Can We Monitor Ecosystem Function Using Keeling Plot Analyses of Nocturnal Cold-Air Drainage?

The carbon isotope signature of ecosystem respiration, δ ¹³C_R, as measured by the Keeling Plot approach, has been related to short-term variations in weather and ecosystem function in several recent studies. In order to obtain an adequate range of [CO₂] and to sample a consistent vegetation type, investigators typically select sampling locations in relatively flat terrain and uniform canopy cover, but these are unusual conditions for many forested ecosystems. In a pilot study, we are collecting samples for Keeling Plot analyses in cold-air drainage systems in small (60-100 ha), deeply-incised watersheds, one covered with old-growth (ca 450-years-old) Douglas-fir/hemlock forest and one covered with young (ca 45-years-old) Douglas-fir forest. We found that the nightly range of [CO₂] was typically 380-460 ppm, sufficient to develop good estimates of δ ¹³C_R. At any point in time there was little variation in [CO₂] with height through the canopy (0.5-30m), so the required range was obtained by sampling over several hours. There was no indication that samples taken from different heights or at different times of night represented sources with different isotopic signatures. The isotopic signature of respired CO₂ in the older watershed averaged about 1 per mil greater than that of the young watershed, and δ ¹³C_R of both locations correlated with modeled stomatal conductance 6 days prior to flask sampling.