How eucalypts cope with heat and drought, and where coping ends
Most of the roughly 800 eucalypt species evolved on a continent where long dry spells and summer heat are normal, not exceptional. The genus carries the marks of that history: leaves that hang vertically to reduce midday sun, thick waxy cuticles, root systems that follow water a long way down, and the ability to shut stomata early and sit out a dry month. It is easy to look at a river red gum on a floodplain and conclude that nothing can hurt it.
Yet the last two decades have produced repeated episodes of what researchers call canopy dieback: whole hillsides of trees dropping their leaves, sometimes recovering from epicormic buds under the bark, sometimes not. The pattern is instructive, because it shows where the well-known adaptations run out.
What the adaptations actually buy
Vertical leaves and reflective surfaces lower leaf temperature by a few degrees relative to a flat, dark leaf in the same sun. That margin matters when air temperature is in the high thirties, because the leaf itself can otherwise climb toward the point where photosynthetic enzymes start to fail. Deep roots keep water flowing through the tree during the dry season, which allows it to keep stomata open and stay cool through transpiration. Early stomatal closure is the fallback: the tree stops losing water, stops cooling itself, and waits.
Each of these mechanisms trades one risk for another. Closed stomata mean no carbon gain, so a long drought is a slow starvation. Deep roots are only useful while there is water at depth, and multi-year droughts in south-eastern and south-western Australia have drained soil profiles that had not been dry in living memory. Transpirational cooling depends on water that the tree may no longer have.
The compound event
Dieback rarely follows heat alone or drought alone. The damaging combination is a dry winter and spring, followed by a heatwave in which night-time temperatures also stay high. The tree enters summer with a depleted water reserve, meets several days above forty degrees, cannot cool its leaves, and suffers hydraulic failure: air bubbles form in the water-conducting vessels and the pathway from root to leaf breaks. Recovery depends on how much of that plumbing survives.
Climate records for southern Australia show both parts of the compound event becoming more frequent. Winter rainfall in the south-west has declined noticeably since the 1970s, and the number of very hot days in the south-east has risen. The trees are not experiencing a slightly warmer version of the climate they evolved in; they are experiencing combinations that were rare.
Which species are exposed
The genus is not uniform. Species from the arid interior, such as some of the mallees, are among the most drought-tolerant woody plants known. Species from the wet forests of Tasmania and Victoria, including mountain ash, the tallest flowering plant on Earth, are far less tolerant and have shown some of the most severe mortality after recent droughts. Plantation species were chosen for growth rate, not for resilience, and blue gum plantations in southern Australia and abroad have had their own dieback episodes.
The practical lesson for anyone planting eucalypts outside their native range is that the label does not tell you the tolerance. Provenance matters, and a tree from a wet coastal population may share a species name with a tree from a dry inland one and behave quite differently.
Further reading
- CSIRO, work on eucalypt physiology and drought mortality
- Bureau of Meteorology, State of the Climate reports
- Western Sydney University, Hawkesbury Institute, heatwave experiments on eucalypts