Thirty-six years of tree population change in an old-growth Pseudotsuga–Tsuga forest

Abstract

Tree populations exhibited considerable individual plant mortality and replacement over a 36-year period in a 500-year-old Pseudotsugamenziesii (Mirb.) Franco var. menziesii (Douglas-fir)–Tsugaheterophylla (Raf.). Sarg. (western hemlock) forest in the Cascade Range of southern Washington, U.S.A. Nearly 22% (113/ha) of the original stems died at an annual rate of 0.75%. This was balanced by recruitment (117/ha) of Tsuga, Abiesamabilis Dougl. ex Forbes (Pacific silver fir), and Taxusbrevifolia Nutt. (Pacific yew) saplings. Diameter distributions and relative species composition were nearly identical at the beginning and end of the 36 years. Compositional changes were slow despite the high turnover; extinction of Pseudotsuga is predicted in 755 years at its current mortality rate. Mortality was generally caused by wind (45.5%) or suppression and unknown causes (39.4%). Additional long-term studies of old-growth forests are needed to understand the direction and rate of successional change.