Objectives
>Students learn to identify signs of dieback in local trees
>Students can identify potential causes of stress in plant communities
>Students describe actions to minimise stressors on local plant communities
Duration
1 hour
Materials required
>Dieback Diagnostic Chart (hyperlink available on Sustaining River Life cd and web versions)
Background
Source: “Dieback in Native Vegetation in the South Australian Murray-Darling Basin: A guide to causes and symptoms” by Department of Environment and Heritage, South Australia (2005)
“’Dieback’ is a common term for the gradual death or long-term decline in the health of vegetation…… Our environment is marked by extremes of drought, flood, salinity and fire. All of these factors can place stress on plants. This can affect their health and their ability to cope with other more acute factors such as insects or pathogens. Fragmentation of vegetation through clearance for intensive agriculture or development, and riverine management, all compound these stresses.”
Recovery often occurs, but plant death is inevitable if the stress factors continue long enough. Death can take weeks, months or even years. Individual plants may be affected, as well as large areas of native vegetation.”
River Red Gum watering trial at Chowilla Floodplain in South Australia, 2004. Lack of soil water can be one of the causes of dieback. Photograph: Bill Phillips
Dieback is not a phenomenon restricted to floodplain or river bank species, however in recent years extensive dieback of River Red Gums along the Murray River has been recorded and largely attributed to lack of soil watering through a combination of the prolonged drought and lack of flooding caused by over-allocation of water and the impact of dams and weirs regulating flows. The photograph above is from a trial watering of Red Gums along the Murray River in 2004 to see if the dieback could be reversed.
Dieback more generally comes about via several factors all impacting on the trees in question. Apart from lack of water in the soil, the reverse, waterlogging, can be a causal factor also. Things like compaction of the soil, or increases in soil salinity or nutrients have also been implicated. The loss of understorey plants can also put stress on trees and this may expose them to the risk of dieback, especially if this happens in concert with one of these other factors.
Mundulla Yellows, another potential contributing factor, is a still somewhat puzzling occurrence. It is usually seen over a wide area and impacts on Eucalypts and a range of other native species such as Banksias and Acacias. Added to these can be pathogens of plant species, with Phytophora root rot being the most serious. It attacks the roots and stems and ultimately reduces the capacity of the tree to absorb water and nutrients.
The first visible signs of dieback are usually insect attack, leading to defoliation, by things such as Sawfly larvae and Chrysomelid beetles. The occurrence of Witches’ Broom is another symptom. Here the twigs become clumped together and the terminal growth shoots appear like a broom. The other early sign is infestation with either Mistletoe or Dodder-laural. These are semi-parasitic plants which use their hosts to gain moisture and nutrients. Mistletoe is generally spread by birds. With both Mistletoe and Dodder-laural trees can survive with a few infestations, but a heavy load, when added to other stress factors, can lead to tree death.
In Eucalypts, the visual symptoms of dieback can be classified into four stages (see below).
Vocabulary
Epicormic growth
Defoliation
Crown
Fragmentation
Water logging
Salinity
Nutrients
Understorey
Soil compaction
Mundulla Yellows
Pathogens
Lesson plan
1. Introduce the concept of tree dieback with the following drawing (see below).
2. Explain that dieback is of great concern, especially along our floodplain ecosystems.
3. Outline the causes and early signs of dieback (see below and Background).
Signs of dieback
>Crown or canopy thinning, beginning at the branch tips and progressing toward the trunk
>Large portions of crown defoliation
>The affected trees may partially recover through new stem and leaf growth from the trunk and branches (epicormic growth) as the tree attempts to replace the lost foliage of the crown
>Finally, all foliage, including epicormic growth dies off, leaving only dead twigs and branches. Tree death occurs.
4. Get the students to think of symptoms of poor tree health they might have seen already.
Symptoms to look for
>Thinning of crown foliage
>Dead branches, often with bare twigs protruding from the tree crown
>Physical damage to, or deformation of, leaves
>Physical damage to trunk and bark
>Yellowing or other discolouration of leaves
>Excessive numbers of insects (e.g. caterpillars), sticky coatings on stems, leaves or under bark, holes and tunnels in timber under bark; and/or lumps or bumps on leaves and stems.
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