HR: 10:20h
AN: B22B-01    [Abstracts]
TI: The Impact of Clouds on Ecosystem-Atmosphere CO¹⁸O Exchanges in the U.S. Great Plains
AU: * Still, C J
EM: still@icess.ucsb.edu
AF: UC Santa Barbara, Geography Department, 3611 Ellison Hall, Santa Barbara, CA 93106 United States
AU: * Still, C J
EM: still@icess.ucsb.edu
AF: UC Santa Barbara, Institute for Computational Earth System Science, Santa Barbara, CA 93106 United States
AU: Riley, W J
EM: wjriley@lbl.gov
AF: Lawrence Berkeley National Lab, Earth Sciences Division, 90-1106, 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Biraud, S C
EM: scbiraud@lbl.gov
AF: Lawrence Berkeley National Lab, Earth Sciences Division, 90-1106, 1 Cyclotron Road, Berkeley, CA 94720 United States
AU: Noone, D C
EM: dcn@colorado.edu
AF: University of Colorado, Program in Atmospheric and Oceanic Sciences, Stadium 255-10, Boulder, CO 80309 United States
AU: Berry, J A
EM: joeberry@globalecology.stanford.edu
AF: Carnegie Institution of Washington, Department of Global Ecology, 260 Panama Street, Stanford, CA 94305-1297 United States
AB: The downward excursion in δ¹⁸O of atmospheric CO₂ observed during the 1990s and the large interannnual variability characteristic of this isotopologue are not understood. We hypothesize that these variations in δ¹⁸O of atmospheric CO₂ may be linked to global-scale variations in cloud cover and its influence on biosphere-atmosphere CO¹⁸O exchanges. Recent work has demonstrated the influence of clouds on canopy photosynthesis through increases in the diffuse radiation fraction and relative humidity, combined with decreases in leaf temperature. In concert, these alterations tend to increase canopy photosynthesis, which should also increase CO¹⁸O fluxes. However, photosynthetic CO¹⁸O fluxes also depend on the δ¹⁸O of leafwater, and enhanced cloudiness should decrease the δ¹⁸O of leafwater by enhancing relative humidity. Thus, the net impact of differing cloud cover on biosphere-atmosphere CO¹⁸O exchanges is difficult to predict. To capture these contrasting effects, we employed a comprehensive ecosystem isotope model (ISOLSM) in the southern great plains region of Oklahoma and Kansas. This region is particularly amenable for such a study because of the density of cloud property and radiation measurements. The region contains natural and agricultural ecosystems representing a variety of photosynthetic pathways and growth forms, including tallgrass prairie pastures, broadleaf forests, and crops. To drive the model across the entire region, we used Mesonet meteorological data collected at 120 stations in 2004, as well as precipitation δ¹⁸O values from the National Atmospheric Deposition Program network. LAI profiles from 2004 were derived from MODIS data. Our results suggest a large impact of clouds on photosynthetic CO₂ and CO¹⁸O fluxes across this region. In an unstressed broadleaf deciduous forest (LAI=6.3), three sequential midsummer days with contrasting cloud cover illustrate this impact. Julian Day 222 is sunny, JD 223 is partly cloudy, and JD 224 is very cloudy. Increasing cloud cover over these days drives increases in relative humidity, the diffuse PAR fraction, and the fraction of canopy photosynthesis from shade leaves. Despite the lower total PAR on JD 223, total canopy photosynthesis is increased over the sunny day, driven by a large increase in shade leaf photosynthesis following the increase in diffuse PAR. Although canopy photosynthesis is higher on the partly cloudy day than the sunny day, the photosynthetic isoflux is lower due to the less enriched leafwater. The very cloudy day (JD 224) is predicted to have a negative isotopic discrimination and thus photosynthetic isoflux due to the lack of leafwater enrichment throughout the daytime. In contrast, discrimination against ¹³CO₂ is predicted to increase slightly over this period, illustrating the myriad impacts that clouds have on biosphere-atmosphere isotope exchanges. Our results also suggest that these effects depend strongly on LAI and photosynthetic pathway (C₃ or C₄) of the vegetation, as well as on the effective water content of canopy leaves and its influence on leafwater δ¹⁸O.
DE: 0321 Cloud/radiation interaction
DE: 0426 Biosphere/atmosphere interactions (0315)
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1851 Plant ecology (0476)
SC: Biogeosciences [B]
MN: Fall Meeting 2005