Inter-annual variation of carbon uptake by a plantation oak woodland in south-eastern England

Abstract. The carbon balance of an 80-yr-old deciduous oak plantation in the temperate oceanic climate of the south-east of Great Britain was measured by eddy covariance over 12 yr (1999–2010). The mean annual net ecosystem productivity (NEP) was 486 g C m⁻² yr⁻¹ (95% CI of ±73 g C m⁻² yr⁻¹), and this was partitioned into a gross primary productivity (GPP) of 2034 ± 145 g C m⁻² yr⁻¹, over a 165 (±6) day growing season, and an annual loss of carbon through respiration and decomposition (ecosystem respiration, R_eco) of 1548 ± 122 g C m⁻² yr⁻¹. Although the maximum variation of NEP between years was large (333 g C m⁻² yr⁻¹), the ratio of R_eco/GPP remained relatively constant (0.76 ± 0.02 CI). Some anomalies in the annual patterns of the carbon balance could be linked to particular weather events, such as low summer solar radiation and low soil moisture content (values below 30% by volume). The European-wide heat wave and drought of 2003 did not reduce the NEP of this woodland because of good water supply from the surface-water gley soil. The inter-annual variation in estimated intercepted radiation only accounted for ~ 47% of the variation in GPP, although a significant relationship (p < 0.001) was found between peak leaf area index and annual GPP, which modified the efficiency with which incident radiation was used in net CO₂ uptake. Whilst the spring start and late autumn end of the net CO₂ uptake period varied substantially (range of 24 and 27 days respectively), annual GPP was not related to growing season length. Severe outbreaks of defoliating moth caterpillars, mostly Tortrix viridana L. and Operophtera brumata L., caused considerable damage to the forest canopy in 2009 and 2010, resulting in reduced GPP in these two years. Inter-annual variation in the sensitivity of R_eco to temperature was found to be strongly related to summer soil moisture content. The eddy covariance estimates of NEP closely matched mensuration-based estimates, demonstrating that this forest was a substantial sink of carbon over the 12-yr measurement period.