Litter decomposition contrasts in second- and old-growth Douglas-fir forests of the Pacific Northwest, USA |
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Authors: | Jeffrey M Klopatek |
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Institution: | (1) School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501, USA |
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Abstract: | Litterfall and its subsequent decomposition are important feedback mechanisms in the intrasystem cycling of nutrients in forest
ecosystems. The amount of litterfall and the rate of decomposition are expected to vary with stand age and climate. Over a
2-year period, decomposition of five litter types were measured in two second-growth forest stands and one old-growth stand
in the Cascade Mountains of southern Washington state, USA. Both second-growth stands were dominated by Douglas-fir Pseudotsuga menziesii (Mirb.,) Franco] but one had a significant proportion of red alder (Alnus rubra Bong.), a nitrogen (N) fixer. The old-growth stand was dominated by Douglas-fir and western hemlock Tsuga heterophylla (Raf.) Sarg.]. All stands had a relatively shallow layer of forest floor mass. The five litter types were placed in each
stand to evaluate decomposition patterns. Despite significant differences in stand age, microclimate and mean residence times
for carbon (C) and N, the rates of litter mass loss varied only slightly between sites. The relative order of species litter
mass loss was: vine maple ≫ salal = western hemlock > Douglas-fir (from the youngest stand) > Douglas-fir (from the N rich
stand with red alder). The initial litter lignin concentration, not lignin:N, was the primary determinant of decomposition
rates, although the initial N concentration was the predictor for mass loss after 2 years in the N rich Douglas-fir-alder
stand. All litter types showed immobilization of N for nearly 2 years. Data for Douglas-fir litter suggest that higher levels
of N may retard decomposition of tissues with greater amounts of lignified material. The retention of N by the litter appeared
influenced by the nutrient capital of the stands as well as the forest floor C:N ratio. Decomposition was minimal during the
cold winter months, but displayed a definitive peak period during early Fall with wet weather, warm soils, and fungal activity.
Thus, long-term climatic change effects on forest floor C storage may depend more on changes in seasonality of precipitation
changes than just temperature changes. |
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Keywords: | Decomposition Litter Lignin Nitrogen C:N ratio Old-growth forest |
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