MODELING PHOTOSYNTHETIC RESPONSES OF A SPRUCE CANOPY TO SO2 EXPOSURE

A net photosynthesis model (ForNePS) was developed to simulate canopy photosynthetic responses of spruce trees to SO2 exposure. This model i ncludes four submodels: (1) a forest light transmission model (ForLit) to calculate photosynthetically active radiation in four leaf-layer c lasses and two leaf-type classes (sunlit and shaded); (2) a forest can opy model (ForCan) to calculate canopy temperature and boundary layer conductance; (3) an optimum control model (ForOpt) to calculate stomat al conductance, SO2 absorption, and evapotranspiration; (4) a net phot osynthesis model (ForNeP) to calculate net photosynthesis. Final outpu t of the ForNePS model provides cumulative SO2 absorption, and cumulat ive CO2 fixation and evapotranspiration as affected by cumulative SO2 absorption. Model parameters were mostly obtained from literature. Key parameters related to water use efficiency and water transport from s oil to foliage were obtained through model calibration with net photos ynthesis, stomatal conductance and dark respiration determinations for 2-year-old black spruce saplings. Model simulations were further cali brated with net photosynthesis and stomatal conductance determinations for foliage on mature spruce trees subjected to SO2 treatments by way of branch chamber experiments. Treatments were conducted in late summ er for 6 weeks. Model simulations for net photosynthesis were compared with mid-summer photosynthesis determinations for spruce foliage on m ature trees subjected to low (background level) and high rates of atmo spheric SO2 deposition for several decades. Annual rates of atmospheri c SO2 deposition within the area ranged from 2 to 70 kg ha-1 year-1. O bservations and ForNePS simulations suggested that, in relative terms, SO2-induced reductions of cumulative CO2 fixation were greater than c orresponding reductions of cumulative water losses.