Regional and annual trends in Douglas-fir foliage retention: Correlations with climatic variables

Swiss needle cast imposes strong geographical patterns in Douglas-fir needle retention throughout the Coast Ranges of Oregon and southwestern Washington. These geographical patterns in foliage retention have been related to the spatial variability in average climatic conditions, with climate presumed a major causal factor in the spread and intensification of the fungus that causes Swiss needle cast. Annual fluctuations in foliage retention have likewise been hypothesized to follow fluctuations in annual climatic conditions. The objective of this analysis was to test a full suite of climatic variables for their ability to predict regional and annual patterns in Douglas-fir foliage retention on 296 permanent sample plots comprising six different Swiss needle cast studies. Foliage retention was estimated annually from 1996 to 2009 and climatic data were generated from the PRISM website through ClimateWNA (Wang et al., 2006). Among the 85 annual, seasonal, and monthly climate variables explored, average foliage retention was predicted most consistently by a temperature-based continentality index, mean annual precipitation, winter temperature, summer temperature, and spring or summer precipitation. The same 85 variables were tested for predicting annual fluctuations in foliage retention, allowing for lagged effects of climatic conditions 1–4 years prior to each year of observation. The annual foliage retention models had climate variables similar to the periodic average foliage retention models, but with a variety of lagged effects. The periodic average foliage retention model suggested that under future climate scenarios foliage retention would increase.

► We model Douglas-fir foliage retention by testing a full suite of climatic variables.
► Continentality index is another useful variable affects foliage retention.
► Annual foliage retention models had similar climate variables, with a variety of lagged effects.
► Model application suggested scenarios foliage retention would increase in the future.