Native tree decline

Native trees are important in maintaining the delicate balance between the natural and physical environments that we depend on for our survival. They protect our groundwater and soil, absorb greenhouse gases, and provide food and shelter for birds, insects and other wildlife.

Native tree decline is often caused by a combination of factors based on climate, soil, water issues, pests and pathogens, poisons and pruning. 

It can take months or years for trees to start showing signs of stress and decline. If decline or death occurs rapidly massive root damage, soil contamination or poisoning may be the cause.

How to tell if a tree is stressed

A stressed tree will commonly have the following characteristics:

  • Leaf discolouration (yellow, purple or brown) and/or excessive leaf loss.
  • Excessive leaf damage from insects.
  • A build up of wood shavings or soil around trunk from ants and termites.
  • Holes in bark and wood from wood borers, termites or other insects.
  • An over or under production of sap.
  • Abnormal amount of bark cracking, shedding or lesions.
  • Fungus growth at the base of the trunk or on stems.
  • Cracked, brittle or fallen limbs.
  • Stem and root rot.

What does a healthy tree look like? 

There should be a presence of insects – they are an important part of a healthy environment and provide a food source for many native birds and animals. However, if insect numbers seem high, and damage is excessive and causing defoliation, this can indicate a problem.

The bark texture and colour on the trunk and branches should also be fairly uniform. It’s normal for healthy trees to produce kino, a sticky sap that engulfs and kills insects attacking the wood. Excessive or no kino may indicate stress.

Bark and branch shedding is also normal, however excessive splitting or shedding may indicate stress.

A healthy native tree should have a dense crown of foliage and the leaves be a uniform green. Some leaf damage from chewing and sap sucking insects is normal, especially in eucalypts.

Commonly affected species

  • Banksias
  • Marri/Red Gums
  • Flooded Gum
  • Peppermint Trees
  • Jarrah
  • Sheoak
  • Tuarts
  • Wandoo.

How to prevent native tree decline

    • Leave bushland around trees intact and revegetate.
    • Control pests.
    • Use water cautiously.
    • Improve the soil.
    • Minimise soil compaction, root disturbance and trunk damage.
    • Avoid poor pruning.
    • Prevent and control disease.
    • Monitor your native trees for any changes.

    Native tree treatments

    Two potential treatments are phosphite and nutrient implant treatments. Both are applied systemically by drilling into the trunk of the tree, then injected. 

    Research on treatments is in its early stages and it isn’t yet understood how phosphite and nutrient applications fully work. Phosphite injections appear to stimulate the tree’s natural immune response. Nutrient injections have been shown to fix nutrient deficiencies, resulting in improved tree vigour and heath.

    Individual trees respond to these treatments differently, so all treatments should be used with a degree of caution.

    The long-term solution

    While it may not be possible to save every native tree suffering from decline, a lot can be done to protect healthy trees. If your trees do die, plant more to replace them or consider leaving the dead trees in place as they too provide valuable habitat.

    Native trees have a much better chance of survival in an environment that is as ‘natural’ as possible. Wildlife that depends on trees will also benefit enormously.

    Ask us or your local landcare group or nursery for more information. Encourage your neighbours to get involved as the more landowners that work together the greater the chance for success. This is particularly important for establishing wildlife corridors that run across many properties to nearby reserves and bushland.

    Flooded Gum decline

    Flooded gums (Eucalyptus rudis) are one of the most important trees on the Swan Coastal Plain. Found mostly along rivers and bordering wetlands, Flooded gums support birds, insects and other fauna, as well as stabilise the soil in areas where erosion is likely to occur. 

    Many Flooded gums in the area are experiencing heavy insect attack and a decline in health. This is due to extensive clearing throughout the South West of WA, which has resulted in a significant loss of biodiversity. 

    One of the many consequences of reduced biodiversity is an increase in pest insect populations. While these pests are native, their numbers have increased drastically due to a reduction in predators, such as birds and other insects, which once kept them at bay.

    Of the insects that occur on Flooded gums, the three main species are psyllids (lerps), the Jarrah leafminer, and Leafblister sawfly. Flooded gums may also be attacked by scale insects, leaf galling insects and chewing insects, such as spitfires.