Eucalyptus obliqua tall forest in cool, temperate Tasmania becomes a carbon source during a protracted warm spell in November 2017.

Tasmania experienced a protracted warm spell in November 2017. Temperatures were lower than those usually characterising heatwaves. Nonetheless the warm spell represented an extreme anomaly based on the historical local climate. Eddy covariance measurements of fluxes in a Eucalyptus obliqua tall forest at Warra, southern Tasmania during the warm spell were compared with measurements in the same period of the previous year when temperatures were closer to average. Compared with previous year, the warm spell resulted in 31% lower gross primary productivity (GPP), 58% higher ecosystem respiration (ER) and the forest switching from a carbon sink to a source. Significantly higher net radiation received during the warm spell was dissipated by increased latent heat flux, while canopy conductance was comparable with the previous year. Stomatal regulation to limit water loss was therefore unlikely as the reason for the lower GPP during the warm spell. Temperatures during the warm spell were supra-optimal for GPP for 75% of the daylight hours. The decline in GPP at Warra during the warm spell was therefore most likely due to temperatures exceeding the optimum for GPP. All else being equal, these forests will be weaker carbon sinks if, as predicted, warming events become more common.