INFLUENCE OF VEGETATION AND SOIL CO2 EXCHANGE ON THE CONCENTRATION AND STABLE OXYGEN-ISOTOPE RATIO OF ATMOSPHERIC CO2 WITHIN A PINUS-RESINOSA CANOPY

Measurements were made of the concentration and stable oxygen isotopic ratio of carbon dioxide in air samples collected on a diurnal basis a t two heights within a Pinus resinosa canopy. Large changes in CO2 con centration and isotopic composition were observed during diurnal time courses on all three symple dates. In addition, there was strong verti cal stratification in the forest canopy, with higher CO2 concentration s and more negative delta(18)O values observed closer to the soil surf ace. The observed daily increases in delta(18)O values of forest CO2 w ere dependent on relative humidity consistent with the modelled predic tions of isotopic fractionation during photosynthetic gas exchange. Du ring photosynthetic gas exchange, a portion of the CO2 that enters the leaf and equilibrates with leaf water is not fixed and diffuses back out of the leaf with an altered oxygen isotopic ratio. The oxygen isot ope ratio of CO2 diffusing out of a leaf depends primarily on the O-18 content of leaf water which changes in response to relative humidity. In contrast, soil respiration caused a decline in the delta(18)O valu es of forest CO2 at night, because CO2 released from the soil has equi librated with soil water which has a lower O-18 content than leaf wate r. The observed relationship between diurnal changes in CO2 concentrat ion and oxygen isotopic composition in the forest environment were con sistent with a gas mixing model that considered the relative magnitude s of CO2 fluxes associated with photosynthesis, respiration and turbul ent exchange between the forest and the bulk atmosphere.