Tree-ring analysis of the fungal disease Swiss needle cast in the Western Oregon coast

Background/Question/Methods

Swiss needle cast disease is specific to Douglas-fir (Pseudotsuga menziesii) and is endemic to the Pacific Northwest.  The disease is caused by the fungus Phaeocryptopus gaeumannii and has been found to occur primarily in sites where mild winters and wet springs and summers favor the pathogen’s growth and reproduction.  The disease has been dated to as early as the 1950s in the western Oregon Coast Range and has significantly lowered the productivity of Douglas-fir forests in the past 30 years.  The goal of this study was to reconstruct the history of the disease and determine the climatic conditions that influence the disease and its impact on radial growth of Douglas-fir in western Oregon.  Tree-ring data from five late-successional Douglas-fir stands in the western Oregon Coast Range were used to quantify the growth suppression patterns associated with the disease and to assess the climate-radial growth relationships.  Correlation and time-series analyses were performed using indexed annual ring width, and mean monthly precipitation and temperature data.

Results/Conclusions

Growth suppression patterns associated with Swiss needle cast disease were consistent across the five sites, indicating that the fungal disease and its impacts on radial growth of Douglas-fir were largely influenced by climatic conditions.  At each site, reductions in radial growth due to the disease increased steadily over time with warming temperatures and displayed an irregular four-year-cycle associated with the maturation cycle of the fungus.  The peak growth suppression years occurred in the final year of each four-year cycle and were significantly correlated with winter temperature and spring and summer precipitation four years in the past and warm temperatures in the following three years.  That is, each four-year cycle of the pathogen began with a year with a warm winter and above average precipitation in May and June, which favored the reproduction of the fungus.  Growth suppression increased consistently within each four-year cycle, due to increasing impairment of stomatal function and early needle abscission in the final year.  The impact of the disease was greater in latewood than in earlywood or annual ring width.  Our results indicate that the disease dates back to as early as the 1590s, which was the earliest record in our dendritic data.