Management of Nutrition in Forests under Stress

During the 1970s and 1980s, Sitka spruce (Picea sitchensis, Bong. Carr. ) was planted extensively on northern Vancouver Island (B. C. , Canada) to regenerate slashburned clearcuts previously occupied by old-growth Thuja p1icata, Donn, Tsuga heterophy11a, Raf. Sarg. , and an ericaceous unders tory shrub salal (Gaul ther ia sha11on, Pursh [CH sites 1 ) . The planted Sitka spruce grew well initially on these sites, but experienced nutritional stress and reduced growth 8 to 14 yr after planting (Germain, 1985; Weetman et a1. , 1990a,b). Accompanying the onset of the nutritional stress was the reestablishment of a complete ground cover of salal, and it has been suggested that there is a causal connection between these two temporally synchronous events (Weetman et al. , 1990a,b). Other ericaceous species have been implicated in nutritional stress in conifer plantations (Mallik, 1987; Robinson, 1972; Handley, 1963; Rose et a1. , 1983).
Three hypotheses to explain this nutritional stress were tested in the study reported in this paper: (1) that salal competition for N can provide an adequate explanation for the observed nutritional stress; (2) that salal inhibits the availability of nutrients to seedlings by interfering with their mycorrhizae; and (3) that the fertility of these CH sites declines after 8 yr following clearcutting and slashburning due to the termination of the flush of nutrients (or “assart effect”) associated with this disturbance. A series of pot and field experiments was carried out to test these hypotheses. 1 2.