It is the bigger and older trees that provide resources in the abundance required by numerous animals. It may take a tree one or two decades before they begin to flower and set seed, which they produce in increasing abundance as they mature. Numerous species of invertebrates, many birds, and a variety of mammals feed on these flowers and seeds. As they mature their trunks and leaves also exude a variety of sweet substances used by many species. Invertebrates harbour within their rough and shedding bark where they are eagerly sought out for food. Yellow-bellied and Squirrel Gliders chew channels through their bark to tap trees for sap. As the trunks and branches thicken the trees provide more stable nesting and roosting sites, while enabling Koalas to hug them on hot days to keep cool.
Once a eucalypt tree is over 120-180 years old they may start to develop hollows in their branches and trunks. In NSW at least 46 mammals, 81 birds, 31 reptiles and 16 frogs, are reliant on tree hollows for shelter and nests. As the trees get bigger so do their hollows, and it may not be until they are over 220 years old that they develop hollows big enough for the largest species. Most eucalypts may only live for 300-500 years, though some are reputed to live for over 1,000 years (see The Importance of Old Trees).
photo: Dailan Pugh OAM
Crown of a Sydney Blue Gum (Koreelah SF) hundreds of years old showing the numerous broken branches and large hollows necessary for large-hollow dependent fauna
Natural forests may support 13–27 hollow-bearing trees per hectare, with numbers varying between species, and increasing on more productive, moister and flatter sites. On agricultural lands the numbers of hollow-bearing trees have been drastically reduced. Similarly they have been significantly reduced throughout the remnant forests by logging, prescribed burning and by culling in Timber Stand Improvement operations.
In State forests in north-east NSW logging prescriptions now require the retention of an average of 8 hollow-bearing trees per hectare within logging areas, though numbers have already been reduced below this level in many forests. Where retained, hollow-bearing trees continue to decline with each logging due to token implementation of prescriptions, poor tree selection, inadequate protection, damage during logging and in post-logging burns, and lax enforcement. (see Protecting Habitat Trees)
Natural forests are generally multi-aged, so that as existing hollow-bearing trees die and collapse there are new trees with developing hollows to replace them (see The Importance of Old Trees). To account for this, up until 2018 the logging prescriptions required the retention of an additional sound and healthy mature tree for each retained hollow-bearing tree as a recruitment tree to be able to develop into the hollow-bearing trees of the future. Trees meeting this definition are also high-quality sawlogs so the Forestry Corporation used to go to extremes to avoid their obligations to protect them, and were finally successful in having the requirement to protect recruitment trees removed in the 2018 Coastal Integrated Forestry Operations Approval. This up and coming cohort of future hollow bearing trees is rapidly declining due to natural mortality and logging. (see Protecting Habitat Trees)
If we are to minimise the hiatus in the availability of hollows for a plethora of native species we must act now to protect, as far as possible, all large old trees, along with sufficient recruitment habit trees to replace existing hollow-bearing trees as they die and to restore hollow-bearing trees throughout native forests.