50 9 Crown structure in Araucariaceae P. B. Tomlinson Harvard Forest, Petersham MA and National Tropical Botanical Garden, Kalaheo, HI, USA Summary: The Araucariaceae is the most ancient extant family of seed plants whose persistence may, in part, be the result of a successful way of making a tree canopy. Deterministic components include: mo- nopodial habit, rhythmic growth, pronounced axis polymorphism, with up to three discrete branch orders, and cones always on ultimate deciduous axes. Opportunistic components include reiteration of trunk axes (total) and first -order branch axes (partial) either from "detached meristems" (cf. Fink, Burrows), as in Arau- caria , or de-differentiation of trunk axes, as in Agathis . Wollemia may represent one end of a spectrum of possibilities within Araucaria . The crown can be presented as a repairable framework (of "explorative axes") bearing ultimate long lived but deterministic photosynthetic units (as "exploitative axes"). Many of these features may have existed in ancestors - notably Archeopteris . In its opportunistic mode, total crown repair is possible on an existing trunk, with trunk regeneration restricted in varying degrees. In high winds branches may be stripped from all but the leeward side of the tree to produce a "banner tree", but the canopy can be replaced in toto, maintaining the distinctive shape (e.g. A. columnaris ). This property seems adaptive in cy- clone -prone habitats and may account for the adaptive radiation in New Caledonia. Future requirements are for quantitative analysis and experimental manipulation. Introduction Araucaria and Agathis. In view of this diver- sity the following account can only be pre- Trees in the family Araucariaceae, as exem- liminary. We do have an exemplary descrip- plified by the commonly cultivated Araucaria tion of Araucaria in New Caledonia by Veil- heterophylla (Norfolk Island pine), have a dis- lon (1978, 1980), which is closely followed tinctive crown form that makes them easily here. Opportunity to study the problem at recognized. What is the ecological signifi- first-hand has been provided by the cultiva- cance of this form? The question can only be tion of Araucaria in the tropics and sub- answered with knowledge about the devel- tropics outside its natural range. Araucaria opment of the crown. At the same time it is araucana (monkey puzzle) is a frequent Vic- clear that the three genera in the Araucari- torian relict in British and Irish gardens, aceae are distinctly different in the method while the recent IDS excursion to New Cale- of crown construction. Again one might ask donia allowed the study of Agathis and Arau- “what ecological inferences can be gained caria species in natural environments and in from a comparison of Agathis, Araucaria and cultivation. Agathis australis has been studied Wollemia?” Agathis is relatively uniform in extensively in New Zealand. crown form but Araucaria is quite diverse, appropriate for a genus with a considerable Architecture geographic range. The diversity of Araucaria Following the principles laid out in Hallé et includes, on the one hand, the spire-like or al. (1978), the mature canopy of the tree can columnar shape of A. columnaris, a form that be perceived as the sum of two processes. excited the crew of James Cook’s First, there is the architectural model of “Endeavour” when they first saw New Cale- the tree, i.e., the growth plan inherent in its donia, and on the other hand trees with a genetic make-up. The visible expression of candelabra habit, as represented by Araucaria this model at any one moment in time is the araucana of the Chilean Andes. Wollemia, in architecture of the tree. Second, there is the so far as it is known, combines features of partial or total repetition of this model (i.e., 9: Crown structure 51 its reiteration) as a response either to unique feature of the tree, based on his very trauma (physical damage) or some change in extensive and detailed observations (Burrows the immediate environment of the tree that 1986, 1987, 1989, 1990b, 1999, and this vol- disrupts the underlying model. The tree ume). Even with a considerable supply of most precisely conforms to its model as a “reserve” meristems of this kind, a contin- well-grown sapling, but reiteration becomes uum of crown structures is realized. Other progressively more important as the tree ages conifers have similar axillary meristems al- because it makes the developmental plan though these may be relatively short-lived more adaptable to changing circumstances. (Fink 1984). Members of the Araucariaceae may consis- Explore and exploit tently be referred to Massart’s model in the Hallé-Oldeman typology (Hallé et al. 1978) In interpreting the crown architecture of a since the trunk axis is monopodial (i.e., with tree as functionally efficient in both photo- a permanent leader) and exhibits rhythmic synthetic ability and mechanical stability, it growth in the form of regular production of is useful to describe the tree in terms of a branch tiers (pseudowhorls) but with vari- permanent mechanical “framework” (derived able spacing. The trunk axis is vertical and from the architectural model), which bears with radial symmetry (orthotropic), whereas relatively ephemeral photosynthetic units as branches are essentially horizontal and have either leaves or shoots. This approach, devel- dorsiventral symmetry (plagiotropic). Some oped by French morphologists (e.g. Edelin extension of this model in terms of Rauh’s 1977) sees the tree as a series of axes that model is included in Veillon’s descriptions initially “explore” the space available for because in some species the distal ends of making a canopy. These “axes of explora- older axes become erect and radially sym- tion” constitute the framework of the tree, metrical. Apart from the relatively constant which supports the “axes of exploitation,” framework of the tree implied in these de- i.e., the ultimate assimilating units. An ex- scriptions, a family character is the high de- ample would be the familiar Ginkgo biloba in gree of axis polymorphism such that axes of cultivation in temperate countries. The axes different branch orders can have distinctive of exploration are the long-shoots, which morphological features, resulting in a highly have a precise disposition according to Mas- efficient photosynthetic system. In leaf sart’s model, producing a framework re- shape the most immediate contrast is be- vealed in the winter leafless condition. The tween the needle-like or awl-shaped leaves of axes of exploitation are the short-shoots that many Araucaria species and the broad, flat- produce most of the leaves clothing the tree tened leaves of Agathis, Wollemia and several in summer. This represents the ultimate de- Araucaria species. All species in the family gree of axis differentiation. Araucaria can be have a highly distinctive juvenile morphol- described readily according to this approach, ogy that is contrasted with the morphology Agathis less easily. of adult shoots. Reiteration in the family is Branch orders expressed either in the replacement of lost orthotropic and plagiotropic axes, together A consistent terminology for axis orders with a dedifferentiaton whereby plagiotropic designates the trunk axis as axo, with succes- axes revert to orthotropy as derived trunk sively higher branch orders as ax1, ax2 …. axes. Replacement axes are developed from etc. This immediately designates branch or- axillary meristems that originate in most leaf ders by the appropriate suffix and allows the axils and may persist throughout the life of comparison of crown structure in different the tree. This abundant supply of reserve trees. A distinction has to be made between meristems is considered by Burrows to be a the topographic order, i.e. the visible 52 Section 2: Morphology, phylogeny, systematics and ecology sequence of axes within the tree, and the tree is then represented by the youngest morphological order, where each axis order branch tier, rather than an unbranched within the architectural model can be recog- leader, with the terminal bud enclosed by nized by its distinctive morphological fea- normal needle-shaped foliage leaves. In Arau- tures without regard to its placement in the caria rhythmic growth of the trunk may be tree. In Araucaria there are usually only three non-seasonal and often irregular, producing morphological axis orders (ax0, ax1, ax2), the several, one or no branch tiers per year, exception being A. cunninghamii, which con- whereas the activity of the higher order sistently has four orders (i.e., adds an ax3). branches can proceed quite independently. Wollemia is distinctive because there are only This disconnection of growth phases in dif- two orders, ax0 and ax1. Agathis essentially ferent parts of the tree is not uncommon in produces two branch orders, but repeats tropical trees, but remains little quantified. first-order branches (ax1) at higher orders of The unit of extension is represented dia- topographic branching. Topographic branch- grammatically in Fig. 5 for comparison with ing of this kind occurs throughout the fam- Agathis. ily, largely by the processes of reiteration. Agathis, in contrast, shows a different con- Units of extension struction of its units of extension because the tier of first-order branches (ax ) is pro- The basic unit of monopodial trunk con- 1 duced at the base of the unit of extension. struction in Araucariaceae may be referred to Here the dormant trunk apex is protected by as the “unit of extension.” Each unit is repre- bud-scales (Fig. 3) developed as a gradual sented by a trunk segment and the series of transition from broad foliage leaves to short plagiotropic branches (ax ) that constitutes 1 bud-scales. The initials of the branch tier each tier produced by rhythmic growth must be present in the dormant bud, because (Figs. 1-6). A similar method of analysis has they extend immediately upon bud burst, been used in the description of Phyllocladus each ax usually being subtended by a scale (Tomlinson et al.