Carbon dioxide exchange of Scots pine shoots as estimated by a biochemical model and cuvette field measurements

Abstract

A biochemical model was used to calculate CO2 fluxes to Scots pine shoots in two boreal measurement stations, Hyytiala in southern Finland (61°51'N, 24°17'E) and Varrio in northern Finland (67°46'N, 29°35'E). The results of the model were compared with cuvette measurements performed in field conditions. A differential equation for change in gas concentration inside a closed cuvette was constructed and solved in order to obtain conductances and fluxes. The results were generally in a good agreement, the correlation coefficients varied from 0.74 to 0.95. Some discrepancies were also found. The model followed more intensively changes in temperature. This could be seen in northern Finland measurements at low temperatures (<18 °C). The modelled temperature response indicated low fluxes at low temperatures, but measurements did not show any decrease. The irradiation response was relatively similar in both measuring sites and according to the model. Cuvette measurements showed slightly smaller quantum yields as a result from shading of the needles. The temperature dependences of the biochemical model parameters J(max) and V(c(max)) were re-evaluated from the field measurements. The results for V(c(max)) agreed well with earlier estimations, while the results for J(max) indicated relatively high values at low temperatures especially in northern Finland. Exponential fitting produced also substomatal concentrations of CO2, which agreed quite well with the model. The daily minimum of substomatal/ambient concentration ratio varied from 0.4 to 0.8.