LAUSR

Abstract Accounting for turbulent temperature and water vapor fluctuations on the vertical flux of CO 2 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 CO 2 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 CO 2 (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  μ mol m − 2  s − 1 at U of 12 m s − 1 (for leaf area index LAI  ≈  2.6 m 2  m − 2 ), (3) for a more dense forest (LAI  ≈  4.8 m 2  m − 2 ), the magnitude of the pressure term at U of 12 m s − 1 was ≈  1  μ mol 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.