1985. Comparison of decomposition models using wood density of Dou g las-fir logs. Can. J. For. Res. 15: 1092-1098. Logs of Pseudotsuga men:iesii (Mirb.) Franco that had been on the ground for up to 313 years were grouped into five decay classes that ranged from I. essentially undecayed, to 5, soft and incorporated into the forest floor but still identifiable. The mean residence times on the forest floor were 7, 17, 33. 82, and 219 years for decay classes I through 5, respectively. The

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... ponential model of litter decomposition was fitted to the density of these lo g s. The summation-exponential model was constructed by summing single-exponential models fitted to lignin, cellulose, and the acid detergent soluble fraction. Both models gave virtually identical, statistically significant fits to the data. Wood density of these Douglas-fir logs decreased more slowly than that of most species other researchers have studied. The single-exponential model gave mineralization rates (k) of 0.0063 and 0.0070 year -' 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 recommended when these constituents are of interest. MEANS, J. E., K. CRON1ACK, JR. et P. C. MACMILLAN. 1985. Comparison of decomposition models using wood density of Douglas-fir logs. Can. J. For. Res. 15: 1092-1098. Des billes de Pseudotsuga menziesii (Mirb.) Franco qui reposaient sur le sol depuis jusqu'ã 313 ans ont etc groupees en cinq classes de decomposition allant de I (les billes essentiellement non décomposees) a 5 (les billes tendres et incorpordes a la couverture morte, mais encore identifiables). Le modele d'exponentielle unique de decomposition de la litiere fut ajuste a la densite de ces billes. Le rnodele d'exponentielle sommative fut construit par la somme des mode.les d'exponentielle simple ajustes a la lignine, a la cellulose et a la fraction soluble dans un deter g ent acide chaud. Les deux modeles traduisent fidelement les valeurs observees. La densite du bois de ces billes de douglas a diminue plus lentement que celle de la plupart des autres especes etudiees par d'autres chercheurs. Lorsque le temps de residence et le temps de residence moyen par classe de decomposition furent utilises comme variable independante, le modele d'exponentielle unique a fourni les taux de mineralisation ( k) de 0,0063 et 0,0070 an -I , respectivement. La lignine s'est decomposee plus lentement que la cellulose ou la fraction soluble dans un detergent acide chauck les taux de decomposition de ces deux dernieres fractions n'etaient pas statistiquement differents. En consequence, le modele d'exponentielle sommative est recommandó lorsqu'on porte un interet particulier a ces deux dernieres fractions. [Traduit par le journal]