- Controls of substrate quality, temperature, and moisture on woody root decomposition in the Pacific Northwest were explored using chronosequences, time series, laboratory incubations, and simulation modeling approaches at three sites: Cascade Head
(CAH), H. J. Andrews (HJA), and Pringle Falls Experimental Forests (PRF). In the chronosequence study, a structural component-oriented approach provided a better estimation of long-term mass loss than initial substrate indices. Western hemlock and ponderosa pine had higher decomposition rate-constants (k = 0.033 to 0.077/year) than Sitka spruce, Douglas-fir, and lodgepole pine (k = 0.011 to 0.03/year). This was mainly due to the presence of root resin cores in the latter species. During the first 2-years of decomposition in a time series experiment, species significantly affected mass loss in fine and small roots (< 1 cm), but not in larger sized roots. No significant difference among sites were observed. Woody root decomposition decreased with increasing root size. Lignin:N and phenols:N ratios were good predictors of k for fine and small roots, respectively, although none of 17 initial substrate indices was a good predictor of k of larger roots. In both chronosequence and time series decomposition studies, dead roots released nitrogen in the early stages of decomposition. In laboratory incubations, dead root respiration was optimum at 30-40 °C. The Q10 of root decomposition was influenced significantly (P < 0.01) by incubation temperature range, but not by species, decay, class or the direction of temperature change Dead root respiration reached an optimum when moisture content was
between 100 and 275%. Exchange of moisture between roots and soils appeared to follow a diffusion process, with larger roots equilibrating more slowly than smaller roots.
A model, ROOTDK, captured the overall mass loss pattern of Sitka spruce, Douglas-fir, and western hemlock but not of lodgepole pine and ponderosa pine. Root decomposition at CAH and PRF is more sensitive to climatic changes than HJA. Thus, even within the Pacific Northwest region the response of root decomposition to an altered climate can be divergent.