Time course of radiation use efficiency in a shortgrass ecosystem: Consequences for remotely sensed estimation of primary production

A reliable estimation of primary production of terrestrial ecosystems is of ten a prerequisite for land survey and management, while being important al so in ecological and climatological studies. At a regional scale, grassland primary production estimates are increasingly being made with the use of s atellite data. In a currently used approach, regional gross, net, and above ground net primary productivity (GPP, NPP, and ANPP) are derived from the p arametric model of Monteith and are calculated as the product of the fracti on of incident photosynthetically active radiation absorbed by the canopy ( f(APAR)) and gross, net, and aboveground net production (radiation-use) eff iciencies (epsilon(g), epsilon(n), and epsilon(an)); f(APAR) being derived from indices calculated from satellite-measured reflectances in the red and near infrared. The accuracy and realism of the primary production values e stimated by this approach therefore largely depend on an accurate estimatio n of epsilon(g), epsilon(n), and epsilon(an). However, data are scarce for production efficiencies of semiarid grasslands, and their time and spatial variations are poorly documented, often leading to large errors for the est imates. In this paper, a modelling approach taking into account relevant ec osystem processes and based on extensive field data was used to estimate ti me variations of epsilon(g), epsilon(n), and epsilon(an) of a shortgrass si te in Arizona. These variations were explained by variations in plant water stress, temperature, leaf aging, and processes such as a respiration and c hanges in allocation pattern between above- and below-ground compartments. Over the 3 study years, averaged values of epsilon(g), epsilon(n), and epsi lon(an) were found to be 1.92, 0.74, and 0.29 g DM (MJ IPAR)(-1), respectiv ely. epsilon(g) and epsilon(n) exhibited large interannual and seasonal var iations mainly due to changes in water limitations during the growing seaso n. Interannual variations of epsilon(an) were much less important. However, for shorter periods, epsilon(an) exhibited very contrasting values from re growth to senescence. The calculation of ANPP seems less prone to errors du e to environmental effects when computed on an annual basis. When estimatin g GPP and NPP, better results are expected if water limitations are taken i nto account. This could be possible through the estimation of water-stress factor by using surface temperature or other indices derived from thermal i nfrared remote sensing data. The limitations due to temporally varying effi ciencies, shown here for shortgrass ecosystems, are also relevant to all dr ought-exposed ecosystems, particularly those with abundant evergreen or per ennial species. Published by Elsevier Science Inc.