Studies on the Growth of Red Mangrove (Rhizophora mangle L.) I. Habit and General Morphology

Abstract

The growth habit of Rhizophora mangle L. in southern Florida is described in a preliminary way. A number of features contrasting with those in temperate trees are emphasized. The seeds are viviparous, and it is a seedling propagule that falls from the tree. The seedling can form adventitious buds, but the mature plant cannot. Branches are formed at intervals within the terminal bud, and on lateral shoots branching changes from monopodial to sympodial. Aerial roots are formed adventitiously and do not develop laterals unless distally anchored in the substrate or injured. Subterranean roots branch endogenously and differ markedly from aerial roots. THE CONCEPT OF normal tree growth is largely based on a study of woody plants of the temperate zone. As a contribution towards a survey of the ecology of mangroves in South Florida we have undertaken an extensive investigation of its three native mangrove species iRhizophora mangle L., Avicennia germinans (L.) Stearn, and Laguncularia racemosa Gaert., which are known locally as red, black, and white mangrove, respectivelyl. These have already revealed a number of features of growth which seem remarkable, at least when viewed in the light of our knowledge of dicotyledonous trees of the temperate zone. Whether, in fact, these features will continue to be regarded as unusual depends on future investigation of the growth of other tropical trees. This is another subject which is being studied extensively at Fairchild Tropical Garden, and it is against this broadening background that our studies specific to mangrove trees are set. The present paper serves to introduce the nonspecialist to general features of growth and morphology of Rhizophora, emphasizing those aspects which may be regarded as unusual. Future accounts will expand the topic of root and shoot growth and seedling establishment. The ultimate object is to prepare a detailed report on Rhizophora which will provide a firm botanical background to ecological and physiological studies on the plant. These studies are being carried out in the vicinity of Fairchild Tropical Garden, Miami, where Rhizophora mangle is toward the northern limit of its distribution. Climate and topography suit it so well, however, that it forms extensive stands. These are much disturbed by real estate development and are subject to natural catastrophes, notably hurricanes. Nevertheless growth of the tree in South Florida is representative for the whole of its range, and abundant material has been available for the investigation of growth responses throughout the year. In this respect we have been more fortunate than most previous investigators who, although they have contributed to the development of an extensive literature on Rhizophora, evidently have not had the opportunity to observe the tree as a growing organism. For this reason a number of the elementary aspects of growth we will describe have remained unobserved. This needs emphasizing, otherwise we may be accused of introducing a topic at an unnecessarily elementary level or of relating information which is readily available already. However, we may point out that much of this earlier information is based on a study of pickled or herbarium specimens and does not concern itself with growth. Future studies in botany are likely to require this phase of re-investigation, at a seemingly elementary level, of living plants before significant advances in our understanding of the growth of tropical plants can be made. Well-grown plants of red mangrove form trees 30-40 feet in height with records in South Florida of over 70 feet. In pure stands trees have a single trunk, with a system of arching aerial roots confined ' Contribution to a study of the growth of native woody plants of South Florida supported by a grant from the Michaux Fund of the American Philosophical Society. Part of this study was carried out while the first author was employed by the Cabot Foundation for Botanical Research of Harvard University. The authors wish to express their thanks for help from both of these sources. BIOTROPICA 1(1): 1-9. 1969 1 This content downloaded from 157.55.39.123 on Fri, 20 May 2016 08:35:11 UTC All use subject to http://about.jstor.org/terms to the base of the plant. Old plants on disturbed or relatively exposed shores tend to develop long low branches supported by aerial roots. This has given rise to erroneous ideas about growth habit which, unfortunately, have passed on as popular knowledge, as is explained below. Under certain circumstances, as outlined below, aerial roots may develop abundantly on high branches. An appreciation of the form of the tree is best obtained from a study of development from the seedling.