Separating The Effects Of Vegetation Phenology And Diffuse Radiation On Land Carbon Uptake
The effects of clouds and aerosols on land carbon uptake may be less than previously thought.
Understanding future climate change requires understanding the full carbon cycle, including how much carbon is taken up by plants over their life cycles, and how that carbon uptake might change with variations in aerosol or cloud conditions. Carbon uptake by plants is often observed to be higher under diffuse radiation associated with clouds and aerosols, implying an effect of these scattering agents on terrestrial gross primary productivity (GPP, which is related to the rate at which photosynthesis occurs). However, the mechanisms underlying the statistical correlation between diffuse radiation and GPP remain uncertain, and the magnitude of the inferred effect varies widely across studies. In this study, scientists showed that the frequently reported enhancement of plant primary productivity by diffuse radiation associated with clouds and aerosols is mainly due to seasonal changes in plant lifecyle (known as phenology) rather than to radiation quality.
Scientists funded by the Department of Energy’s (DOE) Atmospheric System Research program used atmospheric measurements from DOE’s Atmospheric Radiation Measurement (ARM) Climate Research Facility and theoretical modeling to provide new insights into the mechanisms linking diffuse radiation and GPP. They found that diffuse radiation effects on GPP were smaller after accounting for the statistical covariation between diffuse radiation and vegetation phenology. The confounding influence of phenology was confirmed in a canopy photosynthesis and radiative transfer model, suggesting that the effects of diffuse radiation on GPP may have been overestimated in previous studies. These findings address an important land-atmosphere coupling effect, sharpen understanding of the mechanisms linking climate and the carbon cycle, and help inform needed improvements in Earth system models.
GPP has been reported to increase with the fraction of diffuse solar radiation, for a given total irradiance. The correlation between GPP and diffuse radiation suggests there are effects of diffuse radiation on canopy light-use efficiency, but potentially confounding effects of vegetation phenology have not been fully explored. The scientists applied several approaches to control for phenology, using 8 years of eddy-covariance measurements of winter wheat at the ARM Climate Research Facility Southern Great Plains site in Oklahoma. The apparent enhancement of daily GPP due to diffuse radiation was reduced from 260 percent to 75 percent after subsampling over the peak growing season or by subtracting a 15-day moving average of GPP, suggesting that phenology played a role in the apparent diffuse radiation effect. The diffuse radiation effect was further reduced to 22 percent after normalizing GPP by a spectral reflectance index to account for phenological variations in leaf area index and canopy photosynthetic capacity. Canopy photosynthetic capacity covaries with diffuse fraction at a given solar irradiance at this site because both factors are dependent on day of year, or solar zenith angle. Using a two-leaf sun-shade canopy radiative transfer model, the team confirmed that the effects of phenological variations in photosynthetic capacity can appear qualitatively similar to the effects of diffuse radiation on GPP, and therefore can be difficult to distinguish using observations and simple correlations. The importance of controlling for plant phenology when inferring diffuse radiation effects on GPP raises new challenges and opportunities for using radiation measurements to improve carbon cycle models.
Lawrence Berkeley National Laboratory
This research was supported by the Department of Energy (DOE), Office of Science, Office of Biological and Environmental Research under contract number DE-AC02-05CH11231 as part of the Atmospheric System Research and Regional and Global Climate Modeling programs and used data provided by DOE’s Atmospheric Radiation Measurement Climate Research Facility.
Williams, I. N., W. J. Riley, L. M. Kueppers, S. C. Biraud, and M. S. Torn. 2016. “Separating the Effects of Phenology and Diffuse Radiation on Gross Primary Productivity in Winter Wheat,” Journal of Geophysical Research Biogeosciences 121(7), 1903-15. DOI: 10.1002/2015JG003317.