Vertical Foliage Retention in Douglas-Fir Across Environmental Gradients of the Western Oregon Coast Range Influenced by Swiss Needle Cast

Abstract

We investigated the vertical pattern of foliage retention of Douglas-fir (Pseudotsuga menziesii) in the western Oregon Coast Range where Swiss needle cast, a foliage disease caused by Phaeocryptopus gaeumannii, is causing foliage loss and growth impacts. Swiss needle cast reduced foliage retention more in the upper crown than the lower crown within the epidemic area, which is unusual as foliage diseases usually reduce foliage retention most in the lower crown. We hypothesized that as foliage retention increased across environmental gradients that it would also increase in the upper crown at a greater rate than the lower crown. We randomly selected 72 sites from a population of Douglas-fir plantations in the northwest Oregon Coast Range. We estimated foliage retention from the lower, mid and upper crown of 10 trees per plot. We fitted a two-level hierarchical model with tree and stand level predictors to model changes in foliage retention with changing environmental gradient for foliage in each of three vertical crown positions. We found that the vertical pattern of foliage retention was generally similar throughout the study area with lowest retention in the upper crown, and highest retention in the lower crown. Foliage retention increased with increasing distance from the coast, which is correlated with increased elevation and decreased temperature and site productivity. These findings are consistent with our current understanding of conifer foliage retention. No apparent shift occurs from whole crown to lower crown impacts in our study area as foliage retention increases and approaches normal.