Carbon uptake and water use in woodlands and forests in southern Australia during an extreme heat wave event in the “Angry Summer” of 2012/2013

Abstract

Abstract. As a result of climate change warmer temperatures are projected through the 21st century and are already increasing above modelled predictions. Apart from increases in the mean, warm/hot temperature extremes are expected to become more prevalent in the future, along with an increase in the frequency of droughts. It is crucial to better understand the response of terrestrial ecosystems to such temperature extremes for predicting land-surface feedbacks in a changing climate. While land-surface feedbacks in drought conditions and during heat waves have been reported from Europe and the US, direct observations of the impact of such extremes on the carbon and water cycles in Australia have been lacking. During the 2012/2013 summer, Australia experienced a record-breaking heat wave with an exceptional spatial extent that lasted for several weeks. In this study we synthesised eddy-covariance measurements from seven woodlands and one forest site across three biogeographic regions in southern Australia. These observations were combined with model results from BIOS2 (Haverd et al., 2013a, b) to investigate the effect of the summer heat wave on the carbon and water exchange of terrestrial ecosystems which are known for their resilience toward hot and dry conditions. We found that water-limited woodland and energy-limited forest ecosystems responded differently to the heat wave. During the most intense part of the heat wave, the woodlands experienced decreased latent heat flux (23 % of background value), increased Bowen ratio (154 %) and reduced carbon uptake (60 %). At the same time the forest ecosystem showed increased latent heat flux (151 %), reduced Bowen ratio (19 %) and increased carbon uptake (112 %). Higher temperatures caused increased ecosystem respiration at all sites (up to 139 %). During daytime all ecosystems remained carbon sinks, but carbon uptake was reduced in magnitude. The number of hours during which the ecosystem acted as a carbon sink was also reduced, which switched the woodlands into a carbon source on a daily average. Precipitation occurred after the first, most intense part of the heat wave, and the subsequent cooler temperatures in the temperate woodlands led to recovery of the carbon sink, decreased the Bowen ratio (65 %) and hence increased evaporative cooling. Gross primary productivity in the woodlands recovered quickly with precipitation and cooler temperatures but respiration remained high. While the forest proved relatively resilient to this short-term heat extreme the response of the woodlands is the first direct evidence that the carbon sinks of large areas of Australia may not be sustainable in a future climate with an increased number, intensity and duration of heat waves.

Influence of concurrence of extreme drought and heat events on carbon and energy fluxes in dominant ecosystems in the Pacific Northwest region

“…The reduction in NEP was significantly 285 correlated with temperature and VPD but not with soil moisture (Fig. 5 and Table S1), in contrast to other studies demonstrating water scarcity as a primary driver of decreasing carbon sink strength during drought (Allen et al, 2015;Reichstein et al, 2007;Wolf et al, 2016). RE and GPP exhibited a stronger dependence on temperature and VPD than soil moisture.…”

“…warmer and drier summers to the PNW (Dalton et al, 2013;Wolf et al, 2016). Our results suggest that even within 350 the same plant functional type (i.e., evergreen needle leaf) and the same species with different ages, the hotter droughtinduced change in energy process is not uniform due to different interactive effects of specific site environmental and physiological factors (Wilson et al, 2002).…”

“…Under the existing water stress, stomatal down-regulation increased in the ponderosa pines, resulting in a decrease in NEP (Table 3 and in these values led more reduction in sensible heat flux in MP than YP. The pattern of decreasing sensible heat flux at the ponderosa pines differed from other ecosystems, which showed an increased sensible heat flux at dry sites (e.g., Mediterranean and temperate woodlands) in Australia during an extreme heat wave event in summer 2012-2013 (Gorsel et al, 2016).…”

“…The minimal increase in sensible heat flux may be due to evaporative cooling due to the increased latent heat flux suppressing further heating and depleting soil moisture in Douglas-fir (Gorsel et al, 2016).…”

Photosynthetic capacity and leaf nitrogen decline along a controlled climate gradient in provenances of two widely distributed Eucalyptus species

“…In addition to a gradual increase in mean temperatures, climate change is also predicted to increase the intensity and frequency of heatwaves including in species from lower latitudes. Extreme heatwaves can affect important ecosystem functions (Reichstein et al., ; Teskey et al., ; Van Gorsel et al., ) including hydraulic failure (Mitchell, O'grady, Hayes, & Pinkard, ) and leaf damage and/or mortality when thermal thresholds are exceeded (O'Sullivan et al., ). However, it is unknown if and how long‐term acclimation to warming helps with a plant's physiological responses during a heatwave, including the adjustment of CO 2 and H 2 O fluxes during extreme heatwaves (Drake et al., ; Teskey et al., ) and the influence of soil water status (Allen, Breshears, & Mcdowell, ; Ciais et al., ).…”