SUMMARY

Physiological responses to elevated CO2 are discussed at leaf, plant and stand scale in the context of global change and their consequences for water relations at these scales evaluated. A distinction is drawn between short term responses and the responses shown by plants that are fully acclimated by extended periods of growth in a high CO2 environment.

Assimilation of CO2 and stomatal action are the best known processes dependent on CO2 at leaf scale and acclimation reduces their impact on growth, transpiration and water use efficiency.

At plant scale leaf and root growth are generally stimulated in high CO2 but the processes involved are poorly understood. The consequence may be increase in rate of both transpiration and water uptake. Control system based models explicitly including feedbacks offer a means of integrating multiple interacting limiting variables and of analysing sensitivity of processes to increase in CO2 concentration at both leaf and plant scale. To make use of such models to give helpful predictions, better definition of “pressure points” – the processes on which CO2 is known to act – is needed. Response of processes within the plant to elevated atmospheric CO2 is strongly influenced by coupling between leaves and atmosphere.

At the larger stand scale, this should be taken into account explicitly because the degree of coupling depends on the structure of the vegetation. Increase in leaf area will have a larger effect on transpiration from well coupled vegetation (e.g.tall crops, shrubs and trees) but may have little effect on transpiration from poorly coupled vegetation (e.g.short crops, grass lands, dwarf shrubs). […]