VARIABILITY IN LEAF-LEVEL CO2 AND WATER FLUXES IN PINUS-BANKSIANA ANDPICEA-MARIANA IN SASKATCHEWAN

We measured seasonal and canopy-level gas exchange in two stands of ja ck pine (Pinus banksiana Lamb.) and one stand of black spruce (Picea m ariana (Mill.) B.S.P.) on relatively clear days from late May until mi d-September 1994. Field measurements were made with a portable infrare d gas analyzer, and laboratory measurements included photosynthetic ox ygen evolution and needle chemical composition. Seasonally averaged li ght-saturated assimilation rates in the field were 4.0 mu mol m(-2) s( -1) in jack pine and 2.7 mu mol m(-2) s(-1) in black spruce. Rates of assimilation and transpiration were highest in midsummer. The seasonal pattern was especially pronounced for black spruce, probably because cold soil temperatures limited early season gas exchange rates in this species. Among stands, instantaneous water-use efficiency was highest in a young jack pine stand early in the season and higher in the uppe r canopy foliage than in the lower canopy foliage at all sites at the end of the season. Needles of young jack pine exhibited higher photosy nthetic capacity, dark respiration and needle N concentrations than ne edles of trees at the old site. In both species, slight acclimation to shading was manifested by reductions in photosynthetic capacity in th e lower canopy foliage. In both species, first-year needles had greate r photosynthetic capacity than older needles but in situ rates of CO2 assimilation in the field showed little difference among needle age cl asses. In both species, there was a strong correlation between assimil ation and stomatal conductance, indicating that assimilation was highl y stomatal limited and that environmental factors that alter conductan ce (e.g., VPD) have a strong influence on CO2 and water fluxes, especi ally after early season thawing concludes.