Some Characteristics of Wood Decomposition in Forest Ecosystems.
Caroline Preston, Canadian Forest Service, Pacific Forestry Centre, 506 West Burnside Rd., Victoria, BC V8Z 1M5, CANADA and Tony Trofymow, Pacific Forestry Centre, 506 West Burnside Rd., Victoria, BC V8Z 1M5, Canada.
Coarse woody debris is characteristic of older forests with little human disturbance, such as the temperate rainforests of the Pacific Northwest. Large fallen gymnosperm logs typically become dark red and fragment easily. Solid-state 13C NMR shows nonselective mass loss in early stages of decay, or even an increase in the proportion of cellulose, characteristic of white-rot fungi. Later stages are dominated by brown-rot fungi, with faster carbohydrate loss, and increasing proportions of lignin C, although this may not be matched by increasing yield of lignin-derived monomers from CuO oxidation. Total C concentration increases and δ13C decreases, although the latter is difficult to detect against natural variation. Lignin enrichment is favoured by cool, wet conditions, guaiacyl lignin with greater decomposition resistance, and large diameter, and a lignin signature may be seen in forest floor, buried woody horizons, light fractions and coarser mineral soil fractions. However, most of the mineral-associated soil C is in the <50 um fraction, with increasing alkyl C and low aromatic C, not consistent with long-term lignin preservation. White-rot fungi tend to persist in small-diameter wood which typically loses structural integrity, but remains light-coloured without much lignin enrichment. Similar results were found for wood blocks decomposing up to 12 y in litterbags. Thus, it cannot be assumed that lignin is inherently recalcitrant, and white-rot fungi decompose it as fast or faster than cellulose. Radiocarbon analysis of lignin monomers should be useful in determining the lignin role in soil C cycling, although complicated by up to millennial-scale lags between photosynthesis and incorporation into detrital pools. Other open questions include the nature of the various decomposer communities, quantification of microbially-derived C to decomposed wood, changes in monosaccharide yields and composition, alteration of lignin structure and its effect on CuO oxidation yields, and better linkage of wood composition to δ13C.