Mitcham Reserve Tree Conservation

How is Council improving the health of River red gums at Mitcham Reserve?

Council will be mulching around some of the Mitcham Reserve’s River red gums to improve their health and prolong the life expectancy of the trees as well as allow any falling branches to land within the mulch away from lawned recreational areas. 

Large River red gums are significant to the heritage, environment and aesthetics of Mitcham Reserve. While mulching around River red gums will replace 8.7% of the lawn area it will improve the long term health of the tree, eliminate the need for detailed and invasive examinations and manage public safety.

River red gums at Mitcham Reserve

Mitcham Reserve is dominated by large River red gums. Very few old River red gums remain on land managed by Council. The trees on Mitcham Reserve are relatively young, the estimated ages of the oldest specimens (200 to 250 years) being only half to one third of their potential life expectancy under ideal conditions. 

The altered urban environment, degraded soil conditions, irrigation with mains water and competition with invasive weed species impact the health of River red gums and reduce their life expectancy. Five River red gums on Mitcham Reserve are in declining health. The trees have reduced vigour; they grow only a minimal amount of new wood with each annual growth ring. In this condition internal decay progresses more rapidly than growth, so the amount of wood in the tree’s structure diminishes. Risk of branch and trunk failure increases as decay outpaces growth. 

Keeping River red gums growing vigorously requires basic resources such as:

Tree hydration 

Water vapour is lost almost constantly through River red gum leaves. To replace it, trees absorb water through fine, hair-like roots. Large roots provide stability and they transport the water from fine roots to the tree’s stump, but nearly all water absorption is through fine roots. Root growth requires oxygen, so fine root growth is most abundant near the soil surface. Fine roots usually extend well beyond canopy driplines, often growing twice as wide as a tree’s canopy. 

The abundance and spread of fine roots limit a tree’s access to water. Maintaining a broad network of roots over a large area, typically beyond the canopy dripline and possibly into groundwater reserves, is essential to ensuring tree water supply and health in the long term. 

Tree nutrition 

Trees need nutrients to sustain their growth and other biological functions. The elements needed are absorbed in water drawn into the tree through fine roots. Fungi which live in association with fine root networks are mainly responsible for providing nutrients in the soluble form which trees can utilise. 

Active soil biodiversity, including fungi and invertebrates, is essential to tree nutrition. Any reduction in the health and vigour of tree root systems and the biodiversity which naturally occurs in association with fine roots may, in time, diminish tree health. 

Energy for tree growth 

The energy which powers tree growth and other biological functions is provided by carbohydrate in the form of sugar. Sunlight powers the production of sugar in leaves. Restricted access to sunlight, through shading or reduced leaf area, reduces the tree’s sugar production and the energy available for growth. Pruning reduces leaf area and sugar production, which limits growth and impacts on tree health. To maintain optimal growth and tree health, pruning must be kept to a minimum. 

Soil conditions and tree growth 

Mitcham Reserve’s soil has been altered by Kikuyu turf and irrigation. These changed conditions now affect the health and will reduce the lifespan of the River red gums. Restoring soil conditions will improve tree health. 

Kikuyu turf exudes chemicals which can prevent or slow the root growth of other species. Dense masses of Kikuyu roots actively compete for water with fine roots of the River red gums. Increased irrigation is not an answer to competition for water, as mains water can affect tree nutrition. 

Fungi which live in association with roots and which provide nutrients to the trees can be killed by mains water, so tree nutrition can be compromised by irrigation. Regular irrigation can also reduce oxygen diffusion in the soil, which reduces the growth rate and life expectancy of fine roots. Optimal soil and root growth conditions for River red gums can be restored by eliminating Kikuyu turf from their root zones, restoring natural seasonal water cycles free of irrigation, and allowing increased soil biodiversity to re-establish by adding organic carbon to the soil through mulching. 

Seasonal wetting and drying of the surface will restore fine-root growth in shallow soil during the autumn, winter and spring, with deeper growth in summer as the surface dries out. Seasonal wetting and drying of the soil will improve oxygen diffusion, supporting greater root abundance and improving nutrition and water absorption. The alternating seasonal conditions will begin to restore soil health and structure, further supporting improved tree health.

Please refer item 7.5 for a full copy of the report presented to Council on 28 February 2017 Full Council Agenda.pdf(18412 kb)

Mitcham Reserve Heritage Tree Protection Report(2350 kb) 

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