Respiratory fluxes in a mixed conifer forest in northern Idaho: Development of methods and subsequent applications

Isotope theories originally developed for leaves have been applied in studies scaled from the leaf to ecosystem level. Post-photosynthetic changes in delta13C of plants components have been described in the literature, which compromise the application of leaf level theories to other plant C pools and fluxes. Respiratory fluxes are particularly relevant because their delta13C values are used to partition carbon budgets. The delta13C of leaf (deltalr) and stem (deltast-r) respired CO2 have rarely been recorded in the field. The objectives of this research were to: (1) develop methods to collect deltast-r, (2) record values for deltalr and delta st-r from mature trees in the field, and (3) elucidate underlying processes.;Two methods to obtain delta13C of respiratory fluxes with closed chambers were examined: equilibrium and non-equilibrium. Detailed equations for the equilibrium method were developed in addition to a novel chamber design for use in tree stems. These methods were used to evaluate the meaning of deltast-r. Neither the release of root CO2 stored in the stem nor the vertical transport of CO 2 in xylem sap had any influence on delta13C of CO 2 measured in stem efflux. Additionally deltast-r was not contaminated by soil CO2.;Leaf-level gas exchange and isotopic measurements were performed on four conifer species at three canopy positions throughout a growing season. Both vertical and seasonal patterns were described and used to develop canopy level values. These values were combined with delta13C of stem phloem to estimate canopy mesophyll conductance (gm). Neglecting gm in photosynthetic discrimination calculations produced large errors. Phenology affected delta lr as expanding foliage was depleted in delta lr, relative to mature foliage. This was explained by refixation of the leaf CO2 pool during expansion. In mature foliage, respiratory fractionation during decarboxylation (e) was -4 to -7‰. Values were consistent with CO2 derived during glycolysis from a recent-local carbohydrate pool. Findings were supported by: (1) a similar vertical trend in the canopy for deltalr and delta 13C of leaf bulk material, (2) a direct linear relationship among delta 13C of canopy respiration and assimilation, and (3) a constant species-specific e for all sampling months. Next generation of global-scale C models needs to account for both gm and e.