Dispersal Properties of Mangrove Propagules

Abstract

Characteristics of dispersal and establishment for water-borne mangrove propagules, including period and pattern of floating, period of obligate dispersal, time to root firmly, longevity, and vigor, are estimated for six species in Panama (Laguncularia racemosa, Avicennia germinans, A. bicolor, Rhizophora mangle, R. harrisonii, and Pelliciera rhizophorae). Dispersal properties correlate with the spatial distribution of adults within the swamp. Genera whose adults are found on higher ground, on the landward edge of the intertidal zone, have propagules that require a period of freedom from tidal inundation of approximately five days in order to establish firmly in the substrate. Genera whose adults are found on the seaward edge of the swamp, in deeper water, have large, heavy propagules. MANGROVES ARE WOODY PLANTS that grow on quiet seashores in the tropics and subtropics (Walter 1971). Swamps composed of mangroves form an extensive component of tropical vegetation. Because vegetative propagation is an insignificant aspect of mangroves' spread and colonization, a knowledge of the dispersal and establishment of their propagules is a prerequisite to understanding the habitat division that results in adult distributions. This paper presents field and laboratory observations on some aspects of dispersal in four genera of Panamanian mangroves. Mangrove species show a characteristic zonation within the swamp (Watson 1928, Davis 1940, West 1956, Schnell 1971, Walter 1971, Thom et al. 1975). Some species are found predominately on the lower ground or more seaward portions of the swamp; whereas other species are on the higher ground or more landward portions of the swamp. mangrove species on both coasts of the Isthmus of Panama and their zonational tendencies are shown in table 1. These trees constitute the entire higher flora of the swamps, except for epiphytic orchids, bromeliads, and a fern. Most mangrove species share two traits: dispersal in water (van der Pijt 1972) and vivipary. Macnae (1968) describes this latter phenomenon: The zygote, once formed, develops uninterruptedly through the embryo into the seedling without the intervention of any resting stage. Gill and Tomlinson (1969) also discuss . . the continuous development of the embryo without any dormant period. In the usual sense, there is therefore no true seed. Essentially, the dispersal unit is a small tree, for which the term propagule is convenient. continual growth of the embryo during dispersal is a unique aspect of mangrove establishment. NATURAL HISTORY Laguncularia racemosa.-Laguncularia is a monotypic genus in the Combretaceae. L. raceniosa ranges from Florida and the West Indies to Brazil, from northwestern Central America to Peru, and from Senegal to Angola (Exell 1958). Laguncularia propagules are and flattenedobovoid-ellipsoid (lens-shaped) (table 2 and figs. 1 and 2). They are pea-green (Munsell 2.5 7/10) when they fall from the parent tree but turn mnudbrown (Munsell 2.5 3/4) in two days or so. TABLE 1. Mangrove species in Panama. Zone Pacific coast Atlantic coast Family Seaward Rhizophora harrisonii Leechman R. mangle L. Rhizophoraceae (lower ground) (= R. brevistyla Salvoza) Pelliciera rhizophorae Triana & Theaceae Planchon Landward Laguncularia racemosa Gaertn. L. racemosa Gaertn. Combretaceae (higher ground) Avicennia germinans L. A. germinans L. Avicenniaceae A. bicolor Stanley (formerly Verbenaceae) 1 Present address: Division of Biological Science, University of Michigan, Ann Arbor, Michigan 48109, U.S.A. BIOTROPICA 10(1): 47-57 1978 47 This content downloaded from 157.55.39.220 on Fri, 02 Sep 2016 05:18:39 UTC All use subject to http://about.jstor.org/terms pericarp serves as a float and is not shed until the propagule is firmly rooted or nearly dead. propagules germinate while dispersing, the radide pushing out a plug surrounded by the calyx and elongating. Dispersing propagules commonly bear roots. stages by which they take root in the soil are illustrated in figure 3.