Below-canopy and soil CO2 fluxes in a ponderosa pine forest

Below-canopy eddy covariance measurements of CO2 flux (F-cb) and soil surfa ce CO2 flux measurements (F-s) were made seasonally in a ponderosa pine for est in central Oregon in 1996 and 1997. The forest ecosystem has a very ope n canopy, and it is subject to drought and high vapor pressure deficits in summer. Below-canopy flux measurements in March, May, and August 1997 showe d increasing effluxes from the forest floor as soils warmed. In July 1996, daytime F-cb measurements appeared to have been influenced by photosyntheti c uptake of CO2 by ground vegetation. We did not see a similar diurnal tren d in F-cb data in August 1997, probably because photosynthesis may have dec reased with senescence of similar to 1/3 of the pine canopy and the herbace ous species. On 4 days in August 1997, the mean nocturnal F-s (2.6 +/- 0.08 mu mol m(-2) s(-1)) was lower than nocturnal F-cb (3.5 +/- 0.28 mu mol m(- 2) s(-1)) by 26%, and daytime F-s was lower than nocturnal F-cb by 18%, pos sibly because F-cb includes respiration by understory and the lower portion s of trees. The mean nocturnal NEE calculated from above-canopy flux and st orage in the canopy airspace (F-ca + F-stor) at this time was 2.8 +/- 0.40 mu mol m(-2) s(-1), 23% lower than ecosystem respiration calculated from ch amber 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 unde r 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 c onditions (e.g. u* = 0.30 m s(-1)), F-ca begins to become significant and f luxes missed by the above-canopy eddy correlation system degrade the CO2 bu dget. Under windy conditions, the above-canopy eddy correlation measurement is a good approximation and the CO2 budget improves again. Below-canopy fl ux measurements provided useful temporal information for understanding seas onal differences in diel patterns, while the chambers allowed us to charact erize spatial variation in CO2 fluxes. It is important to measure below-can opy fluxes along with above-canopy fluxes throughout the year to understand CO2 exchange components and annual contributions to the carbon budget of o pen canopy forest systems. (C) 1999 Elsevier Science B.V. All rights reserv ed.