothophaeocryptopus 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 growth and physiological responses to biotic and abiotic factors within the Douglas-fir region. WED scientists modeled cellulosic stable carbon and oxygen isotope responses to climate and SNC using isotope data collected at five mature forest stands in western Oregon, USA. WED scientists were show that: 1) stem growth decreased primarily due to decreasing stomatal conductance in response to high vapor pressure deficit at inland sites; 2) for the coastal site, stem growth decreased primarily due to disproportional reduction in assimilation caused by a loss of functioning stomates through early needle abscission and stomatal occlusion by pseudothecia of N. gaeumannii; and 3) summer vapor pressure deficit was the principal climatic variable affecting stem growth and physiological processes of mature Douglas-fir. 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.

Citation

Lee, E Henry, P. Beedlow, Renee J Brooks, D. Tingey, C. Wickham, AND W. Rugh. Physiological responses of Douglas-fir to climate and forest disturbances as detected by cellulosic carbon and oxygen isotope ratios. Heron Publishing, Victoria, B.C, CANADA, 42(1):5-25, (2022). [DOI: 10.1093/treephys/tpab122]

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