The effects of environmental and physiological variables on the gas exchange of forest trees under boreal field conditions

Gas exchange (CO$\sb2$ and water vapor) is one of the most useful approaches to study environmental and physiological responses of photosynthesis under field conditions. The objective of this study was to quantify the gas exchange properties (net CO$\sb2$ assimilation (A), stomatal conductance (gs) and dark respiration (Rd)) of aspen (Populus tremuloides Michx.), balsam poplar (Populus balsamifera L.), hazelnut (Corylus cornuta Marsh.), jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana (Mill) BSP) during the growing season as a function of fluctuations in environmental conditions and foliage development. Field experiments were conducted during the summers of 1994 and 1995 in the boreal forest near Prince Albert, Saskatchewan, Canada. The experiments quantified effects of photosynthetic photon flux density (PPFD), air temperature (Ta), vapor pressure deficit (VPD) and internal CO$\sb2$ concentration (Ci) on gas exchange properties of leaves of aspen and shoots (current-, 1- and 2-year old) of jack pine and black spruce. Photosynthetic photon flux density was the major determinant of leaf and shoot gas exchange under field conditions, although the gas exchange properties were also affected by Ta, VPD and Ci. Under high light, the dominant factors limiting gas exchange were Ta and VPD. The results also revealed diurnal and growing season patterns of gas exchange for these species. Variations in gas exchange during the growing season were correlated with PPFD, Ta, VPD, foliage development, and foliage nitrogen (N) concentration. Diurnal variations in gas exchange further revealed that PPFD was the major environmental variable affecting diurnal variation in A while VPD was the key factor affecting diurnal variation in gs. Different responses of gas exchange to environmental factors among different tree species were also related to soil characteristics