What the flux? High eddy covariance NEP in a dry sclerophyll eucalypt forest is validated using inventory and growth models

© 2016 Alison Claire BennettGlobally forests contribute to climate change mitigation by sequestering about a quarter of anthropogenic fossil fuel CO2 emissions. Understanding their contribution to the global carbon budget is therefore critical. Recent measurements of carbon uptake in temperate eucalypt forests using eddy covariance indicated un-usually high carbon sequestration of these forests. This abnormality could alter our understanding of their carbon uptake. Due to a range of uncertainties in the eddy covariance method and potential for violating flux tower assumptions, validating carbon sequestration is necessary. I validated Net Ecosystem Productivity (NEP) measurements of an eddy covariance flux tower site at a dry sclerophyll eucalypt forest in South Eastern Australia for a three year period (2013 – 2015). A novel approach combining inventory and growth models was used for retrospective prediction. Inventory Net Primary Productivity (NPP) was calculated and a source area adjusted NPP also calculated to account for the wind source area contribution to tower flux measurements. Inventory NPP was 3.19 Mg C ha-1 yr-1 (95% CI: 2.48, 3.88) in 2013, 7.85 Mg C ha-1 yr-1 (95% CI: 6.82, 8.88) in 2014 and 9.48 Mg C ha-1 yr-1 (95% CI: 8.33, 10.6) in 2015. The source area adjusted NPP was 2.90 Mg C ha-1 yr-1 (95% CI: 2.73, 3.06) in 2013, 7.27 Mg C ha-1 yr-1 (95% CI: 6.81, 7.72) in 2014, and 8.91 Mg C ha-1 yr-1 (95% CI: 8.38, 9.45) in 2015, indicating that eddy covariance consistently overestimated carbon flux by 27 – 45%. In the year of low carbon uptake (2013) rates of carbon sequestration were comparable to temperate forests globally, but in years of high uptake (2014 – 2015) sequestration in this forest was comparatively greater. These results demonstrate the importance of validating flux tower measurements and confirm high sequestration rates in this forest type