Habitat Trees
Large, old and dead trees are unique and irreplaceable features of any landscape. Termed ‘habitat trees’, they provide numerous living places and other resources for many kinds of animals and plants.
Habitat trees have hollows, cracks and crevices of various sizes, where animals may live, breed or shelter. Old and dead trees are an essential part of all native forests and can be referred to as ‘nature’s community dwellings’ as they provide an important resource to local wildlife.
Trees and hollows
Hollows are an important element of the Australian landscape and ecosystem processes. The age at which trees develop hollows varies widely. However, large hollows are typically associated with trees that are at least 100 years old. As a tree becomes older, it develops more and larger hollows.
A thriving bushland area in good condition would typically have three to 10 hollow-bearing trees per hectare, each containing as many as 30 hollows of varying sizes and supporting a diverse wildlife population.
Creating a new Habitat Tree
When a large tree’s health is in inevitable decline or has died in Hobsons Bay’s public spaces, we will consider whether it’s appropriate to retain as a habitat tree. The tree will be assessed to check its structural integrity and determine if the location of tree is suitable and does not pose a risk to public or property.
When creating habitat trees, we will typically remove all branches that are less than 20-30cm in diameter. Smaller branches are removed to reduce wind resistance. Artificial hollows may be created in the trunk and branches. The sizes of hollows and their openings are carefully designed to attract micro bats and native bird species commonly found in Hobsons Bay. Nesting boxes may also be added.
Mulch may be added around the base of the tree, along with understory plants to create a healthy and safe haven for wildlife.
Habitat trees are a valuable asset in Hobsons Bay and are included on a cyclic inspection schedule.
Habitat trees provide many resources for wildlife including:
- Cracks, crevices, notches and hollows that provide breeding sites, shelter, refuge and living quarters for a wide range of wildlife species.
- Nesting sites in high branches for birds such as kites, eagles and other raptors.
- Dead branches above the canopy that are utilised by some birds for roosting, sunning, preening and to watch for prey.
- A reliable and plentiful seasonal food supply such as nectar, pollen, exudate (sap), fruit, seeds, leaves, wood and litter for animals of many kinds.
- Large surface areas of trunks and branches that provide extensive hunting sites for animals that feed on organisms that live on and under the bark.
By virtue of their size, old trees provide more food and nesting resources than younger trees. For example, one 300 year old grey box (eucalyptus microcarpa) with a height of 20 metres and a trunk diameter of 1.5m has a bark surface area of approximately 94m2. A 20 year old tree with a trunk diameter of 20cm and a height of 15m has a bark surface area of just 9m2. An animal can therefore forage as profitably on one large tree as on ten smaller trees, at the same time decreasing the risk of predation by not having to travel from one tree to the next.
Dead trees with hollows are just as important for wildlife as live ones. Dead trees may stand for 50 years or more after death and continue to function effectively as habitat trees. When a tree dies, new living spaces are formed as cracks develop and the bark loosens.
Old trees are irreplaceable with many of the ones alive today being at least 200-800 years old. Such trees represent the vestiges of a once-intact ecosystem and provide some sense of what the landscape was like before European arrival. Trees planted today will need two centuries or more before they attain a similar form and position in the landscape.
As part of the ageing process, trees (especially eucalypts) tend to shed branches, both small and large. They also drop large quantities of leaves which serve a number of useful purposes. The shed branches and leaves provide a unique and valuable resource for a vast array of fungi, invertebrates, small mammals, reptiles and frogs. The leaf matter and rotting log and branch detritus add to soil fertility and protects the soil surface.
Some of the functions provided by fallen timber include:
- Breeding and sheltering sites (hollow logs are especially important in this role).
- Foraging sites for insects, birds, mammals and reptiles.
- Basking and hibernation sites.
- Perching sites for birds.
- Nurseries for plants and fungi to germinate and grow.
- Natural traps that help to accumulate leaf litter and soil and create moist, nutrient rich sites.
- A source of lichen and cobweb that is used by birds in nest construction.
Large trees provide shade and protect neighbouring trees from weather extremes. They provide a more stable ‘micro-climate’ in their direct surrounds. The soil beneath old trees tends to be cooler in summer and warmer in winter.
Large eucalypts also draw water from far below the soil and will out-survive younger trees. They do this by a process called hydraulic lift where the phloem or sapwood (vascular tissue) in the trunk of the tree acts like a giant straw drawing water from roots extending far below the surface. This can also be beneficial to surrounding shallower rooted species and helps prevent dryland salinity.
Eucalypts in particular provide food for larger bat species, like the flying fox bats that frequent Hobsons Bay. An interesting feature of many eucalypts is that they produce a high nectar flow between 10pm and 2am, the time when megabats are feeding.
Many native animals such as possums that utilise hollows in habitat trees, feed on and help to control leaf-eating insects that would otherwise contribute to dieback in our urban forests.
Insectivorous bats provide a unique ecosystem service by consuming up to one and a half times their body weight in insects each night. Many of the insectivorous bat species that occur across Victoria prefer to roost in tree hollows. They usually select tree hollows that have entrances not much bigger than their own body size, about 3 cm or less in diameter. Larger colonies, especially those formed when females congregate to give birth, require a large internal hollow with a small entrance. These types of hollows are usually found only in large old trees.
- Retain large old trees (both living and dead), provided the tree is structurally sound to retain. An arborist assessment can determine suitability and reduce some of the smaller branches.
- Retain some fallen woody debris for wildlife.
- Install nest boxes in existing trees.
References and further reading
Learn more about flora and fauna in Hobsons Bay
Gibbons P & Lindermayer DB (2002) Tree Hollows and Wildlife Conservation in Australia. CSIRO Publishing, Melbourne.