Patterns of log decay in old-growth Douglas-fir forests

Abstract

Fallen boles (logs) of Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco), western hemlock (Tsugaheterophylla (Raf.) Sarg.), and western red cedar (Thujaplicata Donn) in old-growth stands of the Cascade Range of western Oregon and Washington were compared with regard to their physical structure, chemistry, and levels of microbial activity. Western hemlock and western red cedar logs disappeared faster than Douglas-fir logs, although decay rate constants based on density change alone were 0.010/year for Douglas-fir, 0.016/year for western hemlock, and 0.009/year for western red cedar. We were unable to locate hemlock or red cedar logs older than 100 years on the ground, but found Douglas-fir logs that had persisted up to nearly 200 years. Wood density decreased to about 0.15 g/cm3 after 60–80 years on the ground, depending on species, then remained nearly constant. Moisture content of logs increased during the first 80 years on the ground, then remained roughly constant at about 250% (dry-weight basis) in summer and at 350% in winter. After logs had lain on the ground for about 80 years, amounts of N, P, and Mg per unit volume exceeded the amount present initially. Amounts of Ca, K, and Na remained fairly constant throughout the 200-year time span that was studied (100-year time span for Na). N:P ratios converged toward 20, irrespective of tree species or wood tissue type. C:N ratios dropped to about 100 in the most decayed logs; net N was mineralized during anaerobic incubation of most samples with a C:N ratio below 250. The ratio of mineralized N to total N increased with advancing decay. Asymbiotic bacteria in fallen logs fixed about 1 kg N ha−1 year−1, a substantial amount relative to system N input from precipitation and dry deposition (2–3 kg ha−1 year−1).

Insect‐mediated nitrogen dynamics in decomposing wood

“…Indeed, inputs of N into decomposing wood by free-living N 2 -fixing bacteria can be substantial (Sollins et al, 1987;Griffiths et al, 1993;Brunner & Kimmins, 2003;Chen & Hicks, 2003). In north-western North America, for instance, Sollins et al (1987) estimated that asymbiotic bacteria in fallen logs fixed about 1 kg N ha −1 year −1 compared with inputs from precipitation and dry deposition of 2-3 kg N ha −1 year −1 . Estimates from north-western Canadian forests were similar, ranging from 1 to 2.1 kg N ha −1 year −1 (Brunner & Kimmins, 2003).…”

Effects of Harvesting Systems and Bole Moisture Loss on Weight Scaling of Douglas-Fir Sawlogs (Pseudotsuga Menziesii var. glauca Franco)

“…Beyond environmental factors present during drying, bark cover and thickness is often assumed to influence drying rates [49,50]. However, when investigating moisture content of branches, Boddy, 1983 [51] did not find drying to be significantly faster in bark-less branches.…”

Abiotic and Biotic Soil Characteristics in Old Growth Forests and Thinned or Unthinned Mature Stands in Three Regions of Oregon

“…Logs in advanced stages of decay have a sponge-like ability to absorb and retain water [63,64], increasing the probability that they would eventually be incorporated into mineral soil rather than being consumed in the next fire. With k values of 0.004 to 0.007 [65], Douglas-fir logs persist in soils for at least 700 to 1250 years [66], implying that death rates of old trees are comparable to or greater than decay rates of downed logs.…”