Douglas B. Mainwaring

Ten-year growth and mortality in young Douglas-fir stands experiencing a range in Swiss needle cast severity

Swiss needle cast, a foliar disease caused by the Ascomycete Phaeocryptopus gaeumannii (T. Rohde) Petr., con- tinues to afflict Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) in north coastal Oregon. Permanent plots were installed in 1998 to assess growth impacts and monitor disease severity. Gross periodic annual increment was measured for three 2- year growth periods and one 4-year growth period and ranged from 0.37 to 31.74 m 3 ·ha -1 ·year -1 . Foliage retention, defined as the average number of annual needle age classes held by a tree, was also estimated as an index of disease severity. As- suming negligible losses in stands with maximum needle retention (approximately 3.9 years), growth losses in net periodic annual increment reached slightly over 50% in stands with the lowest needle retention (approximately 1 year). Mixed-effects regression models supported a consistent relationship between foliage retention and both gross and net periodic annual in- crement among the four growth periods. Periodic annual mortality ranged from 0 to 19.12 m 3 ·ha -1 ·year -1 but was not signif- icantly influenced by Swiss needle cast as measured by average foliage retention. Minimum and maximum foliage retention has fluctuated annually from 1998 to 2008 on the permanent plots, but growth losses at a given level of foliage retention ap- pear to have remained stable. Estimated growth losses are similar to those reported for comparable levels of defoliation by other agents.

Climatic influences on needle cohort survival mediated by Swiss needle cast in coastal Douglas-fir

Swiss needle cast (SNC) severity in Douglas-fir (Pseudotsuga menziesii) has been shown to vary spatially and temporally in response to climatic factors both within its native range and in regions where it has been planted as an exotic species. Survival models were developed for different Douglas-fir needle cohorts to enhance our understanding of how climatic influences on needle longevity are mediated by SNC in the Oregon Coast Range. The climate-based models were based on repeated measurement of 100 plots between 1998 and 2005 coupled with downscaled PRISM climate data. Potential predictors of needle survival by annual cohort were selected from numerous climatic variables at annual, seasonal, and monthly scales. Needle survival probability was positively associated with maximum summer temperature, and negatively associated with minimum winter temperature and spring precipitation. Seasonal climate variables associated with needle longevity are consistent with current epidemiological understanding of Phaeocyrptopus gaeumannii, as well as with previous analyses of climatic influences on SNC severity as measured by average years of foliage retention and frequency of fungal fruiting bodies, or pseudothecia, in stomates.