Toward Using ゴ¹³C of Ecosystem Respiration to Monitor Canopy Physiology in Complex Terrain

• Published in: Oecologia Vol. 158; no. 3; pp. 399 - 410
• Main Authors: Pypker, T. G., Hauck, M., Sulzman, E. W., Unsworth, M. H., Mix, A. C., Kayler, Z., Conklin, D., Kennedy, A. M., Barnard, H. R., Phillips, C., Bond, B. J.
• Format: Journal Article
• Language: English
• Published: Springer 01.12.2008
• Subjects: Carbon dioxide; Carbon isotopes; Forest ecosystems; Forest soils; Fractionation; Physiological Ecology – Original Papers; Respiration; Soil respiration; Soil water; Stomatal conductance; Watersheds
• ISSN: 0029-8549; 1432-1939
• DOI: 10.1007/s00442-008-1154-3

In 2005 and 2006, air samples were collected at the base of a Douglas-fir watershed to monitor seasonal changes in the$(\delta ^{13} CO_{2} )$of ecosystem respiration$(\delta ^{13} C_{ER} )$. The goals of this study were to determine whether variations in$\delta ^{13} C_{ER} $correlated with environmental variables and could be used to predict expected variations in canopy-average stomatal conductance$(G_s )$. Changes in$\delta ^{13} C_{ER} $correlated weakly with changes in vapor pressure deficit (VPD) measured 0 and 3–7 days earlier and significantly with soil matric potential$(\psi _m )$(P value <0.02) measured on the same day. Midday$G_s $was estimated using sapflow measurements (heat-dissipation method) at four plots located at different elevations within the watershed. Values of midday$G_s $from 0 and 3–7 days earlier were correlated with$\delta ^{13} C_{ER} $, with the 5-day lag being significant (P value <0.05). To examine direct relationships between$\delta ^{13} C_{ER} $and recent$G_s $, we used models relating isotope discrimination to stomatal conductance and photosynthetic capacity at the leaf level to estimate values of stomatal conductance ("$G_{s--I} $") that would be expected if respired CO₂ were derived entirely from recent photosynthate. We compared these values with estimates of$G_s $using direct measurement of transpiration at multiple locations in the watershed. Considering that the approach based on isotopes considers only the effect of photosynthetic discrimination on$\delta ^{13} C_{ER} $, the magnitude and range in the two values were surprisingly similar. We conclude that: (1)$\delta ^{13} C_{ER} $is sensitive to variations in weather, and (2)$\delta ^{13} C_{ER} $potentially could be used to directly monitor average, basin-wide variations in$G_s $in complex terrain if further research improves understanding of how$\delta ^{13} C_{ER} $is influenced by post-assimilation fractionation processes.