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We report field observations of oxygen isotope ( 18 O) discrimination during nocturnal foliage respiration ( 18 Δ 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  18 O/ 16 O (δ 18 O) signatures of foliage gas exchange at night. We found that nocturnal stomatal conductance can influence the δ 18 O signature by affecting: (1) the bidirectional diffusion of CO 2  into and out of the leaf (with isotopic equilibration); and (2) the  18 O enrichment of the foliage water with which the CO 2  equilibrates. Both effects were manifest in high apparent  18 Δ R  values and enriched δ 18 O 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 δ 18 O signature of nocturnal foliage respiration, generally thought to decrease the δ 18 O of atmospheric CO 2 . 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 δ 18 O signatures of canopy CO 2  to distinguish foliage and soil respiration, and when modeling the δ 18 O signature of CO 2  exchanged between ecosystems and the atmosphere.

Nocturnal stomatal conductance effects on the δ 18 O signatures of foliage gas exchange observed in two forest ecosystems