Modelling Terrestrial Carbon Exchange and Storage: Evidence and Implications of Functional Convergence in Light-use Efficiency

The practicality of ecological research at a global scale is increasing as a result of the development of satellite remote sensing which enables surface conditions to be inferred for large areas at high spatial and temporal resolutions. Remote sensing of canopy light absorption, for example, enables an important component of canopy carbon assimilation rates to be estimated over extensive areas. Modelling of carbon exchange and net storage may also be facilitated, owing to natural selection for a narrow range of light-use efficiencies among a wide range of plant functional types. This "functional convergence" arises from resource allocation strategies that appear to maximise the benefits of carbon (energy) and nutrients (e.g. mainly nitrogen) relative to the costs of acquisition. Convergence is most evident on a leaf mass per unit ground area basis, a measure that more closely reflects the costs of resource acquisition than does leaf area. We demonstrate, however, that light absorption and utilisation are decoupled so that convergence is to be expected on gross production rather than net production, owing to differences in respiratory costs associated with synthesis and maintenance of plant constituents and associated "payback intervals" on carbon investment in different functional types. A link between fitness and carbon gain is noted in relation to gross primary production. We conclude that, while functional convergence provides a basis for the use of remote sensing of light absorption in measurement of primary production, models driven with light absorption need to include terms that describe the actual respiratory costs of maintenance and synthesis. Quantification of these processes will improve large area primary production models, and enhance the value of the information that can be acquired by remote sensing.