Iain Colin Prentice, Rebecca Thomas
AXA Chair Programme in Biosphere and Climate Impacts, Department of Life Sciences, Imperial College London, UK
Navin Ramankutty
Chair in Global Environmental Change and Food Security, University of British Columbia, Vancouver, Canada
Han Wang
State Key Laboratory of Soil Erosion and Dryland Farming on the Loess Plateau, College of Forestry, Northwest A & F University, Yangling, China
Satellite-based monitoring makes it possible to estimate the fraction of incident photosynthetically active radiation (IPAR) absorbed by green plants (fAPAR) and thus potentially available to power photosynthesis in all kinds of ecosystems. Gross primary production (GPP), the time-integral of total canopy photosynthesis, is proportional to absorbed photosynthetically active radiation – the product of IPAR and fAPAR. The constant of proportionality is the light use efficiency (LUE). Many models predict GPP from IPAR ´ fAPAR, climate, and (often) plant functional type (PFT). These models typically have many empirical coefficients, usually estimated from CO2 flux measurements.
We derive a parsimonious LUE model (‘P’) from first principles, starting with the Farquhar et al. photosynthesis model and two optimality concepts: the ‘least-cost hypothesis’, which predicts an optimum ratio of leaf-internal to ambient CO2, and the ‘co-ordination hypothesis’, which proposes that the electron transport capacity Jmax and the carboxylation capacity Vcmax acclimate to current average daytime conditions so that neither one nor the other is in excess. Our derivation explains the shift from the saturating response of photosynthesis to IPAR, seen in flux measurements over the diurnal cycle, to the linear reponse that applies over longer time scales. Using flux measurements as a benchmark, we show that the new model’s performance rivals or exceeds that of other LUE models – even though it does not distinguish PFTs, apart from C3 versus C4 plants. The model also correctly predicts the effects of rising CO2 on LUE, stomatal conductance, water use efficiency, and the Jmax : Vcmax ratio as measured in Free-Air Carbon Enrichment (FACE) experiments.
LUE models can diagnose spatial patterns and trends of GPP, but depend on observing fAPAR. To project potential GPP into the future requires a quantitative understanding of the controls on fAPAR. Increases in fAPAR during the satellite era have occurred, and have been attributed to warming in high latitudes and more generally to the effect of rising CO2 concentrations on C3 photosynthesis. But increasing CO2 also promotes more efficient water use by plants: this is shown by observed changes in the relationship of maximum fAPAR to precipitation in semi-arid regions. Land use has a more complex influence. Positive effects of land use on fAPAR and GPP may be accounted for in different ways depending on the social and environmental context. In addition to irrigation and the use of artificial fertilizers, these influences may include passive fire suppression and management practices that result in the conservation and/or local redistribution of nutrients.
