Nocturnal air drainage in forest canopies: a new way of studying physiological responses to the weather?

Recent studies show that the amount and isotopic composition of ecosystem-respired CO2 (d13CR) is strongly influenced by recent photosynthesis. Therefore, measurements of d13CR, which may be accomplished through “Keeling Plot” analyses, could provide a powerful way to monitor vegetation function. Unfortunately, many of the world’s most productive forest ecosystems are in mountainous regions that produce complex airflow patterns, resulting in a) uncertainty in the source area (footprint) of the sampled air, and b) reduced nighttime accumulation of CO2, making it difficult to obtain an adequate range of CO2 concentrations for Keeling Plot analysis. However, well-developed night-time cold-air drainage might be used to advantage if the land surface of the drainage could be well-defined and if the drainage were uncoupled from the bulk atmosphere, allowing respired CO2 to accumulate. We studied airflow patterns, CO2 concentrations and d13CR in a small deeply incised watershed in the western Cascade Mountains of Oregon, USA. The watershed is forested with predominantly Douglas fir, about 40 years old.

Katabatic (downslope) flow occurs below the canopy in this watershed typically for at least 12 hours per day in summer. Wind speeds are largest near the ground. During the day, the in-canopy vertical temperature profile is stable, and this leads to stratified flow in which carbon dioxide respired from soil and vegetation is not well mixed in the canopy air-space.

When net radiation above the canopy becomes negative, the in-canopy temperature profile is unstable, and strong mixing of air in the canopy occurs, resulting in very small vertical gradients of CO2. During these periods of strong in-canopy mixing, the horizontal flux of CO2 at the base of the watershed appears to account for less than 10% of the CO2 respired from the whole watershed. Further measurements and modeling will focus on determining whether this small fraction occurs because the source area of respired CO2 detected at our tower is smaller than the entire watershed, or because there is dilution of the in-canopy CO2 by air entrained from above the canopy.