Carbon dioxide fluxes over an ancient broadleaved deciduous woodland in southern England

Abstract. We present results from a study of canopy-atmosphere fluxes of carbon dioxide from 2007 to 2009 above a site in Wytham Woods, an ancient temperate broadleaved deciduous forest in southern England. Gap-filled net ecosystem exchange (NEE) data were partitioned into gross primary productivity (GPP) and ecosystem respiration (R_e) and analysed on daily, monthly and annual timescales. Over the continuous 24 month study period annual GPP was estimated to be 21.1 Mg C ha⁻¹ yr⁻¹ and R_e to be 19.8 Mg C ha⁻¹ yr⁻¹; net ecosystem productivity (NEP) was 1.2 Mg C ha⁻¹ yr⁻¹. These estimates were compared with independent bottom-up estimates derived from net primary productivity (NPP) and flux chamber measurements recorded at a plot within the flux footprint in 2008 (GPP = 26.5 ± 6.8 Mg C ha⁻¹ yr⁻¹, R_e = 24.8 ± 6.8 Mg C ha⁻¹ yr⁻¹, biomass increment = ~1.7 Mg C ha⁻¹ yr⁻¹). Over the two years the difference in seasonal NEP was predominantly caused by changes in ecosystem respiration, whereas GPP remained similar for equivalent months in different years. Although solar radiation was the largest influence on daily values of CO₂ fluxes (R² = 0.53 for the summer months for a linear regression), variation in R_e appeared to be driven by temperature. Our findings suggest that this ancient woodland site is currently a substantial sink for carbon, resulting from continued growth that is probably a legacy of past management practices abandoned over 40 years ago. Our GPP and R_e values are generally higher than other broadleaved temperate deciduous woodlands and may represent the influence of the UK's maritime climate, or the particular species composition of this site. The carbon sink value of Wytham Woods supports the protection and management of temperate deciduous woodlands (including those managed for conservation rather than silvicultural objectives) as a strategy to mitigate atmospheric carbon dioxide increases.