Physiological controls of the carbon balance of boreal forest ecosystems

Mature boreal forest ecosystems in interior Alaska are large annual carbon sinks. Annual tree production is the largest carbon flux. A model of that combined energy, heat, and moisture exchange, tree photosynthesis and respiration, decomposition, and nitrogen mineralization was used to examine the physiological controls of the carbon balance of boreal forests. Simulated annual tree production, forest floor decomposition, nitrogen mineralization, and soil respiration were not significantly different from observed data for nine black spruce (Picea mariana (Mill.) B.S.P.), five white spruce (Picea glauca (Moench) Voss), two quaking aspen (Populus tremuloides Michx.), two paper birch (Betula papyrifera Marsh.), and three balsam poplar (Populus balsamifera L.) forests near Fairbanks, Alaska. The model also reproduced features of observed fertilization, soil warming, and litter transplant experiments. Net carbon uptake during tree growth was the largest simulated carbon flux, and these analyses suggest that differences in the carbon balance of these forests can be explained, in part, through key physiological parameters that link photosynthesis, carbon allocation, nitrogen requirements, litter quality, and foliage longevity. The simulations suggest that the greatest source of variation in these parameters occurs between coniferous and deciduous life-forms not among species. Simulation experiments showed that the coniferous and deciduous physiological parameters maximized annual tree production for coniferous and deciduous forests, respectively, thereby providing a physiological basis for the evolution of the different life history characteristics of deciduous and coniferous species. The strong coherency among physiological parameters allows them to be estimated from easily obtained data and may provide a basis to examine carbon fluxes over large regions.