Trees with big leans

Trees with big leans, as the TCAAA’s Jim McArdle points out, are a phenomenon which eludes most arborists.

Most lay people, non-arborists, would say a tree with a lean is a common threat of failure.

This would only be the case in a solitary tree in moist or saturated ground, or a tree facing the prevailing wind. Some trees lean due to the removal of adjacent trees causing them to compensate a new foraging and support system in order to remain viable.

Why don’t trees lean toward the equator?

With age some do, like Araucaria columnaris (Cook pine). Phototropism is the cellular response to light. For example, most indoor plants on windows to grow toward the sun. Trees have many cells that are opportunistic towards the light, often the sun, or at least towards a gap where they’re exposed to light. The exception is roots which grow away from light (skototropism).


Can a leaning tree be retained?

It would be wise to retain a leaning tree unless it’s hazardous, and the AQF level 3 or level 5 arborist can best ascertain this through testing the roots at the SRZ distance or interface from structural to woody feeder roots. A table of SRZ calculations is given in AS4970 2009.

A tree with a lean is a commonly seen as threatening failure. Sometimes it’s true, but not always. Image: TCAAA


Key areas of concern when assessing or doing a pre-climb questionnaire would be observing a weight-to-balance ratio deficit, slenderness, lift of root plate, root rot, or fracture in the lower or mid stem within the tree, and whether the tree is naturally grown or from a pot, having no tap root; one way of addressing these issues is root test investigation.

We were called to a local job where a tree was moving and the lean toward a house was raising concerns. Within the hierarchy of control there was limited access, so we couldn’t use an EWP, crane or helicopters, and stabilising the tree was a challenge. After tying the tree to an anchoring point and two large and stable trees nearby, the weight-tobalance ratio was corrected and the moving tree was dismantled.

Basal root rot and decay can easily displace a tree and cause it to lean and fail. If saturated for long periods, moisture under the soil tends to move mycelium to up the tree’s vessels towards branches. Unfortunately, it can cause branch failure or lead to root loss and failure or leaning.

Root tests can offer some hope of identifying, root vigour (energy), vibrance (appearance) and vivacity (root tenacity or tendency to spread rootlets and colonise) in the area of support. The issue with identifying species or types of trees is when a vivacious root system overcrowds or outcompetes with a support which could be established as typical or atypical habit. The arborist must know their species and where the zone of rapid taper is located.

Root-plate lift can be established by utilising a foot mallet or probe first, then reviewing the area around the structural root zone.

The foot mallet is simply sounding the ground with the feet or a mallet and determining if any air pockets are found, indicated by lower resonance of the mallet effect. A probe can also be utilised when checking the root zone area for consistency and density in the soil medium, or whether resistance is freely given in the areas of the mallet test – ‘pockets of air’. The structural root zone is the interface of stability and trees would either be windthrown commonly – but not always – at this distance.

When determining compaction, a probe may utilised, but make sure it’s insulated.

More considerations

Hard-surface areas, drainage and moisture collection areas (pools) are examined for their impact on the system and whether they are positive or negative contributors to support. Negative contributors will act like constraining forces on the root system, reducing root-plate support. Positive contributors like other enmeshing tree roots of a similar species will act as increasing support.

Contaminated soils may have a pedal structure and high compaction rate which allow for surface-dwelling roots and poor root-plate adhesion. This is a negative contributor and can cause windthrow or failure.

Competing species in root systems can have a negative contribution on the leaning tree if the roots of the adjacent tree area are invasive and highly charged with opportunistic features. Trees like Celtis sp. (Hackberry), Ficus sp (Strangler Fig), Ficus elastica (Rubber), Ligustrum sp. (Privet) and Erythrinas sp. (Coral), and Ailanthus altissima (Tree of Heaven) are key tree species which will outcompete native vegetation. Native trees which may contribute to competing are Eucalyptus robusta (Swamp Mahogany), Ficus watkinsiana (Nipple Fig) and Casuarina glauca (Swamp Oak).

Where leaning trees are the result of two separate trees competing for root stability on the same point in the landscape, the issue is affectation by the constraining forces. If the tree which is leaning is not the feature tree, then it could be removed. But if it is the feature tree, it would need root-test investigation to maintain the status and to possibly engineer a solution to retain the tree. One simple way of restraining a leaning tree with susceptible root attachment is to buffet the stem against the lean with a stay. The load presses into the timber and the ground may be battened for additional support, particularly in sandy soil or wet soil.

Other options may include engineered cabling with secured anchorage rated to the appropriate tonnage of tension.

Any system introduced would be ideal in increasing support.

One typical large leaning tree the author has observed since childhood always leaned extensively north next to a bustop and toward the road. Image: TCAAA

Wind speeds

On review of the draft AS4790 20024 standards, no additional information is given in the sphere and arborists can change the TPZ to a more subtle oval shape with roots biased towards the lean or away from the lean (note the TPZ is usually a circle and can also be moved if conditions onsite warrant it, including having a cliff one side but still inside a measured TPZ, having a rock platform in the TPZ, or a slab without roots underneath). Major encroachments must have root investigations as per AS4970 2009 by the arborist.

Where the tree roots are leaning with the prevailing wind, the tree has most commonly adapted to the wind force and the windsail area reduces during wind force from weather events on the prevailing wind. Where the lean is into or against the prevailing wind there could be an issue of increasing the wind force in extreme weather and the windsail increases instead of diminishing. The lean has bearing on whether the tree has additional force it can either dissipate or not, hence failure or possible windthrow.

Trees generally weather storms and can lean over to the direction of the prevailing wind, making themselves a smaller target to windforce.

Ken James has an excellent article on wind dynamics and his table indicates typical effects of winds at different speeds on trees.

Note the uprooting of trees at between 89kph and 100kph.

Image: Prime Creative Media


Leaning can be good

Leaning trees within a patch would be typical as they are searching for light, and phototropism can produce artistic leaders and interesting canopies.

One typical large leaning Ironbark tree I’ve observed ever since I was a young child has always leaned extensively north next to my bustop and toward the road.

When investigating the site, the road is on the north with the swale approximately 3.5 metres from the stem, and the south is a field and biodiversity area on the nature-strip curtilage.

From the edge of the patch trees are subjected to additional loading and therefore there is a propensity to take on an increased load which may influence the rootplate and a lean.

Species such as the Araucaria columnaris (Cook Pine) are known to lean toward the north. Large trees are known to lean with different soil-profile densities.

Trees leaning in protected areas are usually acceptable to be retained. Large trees which have shifted from natural conditions may have greater difficulty in re-establishing their position and stabilising. Trees with larger buttress roots are difficult to move, but over time increasing force can shift. Trees on moving soil, slip areas or glacial movements, riverbeds, cliff faces and next to highways where vibration is constant can have difficulty in remaining upright. When trees lean their root system may be exposed, or if they’re supported against a fence or another tree, it may be a case of condemning and replenishing the tree.

In most coastal areas where prevailing salt in the air and constant moisture move a tree in inclement weather, trees can adapt to the conditions or salt injury can take effect on windward side. Some adaptions which are considerable feats include additional ribbing of response wood, additional stem support from advantageous roots, pneumatophores (Mangrove roots) and new epicormic stems which grow vertical and input large reserves of sugars back into the stem.

Historically the hanging tree at Parramatta had a substantial lean and was utilised in hanging convicts. I have to admit that’s one tree I am glad to see a cultural disposition of loss.

The Leaning Trees near Greenough in WA – River Gums (Eucalyptus camaldulensis) have trunks which lie horizontal to the ground. It’s a natural phenomenon caused by airborne salt blown in off the Indian Ocean. Image: Anne Powell/

Deep meaning

Trees leaning into spirituality listed in the Bible include the Acacia, Almond, Almugwood, Balsam tree, Broom tree, Cedar, Cypress tree, Fig, Juniper, Myrtle tree, Oak, Olive trees, Palm, Pine and Aleppo – that’s besides the symbolic trees, including the Tree of Life and the Tree of Knowledge.

The Budda sat under a Ficus religiosa (Bohdi tree) and other trees like the Anandabodhi tree. Birthing trees have similar role in that they present a featured, protected area, usually in their root buttress, allowing for birth to be semi-protected.

The dying stockman from Henry Lawson lay under the shade of a coolabah tree. And it’s thought the Kelly gang had their last stand at Stringybark Creek in Toombullup State Forest near Benalla laid under the Stringybark tree.

The national tree of Australia is the Acacia pycnantha (Golden wattle) but it’s hard to find one flowering at our historic government buildings. It’s also known that Acacia aneura (Mulga wattle) are the most common in Australia, with incredibly dense, heavy and supportive roots. The Ginko biloba (Ginko) is a dinosaur tree that generally leans with a deep taproot and has medicinal qualities improving circulation and keeping memory sharp for the brain, making the nervous system more focussed and synapses responsive. It’s ideal for HSC students with the disclaimer of use for epileptics, heart conditions if pregnant, and so forth.

Designing environments with Australian trees like the Callistemon sp (Bottlebrush), Corymbia ficifolia (Western Red) and Santalum acuminatum (Quandong), where we survey the natural environment for inspiration including trees leaning, or trained to lean into another, to create an avenue, have been utilised as grand walking vistas and entrances.

Trees like the Santalum acuminatum (Quandong), have been utilised as grand walking vistas and entrances. Image: ssmalomuzh/

Icons and memories

Although most of The Leaning Trees scattered in the Greenough area are located on private property, they can’t be overlooked by anyone driving along the Brand Highway. The tree trunks lie horizontal to the ground and have become somewhat of an icon, a bizarre natural phenomenon caused by airborne salt blown in off the Indian Ocean.

Bonsai has also given the preference of leaning – on a microscale – to present a unique artistry in tree habit. The lean is always a feature that can mimic age as some aged and weathered trees can, in difficult environments, have excessive leans.

Trees which have failed and which are lying on the ground are not leaning. Leaning trees within a plantation are not usual as they are difficult to harvest. Monocultures are used in harvesting, although promoting a species can be difficult to control with infectious diseases.

As a child we had a ‘horse’ tree which was over a cliff and we would gladly ‘ride’ it with a brother or playmate. It was anchored into a crevice and would be very difficult to dislodge. This type of tree, although a feature, should be constantly assessed if left in a public school or facility. If removing trees like this on the basis of a leaning tree, it can leave us with less budding climbing arborists.

A common issue is children who like to climb trees find the tree they want to climb the most is close to the school fence, offering a sense of freedom or a joyous escape. Even as a child I was scolded for climbing trees and sent to the principal’s office, and that was after the principal told the school not to climb trees!

Read more from Jim McArdle and the TCAAA at

Image: TCAAA


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