Nocturnal stomatal conductance effects on the δ18O signatures of foliage gas exchange observed in two forest ecosystems

We report field observations of oxygen isotope (O-18) discrimination during nocturnal foliage respiration ((18)Delta(R)) in branch chambers in two forest ecosystems: a Sitka spruce (Picea sitchensis (Bong.) Carr.) plantation in Scotland; and a beech (Fagus sylvatica L.) forest in Germany. We used observations and modeling to examine the impact of nocturnal stomatal conductance on the O-18/(16O) (delta O-18) signatures of foliage gas exchange at night. We found that nocturnal stomatal conductance can influence the delta O-18 signature by affecting: (1) the bidirectional diffusion Of CO2 into and out of the leaf (with isotopic equilibration); and (2) the O-18 enrichment of the foliage water with which the CO2 equilibrates. Both effects were manifest in high apparent (18)Delta(R) values and enriched delta O-18 signatures of foliage water at night. The effects were more pronounced for Sitka spruce because of its higher nocturnal stomatal conductance and higher specific leaf water content compared to beech. We found that taking the effects of nocturnal stomatal conductance into account may change the sign of the delta O-18 signature of nocturnal foliage respiration, generally thought to decrease the delta O-18 of atmospheric CO2. We conclude that nocturnal stomatal exchange can have a profound effect on isotopic exchange depending on species and environmental conditions. These effects can be important when using delta O-18 signatures of canopy CO2 to distinguish foliage and soil respiration, and when modeling the delta O-18 signature of CO2 exchanged between ecosystems and the atmosphere.