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

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 (δ¹³C) of CO₂. Observed patterns in δ¹³C of forest air were associated with photosynthesis, respiration, and atmospheric boundary layer dynamics. Similar relationships between δ¹³C 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 (δ ¹³C_R, calculated from isotopic mixing lines). A systematic bias was identified when estimating δ ¹³C_R from data sets with small range in CO₂ in the samples, leading us to restrict analysis of δ¹³C_R to periods with CO₂ range >40 μmol mol^-1. Values of δ ¹³C_R 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 δ ¹³C_R calculated near the forest floor (<2 m height) versus the upper canopy (5-11 m) on the same nights. δ ¹³C_R 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 δ¹³C_R at night with the analogous quantity calculated during daytime, but only a few daytime periods met the 40 μmol 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 μmol 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 δ¹³C at multiple spatial scales.