Extensive observations of CO2 carbon isotope content in and above a high-elevation subalpine forest -: art. no. GB3023

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