long-term decomposition of Sitka spruce needles in brash

• Published in: Forestry (London) 1999-01, Vol.72 (3), p.207-221
• Main Authors: Titus, B.D, Malcolm, D.C
• Format: Article
• Language: English
• Subjects: calcium; chronosequences; clearcutting; conifer needles; degradation; experimental design; forest litter; magnesium; microhabitats; nitrogen content; nutrient content; phosphorus; Picea sitchensis; potassium; sodium; tree age
• ISSN: 0015-752X; 1464-3626
• DOI: 10.1093/forestry/72.3.207

A chronosequence approach was used to estimate Sitka spruce brash needle decomposition rates over 7 years following clearfelling, by collecting brash needles from forest floors and incubating them in litterbags over a 2-year period on three plots of age 0, 2 and 5 years from the time of harvesting. The data were sequentially fitted to produce a 7-year mass loss curve consisting of four exponential phases: (1) a rapid mass loss phase for labile material over the first 105 days (k = 0.51 a(-1)), (2) a slower second mass loss phase for the first plot up to 2 years (k = 0.33 a(-1)), (3) a third yet slower exponential mass loss phase for the second plot between years 3 and 4 (k = 0.28 a(-1)), and (4) a final rate of k = 0.10 a(-1) for 6-7 years following clearfelling on the third plot. A new approach to analysis of litterbag data was used to demonstrate that if experimental designs use individual collection stations then decomposition rates can be determined for individual microsites as well as for sites as a whole. Microsite decomposition rates varied by up to 300 per cent on each of the three sites examined, but high R2 values (ranging from 0.71 to 0.99) indicate that, despite this large variation between microsites, decomposition within a given microsite proceeds in a consistent manner with time. Further, the decomposition rate for a plot or site can be expressed as k, or the mean of individual microsite decomposition rates. A review of long-term decomposition studies demonstrates that comparing k for individual years is more meaningful than comparing k for longer time periods. Nutritionally, N was retained in the Sitka spruce needles. Phosphorus and Ca content decreased at the same rate as needle mass loss but more labile nutrients such as K, Na and Mg were rapidly lost from the needles. This has implications for long-term nutrient cycling processes on these sites.