Comparison of decomposition models using wood density of Douglas-fir logs

Logs of Pseudotsuga men:iesii (Mirb.) Franco that had been on the ground for up to 313 years were grouped into five decayclasses that ranged from I. essentially undecayed, to 5, soft and incorporated into the forest floor but still identifiable. Themean residence times on the forest floor were 7, 17, 33. 82, and 219 years for decay classes I through 5, respectively. Thesingle-exponential model of litter decomposition was fitted to the density of these logs. The summation-exponential model wasconstructed by summing single-exponential models fitted to lignin, cellulose, and the acid detergent soluble fraction. Bothmodels gave virtually identical, statistically significant fits to the data. Wood density of these Douglas-fir logs decreased moreslowly than that of most species other researchers have studied. The single-exponential model gave mineralization rates (k) of 0.0063 and 0.0070 year when residence time and decay class age (mean residence time of the decay class), respectively, were used as the independent variable. Lignin decayed more slowly than cellulose or the fraction soluble in hot acid detergent,both of which decayed at rates that were not significantly different: thus, the summation-exponential model is recommendedwhen these constituents are of interest.