PARAMETERIZATION AND TESTING OF A COUPLED PHOTOSYNTHESIS STOMATAL CONDUCTANCE MODEL FOR BOREAL TREES

A coupled photosynthesis-stomatal conductance model was parameterized and tested with branches of black spruce (Picea mariana (Mill.) B.S.P. ) and jack pine (Pinus banksiana Lamb.) trees growing in the Northern Study Area of the Boreal Ecosystem-Atmosphere Study (BOREAS) in Manito ba, Canada. Branch samples containing foliage of all age-classes were harvested from a lowland old black spruce (OBS) and an old jack pine ( OJP) stand and the responses of photosynthesis (A(n)) and stomatal con ductance (g(s)) to temperature, CO2, light, and leaf-to-air vapor pres sure difference (VPD) were determined under controlled laboratory cond itions at the beginning, middle, and end of the growing season (Intens ive Field Campaigns (IFC) 1, 2, and 3, respectively). The parameterize d model was then tested against in situ field gas-exchange measurement s in a young jack pine (YJP) and an upland black spruce (UBS) stand as well as in the OBS and OJP stands. Parameterization showed that Rubis co capacity (V-max), apparent quantum yield (alpha') and Q(10) for sin k limitation were the most crucial parameters for the photosynthesis s ub-model and that V-max varied among different measurement series in t he laboratory. Verification of the model against the data used to para meterize it yielded correlation coefficients (r) of 0.97 and 0.93 for black spruce and jack pine, respectively, when IFC-specific parameters were used, and 0.77 and 0.87 when IFC-2 parameters were applied to al l IFCs. For both measured and modeled g(s), the stomatal conductance s ub-model, which linearly relates g(s) to (A(n)h(s))/c(s) (where h(s) a nd c(s) are relative humidity and CO2 mole fraction at the leaf surfac e, respectively), had significantly steeper slopes and higher r values when only the VPD response data were used for parameterization than w hen all of the response data were used for parameterization. Testing t he photosynthesis sub-model against upper canopy field data yielded po or results when laboratory estimates of V-max, were used. Use of the m ean V-max, estimated for all upper canopy branches measured on a given day improved model performance for jack pine (from a nonsignificant c orrelation between measured and modeled A(n) to r = 0.45), but not for black spruce (r = 0.45 for both cases). However, when V-max, was esti mated for each branch sample individually, the model accurately predic ted the 23 to 137% diurnal variation in A(n) for all stands for both t he upper and lower canopy. This was true both when all of the other pa rameters were IFC-specific (r = 0.93 and 0.92 for black spruce and jac k pine, respectively) and when only mid-growing season (IFC-2) values were used (r = 0.92 for both species). Branch-specific V-max estimates also permitted accurate prediction of field g(s) (r = 0.75 and 0.89 f or black spruce and jack pine, respectively), although parameterizatio n with all of the response data overestimated g(s) in the field, where as parameterization with only the VPD response data provided unbiased predictions. Thus, after parameterization with the laboratory data, ac curately modeling the range of A(n) and g(s) encountered in the field for both black spruce and jack pine was reduced to a single unknown pa rameter, V-max.