Tree Architecture A Historical Approach

by Francis Hallé   Overall Tree Form The overall form of a tree, as seen with the naked eye, is controlled by two key factors:  

1) A set of specific rules with a genetic basis which give the young tree its regular ARCHITECTURE. These are easily understood and may be drawn from nature.

2) The tree's environment; the impact of this intensifies with time contributing to the seemingly increasingly chaotic character of an old tree.

 The History of Tree Observation To observe and to draw the overall appearance of trees is by no means a new pastime. On the contrary, there is a venerable European tradition of scientific tree drawing. This particularly can be seen in drawings produced in between the sixteenth and eighteenth centuries .  

Fig. 1.18 Century engraving of Athabasca Lake, Canada.

  Later, field-botanists gave up drawing whole trees, probably under the influence of Linnaeus. By emphasizing plant sexuality, this famous naturalist drew his followers into paying attention to plants vegetative parts.

 In the nineteenth and twentieth centuries, analytical science became increasingly influential and in botany the areas of interest tended to increasingly focus on the minutiae: plant anatomy, meristems, cells, intra-cellular organelles, chromosomes and

   

Mespilus by Dodoens, 1554; Parietaria by Dalechamp, 1653; Pinus in Theophrasle's Historia Plantarum;, 1644; Linnea by Rudbeck 1720; Trees ; Canada, English engraving, 18th century

 

genes. As fascinating as this may be, analytical biology is not in a good position from which to address the problems and understanding of the whole tree. Many botanists are now dealing with DNA nucleotidic sequences and molecular phylogenies and it is unlikely that from that standpoint they will be able to shed much light on architectural determinism.

 For those botanists visiting tropical rainforests, the situation is different, as the diversity of tree forms there is much easier to grasp than in temperate latitudes, even for the most casual observer: the environmental conditions being milder, architecture is revealed more clearly.

 More Recently In 1853, during his Amazonian exploration, Richard Spruce drew architectures of trees in his field book . Extensive experience of South-East Asiatic rainforests allowed E.J.H. Corner to create and render the first system of tree architecture . This had a great influence on my own research. Between 1960 and 1971, I was seconded to several Botany and Forest Biology Services in tropical Africa and I began looking at overall tree form and drawing this; at that time, it was no more than a practical tool to identify large trees in the forest from considerable distance and without the benefit of being able to collect flowers.

 

Fig. 2. E.G.H. Corner, 1949. Ref. 8

  The African rainforests unveiled 21 architectural models (AM), each representing a particular developmental sequence of branching. This approach was altogether new.

       

Spruce, R. On the mode of branching of some Amazon trees. J. Proc. Linn. Soc, London 1861; Madrinan, S. Richard Spruce's pioneering work on tree architecture. 3 Corner, E.J.H. The Durian Theory or the Origin of the Modern Tree. Annals of Botany, 13, no 52:367-415, 1949.

         

Thanks to my friend Roelof A.A. Oldeman, who was working in botanical forestry with IRD in French Guyana, a transatlantic comparison was carried out. This revealed that the same sample of 21 AMs was discovered to occur in the rainforests of tropical America, despite the large floristic differences between the continents.  Important Publications Our joint book (published in Paris) was translated by Benjamin C. Stone, professor of botany at the University of Malaya, and published in 1975 in Kuala Lumpur4. This work describes how the trees of the Asiatic rainforest, although belonging to different genera and families, conformed to the same 21 basic designs; architecture was beginning to look global.  Oldeman added the key-concept of reiteration, a concept which complemented the AMs facilitating the analysis of large trees .  In 1976, P.B. Tomlinson, professor of botany at Harvard University, organized a symposium on tropical tree biology, where great significant was accorded to architecture6. With his extensive experience in Palms, Professor Tomlinson became a keen supporter and advocate of tree architecture in the United States, and the principal author of the next book to be published on the subject . Each AM is named after an appropriate botanist rather than after a tree species, as the latter might not be familiar world-wide. Computerised Studies of Tree Architecture Over the last twenty years of the twentieth century under the influence of the computer as a powerful tool applied in many fields, the simplicity of plant architecture attracted the attention of computer scientists. Simulations of plant growth were performed by Bell in England , others using computer-aided techniques . In France, this approach to tree architecture was developed, and is still being refined by Philippe de Reffye and a group of botanists led by Daniel Barthelemy. Molecular Phylogcnies Molecular phylogenies brought a number of changes in plant systematics: some families (Loganiaceae, Flacouriiaceae) were suppressed, the corresponding species being scattered among other families; the Annonaceae were divided into sub-families; some genera were excluded from their families, for instance Phyllanthus from the Euphorbiaceae, then becoming type-genera for new families, in this case the Phyllanthaceae'u. Interestingly, available architectural data support such changes and the search for convergences with molecular phylogenies is the most recent trend in the study of tree architecture.

Reiteration, The Key Concept Francis Hallé

 

   

1 recall that reiteration was seen and conceptualized by the Dutch botanist Roelof A.A. Oldeman during his stay in French Guyana (1,2); it is now a key-concept in plant architecture. So what exactly is reiteration in a tree? Perhaps the best way to clarify reiteration is to consider that all trees are either unitarian or modular.  

Unitarian Trees Unitarian trees, having no reiteration at all, grow according to only one architectural model; the old tree, being isomorphic to the young one, retains the same silhouette (3). Being very beautiful and frequently used as ornamentals, unitarian trees belong to archaic groups, tree-ferns, Araucaria, Palms or Nutmeg tree. According to say the paleobotanists, reiteration was extremely uncommon in fossil trees (Figure 1).  

Modular Trees Modular trees are those that form, through the reiteration process, colonies of duplicated units growing on top of one another. Whether in Europe (Maple, Oak, Ash) or in the tropics (Mango, Durian, Mahogany), most modern trees are modular; their silhouettes are subject to great change during their life, from pointed to rounded and then flat (3).

                                       

Figure 1: Unitarian tree: Aurucaria. Modular tree: Dipterocarp.  

Defining Reiteration How is reiteration defined? Reiteration is the process through which the tree duplicates its architecture, giving birth to new copies of its architectural unit. Reiteration is a special sort of branching; it produces not branches but 'new trees' growing on top of the original tree.

 This 'new tree' is a reiterated unit (RU); usually complete, it has a trunk, branches, leaves (and even juvenile leaves, if any), flowers, fruits and sometimes a visible root system. RU roots are usually not visible from outside the tree.

Reiteration unit roots take the form of a wood lamina, enveloping the previous wood. It has been suggested that a trunk could be conceived of as an indefinite accumulation of RU root systems (4) (see Figure 2).  As for the mechanism of reiteration, the key point is merely the distance between the merislems. The hierarchy between them results in the apical stem commanding the fate of the lateral stems, if the distance remains short: in this case the various meristems work in a coordinated manner, thus producing an architectural model; which is why young trees are always unitarian.  If the distance increases, the hierarchy fades away; each meristem produces its own copy of the specific architectural model and the whole tree becomes modular: it turns into a 'colony', with the meaning attributed to this term given by zoologists. As the tree grows further, the number of RUs increases, but their size decreases and usually their sexuality becomes more and more efficient (5)-

 Adaptive Value of the Reiteration Process As a modern phenomenon, the reiteration process is of paramount adaptive value:

• It can save the tree's life in case of injury: falling to the ground, tilting due to the wind, cutting of trunk or branch.

• By producing new RUs, the tree increases its surface area, and concomitantly increases its performance as a receiver of solar energy (2, 6).

• The RUs are largely beneficial to the tree, although they are in fierce competition with each other for light, space, water and nutriments. The competition is the same, between RUs within a crown and between trees in a forest.

• Moreover, reiteration lengthens considerably the duration of the tree's life, up to the present record of 43,000 years held by the Tasmanian Lomatia (7). A parallel can be drawn with a coral reef (where elementary polyps have a short life-span, whereas the reef itself, potentially immortal, remains alive through geological timescales.

• At the meristematic level, many trees never reach senescence as their growing points remaining juvenile during the entire life. How is this possible? It has been shown, in trees growing rhythmically like Pines, that all the genes 'extinguished' by methylation are de- methylated as the tree resumes its growth in spring, the genome remaining indefinitely at

the same juvenile stage (8).

                                     

Fig. 2: Root systems of reiterated units.

 Modular or Colonial Trees The following three features of colonial (or modular) trees allow a better understanding of RU's biology: shyness, aggressiveness and intra-crown genomic diversity.  

Crown-Shyness Crown-shyness is a remarkable trait, quite spectacular but not easy to observe as it only occurs in a few families (Pinaceae, Fagaceae, Myrtaceae, Vochysiaceae, Dipterocarpaceae), and between tall conspecific trees.  

Although shyness is not yet well understood, it demonstrates that RUs within the same crown behave like trees in the same population, thus supporting the concept of'coloniality' (3).

                                                 

Fig. 3: Crown shyness, Barneo carrphostree

Aggressiveness Aggressiveness is due to an exceptional abundance and efficiency of the reiteration process; it is typical of those invading plant species called 'pests'. On the contrary, endemic plants in remote oceanic islands are usually unitarian. Where there is competition, the endemics are rapidly overwhelmed and killed, thus demonstrating that reiteration is not only an improving force but is a phylogenetic achievement as well.

 Genomic diversity Genomic Diversity within one crown has been shown to occur in some tree species, either tropical (9) or temperate (10), in which each RU seems to have its own variant of the specific genome, the tree thus becoming a colony of genotypes. The concept is still poorly understood; in other species, such as in Quercus, RUs' genomes appear to be homogeneous. The question is therefore, are there indeed two sorts of trees with two separate forms of genomic behaviour? The answer to this great question lies in the future of tree genetics.

REFERENCES  

l-01deman,R.A.A. L'architecture de la vegetation ripicole des fleuves et criques guyanais. Adansonia, Paris, 12-2 .253-265, 1972  

2-01deman,R.A.A. L'architecture de la foret guyanaise. ORSTOM ed, Paris, Memoire n° 73, 1Q7/I