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

Fallen boles (logs) of Douglas-fir (Pseudotsuga menziesii (Mirb. ) Franco), western hemlock (Tsuga heterophylla ( Raf.)Sarg.), and western red cedar (Thuja plicata Donn) in old-growth stands of the Cascade Range of western Oregon andWashington were compared with regard to their physical structure, chemistry. and levels of microbial activity. Western hemlockand western red cedar logs disappeared faster than Douglas-fir logs, although decay rate constants based on density change alonewere 0.010/year for Douglas-fir, 0.016/year for western hemlock. and 0.009/year for western red cedar. We were unable tolocate hemlock or red cedar logs older than 100 years on the ground, but found Douglas-fir logs that had persisted up to nearly 200years. Wood density decreased to about 0.15 g/cm3 after 60-80 years on the ground. depending on species, then remained nearlyconstant. Moisture content of logs increased during the first 80 years on the ground. then remained roughly constant at about250% (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 throughoutthe 200-year time span that was studied (100-year time span for Na). N:P ratios converged toward 20, irrespective of tree speciesor wood tissue type. C:N ratios dropped to about 100 in the most decayed logs; net N was mineralized during anaerobic incubationof most samples with a C:N ratio below 250. The ratio of mineralized N to total N increased with advancing decay. Asymbioticbacteria in fallen logs fixed about 1 kg N ha I year a substantial amount relative to system N input from precipitation and drydeposition (2-3 kg ha-' year-').