Strangle or be strangled: The harsh reality of figs

High in the canopy of a declining turpentine tree in the heart of Sydney’s Centennial Park, the roots of four baby strangler figs will start to grow and strangle its host tree to death as part of a new horticultural experiment.

The experiment, conducted by the Centennial Parklands’ Arboriculture team, is two years in the making and is attempting to replicate nature to create a new and unique attraction in the urban parkland setting.

If successful, the figs may take up to 20 years to take over its host tree and what it should create is nothing short of nature’s brutal but natural beauty.

Famous examples of the end result can be seen in Cambodia’s Temples of Angkor, which were made famous by Angelina Jolie in the Lara Croft: Tomb Raider (2001) movie.

There are not many examples of this type of tree in the urban Sydney setting, though an exceptional one can be found in the Royal Botanic Garden Sydney.

The host tree was planted many decades ago and is now overgrown with two types of strangler figs: Ficus macrophylla and Ficus henneana.

The tree is located at the end of the Garden’s famous Spring Walk and overhangs the tank stream in the Palm Grove. It is understandable why this Melaleuca styphelioildes has turned into such a successful host for a number of competing figs. The microclimate created by the neighbouring Palm Grove is humid and protected from wind and sun exposure, similar to that of a forest where stranglers are most likely to occur.

The host also has a huge potential for strangler roots to occupy nestled in its branch junctions and behind its papery bark. Despite the constriction by the figs, you can still see remnants of the host tree hanging on for dear life, with its branches and small prickly leaves poking through the engulfing figs.

The truth about stranglers

It’s all about competition and survival of the fittest in the plant kingdom, particularly where adaptations can be the difference between surviving or not. The Ficus genus is one group, which has overcome one of the most challenging aspects of life in the forest.

All figs have the potential to become a strangler, which is a common growth habit that is found in many tropical forest species. This adaptation enables plants to begin life in the top of the existing canopy line where light is more available as opposed to the intense race for light from the depths of the forest floor. These plants are known as hemi-epiphytes, spending the first part of their life without rooting into the ground.

Figs will start their life in nature at the top of a canopy, their seeds are often bird-dispersed and germinate in crevices at the top of other trees. These seedlings slowly grow their adventitious roots downward surviving on the rainwater running down the stems and the forests ambient humidity. Once only using the host tree for support, they grasp and eventually envelop it whilst they grow upward to reach into the sunlight zone above the canopy, where they can accelerate their rate of photosynthesis.

Figs, if successful, will out-compete any host tree and shade it out as they grow. Their primary challenge is the long journey their roots have to make as they extend and elongate bit-by-bit by simply hanging or opportunistically following the rain water courses down the trunk of their host.

Once the root hits the grounds, there is an explosion of nutrients and water uptake. The fig can now access as much as it needs through its plumbing and accelerates its growth at an exponential rate. As the fig sends sugars to its other parts, usually including many more aerial roots to fast-track them to the ground in support, it starts to form its own self-supporting trunk as it lignifies and continues to wrap and constrict its previously supporting host.

This is the point where it starts strangling as its secondary thickening starts to occur, it forms branches and a lattice-type structure often forms as its roots and branches graft and envelop one another. Fig species have adapted in this way, grafting onto themselves to reinforce as one uniting system.

As it thickens it chokes the host tree to death, which in turn breaks down into organic matter for the tree to thrive. From here the strangler fig can resemble a pipe with a hollow central core. The outer shell of a tree is all that is required for structural support, so this is not an issue for its stability. In fact, hollow trees are known to withstand storms and remain healthy long after the core has decayed, as the outer shell contains all of the trees living components.

The sacrificial tree

Choosing a sacrificial tree for the Parklands' experiment was very important. The team chose a turpentine (Syncarpia glomulifera), located in Turpentine Grove near the Centennial Homestead and Bird Sanctuary in Centennial Park. The tree is in an inevitable state of terminal decline for various stress reasons, which is normal for a tree of this type in this environment.  

Most trees that are in decline across the Parklands due their age, health or other various reasons, need to be managed quite carefully as their branches die and become brittle. However, this particular species’ timber has a very high bulk density, which means it doesn’t shed its branches quickly like most other trees. What it also means is that they are able to monitor its decline in a public area without compromising risk to visitor’s safety.

The stringy and spongy bark also has its own microclimate within, which can be utilised by the figs for protection from drying out. The selected tree already has a couple of figs already growing on it that have occurred naturally. They are very small and don’t have the energy or leaf mass to grow at a quick rate just yet like the larger ones we have planted, however in time and if successful they will achieve the same outcome.

The difference with these self-sewn figs is that they are much lower in the canopy, and although their roots have a shorter journey, their position in the tree will remain low so the potential to form a long strangling trunk is limited.

How it will work

Techniques we used to do this experiment were developed by the in-house expertise of the Parklands’ Arboriculture team.

The figs were propagated and sourced from seeds collected from Centennial Parklands and Royal Botanic Garden Sydney and include a mix of Ficus species, such as Ficus macrophylla. Cuttings taken from trees that were specifically selected due to their abundance of aerial roots and genetic predisposition to develop them. They were then grown for 12 months in the Parklands' Nursery.

To create a natural pot for the figs to establish in, which we could then transplant (fig and all) into the host tree, Senior Arborist Peter Butler and his team used birds nest ferns (Asplenium australasicum), which are native to Australia. They are also an epiphyte, which is a plant or living thing that lives out of the ground and uses a tree for support.

It is also a temporary and sacrificial pot, secured using biodegradable hessian ties, that provides a safe place of establishment until the fig is able to fix itself to the tree. The four baby figs were placed around 15 m in the canopy and it is anticipated that the figs should start to take by the end of Autumn.

The one thing that could be detrimental to their survival is lack of rain. As with most trees and plants across Centennial Park, water is the biggest limiting factor to their survival and health.

Figs are very resilient trees, as long as there is enough available water during their establishment – it will allow the little figs to survive. It is a risk sticking them in their new home high off the ground without any watering regime, however they have been hardened off in the Parklands' Nursery without any water for months now so they can survive on rainwater alone.

The ferns should provide a temporary haven for now, but summer will be the ultimate test for them – especially if this drought and lack of rainfall continues. Regardless of the challenges, this is the harsh reality of trees in nature and we are confident the resilience of figs will help this projects success.

Watch the project come to life

Watch this video to see how our arborists installed the four baby strangler figs into the canopy of its host tree recently in Centennial Park.