Toward using δ < sup> 13 C of ecosystem respiration to monitor canopy physiology in complex terrain

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In 2005 and 2006, air samples were collected at the base of a Douglas-fir watershed to monitor seasonal changes in the δ13CO2 of ecosystem respiration (δ13CER). The goals of this study were to determine whether variations in δ13C ER correlated with environmental variables and could be used to predict expected variations in canopy-average stomatal conductance (G s). Changes in δ13CER correlated weakly with changes in vapor pressure deficit (VPD) measured 0 and 3-7 days earlier and significantly with soil matric potential (ψm) (P value < 0.02) measured on the same day. Midday Gs was estimated using sapflow measurements (heat-dissipation method) at four plots located at different elevations within the watershed. Values of midday Gs from 0 and 3-7 days earlier were correlated with δ13CER, with the 5-day lag being significant (P value < 0.05). To examine direct relationships between δ13CER and recent Gs, we used models relating isotope discrimination to stomatal conductance and photosynthetic capacity at the leaf level to estimate values of stomatal conductance ("Gs-I") that would be expected if respired CO2 were derived entirely from recent photosynthate. We compared these values with estimates of Gs using direct measurement of transpiration at multiple locations in the watershed. Considering that the approach based on isotopes considers only the effect of photosynthetic discrimination on δ13CER, the magnitude and range in the two values were surprisingly similar. We conclude that: (1) δ13CER is sensitive to variations in weather, and (2) δ13CER potentially could be used to directly monitor average, basin-wide variations in G s in complex terrain if further research improves understanding of how δ13C ER is influenced by post-assimilation fractionation processes. © 2008 Springer-Verlag.

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