Eucalypt health surveillance in Canberra

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Purpose

For use as a reference collection to monitor Eucalypt health in the ACT. Eucalypts include the following genera: Eucalyptus, Corymbia and Angophora. It complements another citizen science project that monitors the health of snow gums: https://www.saveoursnowgum.org/

Eucalypt health can be affected by climate change, drought, plant pathogens and pests, environmental weeds, and very high severity fires. For example: Bell Miner - psyllid dieback, Phytophthora dieback, drought-induced fissure - longicorn beetle dieback, drought-induced xylem embolism and cavitation (hydraulic conductance failure), ginger tree syndrome, lack of resprouting of epicormic strands due to vascular cambium death from very high severity fires, novel increase in fire intensity from African lovegrass invasion, leaf blight and canker (dead bark, phloem and cambium) from fungal disease, and rural dieback caused by nutrient imbalance.

The condition of the Eucalypt canopy, leaves, buds, and bark (particularly on smooth barks) are good indicators of plant health which can be revealed by the sighting photos. NatureMapr has a timeline function which collates sightings overtime of a species at a location. This can show changes in plant health.

Eucalypts are keystone species for many Australian ecosystems. Planted Eucalypts are also strongly associated with insect and bird diversity and this can assist bird populations in adjacent woodland or forest. Sightings of Eucalypts in cultivation are valuable data for understanding if a non-local species is able to flower outside its natural distribtuion. This helps with understanding the impact of climate change. Hence why this project includes planted Eucalypts as well as those that are naturally occurring.

Eucalypt canopies and fine roots affect soil properties, via rainfall through fall, rainfall stem flow, leaf and bark litter, extensive fine root turnover, and mycorrhizae. This has a significant effect on nutrient cycling and understorey vegetation patterns.

How to add your Eucalypt sightings

Add your Eucalypt sightings to the project collection by selecting the star icon at the top right of the sighting page (to the left of the edit option) on your desktop computer, and scroll thru the collection list to find the collection title 'Eucalypt health surveillance in Canberra'. Don't forget to fill out all the data fields for the Eucalypt sighting: number of the species (i.e. number in the immediate area, usually within 20m, or if uncertain set the number to 1), tree health, height, number of hollows, circumference, canopy radius (if asked), planted or naturally occurring, and use the public comments field for extra plant health information (e.g. disturbances, pests, site history), and for noting different age classes or plant health status of the number recorded.

Helpful id guides: The EUCLID app, and Native Eucalypts of Victoria and Tasmania - South-eastern Australia by Dean Nicolle.

Reviewing the sightings - some interesting patterns

There are enough Eucalypt sightings (3,261) in the ACT to start seeing some patterns with Eucalypt health.

Method: 1,202 of the 3,261 sightings, comprising 60+ species, have been added to this Eucalypt health surveillance project. Most of the sightings are from the last 10 years. The sightings added have a good image of the tree canopy and/or a good image of the trunk (for smooth bark species). Some also have close up images of the condition of the leaves, oil glands and venation. Each sighting was assessed based on the associated data fields, comments, and examining the images.

The sightings show examples of:

Pseudosydowia eucalypti (a leaf fungus parasite) causing dieback in planted E. globulus subsp. biocostata and E. cinerea subsp. cinerea.
'Ginger tree syndrome' in E.lacrimans, and E.pauciflora subsp. pauciflora.
Drought induced trunk fissures and/or longicorn beetle larvae damage in E.pauciflora subsp. pauciflora
Pysllid-lerp infestations in young E.blakelyi.
Post-drought leaf damage in E.macroryhncha subsp. macrorhyncha from leaf blight.
Rural dieback in E.melliodora and E.blakelyi.
Canker in E.pauciflora subsp. pauciflora and E.mannifera subsp. mannifera

Other species specific drought and fire related observations:

Drought tolerance of species sightings consistent with the literature, i.e., lower water potential (Ψ) tolerated in more drought tolerant species. One measure of drought tolerance is based on leaf osmotic potential at zero turgor (Ψo). Species with greater drought tolerance have a more negative Ψo. From most to least drought tolerant based on Ψo: E.microcarpa > E.melliodora > E.globulus (subsp. bicostata?) > E.maculata > E.pauciflora > E.viminalis.
Large branch drop by E.melliodora during the 2018-19 drought "During hot dry conditions, branches with insufficient water become brittle and can fall in windy conditions, especially from old trees. " https://www.science.org.au/curious/earth-environment/story-our-eucalypts Refer to the following sighting as a good example: https://canberra.naturemapr.org/sightings/4575886 Select the timeline icon next to the species name to see the photo sequence.
A naturalised population of the rare and endangered E.parvula has shown to be resilient to very high severity fire (the 2003 fire).
Very high severity fires in early 2020, near the end of the drought killed trunk and branch vascular cambium on some trunk/branch resprouting capable species. This meant the epicormic strands on the branches and trunks of the impacted Eucalypts did not resprout. Those effected Eucalypts were only showing lignotuber resprouting, e.g., E.dives, E.dalrympleana subsp. dalrympleana, and E.viminalis subsp. viminalis. This impact was relatively greater on smaller trees (consistent with the literature).

General observations:

Resistance and resilience of most species to the 2018-19 drought (consistent with the literature: ability of many Eucalypt species to shut-down when drought occurs, i.e., resistance, and ability to rapidly absorb and transpire water when it becomes available again, i.e., resilience).
Patchy branch death (hydraullic failure) in a range of species but mainly at marginal sites for the species.
Tall trees in marginal habitat suffered more patchy branch death from the 2018-19 drought than shorter trees of the same species (consistent with the literature).
Sites with less water stress had less examples of patchy branch death. This included many large trees in urban open space. The exception was large trees in some rural dieback areas.
Subgenus Eucalyptus (with the exception of E.rossii) had more patchy branch death than subgenus Symphomyrtus (consistent with the literature).

Patchy branch death - when is it a problem? :

Some patchy branch death is to be expected due to drought.
Large amounts correspond to a significant loss in the ability to transport water in the xylem, i.e., a large loss in hydraulic conductivity.
As drought worsens, water potential becomes more negative, leading to increased xylem tension, xylem embolism from air seeding, xylem cavitation, and branch death.
Ψ50 is the water potential where there is a 50% loss in hydraulic conductivity.
Ψ88 is the water potential where there is a 88% loss in hydraulic conductivity.
At Ψ50, Eucalypts are in trouble but can recover. At Ψ88 it is lethal.
Ψ50 may not present as 50% of the crown dying. However a large amount of patchy branch death indicates significant hydraulic failure.
Recovery after patchy branch death is via resprouting on unaffected parts of branches, or resprouting on the trunk or lignotuber, depending on the Eucalypt species.

Fire and patchy branch death:

Patchy branch death is also caused by high and very high severity fire if the vascular cambium is killed and epicormic strand growth cannot resprout on the branches.
Then for the Eucalypt to survive it needs to be a species that also resprouts from the trunk, or from the base via a lignotuber.
In very high severity fires even vascular cambium on the trunk of smaller trees can be killed. Then epicormic strand resprouting will fail on trunk resprouting species.

Hollow formation:

Hollow dependent fauna require Eucalypt forests and woodlands with sufficent tree hollows of different sizes and locations.
Note that patchy branch death is not why tree hollows develop in Eucalypts.
Hollow formation in Eucalypts is age related (usually > 110 years old in Southern Australia), due to activity of termites (cellulose breakdown) and fungi (cellulose and lignin breakdown).
Fire damage can assist hollow formation, but high severity fires can lead to the 'chimney effect' and tree collapse in hollow-bearing Eucalypts.

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