Improving ecosystem productivity modeling through spatially explicit estimation of optimal light use efficiency

A com m on assumption o f rem ote sensing-based ligh t use efficiency (LUE) models fo r estimating vegetation gross primary productiv ity (GPP) is tha t plants in a b iom e m atrix operate at the ir photosynthetic capacity under optim a l c iim aticcond itions. A prescribed constant b iom e m axim um ligh t use efficiency parameter (LUEmax) defines the m axim um photosynthetic carbon conversion rate under these conditions and is a large source o f model uncertainty. Here we used tow er eddy covariance measurement-based carbon (CO2) fluxes for spatial estim ation o f op tim a l LUE (LUEopt) across N orth America. LUEopt was estim ated at 62 Flux N etw ork sites using tow e r daily carbon fluxes and m eteoro logy, and satellite observed fractional photosynthetica lly active radiation from the M oderate Resolution im aging Spectroradiom eter. Ageosta tistica i m odel was fitte d to 45 flux tow er-derived LUEopt data points using independen t geospatial environm enta l variables, inc lud ing g lobal p lant traits, soil moisture, terrain aspect, land cover type, and percent tree cover, and va lidated at 17 independent tow e r sites. Estimated LUEopt shows large spatial va riab ility w ith in and am ong d iffe ren t land cover classes ind ica ted from the sparse to w e r netw ork. Leaf n itrogen co n ten t and soil m oisture regim e are m ajor factors exp la in ing LUEopt patterns. GPP derived from estim ated LUEopt shows s ign ifican t corre la tion im p rovem en t against tow e r GPP records (/? ̂= 76.9%; mean root-mean-square error (RMSE) = 257g C m ^^ y r^ * ), relative to alternative GPP estimates derived using biome-specific LUEmax constants (/?̂ = 34.0%; RMSE = 439 g C m ^^y r^ * ). GPP de te rm ined from the LUEopt m ap also explains a 49.4% greater p ro p o rtio n o f to w e r GPP va riab ility at the inde p en d e n t va lida tion sites and shows prom ise fo r im p rov ing understand ing o f LUE patterns and environm enta l contro ls and enhancing regional GPP m on ito ring from satellite rem ote sensing.