Water flux of Eucalyptus regnans: defying summer drought and a record heatwave in 2009.

Making predictions as to how heatwaves will affect forests in the future is a major challenge in ecosystem science, not the least because we have few documented examples of how they respond now. We captured the effects of drought and a record-breaking heatwave on whole-tree water use (Q) in Eucalyptus regnans during the summer drought of 2008/2009 in southeastern Australia. While air temperatures steadily increased, average maximum sap flow (J Smax) declined with progression of the drought prior to the heatwave. In the period approaching the heatwave, Q during daytime (Q d) steadily declined, while nighttime Q (Q n) increased. This pattern was particularly pronounced during nights that followed hot days (>32 °C) where Q n was frequently 20-30 % of Q d. We found clear trends in the relation of Q d to Q n that point to the increasing importance of refilling depleted stem water stores following hot days. On the day the heatwave climaxed (7 February 2009), sap flow (J S) was dramatically low, and declined as weather conditions became increasingly arid (air temperature > 42 °C, vapor pressure deficit >7 kPa). Almost immediately after the heatwave passed J S resumed its common diurnal hysteresis, albeit at slightly slower rates. In the context of prognosticated effects of future climate, our data highlight that depletion and refill of stored water in E. regnans are likely important features for the tree to endure drought- and heat-related climatic extremes. We suggest that elucidating the peculiarity of capacitance and defining its threshold for keystone tree species, such as E. regnans, can add to our understanding of how climatic extremes may affect forests.