PHYSIOLOGICAL-BASIS OF THE LIGHT USE EFFICIENCY MODEL

The observation that, for unstressed plants, light use efficiency of a plant canopy, defined as the ratio of net primary productivity (NPP) to absorbed photosynthetically active radiation (APAR), is approximate ly constant with respect to changes in APAR, implies that NPP can be m odeled using a linear relationship with APAR. However, such a linear r elationship is counter-intuitive because the relationship between leaf photosynthesis and absorbed light is strongly nonlinear. Three argume nts have been advanced to explain the observed linear relationship bet ween NPP and APAR. In this paper, a detailed, physiologically based mo del of canopy radiation absorption and photosynthesis (MAESTRO) was us ed to analyze these arguments. The first argument is that the canopy i s structured so that radiation is distributed throughout the canopy su ch that most leaves are exposed to non-saturating quantum flux density , resulting in a linear response of canopy photosynthesis to APAR. Sim ulations of MAESTRO indicated that this explanation is inadequate, bec ause daily values of canopy photosynthetic light use efficiency calcul ated with MAESTRO were highly variable regardless of canopy structure. The second argument is that variability in light use efficiency decre ases with increasing time scale. The simulations showed that this is t rue to some extent, although simulated annual canopy photosynthetic li ght use efficiency still varies across sites with different LAI or lig ht climate. The third argument is that changes in canopy nitrogen cont ent act both to maximize net canopy photosynthesis and to keep light u se efficiency constant. This argument could not be tested with the mod el, but the failure of the first two explanations suggests that this t hird explanation deserves closer attention.