The effect of static pressure-wind covariance on vertical carbon dioxide exchange at a windy subalpine forest site

Accounting for turbulent temperature and water vapor fluctuations on the vertical flux of CO₂ measured by an open-path infrared gas analyzer are commonly known as the Webb–Pearman–Leuning (WPL) corrections. Static pressure fluctuations also affect air sample density, but the magnitude and effect of these changes on CO₂ flux is not usually considered. In our study, the turbulent static pressure p and the vertical wind component w covariance w^′p^′¯ from just-above a Rocky Mountain subalpine forest are examined. The magnitude of w^′p^′¯ was highly correlated to the mean horizontal wind speed U with similar characteristics during the daytime and nighttime. The pressure term was calculated from w^′p^′¯ and compared to other terms in the equation for the vertical net ecosystem exchange of CO₂ (NEE). We found the following: (1) for U≲6 m s⁻¹, the pressure term was small, (2) as U increased beyond 6 m s⁻¹, the pressure term became more and more important reaching a magnitude of ≈ 2 μmol m⁻² s⁻¹ at U of 12 m s⁻¹ (for leaf area index LAI ≈ 2.6 m² m⁻²), (3) for a more dense forest (LAI ≈ 4.8 m² m⁻²), the magnitude of the pressure term at U of 12 m s⁻¹ was ≈ 1 μmol m⁻² s⁻¹, or about half the open forest value, and (4) based on 14 years of measurements, the interannual mean and standard deviation of the yearly cumulative pressure and NEE terms were −32.8± 24.5 g C m⁻² year⁻¹ and −147.4± 231.7 g C m⁻² year⁻¹, respectively. Therefore, on average, including the pressure term in the NEE calculation reduces the annual carbon uptake by the GLEES forest from 147.4 g C m⁻² year⁻¹ to 114.6 g C m⁻² year⁻¹. This implies that carbon uptake by forests in windy locations that ignore the pressure term can be overestimated by as much as 20%. However, the year-to-year variability of both NEE and the pressure term was large due to changes in the forest structure and LAI from beetle attack and tree die-off. Finally, we present results from sensor-manipulation experiments examining the effect of tilting the quad-disk pressure ports on the turbulent static pressure.