The circle of (a tree) life

The longevity of a tree is hugely dependant on the species and the environmental factors in which they grow. In a natural setting some trees live 5-10 years, such as some Acacia species, whilst others can live up to 5,000. The oldest currently known living tree is a Great Basin bristlecone pine, found in the White Mountains of California that has been aged at 5,067 years. In Australia, our veteran eucalypts and hard woods can reach 400-500 years before succumbing to wood decay fungi, major storms or bushfires. They have a significantly shorter life than that of some conifers as our climate is more favourable to a range of wood decay fungi, which exist ubiquitously.

Within the Parklands, our primary tree species are the paperbarks, maritime pines, eucalypts, she-oaks, evergreen oaks and of course, the iconic Australian figs. Each species has a different expected life span, however even though it is a relatively green space in our city living, the trees in our Parklands do have to endure some residual challenges with the urban environment.

Example of the life stages of a tree: Eucalyptus

Our Arborists work to ensure that our trees reach their full potential. Trees are sourced and grown for years ahead of time in a specialist advanced tree nursery where they are propagated and grown to the highest possible standard, crucial for their establishment and longevity as a Parkland tree.

At the beginning of a newly planted tree’s life cycle, it receives an intensive aftercare regime, which aids them through their initial establishment into their new environment and through their most critical developmental stages. This includes an intense watering, mulching and fertilising program, with annual monitoring, weed control and formative pruning to ensure that the tree’s canopy grows to suit its space and to produce a form which is ideal for the trees’ structure and maintenance requirements.

A before and after photo of the formative pruning of a Ficus microcarpa tree

Just like with people, illness isn’t always visible on the surface and the impacts of age are often more internal, such as the weakening and breaking of bones. Particularly with trees, issues can arise and develop deep below the surface such as under the ground within the root system or internal structures in the above ground parts, such as excess decay of living tissues causing structural weakness.

To passers-by, a tree may seem healthy, however our highly skilled and experienced arborists are trained to identify changes or even the tiniest of issues through routine inspections. Every tree in the Centennial Parklands is individually assessed on an annual basis – yep, they check every single one of the 16,000 trees! You may often see trees marked with white or red tape, which indicates that the tree requires extra watering over summer months.

A newly planted tree that is marked to ensure it has extra watering

However, if a tree becomes unhealthy or poor in structure sometimes, there is just nothing that our Arborists can do to bring it back to its full bill of health or retain it safely. As a result of disease, storms or just time, trees can begin to show signs of decline. Examples of concerning signs our Arborists look out for include:

• A change in the space between neighbouring trees or the disorientation of leaves – may indicate a branch fracture or hanger, or even an entire tree and its root plate heaving in the ground
• A sudden change in site conditions – this may be in the form of a newly exposed tree to prevailing winds following a tree removal or a change in hydrology, where gutting can change the infiltration of rainwater
• Bark inclusion – a genetically inferior structural defect when bark is incorporated into the stem or branch union causing a weakened connection or incremental growth causes their eventual separation in some cases
• Signs of decay, fungal infections or root damage – sparse canopies, an over production of dead wood, branch retrenchment or cavities can indicate any of these factors
• Discolouration of tree parts or changes in optimal leaf size and colour – can indicate ‘stress’ from a variety of factors such as soil toxicities, nutrient deficiency or abiotic factors
• Any changes to the timing of a trees normal processes such as flowering or new leaf production – can indicate stress that may be obvious or require further investigation

A Ficus microcarpa showing signs of decline, such as loss of canopy and a decrease in leaf production

Trees do in fact die of old age, but the term ‘old age’ tends to be a secondary cause to a variety of inhibiting factors. When a tree grows and increases in size, it needs to produce more sugars to feed its parts to maintain them and keep growing. Unlike in humans, trees never stop growing as their large and complex ‘factory’ of processes keep spending sugars created through photosynthesis. A tree is always prioritising its sugar budget to allocate its resources into areas such as growth, reproduction (flowering), disease resistance and storage.

Once the tree ages to a point or the external factors overcome the trees natural systems, its loss of vitality causes things such as leaf production and new wood development to slow down. Trees sometimes enter this ‘mortality spiral’ where they can become susceptible to environmental factors, pathogen damage or eventual death. They can also cope remarkably well through this stage as they are very resilient but may do this by forming new canopies (with shorter distances to travel), retrench existing live branches or turn these into dead parts. Just as when humans age, their immune systems weaken and we make life easier for ourselves by slowing down.

The environment of Centennial Parklands also puts pressure on trees. Our parks naturally have fast draining and sandy soil, which is great for drainage and high traffic as it is resistant to compaction, however it is naturally poor in nutrients and its water holding capacity. By far, water availability is our biggest challenge and limiting factor for our trees here but every effort is made to prioritise our watering and establish drought tolerant tree species that can cope.

Centennial Park also gets a large amount of coastal exposure due to our proximity to the ocean so strong winds and lightning strikes are common. In contrast to a natural environment, growing in a Parkland has its own limiting factors and stress causing factors, such as a restricted cycle of nutrients that would usually be sustained within a plant community, compaction by vehicles or pruning required for public access such as roads.

Despite all these challenges, trees are very resilient. With the right tree management practices their health and longevity in an urban environment is critical to maintain a good relationship with trees and enjoy everything they provide.

An example of a tree infected with fungus

A large amount of careful planning and decision making is made before we decide to remove a tree. This is because we only remove trees if completely necessary, and every effort is put into preserving our trees as long as possible. Although sad, it is a vital part of tree management and being part of a dynamic Parkland, where we are constantly evolving to improve facilities and create areas that will be appreciated by all. Trees are a significant part of the Parklands and they are not removed lightly.

Our Arborists carry out tree inspections periodically over the entire site and schedule work according to many factors. Tree risk assessments and inspection practices are carried out to a high industry level standard where all skills and experience of our arborists are utilised. Quite often trees need further investigation where we carry out advanced assessments when necessary, such as climbing aerial assessments for a closer look, scientific analysis or utilising modern technical equipment such as a Picus Tomograph to monitor our trees using sound waves. The Picus Tomograph can help us see the extent of any internal decay and if the tree’s structure is compromised.

An example of a Split Monitor, put in place to monitor the size a new split that appeared in the trunk of a Ficus microcarpa

A trees’ terminal decline or death, if not managed correctly can be a threat to public safety. A tree may look healthy on the outside, but if its internal structure is compromised, this can lead to branch failure and the potential to cause serious damage. Once a tree hads failed, a team are brought in to remove the remains of the dying tree to mitigate any further hazards the tree may pose.

Our three parks receive 30 million visits annually so public safety is our top priority.

Tree failure at Learners Cycleway

The Parklands is a dynamic and ever-changing environment. When a tree is removed, it inevitably leaves an obvious gap in the landscape.

We strive to replant every tree felled with the same, or a more appropriate tree species, if necessary. The availability of advanced trees grown to our demanding standards often delays this process, in some cases it can take us years to grow a replacement tree that will be sure to endure. Our Arborists plan years ahead to grow trees available for planting each year, however some tree removals can catch us unaware at times where species replacement is required.

Consideration must be made to the success in establishing a young tree in areas where competition for light, space, water and nutrients with other trees is inevitable. Dense avenue replacements are notoriously hard to achieve this; however, a Grand Drive Tree Succession Plan is already underway to replace many of the trees we have lost over the years in stages to repopulate the iconic avenue with the same species it is so well known and loved for.

The Centennial Parklands’ Tree Master Plan sets out strategies for conserving existing trees as well as integrating new plantings into the Parkland’s historic fabric through our tree replacement program. We use this combined with many other considerations specific to each area to maintain and add to our diverse tree collection of 234 tree species into the future.

In 2021, over 200 trees were planted throughout the Parklands. Similar numbers are aimed for each year to for our Annual Tree Planting program.

The effects of drought, old age and the urban environment has taken its toll on our trees. In the next 40 years, a large percentage of our tree population, especially those that are already mature or over-mature will need to be replaced due to their terminal decline. To combat this, we are aiming towards planting 3,000 trees over the next 10 years to ensure that the Centennial Parklands remains one of Sydney’s best loved and healthy green spaces.

Newly planted Quercus ilex in Centennial Park