Studies on the Growth of Red Mangrove (Rhizophora mangle L.) 3. Phenology of the Shoot

An analysis of shoot extension growth of Rhizophora mangle is presented, based on the continuous observation of a small population over a period of two years. This provides phenological information for a tree which fits the general category of "evergrowing" although in South Florida the climate is distinctly seasonal. Environment has a strong governing influence, but the general progression of events is mediated endogenously. Growth is morphologically non-articulate, without obvious flushes, so that the unit of growth is regarded as the single internode. The apical bud has a uniform construction throughout the year, consisting of three leaf primordia and associated stipules, the expansion of the oldest pair of leaf primordia coinciding with the initiation of a new pair. Consequently environmental control of growth is effected through developmental processes in the apical bud. The rate of leaf expansion and leaf fall is at a peak in summer when temperatures and radiation levels are highest. Leaf fall is closely correlated with leaf expansion so that there is a relatively constant number of leaves on a shoot. However, there is some evidence that propagule abscission also affects leaf fall. All lateral organs (vegetative branches, inflorescences, and suppressed axillary buds) are developed within the apical bud, the types of organ differentiated at any one node apparently depending on plastochron length. This is most evident with regard to precocious (sylleptic) branches which are produced periodically in groups of from one to three. Suppressed axillary buds are very reduced and subject to apical dominance. When released from this. by pruning for example, they can reach a size which permits them to grow out as normal apical buds. Under continued apical dominance the life span of their potential for further growth is limited. Rhizophora mangle is, in a general sense, "everflowering" but expansion of flower buds is most frequent in summer, leading to a peak of fruit production about 4-7 months later and the largest crop of propagules 4-6 months later still. These observations are summarized in the form of a model which indicates how a single reproductive shoot might appear during this cycle, and how the population, as a whole, behaves. A hypothetical model of interrelationship between phenological phenomena is also presented as a basis for further work. The models serve to illustrate how shoot growth occurs in this tropical species, and how this is in marked contrast to the more familiar patterns of shoot growth in temperate trees. THE MOST DIVERSE floras of the world occur in the tropics where trees are usually the dominant life form. Despite this the study of the growth and development of tropical trees has been neglected (Tomlinson and Gill 1971) and present heories of tree behavior are based almost entirely on a few relatively well-studied north temperate xamples. Investigations of a wider range of tropical species should be made in order to extend our knowledge and add perspective to current theories of tree growth. This present study of shoot growth was undertaken with this ideal in mind using a widespread tropical species Rhizophora mangle L. which extends into subtropical Florida. The study continues the series of papers dealing with various aspects of the growth and anatomy of this species (Gill and Tomlinson 1969, 1971). Twig morphology of north temperate trees is characterized by segments delimited by series of bud scale scars. These articulations occur at the bases of branches and at intervals along them in such a way that they reflect he seasonal cessation of growth imposed by the severe winter climate. In contrast o this, Rhizophora shows no obvious articulation in the shoot system, an observation which led Koriba (1958) to place the Singapore species R. conjugata and R. macronata in his "evergrowing" category. South Florida has a seasonal climate, in contrast o the uniform climate of Singapore, and one might not expect the term "evergrowing" to apply to a species in a -non-uniform climate. The ability of an "evergrowing" species to flourish in a seasonal climate is investigated in this present article. A number of recent studies of north temperate trees have related the type of extension growth of the tree to the composition of its terminal buds (e.g., Critchfield 1960, Gill 1971). A few similar studies have attempted to do this for individual tropiical tree species (Bond 1942, 1945) but not one of these studies has involved a species which has a non-articulate morphology and is, therefore, apparently "evergrowing. " Known examples of growth types 1 Contribution to a study of the growth of native woody plants of South Florida supported by a grant to Dr. P. B. Tomlinson from the Michaux Fund of the American Philosophical Society. Diagrams are the work of Priscilla Fawcett, Botanical Illustrator, Fairchild Tropical Garden. 2 Present address: Division of Plant Industry, C.S. I.R. O. , P.O. Box 109, Canberra A.C. T. Australia 2601. 31Present address: Harvard Forest, Petersham, Massachusetts 01366 U.S.A. BIOTROPICA 3(2): 109-124 1971 109 in trees as a whole have been considered in a recent review by Tomlinson and Gill (1971) in a very general way where the suggestion is made that there are unlimited ways in which growth can be expressed in tropical trees. An earlier paper in this series on Rhizophora (Gill and Tomlinson 1969) described and illustrated the general morphology of R. mwngle and this provides a background for the present study in which extension growth and reproductive events are considered in relation to the seasonal changes in the environment. Radial growth has not been investigated despite its possible relationship to other phenological events. MATERIAL AND METHODS The source of material was a population of red mangrove in the vicinity of Fairchild Tropical Garden, Miami, Florida on the edge of Biscayne Bay. Climatically, the area is characterized by a warm humid summer and a relatively cool dry winter. Both maximum and minimum temperatures are high in summer when insolation reaches its highest value. Representative values for the period of observation are shown in figure 1. In winter, maximum temperatures may be high but minimum temperatures can fall close to freezing. Rainfall is spoiradic in winter and long periods without rain are common. In contrast, rain in summer is frequent and heavy. The annual average is 58 in (153 cm). Soils in the mangrove zone always remain wet and salinity varies according to tide, rainfall, and evaporation. The plants studied were 0.5-4 m tall and occurred in an area of mineral soil inundated by brackish water. All reproductive stages may be observed at any time of the year. Consequently observations were begun at different times on three different sets of shoots showing a variety of reproductive stages. One of these sets (shoots 22-41) was chosen because of the apparent high vigor of its shoots and their abundant branching. Measurements were begun on shoots of set one (1-21) in November 1968; on shoots of set two (22-41) in December 1968; on shoots of set three (42-58) in March 1969. Observations on all shoots continued until December 1970. The 58 shoots measured belonged to a total of 22 plants.