The Carbon Cycle of a Maritime Ancient Temperate Broadleaved Woodland at Seasonal and Annual Scales

Abstract

This study compares different approaches to quantifying the carbon cycle in a temperate deciduous forest at Wytham Woods in England, which is unusual in its maritime climate and mixed age structure, reflecting low levels of past management. We tested whether eddy covariance and biometric measurements gave consistent estimates of woodland productivity and ecosystem respiration at monthly and annual timescales. Biometric methods estimated gross primary productivity (GPP) as 22.0 ± 1.6 Mg C ha−1 y−1, close to the eddy covariance GPP value of 21.1 Mg C ha−1 y−1. Annual ecosystem respiration (R ECO) was similar, at 20.3 ± 1.5 Mg C ha−1 y−1 for biometric and 19.8 Mg C ha−1 y−1for eddy covariance. The seasonal cycle of monthly biometric and eddy covariance R ECO estimates also closely matched. Net primary productivity (NPP) was 7.0 ± 0.8 Mg C ha−1 y−1, 37% of which was allocated below ground. Leaf fluxes were the greatest component of NPP and R ECO. Ecosystem carbon-use efficiency (CUE = NPP/GPP) was 0.32 ± 0.04; low compared to many temperate broadleaved sites but close to values for old-growth sites. This may reflect the age of some trees, and/or the oceanic climate with relatively mild winters during which there can be substantial autotrophic maintenance respiration in winter but negligible growth. This study demonstrates that biometric measurements can provide robust estimates of site productivity and respiration and that eddy covariance and bottom-up measurements can be combined on seasonal and interannual timescales to enable a detailed understanding of the forest carbon cycle.