Article: Tree Welding & Natural Grafts - The Magic of Myrtaceae

 

 

Tree Welding - Natural Grafts a form of Self-Optimisation of Woody Plants

 

Anyone that knows my work knows that study of tree mechanics of SE Qld trees is an ongoing personal/business interest and passion. I have been observing and gathering data on natural grafts for four years with a view to this article. According to Science natural grafting is common place with lower plants (non vascular plants), this project is a study of natural grafts amongst higher plants (with study of the family Myrtaceae) which according to my reviewer has limited scientific record.

 

 

This is my second article written subject to review by a professional associate, the reviewer for this article is Scott McKenzie a Sydney based consulting Botanist BSC-Biological, ADV Cert Hort (Urban). Scott like me is a keen Shigoist and is a very seasoned consultant and report writer for a Sydney based arboricultural company, he is an experienced Resistograph user and is a rare operator in that as well as being a qualified horticulturist he is a scientist, making his report writing skills and ability to quantify scientific information be considerably beyond the scope of an arborist. Scott has been reviewing my Tree Culture Course Manual since its conception in 2004, his encouragement and support has been integral to my personal growth in the area of science and as an arborist.

 

In honour of my mentor (Shigo) and reason for developing a scientific brain readers may notice my articles are becoming more scientific in their content, though I do not wish to alienate some readers this is necessary and is evidence of an arborist following the fathers (Shigo) call to develop brain to match the muscle, like Shigo I encourage all arborists to do the same.

 

 

As a result of my study of the body language of trees and the science of Shigo and Mattheck I believe that the genuses Eucalyptus and Corymbia are doing some unusual things with their bodies (in reference to VTA), unusual in the context of current science, but normal in the context of the evolution of biological systems or plants. For the sake of this article I collectively refer to these genuses

as `gums’.

 

Grafting of vegetation as a horticultural craft was born as a result of natural grafts observed in species such as Citrus and Prunus.

I have seen little data on natural grafts and after a limited response following my posting photographs of a natural graft (between two different species of Eucalypt) on a world wide forum I was inspired to write this article and to also introduce a rather astonishing tree to our profession, a tree nick-named the Lionel tree.

 

We know that trees are reactive generating organisms that are products of their environment, made of elements and designed by the elements.

As self optimising organisms trees adapt into a range of environments from forest giant to bonsai.

 

Science (Shigo 91/Mattheck 94) has documented a range of symptoms that quantify a trees mechanical status and are indicators of reaction wood development.

Mechanically successful trees with or without mechanical constraints are successful because of their wood production or `muscle’.

Reaction wood production with symptoms such as ribs, ropes, pillars, growth striations, increment strips, welds, feet and grafts are all created by trees in response to their distribution of load bearing stress (compressive and tensile) and are evidence of woody muscle.

 

This article and the photographic evidence within is an example of what Australian trees are doing within their structures – or for the simplicity of this article to be referred to as their bodies. I see natural grafts as being evidence of self optimisation or reaction wood development and believe that arborists and tree managers should be considering working with trees as a means to strengthen mechanical constraints via encouraging natural grafting to take place, this concept is to be covered further in a future article.

 

Trees with crown structures which cross over and touch have the capacity to fuse together, this occurs as a result of the generation of incremental layers of sapwood.

Generally (amongst arborists) we see reaction wood as a being reactive wood tissue formed by the cambium in response to load bearing stress (usually in ribs, ropes or pillars). I believe that trees are reactive with triggers other than just load as a catalyst with grafting being a probable example.

 

Before we can comprehend grafts as a form of optimisation we must first consider and comprehend wood.

 

 

    1.  Cambium layer – trunk meristem - cell generator producing xylem (on the inside) and phloem cells (on the outside), the cambium at the centre of the symplast also acts as the chemical `brain’ of the tree.

 

    2.  Phloem & Xylem – phloem cells transport simple sugars (glucose) as carbohydrate, with the woody xylem cells transporting water and elements as well as make up tree structure (deal with tensile/compressive load) via thickened cell walls.

 

    3.  Cork cambium – suberises outer bark.

 

    4.  Bark - outer layer of spent suberised phloem cells for waterproofing and protection.

 

    5.  Wood is formed by the cambium layer as sapwood in layers or increments, as sapwood cells age they lignify (cell walls become infused with lignin), the primary role of new sapwood or the most current tree or NOW tree (to quote Cowan – ISAAC Brisbane Conference 2008) is for transporting water/elements. As sapwood ages its primary role is support though it still also pipes water The now tree as the most recent layer of sapwood is laid down by the cambium in response to growth (the synthesis of carbohydrate, water & elements) and the latest environmental loading.

 

    6.  With sequential growth increments, time, age and wood differentiation sapwood becomes heartwood (according to Shigo there are a number of types ofhardwood). As trees reach an advanced age they become hindered by previous increments (past trees) which detract from the mechanics of the most recently designed tree - the now tree. It is widely understood that a cylinder (assuming it has sufficient wall thickness) is stronger than a solid. Though heartwood still plays a functional role (transporting water/minerals) as trees become of an advanced age it is beneficial for them to shed their ballast. Trees achieve this via forming columns of decay through their heartwood (as with mychorrizal nutrient absorption this is achieved via fungal association).

 

    7.  Medullary rays are responsible for conveying information (from the cambium layer) and substances laterally through wood, wood rays also mechanically strengthen wood by helping bind wood increments together.

 

There are many examples of trees as generators growing `their bodies’ over other bodies such as other woody plants, fence palings and even motorbikes (Figs 2-7).

 

 

In the course of my career having worked in Britain, Germany and Australia as an arborist I have observed only one genus of tree that can achieve the act of `regeneration’ (a play on words - trees are generators and do not regenerate) over their felled stumps via a stump sprout – Eucalyptus.

 

 

I have observed forest red gum (E. tereticornis), white mahogany (E. umbra), grey gum (E. propinqua), Gum topped box E, moluccana and tallowood (E. microcorys), though suspect other species of Eucalypt and species of Corymbia capable of stump sprout generation. In both these trees you can see the original tree (stump) in the body of the tree.

 

It is a natural part of a trees growth to use whatever is available in its environment to succeed biochemically and biomechanically. Like a modern defence force has a range of strategies available to deal with conflict, in a sense a tree has a range of strategies to maintain a state of optimisation, of which I see grafting as being one, however in reality trees have one strategy that is they grow reaction wood, I see natural grafting as a form of adapted reaction wood development.

Because trees (like any life form) are evolving organisms some species and some individuals (as with plant genera and plant families) are more successful at grafting than others, and the family Myrtaceae with the Genuses Corymbia and Eucalyptus appears to be leading the way.

 

Grafting occurs as a portion of a tree such as a branch crosses over another stem or branch then touches and abrades `itself’ (its neighbour or neighbouring portion of itself) causing wounding both to the thickening limb and the thickening stem.

As these portions continue to thicken (make incremental growth) and abrade sapwood to sapwood they squeak and rub until (due to compression between their surfaces) they can move no more. The cambium layers of the branch and stem then generate sufficient woundwood over both wounds to fuse and grow together. In the case of a successful graft the fused parties compartmentalise their wounds and share photosynthate. And at a more advanced stage of wood generation the grafted branch and stem also share mechanical load.

 

 

Fig 1 and Fig 12 are examples of Corymbia citriodor a maculata (spotted gums) which are at advanced stages of grafting where they are now sharing mechanical load as well as have the ability to share common properties such as water and nutrients.

 

Generative optimisation of rootplates – Buttress and root grafts

 

It is widely recognised by science, the horticultural and arboricultural professions that root grafting occurs between trees. Root grafting between trees is a brilliant means for trees to greatly increase their surface area for nutrient absorption. Woody roots are also mechanical optimisers and are known to use rocks as underground bollards (Mattheck 04) by growing slings of wood around them. Grafting below ground likewise plays a significant mechanical role.

 

 

Thanks to a presentation (ISAAC Brisbane 08) run by UK arborist/researcher/ecologist Andrew Cowan of Arbor Ecology I had my attention bought to the phenomenon of the development of the `cone’ as a means for trees to increase their mechanical footprint.

This occurs when trees reaching an advanced age generate interconnecting wood tissue over and between buttresses and roots – this involves grafting and is typical of the root plates of advanced age Brush Box (Lophostemon confertus) and trees of the genera Ficus.

 

Fig 13 – has grafted a curtain of roots into one mass of wood, generating a huge footprint to better manage compressive loading, this tree like its neighbours is growing in boulders in

a dry creek bed.

 

 

Figs 14 and 15 are of a spotted gum (C.citriodora maculata) with an optimal shear root plate (soil controlled). Lateral roots where entirely severed on three sides when the road was graded. The tree generated a new root system over its damaged root plate which over time has grafted together to form a woody cone which is still growing. This tree is a good 35m tall with a spread of 20m

growing in a very exposed area and is a testament to the mechanical optimisation of the gum.

 

 

A naturally grafted prop on a multi-trunked Moreton bay ash Corymbia tessellaris at Mackay Botanical Gardens

 

 

I see all successful grafts as astonishing acts of self optimisation, this example is brilliant. The stem (Fig 16) marked with a red arrow was a leader which became grafted to its neighbouring stem (blue arrow) its growing point was then removed with the wound becoming occluded (closed following incremental generation of wound wood).

 

The stem though having lost its crown still receives energy as it is now supplied by its neighbouring stem whilst drawing on its own supply of water and elements. This may or may not be seen as a form of optimisation by us, but it is a classic example of a tree using its resources to maintain a state of sustainability (bio-chemically and bio-mechanically). When we consider Shigo’s tree as a giant oscillating pump (or his model for dynamic equilibrium) then we have to be impressed with this tree.

 

This example is living evidence of mechanical adaptation with the growth of a natural graft, in the course of my work I have been finding and seeking out trees which use self grafting as a means to strengthen mechanical constraints (I prefer this term to the term `defect’). My most significant finds so far (like rolling ribs – turnbuckles/horseshoes) are gums which have formed grafts directly above V forks. I refer to these grafts as natural bridge grafts.

 

 

Natural bridge graft unions above V forks – assisting gum trees to stabilise, compartmentalise and transform V forks into U forks

 

 

These samples of Corymbia (species uncertain from Rockhampton), Lophostemon (confertus – brush box – Sunshine Coast) and Melaleuca (bracteata – the black tea tree - Moggill) all have grown/are growing grafts within 1-2m above V forks.

 

 

The Melaleuca (both sides of V fork) with a natural bridge graft on one side and a rib on the other.

 

 

Natural Bridge graft above a V fork on a Sydney Blue gum Eucalyptus saligna Ocean View Qld

 

 

 

Scribbley gum - Eucalyptus racemosa - with evidence of a natural bridge graft in process directly above a V fork.