The sensitivity of stand-scale photosynthesis and transpiration to changes in atmospheric CO2 concentration and climate

摘要

The 3-dimensional forest model MAESTRO was used to simulatedaily and annual photosynthesis and transpiration fluxes of forest stands and thesensitivity of these fluxes to potential changes in atmospheric CO₂concentration ([CO₂]), temperature, water stress and phenology. The effects ofpossible feed-backs from increased leaf area and limitations to leaf nutrition weresimulated by imposing changes in leaf area and nitrogen content. Two different treespecies were considered: Picea sitchensis (Bong.) Carr., a conifer with longneedle longevity and large leaf area, and Betula pendula Roth., a broad-leaveddeciduous species with an open canopy and small leaf area. Canopy photosyntheticproduction in trees was predicted to increase with atmospheric [CO₂] and lengthof the growing season and to decrease with increased water stress. Associated increases inleaf area increased production further only in the B. pendula canopy, where theoriginal leaf area was relatively small. Assumed limitations in N uptake affected B. pendula more than P. sitchensis. The effect of increased temperature wasshown to depend on leaf area and nitrogen content. The different sensitivities of the twospecies were related to their very different canopy structure. Increased [CO₂]reduced transpiration, but larger leaf area, early leaf growth, and higher temperature allled to increased water use. These effects were limited by feedbacks from soil waterstress. The simulations suggest that, with the projected climate change, there is someincrease in stand annual `water use efficiency', but the actual water losses to theatmosphere may not always decrease.