Major limb failure

Fig 1: Near Map image, 21/6/23, the limb failure occurred on 24/7/23, the image taken just over four weeks prior to its failure. A partially filled crease (red arrow) can be seen in the tree’s crown where the limb subsided following failure of the cabling system. This, followed by subsequent canopy growth, bridged the gap. I don’t know the date of the cabling failure, though I’d suggest the growth into the crease is at least 24 months old. Another year or two of growth and the crease would have disappeared.

Cassian Humphreys goes out on a limb to deal with major limb failure.

Tree climbers, martial artists, motorcycle riders and for that matter human beings, have a good understanding of the circle verses the straight. Whether we’re standing full stretch on a cylindrical limb supported via a cylindrical rope on a straight line, or we’re stepping offline to encounter a closed fist, accelerating a partially pressurised tubular tyre into a corrugated dirt road, or just paying attention to the circles, the straights and the angles in our own bodies, we all have an understanding of the limits posed by the angle versus the curve. So it is with a tree, it’s the angles trees have to overcome to mitigate failure, with the circle being the means to deal with load-bearing stress. It’s the non-optimised hard angles that cause tree failure. So why do arborists install hard angled cabling systems into well curved trees?

While on the subject of Tree-Health-Care, on behalf of Gold Leaf Canberra I had the great honour to assess and write a status-report and projectproposal on a heritage tree. This article is based on the report I did on behalf of a significant, 200-year-old estate.

The reason behind the trees being bought to arboricultural attention was a worthy study in itself.

To be effective, proactive tree management must involve treecultural practice. Management strategies are only useful if the works that follow extend tree life. Some arboriculturists have feet in the oldworld of reactivity and the new-world of proactivity, which is about New-Earth values. This, I believe, is where we really want to be.

The exceptional specimen that is the feature of this article signifies the bridge between Australian heritage and Australian land care. This article explores the reason behind the trigger for the work, with a study of a recent major-limb failure.

Elbow limb formation

This study involves historic cabling of crown structure that failed to optimise due to over support from cabling – support that was subsequently lost when the cabling failed leaving a heavily weighted elbow limb.

I have witnessed a number of cabled trees over the years which have been cabled unnecessarily.

Any tree with wide open forks and curvaceous, well-tapered limbs has a biomechanical advantage and is best left to the natural environment to sustain and generate natural strength. If we are true to arboriculture, all we need to prescribe is tree-health-care to support tree biomechanics.

This tree was set up for failure by over-pruning and cabling. I am reminded of the 1994 Robert Zemeckis drama-comedy movie Forest Gump, with Tom Hanks and 1920’s children prescribed leg callipers like common pharma-pills.

I have been writing on the reiteration of tree crown structure for years (Epicormic/Endocormic – since 2008), along with US North-Carolinian arboricultural-writer Guy Meilleur. To recognise the transition trees make following retrenchment, topping or limb shortening you have to understand how trees reiterate or grow anew following the loss of a leader. We arborists have all grown up with the term ‘epicormic’ (Latin for ‘upon the stem’), I created the term ‘endocormic’ to describe the opposite.

Fig 2: From my original August 2023 report, the estate’s iconic Cedar of Lebanon – Cedrus libani – with a hole in its crown revealing its recently exposed internal crown structure following the failure. The image above reveals a squat lower crown on the house side of the image, due to past limb removal and limb reduction to accommodate the building. The shortening of the tree’s crown set tree limbs up for reiteration and elbow-limb development. The failed limb was a cabled reiteration that had grown out of a lateral limb that had been shortened.

So how is an elbow-limb formed?

Fig 5 (top) and fig 6. Examples of acute angled vertical reiterations,on the Cedar, either shoots or dormant buds. Both limbs have made
significant extension and incremental growth. In the case of the estate’s Cedar there were three such limbs, the largest of which failed.
The branch break-out failure, with an adjacent remaining elbow-limb has not been cabled. Because of the poor limb-to-trunk buttressing
and incremental growth it can be seen the failure point had insufficient growth increments in proportion to branch diameter. I wasn’t
concerned with the remaining elbow limbs as they are protected within the tree’s outer-canopy.

 

To be true I only make reference to elbowlimbs to describe reiterated limbs with acute angles. Coming back to the Cedar tree with very acute angled reiterations, my observations are simple: following limb shortening, either a dormant bud, a pre-existing shoot or limb took on a dominant role. Initially this was triggered by the loss of auxin in what was the original terminal growing point, now removed. The same process applies for topped limbs, harp trees and retrenching trees.

For a greater introduction into elbow limb reiteration or the transition of epicormic to endocormic I encourage the reader to read my past AAA Apr/May 2021 article Epicormic, Endocormic, Adapting Veteran Trees And Arborists Minds.

Another significant element of the limb’s failure involved drought stress. Drought-induced wood-embrittlement is a subject I have reported on multiple times. Because few arborists are professionally concerned with administering health to trees, we miss out on one of the simplest means to mitigate such failures: watering.

It has taken humankind’s imposition, albeit experimental, with cabling trees to impair nature’s optimisation. Australian arborists need to face the fact that the only crown structure we should ever seek to support is one which is mechanically impaired or defective to the point of failing. Supporting well optimised crown structure weakens it. Far better to support tree vitality (Shigo’s definition) to assist biomechanical-optimisation, such as enabling soils to better receive and hold water.

Fig 3: This image was supplied by the client prior to its the clean-up and removal. l was able to access and inspect the failed reiteration in the site’s yard. This limb, around 45cm diameter close to its branch break-out failure point (a 75cmm long-tear), was approximately nine metres in length. Branch break-out failures are limbs or leaders that fail at the trunk, leaving holes.

 

Fig 4: Reiteration of a Spotted Gum’s crown following topping.

Treatment for accelerated wound occlusion

Based on Shigo’s Modern Arboriculture reference to a tree’s capacity for cambial-cellular differentiation, for some years I’ve been experimenting with creating sphagnum patches on lesions on trees, mainly Ficus species in SE Queensland, with great success, achieving more rapid than normal wound closure over mechanical injuries and lesions. This process involves the placement of sphagnum-moss over the injury site, irrigating, then wrapping the moss/injury/branch or trunk with hessian and clipping into place. With regard to the injury site on the remaining limb on the Cedar tree, I recommended such treatment (ref: to fig: 5). I also advised acoustic testing of the limb over time, subsequent to wound closure, as a means to determine decay propagation. Knowing the genus and its wood resilience I expect full closure and compartmentalisation, without need to remove the remaining elbow limb.

Pruning for subordination and greater wind resilience

Another way to benefit the tree’s longevity is via pruning to subordinate the tree’s crown. This involves hedging the canopy periphery via volume reduction, or pruning trees at bud and twig level to mimic environmental subordination. Growth suppression in the outer canopy by environmental conditions is obvious to those who study trees, over erroneous mathematical formulas that only support the idea of the tree.

For a more detailed coverage on this topic read my AAA Oct/Nov 2022 article Sympathetic Pruning Downunder, Lessons Learned from Australian Red Cedar and Tree Morphogenesis. Then read the co-written 2022 articles AAA Oct/Nov UK David Lloyd-Jones Sympathetic Tree Management Working, With Tree Morphogenesis and AAA Aug/ Sept CAN Ryan Redvers Working with Nature & Maths for Longevity. With regard to the Cedar tree, I advised on a series of volume reductions to help subordinate the tree’s crown in pruningcycles over the next decade, the aim being to better proof the tree against storm events as well as support canopy closure over the current hole since the limb failure. Based on experience of environmental subordination of trees, I explained volume reductions with pruning wounds being in symplastic tissue under 2.5cm in diameter (in the case of the Cedar much less) do little to impact tree vitality, as the trees in question sustain full photosynthetic capacity while making mechanical gains. The age of arboriculture.

Fig 8 (above) and fig 9 (below): Before and after pictures of the Gold Leaf volume reduction, stage 1 of a proposed series. This would be a textbook volume reduction if subordination pruning was actually in the national and international arboricultural repertoire. Those of us who do such works are pioneering arboriculturists.

 

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