Decomposition of Small Woody Debris of California Red Fir

Abies forests cover the subalpine region from latitude 36°N in Sierra Nevada, California, to 55°N in British Columbia, Canada. Because of slow nutrient cycling, Abies forests usually are nutrient‐stressed. This study determined the decomposition rates, changes in the C/N ratio, and the dynamics of N, P, and Mn of bark and wood of small diameter debris of California red fir ( Abies magnifica A. Murr.). Triplicate 0.06‐ha plots were established with four combinations of N‐fertilization and thinning treatments: Control (16 687 trees ha −1 ), Thinned (1141 trees ha −1 ), N fertilized (300 kg N ha −1 , as urea), and Thinned + N fertilized (1141 trees ha −1 , plus 300 kg N ha −1 ). Individually labeled samples, 0.5‐ to 5‐cm diameter and 15‐ to 30‐cm length, were scattered in the plots and retrieved after 3 and 17 yr. Bark and wood were then separated and analyzed for total dry mass, and the concentration and mass of N, P, and Mn. For bark there were few notable effects of the treatments at 3 or 17 yr. Over the 17‐yr period, total dry mass of bark decreased by 62% (because of decomposition and sloughing); the C/N ratio dropped from 72/1 to 48/1; concentrations of N and Mn increased, but P concentration decreased; masses of N and P decreased, but Mn mass increased. For wood, there were a few significant effects of the treatments at 3 yr and none at 17 yr. At 3 yr, thinning reduced the C/N ratio; all the treatments lost more total dry mass than the Control; N and P mass losses differed slightly between treatments, but Mn mass loss did not differ between treatments. Over the 17‐yr period, total dry mass of wood decreased by 37%; the C/N ratio dropped from 217/1 to 177/1; concentrations of N and Mn increased, but P concentration decreased; masses of N and P decreased, but mass of Mn remained the same. White‐rot fungi require Mn to decompose lignin, and maintain relatively high levels of Mn in decomposing bark and wood.