Tasmanian eucalypts in trouble

Eucalypts come in all shapes and sizes and are found in almost all environments throughout Australia. Their shaggy forms, distinctive smell and delicate blossoms are a fundamental part of our Australian identity.

In Tasmania, we have 29 species of eucalypt, with 16 species found nowhere else. Our species vary from the tallest flowering plant, the mountain ash (Eucalyptus regnans), to the smallest eucalypt species, the varnished gum (Eucalyptus vernicosa), that grows on the wind-swept mountain tops of western Tasmania.

Morrisby’s gum blossom

Tasmanian eucalypts are incredibly important. They are a key component of global biodiversity, provide habitat that is critical for threatened species (such as the forty-spotted pardalote, swift parrot, and masked owl).

Eucalypts also hold significant cultural heritage value, for example the cider gums, from which the Tasmanian Aboriginal people were recorded drinking the rich sugary sap or ‘cider’ that leaked from crevices in the trunk and which were tapped to make the sap flow better.

Currently there are two Tasmanian endemic white gum species under significant threat of extinction, Morrisby’s gum (Eucalyptus morrisbyi) and the Miena cider gum (Eucalyptus gunnii subsp. divaricata). These closely related species are unlikely cousins, occurring in very contrasting environments. Morrisby’s gum is found in coastal pockets in Tasmania’s south-east while Miena cider gums occur on the edge of frost hollows on the Central Plateau. Despite their differences, these white gums are united by the challenges they face.

Downward trends in population numbers of both species are alarming. Only two populations of Morrisby’s gum occur in the wild, with tree numbers dwindling from around 2000 to fewer than 50 in the last few years. Miena cider gums are more widely dispersed, found at a handful of sites on the Central Plateau. However, the health of trees within these populations has been severely impacted by drought and then wildfires in 2019.

Threats

Drivers of the collapse of these species are multiple and interlinked. Unsurprisingly, climate change is playing a part. Climate change impacts these species directly, with rising temperatures and changes in rainfall causing drought stress in trees. Stressed trees produce fewer chemical defenses, making them more vulnerable to attack from insects and vertebrates. In turn, warmer temperatures foster larger insect populations, leading to an increase in insect attack when trees are most vulnerable. Recent occurrences of widespread dieback in alpine snow gums in the Australian alps is a warning to the devastating impact increases in insect attack can have on eucalypt species.

Climate change is also increasing the likelihood of wildfire, with negative effects on many Miena cider gums. This species is the most frost tolerant eucalypt, but unlike most eucalypts, is fire sensitive. A combination of dry lightning strikes, increasingly hot days and dry spells has seen unprecedented wildfires on Tasmania’s Central Plateau.

If this was not enough, the stressed and vulnerable trees are also faced with vertebrate herbivore attack, particularly from possums. The leaves of Morrisby’s and Miena cider gums are highly palatable to possums, which readily defoliate mature trees and browse seedlings.

Combined, these pressures are leading to Morrisby’s and Miena cider gum death and population collapse. Unabated, these pressures threaten the long-term survival of these species.

Solutions

The challenges faced by these species are daunting. However, we believe that they are not insurmountable. Community awareness combined with collaboration between management organisations to reduce some of the more manageable pressures on the species can help to improve their conservation outcomes.

Here at Enviro-dynamics, we have conservation programs working to increase resilience in Morrisby’s and Miena cider gum populations (the latter through the Derwent Catchment Project). We aim to reduce herbivory pressure by individually caging small trees, banding mature trees and fencing discreet stands. We are mitigating fire impacts by developing fire management plans in conjunction with landholders and managers. Additionally, we are working with the Tasmanian Seed Conservation Centre to bolster seed banks, and working with the local community to plant Morrisby’s gum seedlings. Our programs are showing positive outcomes, though there is still much work to be done. We continue to work with others to help these struggling species stay in the landscape for as long as possible.

The Morrisby’s gum project is funded by the National Landcare Program through NRM South. Funding for the Miena cider gum conservation program is supported by Hydro Tasmania and the Australian Government Bushfire Recovery Program 2020 through the Tasmanian State Department of Natural Resources and Environment. Additional funding for our Morrisby’s gum program is provided by the Department of State Growth and Clarence City Council.

Enviro-dynamics ecologist, Laurie Kerr, grateful to find a live Miena cider gum post fire, 2021.