Extensive observations of CO2 carbon isotope content in and above a high-elevation subalpine forest

[1] The dynamics of forest-atmosphere CO2 carbon isotope exchange were examined in a coniferous forest in Colorado, United States. Tunable diode laser absorption spectrometry provided extensive characterization of the carbon isotope content (dC) of CO2. Observed patterns in d C of forest air were associated with photosynthesis, respiration, and atmospheric boundary layer dynamics. Similar relationships between dC and CO2 were observed at all forest heights and confined to a relatively narrow envelope. Substantial variation was observed in the isotope ratio of nocturnal ecosystem respiration (dCR, calculated from isotopic mixing lines). A systematic bias was identified when estimating dCR from data sets with small range in CO2 in the samples, leading us to restrict analysis of dCR to periods with CO2 range >40 mmol mol . Values of dCR varied from 28.1 to 25.2%, with variation from one night to the next as large as 1.7%. A consistent difference was observed between dCR calculated near the forest floor (<2 m height) versus the upper canopy (5–11 m) on the same nights. dCR was more enriched in the upper canopy than near the ground on 34 of 43 nights, with a mean enrichment of 0.6% and a maximum of 2.3%. A similar pattern was observed comparing dCR at night with the analogous quantity calculated during daytime, but only a few daytime periods met the 40 mmol mol 1 criterion. Comparisons between air samples measured (1) 10 m above the forest canopy, (2) 3 km away, and (3) within the convective boundary layer 125 km distant showed CO2 differences between sites as large as 5–6 mmol mol 1 even at midday. These results suggest that attempts to use flask measurements at remote monitoring stations as a proxy for the air directly interacting with a vegetation canopy should be made with caution. However, our results also suggest that substantial information about biosphere-atmosphere isotopic exchange can be obtained by simultaneous examination of CO2 and d C at multiple spatial scales.