Autumn warming and carbon balance of a boreal Scots pine forest in Southern Finland.

We analyzed the dynamics of carbon balance components: gross primary production (GPP) and total ecosystem respiration (TER), of a boreal Scots pine forest in Southern Finland. Our aim was to study how these dynamics are related to different environmental conditions and how they affect the inter-annual variation in the carbon balance in autumn (September-December). We used standard micrometeorological data and CO₂ exchange measurements collected by the eddy covariance (EC) technique over 11 years. The intra-annual relationships between the carbon balance components and the environmental factors were studied by the correlation analysis. Two models, a stand photosynthesis model and a generic dynamic vegetation model (ORCHIDEE), were also applied in the analysis. EC data revealed that increasing autumn temperature significantly enhances TER: the temperature sensitivity was 9.5 gCm^-2 °C^-1 for the period September-October (early autumn when high radiation levels still occur) and 3.8 gCm^-2 °C^-1 for November-December (late autumn with suppressed radiation 15 level). The cumulative GPP was practically independent of the temperature in early autumn. In late autumn, air temperature could explain part of the variation in GPP but the temperature sensitivity was very weak, less than 1 gCm^-2 °C^-1. The stand photosynthesis model predicted that under a predescribed 3-6°C temperature increase, the temperature sensitivity of 4-5 gCm^-2 °C-1 in GPP may appear in early autumn. The 20 TER and GPP sensitivities, produced by the ORCHIDEE model, were similar to observed ones when the site level 1/2 h time step was applied, but the results calculated by using daily meteorological forcing, interpolated to 1/2 h time step, were biased stemming from the nonlinear relationship between the processes and the environmental factors. [ABSTRACT FROM AUTHOR]