Causes and amelioration of nutrient deficiencies in cutovers of cedar-hemlock forests in coastal British Columbia

Abstract

Poor growth of conifer regeneration occurred on sites formerly occupied by old-growth cedar-hemlock (CH) forests in coastal British Columbia, 5–8 years after clearcutting and slashburning. Symptoms included chlorotic foliage and growth check of Sitka spruce, western hemlock, western red cedar and amabilis fir, coincident with the expansion of the ericaceous shrub, salal, on the cutovers. Fertilization trials identified N and P deficiencies as the cause of the growth check of conifers, and additions of 300 kg N ha−1 and 100 kg P ha−1 significantly improved tree growth rates. Equivalent growth responses were achieved with additions of sewage sludge and fish silage. Burning, cultivating, liming, higher planting densities or herbicide application, were less effective in promoting conifer growth. The nutrient deficiencies in conifers on CH cutovers were the result of two factors: low nutrient availability in soil and humus, and competition and interference from salal. Salal immobilized substantial amounts of N in biomass and an in vitro study suggested it was able to use organic forms of N through its mycorrhizal fungi. The mycorrhizae of salal also interfered with those of hemlock, which further reduced their ability to take up nutrients. High concentrations of phenolic acids were associated with salal, which interfere with mineralization and uptake of N. The low availability of N and P in CH cutovers originated in forest floors of the old-growth forests prior to clearcutting. Nutrient availability was low in all layers of the forest floor in CH forests, and this appeared to result from three main factors. First, cedar litter contains less N and more decay-resistant material than other species, and produces forest floors with low rates of N mineralization. Second, the forest floors in CH forests are wetter and have less soil fauna than in HA forests, leading to incomplete decomposition and mineralization of N. Third, the salal understorey in CH forests interferes with mineralization of N through the production of tannins. Key words: growth check, fertilization, herbicide, ericaceous shrub, sewage sludge, mycorrhizae, allelopathy, nitrogen availability, nutrient cycling, NMR analysis