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

The 3-dimensional forest model MAESTRO was used to simulate daily and annua l photosynthesis and transpiration fluxes of forest stands and the sensitiv ity of these flutes to potential changes in atmospheric CO2 concentration ( [CO2]), temperature, water stress and phenology. The effects of possible fe ed-backs from increased leaf area and limitations to leaf nutrition were si mulated by imposing changes in leaf area and nitrogen content. Two differen t tree species were considered: Picea sitchensis (Bong.) Carr., a conifer w ith long needle longevity and large leaf area, and Betula pendula Roth., a broad-leaved deciduous species with an open canopy and small leaf area. Canopy photosynthetic production in trees was predicted to increase with at mospheric [CO2] and length of the growing season and to decrease with incre ased water stress. Associated increases in leaf area increased production f urther only in the B. pendula canopy, where the original leaf area was rela tively small. Assumed limitations in N uptake affected B. pendula more than P. sitchensis. The effect of increased temperature was shown to depend on leaf area and nitrogen content. The different sensitivities of the two spec ies were related to their very different canopy structure. increased [CO2] reduced transpiration, but larger leaf area, early leaf growth, and higher temperature all led to increased water use. These effects were limited by f eedbacks from soil water stress. The simulations suggest that, with the pro jected climate change, there is some increase in stand annual 'water use ef ficiency', but that actual water losses to the atmosphere may not always de crease.