The influence of clouds and diffuse radiation on ecosystem-atmosphere CO2 and CO18O exhanges

Journal Article · Fri May 01 04:00:00 EDT 2009 · JGR-Biosciences

OSTI ID:964383

Still, C J; Riley, W J; Biraud, S C; Noone, D C; Buenning, N H; Randerson, J T; Torn, M S; Welker, J; White, J W.C.; Vachon, R; Farquhar, G D; Berry, J A

This study evaluates the potential impact of clouds on ecosystem CO{sub 2} and CO{sub 2} isotope fluxes ('isofluxes') in two contrasting ecosystems (a broadleaf deciduous forest and a C{sub 4} grassland), in a region for which cloud cover, meteorological, and isotope data are available for driving the isotope-enabled land surface model, ISOLSM. Our model results indicate a large impact of clouds on ecosystem CO{sub 2} fluxes and isofluxes. Despite lower irradiance on partly cloudy and cloudy days, predicted forest canopy photosynthesis was substantially higher than on clear, sunny days, and the highest carbon uptake was achieved on the cloudiest day. This effect was driven by a large increase in light-limited shade leaf photosynthesis following an increase in the diffuse fraction of irradiance. Photosynthetic isofluxes, by contrast, were largest on partly cloudy days, as leaf water isotopic composition was only slightly depleted and photosynthesis was enhanced, as compared to adjacent clear sky days. On the cloudiest day, the forest exhibited intermediate isofluxes: although photosynthesis was highest on this day, leaf-to-atmosphere isofluxes were reduced from a feedback of transpiration on canopy relative humidity and leaf water. Photosynthesis and isofluxes were both reduced in the C{sub 4} grass canopy with increasing cloud cover and diffuse fraction as a result of near-constant light limitation of photosynthesis. These results suggest that some of the unexplained variation in global mean {delta}{sup 18}O of CO{sub 2} may be driven by large-scale changes in clouds and aerosols and their impacts on diffuse radiation, photosynthesis, and relative humidity.

• Research Organization: Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US)
• Sponsoring Organization: Earth Sciences Division
• DOE Contract Number: AC02-05CH11231
• OSTI ID: 964383
• Report Number(s): LBNL-2155E
• Journal Information: JGR-Biosciences, Journal Name: JGR-Biosciences Vol. 114
• Country of Publication: United States
• Language: English

Related Subjects

54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
AEROSOLS
CARBON
CLOUD COVER
CLOUDS
ECOSYSTEMS
FEEDBACK
FORESTS
GRAMINEAE
HUMIDITY
PHOTOSYNTHESIS
RADIANT FLUX DENSITY
RADIATIONS
SKY
TRANSPIRATION
WATER