An interannual assessment of the relationship between the stable carbon isotopic composition of ecosystem respiration and climate in a high-elevation subalpine forest

We measured the carbon isotopic composition (δ¹³C) of ecosystem respiration (δ¹³C_R) in a subalpine forest across four growing seasons to examine whether patterns in δ ¹³C_R were consistent with those expected based on leaf-level gas-exchange theory, and in agreement with past studies of the relation between δ¹³C_R and climate conducted across broad geographic regions. Conventional trends (i.e., less negative δ¹³C_R with increased vapor pressure deficit (VPD) and air temperature (T_AIR), and decreased soil moisture (θ)) were observed when we focused on the driest portions of average-wetness years and when δ¹³C_R was positively correlated with nighttime ecosystem respiration (R_E). Nonconventional trends (i.e., more negative δ¹³C_R with decreased θ, and increased VPD and T_AIR) were observed under specific climatic conditions (e.g., late snowmelt; extreme T_AIR late in the growing season), and when δ¹³C_R was negatively correlated with R_E. These nonconventional trends were independently corroborated using δ¹³C of extracted sugars from needles of dominant tree species at the site. Our results clearly demonstrate that the commonly reported relations between δ¹³C_R and climate may break down depending on the interactions among environmental conditions. Efforts to model and predict the variability of δ¹³C_R under changing climatic variables must characterize and parameterize the effects of unique combinations of weather conditions and variable hydrologic regimes, in combination with the susceptibility of photosynthetic isotope discrimination to extreme air temperatures.