Possibilities of estimating the annual carbon budget from measurements of CO2 exchange applied to a simple model of tree structure are discussed. It is suggested that this approach is particularly useful where estimates of root production are required. The accuracy of such an approach depends upon the number of samples of carbon dioxide exchange which are taken and it is discussed how these may be restricted in time and space.
Almost continuous measurements of CO2 exchange were made on shoots of Scots pine by the use of temperature controlled assimilation chambers. The errors introduced for reduced sampling rates could then be estimated. There was a linear relationship between the number of days measured per week and the deviation between measured and estimated annual photosynthesis. From April to November two days of measurements per week were required to keep the error below ten per cent, five days per week to keep errors below five per cent. Measurement frequency can be reduced by using simple curve-fitting models to fill in days where measurements are lacking.
The spatial distribution of samples within trees is discussed. It is shown that more samples must be taken in denser grown stands because of the effects of the light climate upon the photosynthetic efficiency within the crown.
The influence of stem and coarse-root respiration upon the annual carbon balance is discussed and examples are given of the errors introduced in the estimates of an annual carbon budget for a young Scots pine if the period of measurements was shortened from a year to the six warmest months of the year.
