Including the effects of water stress on decomposition in the Carbon Budget Model of the Canadian Forest Sector CBM-CFS3

Decomposition of plant detritus and humified organic matter in terrestrial ecosystems is a primary source of atmospheric carbon dioxide (CO2), yet the dynamics of decomposition are not well understood, particularly their response to climate change. The Carbon Budget Model of the Canadian Forest Sector (CBM-CFS3) presently includes a sub-model to simulate the decomposition of dead organic matter carbon pools using base decay rates modified by temperature quotients. In this study, representation of litter decomposition was improved in the CBM-CFS3 by reducing decay rates under limited moisture conditions. Water stress effects were determined from comparisons of model predictions with data from a 12-year national litterbag decomposition study—the Canadian Intersite Decomposition Experiment (CIDET). Several simple water-stress modifiers based on precipitation and potential evapotranspiration were tested, and parameters were simultaneously fit by minimizing the least-squared error. The best-fitting formulation used the annual average of the ratio of monthly precipitation to monthly potential evapotranspiration, and increased the explained variance by 8%. Water-stress modifiers were applied to decay rates to predict carbon stocks at 516 ground plots from a national soil plot database. The addition of the water-stress modifier modestly increased litter and humified organic matter carbon stocks at dry locations and decreased these carbon stocks at non-water-stressed locations. The new ability to lower decay rates of certain dead organic matter pools under limited moisture conditions in the CBM-CFS3 has the potential to reduce bias in carbon flux predictions for those regions and for future climate change scenarios where moisture limits decomposition processes.