Woody debris decay rates and biomass loss: New findings from old logs

Background/Question/Methods

Woody debris constitutes a substantial carbon pool in forests worldwide. Woody debris decay rates have recently received much attention because of the need to quantify temporal changes in forest biomass and carbon stocks. Given the paucity of biomass depletion curves, researchers often must rely on published decay constants derived from wood density depletion to characterize deadwood biomass and carbon dynamics. However, density decay rates have long been known to underestimate biomass and carbon depletion, because they fail to account for volume depletion as decay progresses. Using a chronosequence approach, we present a method to simultaneously assess wood density, volume, biomass, and carbon depletions of decaying logs. The method is based on the observation, confirmed herein, that decaying logs have a collapse ratio (cross-sectional height/width) that diminishes through time and thus can be used as a surrogate for volume remaining. Combining volume depletion with wood density depletion from these same logs allowed us to quantify biomass, and hence carbon, depletion for three study species.

Results/Conclusions

Results clearly show that density, volume, and biomass follow distinct depletion curves, none of which was well described by the commonly assumed negative exponential form (based on AICc rankings). Volume depletion showed an initial lag period, even while wood density was declining. However, once volume depletion began, biomass loss (the product of density and volume depletion) occurred much more rapidly than density depletion alone. At the temporal limit of our data, the proportion of initial biomass remaining was roughly half that of density remaining. The adoption of the volume-depletion approach used in this study would result in more realistic and comprehensive characterization of deadwood decomposition, thereby improving biomass-loss and carbon-accounting models.