CAUSES AND AMELIORATION OF NUTRIENT DEFICIENCIES IN CUTOVERS OF CEDAR-HEMLOCK FORESTS IN COASTAL BRITISH-COLUMBIA

Poor growth of conifer regeneration occurred on sites formerly occupie d by old-growth cedar-hemlock (CH) forests in coastal British Columbia , 5-8 years after clearcutting and slashburning. Symptoms included chl orotic foliage and growth check of Sitka spruce, western hemlock, west ern red cedar and amabilis fir, coincident with the expansion of the e ricaceous shrub, salal, on the cutovers. Fertilization trials identifi ed 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 imp roved tree growth rates. Equivalent growth responses were achieved wit h additions of sewage sludge and fish silage. Burning, cultivating, li ming, higher planting densities or herbicide application, were less ef fective in promoting conifer growth. The nutrient deficiencies in coni fers on CH cutovers were the result of two factors: low nutrient avail ability in soil and humus, and competition and interference from salal . Salal immobilized substantial amounts of N in biomass and an in vitr o study suggested it was able to use organic forms of N through its my corrhizal fungi. The mycorrhizae of salal also interfered with those o f hemlock, which further reduced their ability to take up nutrients. H igh 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 la yers of the forest floor in CH forests, and this appeared to result fr om three main factors. First, cedar litter contains less N and more de cay-resistant material than other species, and produces forest floors with low rates of N mineralization. Second, the forest floors in CH fo rests 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.