Below-Canopy And Soil CO2 Fluxes In A Ponderosa Pine Forest

Below-canopy eddy covariance measurements of CO₂ flux (F_cb) and soil surface CO₂flux 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 CO₂ 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 CO₂ 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 CO₂ budget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO₂ 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 CO₂ fluxes. It is important to measure below-canopy fluxes along with above-canopy fluxes throughout the year to understand CO₂ exchange components and annual contributions to the carbon budget of open canopy forest systems.