Below-canopy and soil CO 2 fluxes in a ponderosa pine forest

Abstract

Below-canopy eddy covariance measurements of CO 2 flux ( F cb) and soil surface CO 2 flux measurements ( F s) were made seasonally in a ponderosa pine forest in central Oregon in 1996 and 1997. The forest ecosystem has a very open canopy, and it is subject to drought and high vapor pressure deficits in summer. Below-canopy flux measurements in March, May, and August 1997 showed increasing effluxes from the forest floor as soils warmed. In July 1996, daytime F cb measurements appeared to have been influenced by photosynthetic uptake of CO 2 by ground vegetation. We did not see a similar diurnal trend in F cb data in August 1997, probably because photosynthesis may have decreased with senescence of ∼1/3 of the pine canopy and the herbaceous species. On 4 days in August 1997, the mean nocturnal F s (2.6 ± 0.08 μmol m −2 s −1) was lower than nocturnal F cb (3.5 ± 0.28 μmol m −2 s −1) by 26%, and daytime F s was lower than nocturnal F cb by 18%, possibly because F cb includes respiration by understory and the lower portions of trees. The mean nocturnal NEE calculated from above-canopy flux and storage in the canopy airspace ( F ca + F stor) at this time was 2.8 ± 0.40 μmol m −2 s −1, 23% lower than ecosystem respiration calculated from chamber measurements on soils, wood, and foliage. The largest difference was observed on a more a turbulent night ( u * = 0.30 m s −1) when F ca + F stor was even significantly less than F cb and F s. Our hypothesis is that under calm conditions (e.g. u * < 0.15 m s −1 as observed on three of the nights), F ca is negligible and has no impact on the CO 2 budget. Under weak wind conditions (e.g. u * = 0.30 m s −1), F ca begins to become significant and fluxes missed by the above-canopy eddy correlation system degrade the CO 2 budget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO 2 budget improves again. Below-canopy flux measurements provided useful temporal information for understanding seasonal differences in diel patterns, while the chambers allowed us to characterize spatial variation in CO 2 fluxes. It is important to measure below-canopy fluxes along with above-canopy fluxes throughout the year to understand CO 2 exchange components and annual contributions to the carbon budget of open canopy forest systems.