ARS | Publication request: Variability in light-use efficiency for gross primary productivity on Great Plains grasslands

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 20, 2010
Publication Date: January 27, 2011
Citation: Polley, H.W., Phillips, B.L., Frank, A.B., Bradford, J.A., Sims, P.L., Morgan, J.A., Kiniry, J.R. 2011. Variability in light-use efficiency for gross primary productivity on Great Plains grasslands. Ecosystems. 14:15-27.

Interpretive Summary: The amount of carbon (C) that plants capture via the process of photosynthesis (gross primary productivity; GPP) determines plant productivity and affects the amount of C that is stored in terrestrial ecosystems. GPP often is estimated at regional and global scales by multiplying the amount of light absorbed by the plant canopy by a value of light use efficiency, C uptake per unit of absorbed light. Light use efficiency usually is assumed either to be constant or to change predictably as temperature and other environmental conditions change. An implicit assumption of this approach is that environmental effects on light use efficiency do not differ from year-to-year or among ecosystems. We used measurements of C uptake and release from 3 native grasslands in the central Great Plains of the USA to determine whether environmental effects on light use efficiency varied among years and grasslands. Light use efficiency was measured as the slope of the linear relationship between GPP and the amount of light that plants absorbed. Light use efficiency declined as the amount of absorbed light increased and varied among grasslands and among years on each of the grasslands. Light use efficiency was greater during years when the mean air temperature was high than low. Our results indicate that we must account for site-to-site variation in plant and environmental variables in order to accurately predict GPP.

Technical Abstract: Gross primary productivity (GPP) often is estimated at regional and global scales by multiplying the amount of photosynthetically active radiation (PAR) absorbed by the plant canopy (PARa) by a light-use efficiency (eg) which is modeled as a function of air temperature (Ta) and other environmental conditions. An implicit assumption of this approach is that environmental effects on eg or GPP are repeatable among years. We used CO2 flux measurements from three native grasslands in the Great Plains of the USA to determine whether environmental effects on GPP and eg varied among years and grasslands. Growing season values of GPP increased as annual precipitation increased on grasslands apparently because greater precipitation increased leaf area and PARa. This positive effect of increased precipitation on GPP was partly offset by a decline in eg, measured as the slope of the GPP-PARa relationship, as LAI and light capture increased. The relationship between GPP and PARa varied among grasslands and differed among years on a given grassland because of differences in the coupling between Ta and PARa. GPP responded more to a given increase in PARa during years when maximum values of LAI were limited and Ta at a given PARa was high. Results imply that we must account for among-site and among-year differences in seasonal trends in Ta and canopy development in order to accurately predict GPP.