No abstract

We studied snags in 30 stands, 5-445 years old, of unmanaged and managed Douglas-fir (Pseudotsuga menziesii) in western Oregon to gain information about snag populations and status after logging. As snag production rates (snags/ha/year) declined from about 100 to 1, mean snag density decreased from 190 to 18/ha in age-classes 35 and 200+, respectively; remnant snags (formed in previous stands) represented 5-14% of current densities. Meanwhile, average snag dbh increased from 13 to 72 cm, and as dbh increased, snags stood longer. Douglas-fir was the dominant species among snags in all forest age-classes. Linear regression analysis showed a correlation (P < 0.001) between snag age and deterioration; populations consisted of fewer young (sound) and old (highly decayed) than middle-aged (partially decayed) snags. Cluster analysis revealed 5 stages of deterioration based upon snag size and decay condition. In unmanaged stands, most (62%) snag populations were distributed randomly, but patches of snags were found in all age-classes. Fewer snags (P < 0.001) remained after thinning and clear-cutting unmanaged forests, and natural snag production was disrupted. Large snags should be retained within forests managed over long (>200-year) rotations; in riparian forests; in extensively managed, slow-growing forests; and within intensively managed forests, safety permitting.

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).

No abstract

Free-living microaerophiles fixed 15N2 and reduced acetylene in fallen tree boles at two old-growth Pseudotsugamenziesii stands in western Oregon. Acetylene reduction was most rapid under an atmosphere of 2–10% O2, whereas under prolonged anaerobic conditions it was at or below detection limits. Acetylene reduction rates increased up to fourfold during long-term incubations in acetylene (> 12 h). Ratios of acetylene reduction to N2 fixation frequently exceeded 6.0 during such long-term incubations but averaged 3.5 when samples were incubated < 7 h; consequently, long-term incubation of low-activity material in acetylene should be avoided. A preliminary survey indicated that N2 fixation by free-living organisms in fallen boles was less than other potential N inputs to fallen boles and to the forest ecosystem.