Báo cáo khoa học: "Some architectural aspects of tree ageing"

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Some architectural aspects of tree ageing Barthelemy, C. Edelin F. Hallé D. du A. Broussonet, 34000 Laboratoire de Botanique, Institut CNRS), 16’3, Botanique (UA327 rue Montpellier, France sequence of genetically determined Introduction events. For instance, in Virola surinamen- sis (Roland.) Warb. (Myristicaceae), a Despite the numerous investigations on South American tropical tree which ageing, among which the work of tree conforms to Massart’s model, the first Schaffalitzky de Muckadell (1959) is cer- phase of growth consists of the develop- tainly the most famous, we are still far ment of tiers of plagiotropic branches on from being able to give a definition of this the orthotropic trunk, a very simple archi- general process. This, we believe, mainly tecture which corresponds to the architec- results from the difficulty to identify precise tural unit (Edelin, 1977) of this species. markers of development and of the phy- The second phase, which starts when the siological state of an old tree. Using the organism is 5-7 m high, is marked by the concepts of architectural model and reit- development of forks at the extremity of eration al., 1978), architectural (Halle et the branches; each axis of this fork is a studies, along with other sectors of re- partial reiterated complex. The third phase search, may contribute to increasing our begins when the tree is 15-20 m tall: total knowledge of tree ageing, by analyzing reiterated complexes grow out vertically at and describing the successive morphoge- the tip of the branches. These reiterated netic processes that occur between crown perennial and, together complexes are construction and the death of woody with the branches from which they are plants. We have only a few data on this issued, they build up the framework of the problem, but recent observations lead us crown. Further, we observe that the new to distinguish 3 major kinds of architec- branches growing out of the trunk no long- tural events during this period. er support total reiterated complexes, but they still produce terminal forks. Higher on the trunk, before it stops growing defini- The reversion to archi- juvenile-like tively, the last branches developed do not a tecture bear any kind of reiterated complex (Fig. 1. ) Between germination and crown construc- Thus, reiteration seems to characterize tion, the tree shows a series of architec- momentary and relatively short phase of a tures that arise according to an invariable tree development, after which, the or-
L. f. (Clusiaceae) ganism develops the architecture Symphonia gtobulifera same as whose architecture that the juvenile period, but is tropical tree during seen a conforms to Massart’s model (Fig. 2): a an inverted sequence of events. following monopodial, orthotropic trunk bearing tiers of plagiotropic branches. Flowers are sup- ported by order 5 axes, which are short Invasion by flowering shoots. During ontogenesis, the number of growth units between the point of insertion of a branch and its first flowering short The ability to flower is used by several shoot decreases from one tier to the fol- authors as a criterion to define the transi- lowing one. In other words, as the tree tion between the juvenile and the adult grows older, its new branches are able to condition. Recent observations (Barthele- flower more and more precociously. my, 1988} have shown that the location of Another example is given by a pioneer flowers and inflorescences within the native to tropical South America: tree architecture of a plant may move progres- Isertia coccinea Vahl. (Rubiaceae). This sively during its development. This inva- tree conforms to Scarrone’s model (Fig. sion by flowering will be illustrated by two 3a). The orthotropic, monopodial trunk examples.
supports tiers of orthotropic branches which grow sympodially by virtue of termi- nal flowering. If we compare the length of branches at first flowering (Fig. 3b), we observe that the number of nodes below terminal inflorescences decreases ac- cording to the level of the branch on the trunk: the higher the branch, the smaller is the number of nodes. Then, as the tree grows older, it develops branches able to flower more and more precociously and after the formation of a decreasing num- ber of nodes. These two examples show that, during growth, flowering is progressively tree extended to all the vegetative structures, according to an acropetal flowering gra- dient, that underlie tree ageing. The proleptic reiterative process proleptic reiterated The of occurrence of an old tree has in the complexes crown been described by Oldeman (1972). It varies followin!g various modalities in time and space, according to species, but recent investigations reveal the existence of a continuum between all these modali- ties. This will be demonstrated by 3 cases. In Humiriastrum subcrenatum (Humiria- French Guiana), reiterated com- ceae, plexes grow out on the upper side of the whole length of the limbs, when they are still growing. Such small ’individuals’ develop in the crown and fill up the avail- able volume.
phogenetic events which occur simulta- In Qualea rosea Aubl. (Vochysiaceae, neously and progressively, according to a French Guiana) the proleptic reiterated complexes occur only at the base of the sequence that is specific to each species. They lead us to distinguish numerous limbs, near the trunk, when the crown has growth stages which punctuate tree completed its development and is going to ageing. In return, the knowledge of these die. The simultaneous and probably coor- stages enables us to determine with very dinated development of the reiterated complexes leads to the building up of a high precision the true physiological states reached by an old tree, and these events new homogeneous crown which replaces can be used as markers of tree ageing the former one. and senescence. In Eperua falcata Aubl. (Caesalpinia- ceae, French Guiana), the reiterated com- plexes, also appear at the base of the main branches and when the crown Acknowledgments to lose its limbs, but their develop- begins ment is very delayed in space and time: the first complexes occur at the top of the This research has been financially supported by crown, the following ones half-way on the the CNRS (ATP ’Physiologie de la croissance et du développement des v6g6taux ligneux’). trunk, and the last ones, some years be- fore the tree dies, develop near the base of the bole. Each of these species develops nu- References proleptic reiterated complexes merous when ageing, but it is clear that crown Barthelemy D. (1 (88) Architecture et sexualit6 architecture of the old tree evolves in diffe- chez quelques plantes tropicales: le concept de rent directions according to complex loca- floraison automatique. Ph.D. Thesis, Universite tion and the mument of their appearance: Montpellier II, France it can lead to a reinforcement of the Edelin C. (1977) Images de I’architecture des crown, to a lowering of the crown or to its conif6res. Ph.D. Thesis, Université Montpellier complete replacement. II, France Halle F., Oldeman R.A.A. & Tomlinson P.B. (1978) In: Tropical’ Trees and Forests. Springer- Verlag, Berlin, pp. 441 Conclusion Oldeman R.A.A. (1972) L’architecture de la foret guyanaise. Ph.D. Thesis, Université Mont- pellier II, France The reversion to a juvenile-like architec- Schaffalitzky de Muckadell M. (1959) Investiga- ture, the invasion of the vegetative struc- on ageing of apical meristems in woody tions tures by flowering and the development of plants and its importance in sylviculture. Forsti. proleptic reiterated complexes are mor- Forsoegsvaes. Dan. 25, 310-455