Variations in the influence of diffuse light on gross primary productivity in temperate ecosystems
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Agricultural and Forest Meteorology
The carbon storage potential of terrestrial ecosystems depends in part on how atmospheric conditionsinfluence the type and amount of surface radiation available for photosynthesis. Diffuse light, result-ing from interactions between incident solar radiation and atmospheric aerosols and clouds, has beenpostulated to increase carbon uptake in terrestrial ecosystems. However, the magnitude of the diffuselight effect is unclear because existing studies use different methods to derive above-canopy diffuse lightconditions. We used site-based, above-canopy measurements of diffuse light and gross primary produc-tivity (GPP) from 10 temperate ecosystems (including mixed conifer forests, deciduous broadleaf forests,and croplands) to quantify the GPP variation explained by diffuse photosynthetically active radiation(PAR) and to calculate increases in GPP as a function of diffuse light. Our analyses show that diffuse PARexplained up to 41% of variation in GPP in croplands and up to 17% in forests, independent of directlight levels. Carbon enhancement rates in response to diffuse PAR (calculated after accounting for vaporpressure deficit and air temperature) were also higher in croplands (0.011–0.050 µmol CO2 per molphotons of diffuse PAR) than in forests (0.003–0.018 µmol CO2 per mol photons of diffuse PAR). Theamount of variation in GPP and carbon enhancement rate both differed with solar zenith angle and acrosssites for the same plant functional type. At crop sites, diffuse PAR had the strongest influence and thelargest carbon enhancement rate during early mornings and late afternoons when zenith angles werelarge, with greater enhancement in the afternoons. In forests, diffuse PAR had the strongest influenceat small zenith angles, but the largest carbon enhancement rate at large zenith angles, with a trend inecosystem-specific responses. These results highlight the influence of zenith angle and the role of plantcommunity composition in modifying diffuse light enhancement in terrestrial ecosystems, which will beimportant in scaling this effect from individual sites to the globe.
Cheng, Susan J.; Bohrer, Gil; Steiner, Allison L.; Hollinger, David Y.; Suyker, Andrew; Phillips, Richard P.; Nadelhoffer, Knute J. 2015. Variations in the influence of diffuse light on gross primary productivity in temperate ecosystems. Agricultural and Forest Meteorology. 201: 98-110. https://doi.org/10.1016/j.agrformet.2014.11.002.