Leading Arboricultural and Ecological Consultants

Tree MORPHOLOGY & diagnostics

Seminar V

Report on the third Treeworks Environmental Practice/AA Seminar held on 23rd & 24th March 2006

Originally published in essentialARB magazine issue 16 by Andrew Cowen and Neville Fay

As we say goodbye to the last of the autumn colour left on bare winter branches it is time to admire the structural beauty of trees. Beyond this aesthetic interest there is also something special to learn about ageing and morphology in trees. Andrew Cowan & Neville Fay consider some ideas about the ageing process of trees derived from the work of Pierre Raimbault.

Although Raimbault’s idea were first presented in English at the second European congress of Arboriculture in Versailles 1995, they have not gained a broad understanding in Britain. So in response to the view that tree morphology needs wider appreciation in arboricultural circles Treework Environmental Practice will be introducing Raimbault and his morphological studies to a UK audience on March 23-24, 2006 as part of the ‘Innovations in Arboriculture’ seminar series.

Morphology fundamentally relates the form and structure of an organism. In trees the study of morphology has been linked to ageing and the physiological processes associated with different developmental stages.

Tree age classification is a valuable tool for tree recording and monitoring. However, the use of a series of labels (such as ‘young’, ‘middle-aged’, ‘mature’ to ‘over-mature’) as age classification can lead to an oversimplified structural view, while the subjectivity of its use may be prone to inconsistencies and in turn to misconceptions about how trees age.

Where there is poor understanding of the ageing process in trees those involved in surveying and management may build-in inconsistencies in characterising, recording and reporting their condition. This may apply where conclusions are drawn regarding tree condition in the context of observed stresses, particularly if the ageing process is reduced from a dynamic developmental progression to viewing the tree in terms of aged-states, each representing a static condition.

Understanding the ageing process can lead to an enhanced appreciation of the life-cycle of trees and significantly contribute to arboricultural management decisions. Such knowledge takes into consideration tree species’ growth characteristics and applies these to morphological and physiological changes that occur during the developmental stages of the Tree-ageing process.

Tree age is when the germination date is known and can be expressed as a chronological measurement. However this can be deceptive when considered in terms of the ageing developmental process as different species grow and mature at different rates.

Tree growth is influenced by genetic factors and the growth context, to the extent that physiological and morphological processes will determine the increasing complexity of the organism from establishment to maturity. As a result, for example, a stressful growing environment can influence the early developmental (establishment) phase leading to a prematurely ‘aged’ tree, while competition between adjacent trees may prolong the establishment phase, leading to slower, more gradual early-stage changes.

The architectural morphology of trees contributes to their aesthetic appeal. However from an arboricultural viewpoint an understanding of morphological stages can be used as an aid to the observation and diagnosis of stress. Pierre Raimbault’s concept of tree morphology, in viewing the tree in terms of a series of standard developmental stages, studies how morphology operates, and, from such observations, he contends that it is possible to provides a systematic approach to understanding ‘normal’ and ‘abnormal’ development.

This is based on the premise that the physiological and environmental history of the tree is somehow ‘written’ in its form and that variations from a developmental norm may be interpreted as ‘perturbations’ from normal physiological status. In order to make this manageable Raimbault in identifying ten distinct stages in the life-cycle of trees, seeks to show complexity and changes in both the above ground and below-ground architecture of the tree during the passage through these stages.

Thus Raimbault’s system considers a diagnostic approach derived from morphological parameters and compares these with a normative ‘standard’. This allows observations to gauge whether the current state of the tree conforms to the established standard so that the observer is able to place the tree in one of the stages on the standard developmental scale.

On the other hand, if certain observed criteria do not match, then this can direct the arborist to regard the tree’s condition as physiological perturbed. To make informed arboricultural management decision it is therefore important to be familiar with the method for morphologically-based physiological diagnosis to be able to ascribe, understand and interpret causes to these differences.

Raimbault refers to the concept of ‘plant memory’ and idea which dates back to such venerable thinkers as Leonardo da Vinci and Wolfgang Goethe who observed plant and tree morphology, structure and function. He visualises their physical form as a record of their development. Interpreting this ‘memorised’ physiognomy helps to identify both current stresses and traumatic events in the past growth processes of the tree.

Stages 1 to 5 (the first sequence of Raimbault’s ten stages from seed to eventual senility) relate to the development from seedling to early maturity. During these early stages there is progressive branch differentiation and complexity in crown structure in the process of optimising leaf area, while lateral development is suppressed by apical dominance.

The root system also begins to develop a tiered structure optimising its capacity to fulfil the growing demands of the expanding leaf canopy (Fig 1).
During stages 6 through to 8, apical dominance is progressively reduced resulting in an increasingly rounded crown, some loss of peripheral vitality and a gradually progressive loss of under-branch sub-lateral growth (abaxial mortality).

Reiteration (replication of the tree’s form in individual epicormic growths) begins to initiate, and when this coincides with occasional branch loss dysfunctional tissue gradually and increasingly develops from fungal colonisation. This process instigates growth about the inner, lower crown region in response to the declining, retrenching outer crown canopy (basifugal mortality).

Raimbault’s stage 8 is characterised by a corresponding retraction in the root system. Where the oldest roots die off internal fungal decay of the lower trunk may also then occur and introduce decay into the centre of the base of the trunk.

Raimbault considers that some trees will die at stage 8, but there are those that will survive beyond this and progress through the subsequent two stages (9 and 10) and beyond. In stages 9 and 10 he describes how the living crown condenses to form a lower canopy and there is a corresponding retrenchment of the root system. The tree’s resources during these stages begin to be centrifugally concentrated and this coincides with initiation of growth from dormant and adventitious buds on the trunk and main branches.

As vascular dysfunction progresses the tree is increasingly hollowed by fungal agencies and when cambial dysfunction also occurs this may also result in the bark circumference becoming discontinuous. In the latter stages the survival of the tree will depend on the mechanincal stability of the trunk and main root system.

However, its effective longevity can be sustained by the viability of vascular columns on the trunk that directly link sections of the root system with fragmented upper parts of the crown or trunk. These cambial columns can form distinct but complete units as the parents tree starts to disintegrate. Being directly linked to their own root system the columns have the capacity to break free from the corporate system, rejuvenate and repeat some or all cycles from stages 1 to 10 (re-generating as phoenix trees).

Raimbault contends that this physiomorphological diagnosis can be applied to determine maintenance operations, independently from all actions that might typically be considered in response to pathological or mechanical conditions. He proposes remedial pruning that is informed by observation of the crown architecture in later stage trees (stages 6 to 10) to manage crowns that simulate trees in stages 1 to 4.

Further examples of this approach suggest sympathetic pruning work to enhance the form of young adults at stages 5 and 6 and refresh those at stage 7.Perhaps the most important aspect of this morphological diagnostic and management perspective is that the methodology leads to increased awareness and consideration of trees in the later stages of ageing (stage 8 and beyond) which leads on to crown contraction at stage 9 (sometimes termed retrenchment). It is here that the dynamic survival strategy of the tree is most evident.

Stage 10 can be interpreted as a turning point in a cyclic ageing process where the tree may be ‘reborn’ showing the potential for re-juvenation through ‘phoenix’ growth forms (as when, for example, a tree layers following collapse or when regenerating from internal adventitious rooting) so that the life cycle of the tree may begin again as a young establishing tree. In such cases the ‘young’ tree is paradoxically part of the original ancient stage tree, which also shows all the characteristics of the young establishing phase.

For further information a more comprehensive review of the tree ageing process and tree management implications from a morphological perspective can be found in the paper ‘Environmental Arboriculture, Tree Ecology and Veteran Tree Management’, by Neville Fay, Arboricultural Journal (2002) No. 26