Using nocturnal cold air drainage flow to monitor ecosystem processes in complex terrain

This paper presents initial investigations of a new approach to monitor ecosystem processes in complex terrain on large scales. Metabolic processes in mountainous ecosystems are poorly represented in current ecosystem monitoring campaigns because the methods used for monitoring metabolism at the ecosystem scale (e.g., eddy covariance) require flat study sites. Our goal was to investigate the potential for using nocturnal down-valley winds (cold air drainage) for monitoring ecosystem processes in mountainous terrain from two perspectives: measurements of the isotopic composition of ecosystem-respired CO₂ $\left( {{\rm{\delta }}^{13} {\rm{C}}_{{\rm{ER}}} } \right)$ and estimates of fluxes of CO₂ transported in the drainage flow. To test if this approach is plausible, we monitored the wind patterns, CO₂ concentrations, and the carbon isotopic composition of the air as it exited the base of a young (~40 yr-old) and an old (>450 yr-old) steeply sided Douglas-fir watershed. Nocturnal cold air drainage within these watersheds was strong, deep, and occurred on more than 80% of summer nights. The depth of cold air drainage rapidly increased to tower height or greater when the net radiation at the top of the tower approached zero. The carbon isotope composition of CO₂ in the drainage system holds promise as an indicator of variation in basin-scale physiological processes. Although there was little vertical variation in CO₂ concentration at any point in time, we found that the range of CO₂ concentration over a single evening was sufficient to estimate ${\rm{\delta }}^{13} {\rm{C}}_{{\rm{ER}}} $ from Keeling plot analyses. The seasonal variation in ${\rm{\delta }}^{13} {\rm{C}}_{{\rm{ER}}} $ followed expected trends: during the summer dry season ${\rm{\delta }}^{13} {\rm{C}}_{{\rm{ER}}} $ became less negative (more enriched in ¹³C), but once rain returned in the fall, ${\rm{\delta }}^{13} {\rm{C}}_{{\rm{ER}}} $ decreased. However, we found no correlation between recent weather (e.g., vapor pressure deficit) and ${\rm{\delta }}^{13} {\rm{C}}_{{\rm{ER}}} $ either concurrently or with up to a one-week lag. Preliminary estimates suggest that the nocturnal CO₂ flux advecting past the 28-m tower is a rather small fraction (