Western hemlock growth response to increasing intensity of Swiss needle cast on Douglas-fir: Changes in the dynamics of mixed-species stands

Abstract

Western hemlock (Tsuga heterophylla (Raf.) Sarg.) is a majorcommercial tree species in western Oregon andWashington andis often associatedwith coast Douglas-fir (Pseudotsuga menziesii (Mirb.) Francovar menziesii) andother species in coniferous forests of the Coast Ranges and Cascade Mountains. Growth of Douglas-fir in many coastal forests has been negatively affected by Swiss needle cast (SNC), a foliar disease caused by the ascomycete Phaeocryptopus gaeumannii (T. Rohde) Petr. and characterized by premature foliage loss on severely infected Douglas-fir trees. The effect of SNCon stand dynamics in mixed Douglas-fir-western hemlock standswas tested by constructing a diameter increment model forwestern hemlock that quantified its growth response to varying SNC severity in Douglas-fir. Diameter increment of western hemlock in any given growth period increased with increasing initial SNC severity as measured by Douglas-fir foliage retention (FR), here defined as the number of annual needle cohorts held by the tree. Furthermore, a decline in Douglas-fir FR during the growth period was associated with an additional increase in diameter increment of western hemlock trees. Assuming no change in FR over the growth period,western hemlock trees in stands with severely impacted Douglas-fir (initial FR≥1.5 years) averaged 79 per cent greater diameter growth than that in relatively healthy stands (initial Douglas-fir FR≥3.5 years). The implied annual diameter growth response of a western hemlock with initial diameter at breast height of 10, 20, 30 or 40 cm was 0.29, 0.52, 0.65 and 0.68 cm year-1, respectively. Compensatory growth by western hemlock in mixed-species stands alters stand dynamics by allowing this species to surpass the growth of Douglas-fir experiencing severe SNC.