Thinning Mixed-Species Stands of Douglas-Fir and Western Hemlock in the Presence of Swiss Needle Cast: Guidelines Based on Relative Basal Area Growth of Individual Trees

37 In coastal forests of the Pacific Northwest, young coniferous plantations typically contain a 38 mixture of planted and natural Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga 39 heterophylla). Swiss needle cast (SNC) disease inhibits the growth of Douglas-fir to varying 40 degrees in these stands, depending on SNC severity. In addition to the value differential between 41 Douglas-fir and western hemlock, foresters must account for differences in growth potential (tree 42 size, competitive position, site characteristics, disease pressure) when selecting trees for retention 43 during thinning operations. Diameter increment models for Douglas-fir and western hemlock 44 were developed from permanent plot data collected for the SNC growth impact study (GIS), Pre45 commercial Thinning Study (PCT), Commercial Thinning Study (CT), and Retrospective 46 Commercial Thinning Study (RCT). Predictor variables represent tree size, competitive position, 47 site characteristics, and SNC severity. SNC severity was indexed by foliage retention, defined as 48 the number of annual needle cohorts held by a tree. Foliage retention was positively correlated 49 with Douglas-fir diameter increment and negatively correlated with western hemlock diameter 50 increment. Charts developed from the diameter growth models provide a field tool for assessing 51 the relative basal area growth of adjacent Douglas-fir and western hemlock of a given initial 52 diameter in a stand of given SNC severity. In a stand with severe SNC (foliage retention=1.5 53 years) the basal area growth of a 6-inch western hemlock tree will exceed the basal area growth 54 of any Douglas-fir tree up to 7.7 inches in DBH. In a relatively healthy stand (foliage 55 retention=3.0 years) the basal area growth of 6-inch Douglas-fir and western hemlock trees will 56 be approximately equivalent. 57