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

Timothy J Wardlaw

doi:10.1038/s41598-022-06674-x

Abstract

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.

Highlights

Tasmania experienced a protracted warm spell in November 2017
Patterns of carbon and energy fluxes during the heatwave event reported by van Gorsel, et al.[3] were consistent with photosynthesis and transpiration being coupled through mechanisms of stomatal controls articulated in[6] that regulate water loss and maintenance of intercellular C O2 concentrations
Over nearly a century of measurements, 99 in-growing-season (Austral spring–summer) heatwave events have been recorded at Cape Bruny Lighthouse (59 km southeast of Warra Supersite)

Summary

Introduction

Tasmania experienced a protracted warm spell in November 2017. Temperatures were lower than those usually characterising heatwaves. During one heatwave event in Australia for example, van Gorsel, et al.[3] found the pattern of responses in carbon and energy fluxes in eucalypt dry sclerophyll forests and woodlands across southern Australia differed from that of a tall eucalypt forest in south-eastern Australia. De Kauwe, et al.[8] were unable to show conclusively that carbon and energy fluxes decoupled in forest ecosystems experiencing extreme heatwaves (consecutive days with maximum temperatures > 35 °C). Against describing extreme heatwave events in terms of consecutive days above a threshold temperature because they do not properly account for the regional differences in historical climates. Instead, they advocate regionally adjusted metrics such as consecutive days that exceed the 90th percentile value for the calendar day. Such regionally adjusted metrics should better account for any local adaptation by the forest to the regional climate

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