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

T Vesala,P Ciais,P Kaski,E Nikinmaa,P Hari,S Sevanto,P Kolari,S Launiainen,J Pumpanen,H Mannila,E Ukkonen,S L Piao

doi:10.5194/bg-7-163-2010

Abstract

Abstract. 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. The main focus is on investigations of environmental drivers of GPP and TER and how they affect the inter-annual variation in the carbon balance in autumn (September–December). We used standard climate data and CO2 exchange measurements collected by the eddy covariance (EC) technique over 11 years. EC data revealed that increasing autumn temperature significantly enhances TER: the temperature sensitivity was 9.5 gC m−2 °C−1 for the period September–October (early autumn when high radiation levels still occur) and 3.8 gC m−2 °C−1 for November–December (late autumn with suppressed radiation 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 gC m−2 °C−1. Two models, a stand photosynthesis model (COCA) and a global vegetation model (ORCHIDEE), were used for estimating stand GPP and its sensitivity to the temperature. The ORCHIDEE model was tested against the observations of GPP derived from EC data. The stand photosynthesis model COCA predicted that under a predescribed 3–6 °C temperature increase, the temperature sensitivity of 4–5 gC m−2 °C−1 in GPP may appear in early autumn. The analysis by the ORCHIDEE model revealed the model sensitivity to the temporal treatment of meteorological forcing. The model predictions were similar to observed ones when the site level 1/2-hourly time step was applied, but the results calculated by using daily meteorological forcing, interpolated to 1/2-hourly time step, were biased. This is due to the nonlinear relationship between the processes and the environmental factors.

Highlights

The carbon balance of boreal forest ecosystems is sensitive to prevailing weather conditions
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
The results revealed that the correlation between GPP and TER with the driving environmental factors changes intraannually, during the autumn (September–December) when the air temperature and insolation drastically decrease

Summary

Introduction

The carbon balance of boreal forest ecosystems is sensitive to prevailing weather conditions. In the summertime these ecosystems are clear sinks of carbon, but in the winter they become small carbon sources because carbon uptake via photosynthesis decreases more than respiration (e.g. Suni et al, 2003; Lagergren et al, 2008). The annual balance is affected especially by the temperature in the autumns and springs (Mäkelä et al, 2006). During those periods, the difference in temperature sensitivity of photosynthesis and respiration processes is an important controller of the carbon balance. According to the climate simulations, the mean annual air temperatures in northern Europe are expected to increase between 2–6 ◦ C during this century and the increase is likely to be strongest

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Discussion

Conclusion