Spring initiation and autumn cessation of boreal coniferous forest CO2 exchange assessed by meteorological and biological variables

We studied the commencement and finishing of the growing season using different air temperature indices, the surface albedo, the chlorophyll fluorescence (Fv/Fm) and the carbon dioxide (CO₂) tropospheric concentration, together with eddy covariance measurements of CO₂ flux. We used CO₂ flux data from four boreal coniferous forest sites covering a wide latitudinal range, and CO₂ concentration measurements from Sammaltunturi in Pallas. The CO₂ gas exchange was taken as the primary determinant for the growing season to which other methods were compared.

Indices based on the cumulative temperature sum and the variation in daily mean temperature were successfully used for approximating the start and cessation of the growing season. The beginning of snow melt was a successful predictor of the onset of the growing season. The chlorophyll fluorescence parameter Fv/Fm and the CO₂ concentration were good indicators of both the commencement and cessation of the growing season. By a derivative estimation method for the CO₂ concentration, we were also able to capture the larger-scale spring recovery. The trends of the CO₂ concentration and temperature indices at Pallas/Sammaltunturi were studied over an 11-yr time period, and a significant tendency towards an earlier spring was observed. This tendency was not observed at the other sites.