Carbon dynamics of southern Rocky Mountain fens

Abstract

The objectives of my dissertation were to determine: (1) the current carbon accumulation rates in Rocky Mountain National Park pristine fens, (2) whether the Grand Ditch water diversion in Rocky Mountain National Park decreased carbon accumulation in fens, (3) how much the water table must be lowered in fens before significant changes in gas efflux occurs and (4) how well the CENTURY ecosystem model can be used to simulate long-term carbon accumulation. Carbon balances were calculated during 1997 and 1998 to quantify carbon accumulation rates in three pristine fens and two fens beneath the Grand Ditch. Site hydrologic regime was found to directly control carbon accumulation in the five study fens. All fens accumulated carbon when the water table remained at or above the soil surface. However, both pristine fens and fens beneath the Grand Ditch lost carbon during years when the water table dropped beneath the soil surface for more than three weeks during the summer. Microcosms were installed in a Colorado fen to manipulate water levels and measure the response in CO2 and CH4 efflux. The experiment showed that CO2 efflux was lowest when the water table was above the soil surface, but efflux rates doubled when the water table dropped beneath the soil surface. However, further lowering of the water table beneath the soil surface had little additional effect on CO2 efflux. The highest CH4 efflux occurred when the water table was just above the soil surface and decreased when the water table was either deeper or more ponded. CENTURY was able to simulate carbon cycling in peatlands by altering three anaerobic variables. However, CENTURY was unable to properly simulate carbon accumulation in an uncalibrated peatland because of limitations in how anaerobic conditions are created in CENTURY. Nevertheless, once calibrated, the usefulness using an ecosystem model for peatland carbon budget analyses became apparent as it allowed predictions to be made of peat composition and the consequences of exposing peat bodies to aerobic conditions. CENTURY predicted that most of the fen peat stored came from root material, which was easily decomposed when exposed to aerobic conditions.