GAS-EXCHANGE PROCESSES OF YELLOW-CEDAR (CHAMAECYPARIS-NOOTKATENSIS) IN RESPONSE TO ENVIRONMENTAL VARIABLES

Yellow-cedar (Chamaecyparis nootkatensis (D. Don) Spach) gas exchange processes were measured in response to the following primary environmental variables: photosynthetically active radiation, vapour pressure deficit, root temperature, and soil moisture. Under nonlimiting edaphic conditions, maximum stomatal conductance and maximum CO2 assimilation increased rapidly as photosynthetically active radiation increased from 0 to 200-mu-mol.m-2.s-1 and from 0 to 500-mu-mol.m-2.s-1, respectively. Thereafter, greater photosyndietically active radiation levels only resulted in minor increases in stomatal conductance and CO2 assimilation. Maximum stomatal conductance and maximum CO2 assimilation declined in a concave manner as vapour pressure deficit increased from 1 to 5 kPa. Response surface model for stomatal conductance showed vapour pressure deficit was the primary influence after light had caused initial stomatal opening. Response surface modeling approach showed CO2 assimilation increased as photosynthetically active radiation increased, but increased vapour pressure deficit resulted in a suppression of CO2 assimilation. Response surface model showed internal CO2 concentration declined sharply as photosynthetically active radiation increased from 0 to 500-mu-mol.m-2.s-1, but it remained constant with increasing vapour pressure deficit. Decreasing root temperature resulted in a continual decline in CO2 assimilation and stomatal conductance from 22 to 1-degrees-C, while internal CO2 concentration declined from 22 to 13-degrees-C with little change between 13 and 1-degrees-C. As predawn shoot water potential decreased from -0.5 to -2.0 MPa, CO2 assimilation declined in a linear manner, while stomatal conductance and internal CO2 concentration declined in a concave manner.