Consequences of elevated CO2 and O3 on birch canopy structure: Implementation of a canopy growth model

We used a canopy growth model as a scaling tool to make canopy level predictions on leaf area index (LAI) and leaf area distribution from short-term measurements of 8-year-old Betula pendula Roth trees of two clones in an open-top chamber experiment with elevated CO2 and ozone treatments. The model approximates the canopy as a population of growing and propagating shoots where growth and ramification parameters are dependent on local light environment. The model was parameterised in field investigations and validated using measured leaf litter area. The model describes well the changes in leaf area caused by different treatments and allows to track the mechanisms behind these changes. The two clones responded similarly to elevated ozone concentration with a slight decrease in leaf area and increased allocation of foliage towards the top of the trees. Elevated CO2 caused considerable increase in steady-state LAI with more foliage in the lower part of the canopy in clone 80 but a slight decrease in LAI of clone 4. However, there appeared a strong ameliorative effect of CO2 in CO2 × O3 treatment leading to the disappearance of the negative effect of ozone and causing a slight increase in LAI compared to CO2 treatment. The model appeared to be very sensitive to the radiation model parameters. This is expressed in a large range of LAIs generated by the model. Our study stresses the importance of detailed investigation of leaf area distribution because change in the distribution as a response to environmental factors may be independent of changes in total leaf area and because this change may alter species competitive ability and shade tolerance.