Decomposition and nutrient release in Douglas-fir needle litter in relation to stand development

Abstract

Decomposition rates and changes in the nutrient content of needle litter were examined in an age sequence of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) stands in western Washington. The stands at the initiation of the study were 11, 24, 44, 75, and 97 years old. Nylon litter bags (1 mm mesh) containing needles from the 44-year-old stand were placed in the stands in February 1975. Bags were collected after 3, 6, 12, and 24 months, weighed, and analyzed for N, P, K, Ca, Mg, Mn, and lignin. Decomposition constants (k values) were determined. After 2 years, maximum decomposition rate occurred in the 24-year-old stand, where temperature and moisture conditions were most favorable. Low litter moisture tended to inhibit decomposition in summer. Values of k determined from 1-year weight loss data and percent needle lignin after 2 years were significantly different between stands. Values of k determined from weight loss were greater than those determined from litter fall weight: forest floor weight ratios. Loss of elements from litter bags after 2 years was in the following sequence in all stands, N < Mn < Ca < Mg < P < K. Annual stand net productivity was strongly correlated to N + K loss from litter bags (r = 0.96).