More realistic CO2 fertilisation in a revised big-leaf model

The observed increase in forest productivity over the industrial period is largely attributed to a stimulation of photosynthesis by increasing atmospheric CO₂ concentrations. The extent to which this fertilisation effect will persist in the future, however, is uncertain as competing limitations such as the availability of nutrients and soil moisture may prevent plants from making use of the carbon they assimilate, which may dominate the future photosynthesis response.

Many early studies of the CO₂ fertilisation effect use simulations that make use of the classical ‘big-leaf’ approach for scaling leaf photosynthesis to the canopy described by Sellers et al., (1992), which has been shown to produce unrealistically low sensitivities to light. Light limitation has the potential to significantly weaken the CO2 fertilisation effect relative to these early predictions. More realistic multi-layer canopy schemes have since been developed that produce more accurate light responses, however, the impact that this has on predicted CO₂ fertilisation seems to have gone unstudied.

In this presentation we present a modified version of the big-leaf canopy scheme. This new version produces more realistic sensitivities to light while remaining more computationally efficient than modern multi-layer canopy schemes. We examine the effect that the new big-leaf scheme has on predictions of future CO2 fertilisation and demonstrate the importance of correctly simulating the dominant limitations of photosynthesis.