In the hills to the east of Melbourne I am seeing large old trees being retained in greatly disturbed surroundings with new developments pushing in and clearing occurring primarily for house lots and to reduce fire fuel loads. It is rare not to hear mowers, chainsaws and blowers nearby obsessively clearing around and beneath the trees.
As a result of this I am frequently observing the denuded condition of the root zone of retained trees and a lack of under-storey vegetation.
A recent example I observed was in the outer east of Melbourne where remnant Messmate Stringybark (Eucalyptus obliqua) trees were dying off on a large bush block because of aggressive clearing of the under-storey for fire fuel management. The trees within the site generally displayed poor health characterised by dead trees or trees with sparse foliage density, dead wood in the crown, dieback and epicormic sprouts along internal branches.
These symptoms can be partly attributed to age related decline in some of the larger trees but the general malaise was primarily associated with the regimen of fuel reduction clearing which was evident as removal of the under storey vegetation, surface root damage and decay from repeated mower impacts and subsequent degradation of soil micro flora and fauna, loss of natural accumulated debris and mulch, soil compaction and increased rates of soil moisture evaporation.
There are many benefits to maintaining or reintroducing under-storey plants into the area surrounding trees and beneath the tree canopy including;
- Acting as an insulating layer that can protect the soil from extremes of heat, cold and strong winds.
- Contributing to the accumulated mulch layer as well as intercepting leaves and debris from over-storey trees as a mulch layer that might otherwise be blown away.
- Improving soil porosity with finer stems and roots and lower branching often to ground level tickling the soil surface.
- Hosting an increased range of mycorrhizal fungi on their roots that extend the nutrient and water absorbing capabilities of all plants and is especially important to the canopy trees.
- Providing greater diversity of species and habitats for small native birds and mammals that are important to the trees in terms of pest and insect control as well as pollination.
- Providing shelter for fauna that prefer to forage within the understorey rather than in open areas, where they are more vulnerable to disturbance or attack.
- Softening the impact of heavy rain and drips from over-storey trees that can erode soil and leach soil nutrients.
- Intercepting and slow surface runoff allowing it to penetrate the soil at a more steady rate and retain a more humid environment at ground level to assist mulch to decompose and slow evaporation and preventing formation of a waxy cuticle that can sometimes form on exposed and undisturbed mulch layers.
- Frequently includes pioneer species such as wattles that contribute to improving soil fertility by fixing nitrogen in the soil.
The understorey plants are therefore both a source of organic material but also helps the processes that allow nutrients to be returned to the soil.
Introducing or maintaining a healthy understorey assists in conserving and protecting the essential elements of soil and water as well as contributing to habitat, a source of food and nesting materials, breeding sites and shelter for many native animals which ultimately are beneficial to the ongoing health of the retained over-storey trees.
These benefits are not limited only to bush sites but the same principles can be applied to larger amenity trees in the urban setting. Considering the use of small ornamental shrubs and groundcovers, as opposed to a lawn, can provide benefits in improving soil health and moisture retention.