Tree Morphology - a continuing story....

This seminar, presented by Treework Environmental Practice in association with the Arboricultural Association, continued on the good work of an initial two-day Morphology seminar held in March 2006. This day marked Seminar VII in the ongoing series exploring the outer reaches of arboriculture brought to us by TEP’s supremo Neville Fay.

The aging process is both an ontogenetical and a physiological response - and combines patterns of ‘prescribed norms’ with the body language of adaptive growth. Our study of perturbations to the norm can make a powerful diagnostic tool.

By appreciating and enjoying these adaptations we can learn to challenge traditional concepts of form and formality. What was once considered valueless tells a tale of history and regeneration, once our eyes are unlocked. This applies not just to the ancient pollard but also the less conventionally perfect specimen, rarely allowed to survive in a heavily managed tree population (I put a good part of the blame for this on the British Standards for nursery stock! It seemed such a good idea at the time...but then so did dead wooding and heavy thinning and hopefully we have educated ourselves out of that).

The “Urpflanze” - Goethe’s archetypal plant concept form, from which all plants are derived - a morphological plant patent?

During a guided tour through the history of tree morphology, while reviewing aspects of the previous seminar it was a pleasure to revisit Leonardo and Goethe, the Golden Ratio and the Fibonacci sequence. The latter can be found on a limitless scale, from the double helix of DNA to the working of constellations. It is a growth form not a fixed bilateral symmetry, and well illustrated by the spiralling contortions that many plants display in their leaf angle (137.5 degrees) to maximise sunlight collection.

Francis Hallé

Neville then introduced the legendary Professor Francis Hallé, Professor of Botany at the University of Montpellier.  Famous for landing inflatable contraptions on the canopy of tropical rainforest trees and having worked in that environment for 40 years, his first session was a historical approach to tree architecture, paying tribute to the British and European tradition of scientific tree drawing from the 16th – 18th centuries.  

Early plant illustration gave way to an ever closer examination of vegetative parts (flowers and fruits), then anatomy, meristems and cells - finally down to chromosomes and genes.  The smaller the focus the less obvious the tree architecture, and for Hallé the macro view of tropical trees, less complicated being in a relatively benign growing environment, illustrates his subject best.    

Expanding on the ontogenetical v physiological theme, Hallé pointed out that a young tree displays its genetic inheritance perfectly, showing a regular and easily illustrated architecture that in time becomes chaotic visually as the likes of gravity, frost, fire and all the other environmental factors make their mark.

Along with Roelof Oldeman, Hallé concluded earlier in his career that all trees conform to one of some 20 architectural models; a theme developed later the second speaker.

Hallé: Reiteration - Unitarian (left) and Modular

The important concept of reiteration was clarified and according to Hallé all trees are either unitarian or modular.  Unitarian trees being historically older have simpler, often very beautiful and symmetrical form retained throughout the aging process (Araucaria and Palms are obvious examples with nearly all very young trees conforming to this model).  Modular trees form duplicated units growing one on the other, comprising most ‘modern’ trees whether tropical or otherwise, from Mahogany to Maple.  This reiteration involves self-replicated new units of the tree’s architecture, and his vision one of the tree producing not so much branches as new trees growing higher up on the original tree; these await their opportunity when older crown parts die.  Once your eye is in, this reiterated unit can often be seen to have a trunk, branches, leaves, flowers and fruit – and even roots.  This gives rise to the fascinating prospect that a trunk could be seen as an accumulation of the root systems of the reiterated units above.

Reiteration is seen as an adaptive system that can respond to trauma and change; it can increase solar intake, constantly rejuvenate the crown extremities and generally extend life (exemplified in Westonbirt arboretum’s 2000 year-old small leaved lime and even more so in the case of Tasmania’s record-holding, apparently 43,000 year old Lomatia!).  Comparisons were made with the growth of a coral reef where, although the individual units may be in intense competition, the organism as a whole prospers from the integration of components into old age.

Modular characteristics include crown shyness (whereby units within a canopy keep separate from one another - a different concept from the physical response when adjacent branches collide); aggressiveness of invasive species with especially high level of reiteration.  Let loose in a different environment, these species can quickly overwhelm unitarian endemic species.  Thirdly, and excitingly, is the prospect of genomic diversity whereby different reiterative units in the same tree appear to be variants of the parent genome, forming genetically distinguishable units in the larger ‘colony’!  Expect much more on that one in the future.

Professor Hallé would not have been allowed to leave us without some reference to his seductive life in the tropical rainforest canopy, and we had some tantalising pictures of the blimp, helium balloon and canopy platform that enables unprecedented access to this other world, full of its own species and ecosystems.  Sadly, an offer by one delegate to come along and make the tea was turned down – the participants have to earn their place in the trees with a rather more scientific contribution (but perhaps this explains continental tea making..!).

Pierre Raimbault

Dr Pierre Raimbault, Institut National Horticole in Angers, France, who had presented a paper at the previous seminar and conducted an excellent outdoor session, re-acquainted us with the principle that morphological study aids tree assessment.  Lack of space prohibits covering this ground however to summarise he reviewed types of growth forms. Continuous growth (such as with Cupressus) was contrasted with rhythmic growth (long, apically dominant shoots that may have several annual flushes of growth).  Pruning has a great impact on growth; auxins from the leaves of the tip block the development of side buds, the latter having to wait until the following spring for their window of opportunity before being again suppressed by apical leaves.  Mid summer can be a turning point in the growth pattern; pinching out a shoot in August rarely produces immediate side growth, and is better left until the following year  (I have found this out the hard way when developing a young Apricot tree).

Dr Raimbault then illustrated the 25 models of tree architecture, all built on 6 main characters: one or several trunks, co-dominant stems or differentiated trunks & branches, rhythmic or continuous growth, monopodial or sympodial growth and lateral or terminal flowering.  This is a definite field of study in itself, however two examples would be the Koriba where one lateral (produced after flowering) takes dominance, as with Prunus and Ailanthus, and Leeuwenburg where the terminal shoot, on flowering, is replaced by two co-dominant shoots (as with Lilac, Rhododendron).

Figure of nine architectural models: Each model named after a botanist (e.g. Attims: cypress; Rauh: ash; Massart: fir: Scarrone: horse chestnut; Troll: elm.)

The tree aging process was explained in detail, with early maturity seeing the development of biologically independent units in the crown and the final stage for most trees seeing the death of major branches, large reiterations arising closer to the trunk, and (unless rejuvenation occurs) the eventual death of the tree. A few will continue with these reiterations forming distinct columns of wood and bark - even roots growing in the decaying trunk; these units are completely biologically independent from each other and the parent tree: an expression of the ‘phoenix’ survival process in veteran tree parlance.

Figure showing mature and post-mature stages of the root system

Taking the analysis below ground to the stages of root development proved most illuminating. The tap root becomes morphologically and functionally insignificant as the tree matures with reiterations of the fascicled root system extending horizontally and vertically (then horizontally again). In late maturity the death of deepest and furthest root occurs, with reiterated, new exploratory, fine roots providing a more superficial system, especially vulnerable to climate and soil changes. As with the phoenix aerial parts, some survivors will develop reiterated roots from reactivated cambium at the base of the trunk or within the hollow stem.

We were reminded that severe crown pruning at the later stages often leads to death in a year or two due to loss of foliar-generated carbohydrate starving the roots. This can apply to individual, independent reiterations as well as the tree as a whole. Other principles of pruning were touched on, mostly confirming accepted good practice. In particular, shortening all axes (‘heading’) stimulates a chaotic, bushy response due to the morphological reaction to hormone disruption. One should be guided by the tree morphologically but also of course have regard to structural safety, no doubt sometimes leading to compromises in ideal outcomes.

The session was deprived of further focus on the management implications of all the theory, due to lack of time. This was perhaps the shortcoming of the day, as the previous seminar had left a tantalising thirst for useful, practical outcomes that has still to be satisfied. This subject will always be, to an extent, work in progress but we look forward to learning more in the future about the ways our day to day, practical tree management can be better informed by the study of morphology. To this extent Nev Fay is proposing to organise a two-day field based master class with Pierre Raimbault in 2008 with a small invited group to review conventional and veteran tree management practices at selected key sites.

Geoff March
Registered consultant

Review of SEMINAR IX of the series which took place on 15th and 16th November 2007 will be available soon.
A two-day master-class on Integrated European Tree Assessment Approaches to Highway & Amenity Tree Inspection: looking at fungal decay patterns using non-invasive tree hazard inspection methods for risk decision making. It is based on interpreting decay using Michael Schlag’s system for assessing the steady state balance between fungal decay and adaptive growth. How hollow can a tree be and still be expected not to fail. The seminar will also look at new cable bracing methods.