Temporal and spatial effects on the isotopic composition of ecosystem-respired CO2 in a subalpine forest

Stable isotopes provide insight into ecosystem carbon cycling, plant physiological processes, and atmospheric boundary-layer dynamics, and are useful in integration of processes over multiple scales.  Of particular interest is the δ¹³C of nighttime ecosystem respired CO₂ (δ¹³C_R).  Recent advances in technology have made it possible to examine the variation in δ¹³C_R within a forest canopy over wide temporal (hours to years) and spatial (meters) ranges.  We used tunable diode laser spectroscopy to examine δ¹³C_R at within- and below-canopy spatial locations in a subalpine forest at 0.5 h intervals over the entire growing season.  The topography and presence of sustained nightly drainage flows at our site suggests that, on nights with stable atmospheric conditions, there is little mixing of air near the ground (from 0.10 to 2m, δ¹³C_R-g) with that in the canopy (6 to 11 m, δ¹³C_R-c).  On these stable nights, δ¹³C_R-g varied from -26.1 ± 0.6 to -23.8 ± 0.6‰ and δ¹³C_R-c varied from -26.9 ± 0.6 to -24.1 ± 0.6‰.  We also found a systematic depletion of 0.81 ± 0.74‰ of δ¹³C_R-g compared to δ¹³C_R-c.   This observed difference may be due to differences in isotopic fraction between leaf and root/rhizosphere respiration, or to differences in the isotopic composition of the substrates being respired by leaves and the soil.