Nutrient Cycling in an Age Sequence of Western Washington Douglas-fir Stands

Abstract

The cycling of nitrogen, phosphorus, calcium, magnesium and potassium in a series of western Washington Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] stands ranging in age from 9 to 95 years has been described. The stands were of relatively low productivity being limited by low nitrogen. The content of nitrogen, phosphorus, magnesium and potassium in tree foliage all tended to stabilize at about 40 years whereas calcium continued to increase. The content of all nutrients in the wood continued to increase with stand age. Nitrogen in the forest floor accumulated constantly at about 5.7 kg ha⁻¹ year⁻¹ and this together with the above-ground tree accumulation meant about 10.5 kg ha⁻¹ year⁻¹ nitrogen was immobilized. Calcium also increased with time in the forest floor with age whereas the other nutrients were fairly constant after about 30 years. Understorey nutrient content reached a peak at about 20 years, while understorey litter-fall was significant throughout the age sequence. Internal redistribution, especially of nitrogen, represented an increasingly greater proportion of stand requirement with increasing stand maturity.