Maximum Longevities of Rhizophora apiculata and R. mucronata Propagules

Abstract

The longevity of viviparous mangrove seedlings (propagules) in seawater is a key factor determining their ability to survive dispersal both locally and across large expanses of ocean. The purpose of this experiment was to determine the maximum longevities of propagules from two common Pacific mangrove species: Rhizophora mucronata Lamk. and Rhizophora apiculata Bl. Propagules from each of these species were placed in outdoor tubs with continuously flowing seawater. The condition of each propagule was monitored until it sank or started to rot. Propagules were then planted to determine viability. After planting, 50% of R. apiculata propagules and 21% of R. mucronata propagules were viable. For both species, mortality of propagules was strongly related to the length of the floating interval. Maximum longevities for R. mucronata and R. apiculata propagules were 150 (median = 70) and 89 days (median = 7), respectively. Rhizophora mucronata propagules appeared to be better equipped for long-distance dispersal, yet had low survivorship that would decrease overall dispersal opportunities. In comparison, R. apiculata propagules had higher survivorship yet shorter longevity and, thus, appeared to be better equipped for shorter distance dispersal.

Several Species of mangroves produce viviparous seedlings (propagules) that are buoyant and can disperse in seawater. The limits for such dispersal have long been the topic of speculation, especially among those seeking to explain the presence of closely related species in distant areas or the total absence of mangroves from suitable habitats (Guppy 1906, Ridley 1930, Tomlinson 1986, Duke 1992).

One of the chief factors controlling the dispersal of viviparous mangroves is longevity, the number of days propagules remain buoyant and viable in seawater. Longevity of mangrove propagules varies between different species. Rhizophora mangle propagules can remain viable in laboratory vessels for over 12 months (Davis 1940) and can float throughout a 3-month period (Ellison 1996). Rhizophora harrisonii (now referred to as R. racemosa [pers. comm., N. Duke, Department of Botany, University of Queensland, St. Lucia, Australia]) and Pelliciera rhizophorae propagules are capable of regaining buoyancy after an initial sinking stage and can survive 3 months floating in seawater with no mortality (Rabinowitz 1978). Avicennia marina propagules may remain viable, either exposed by tides or totally submerged (in enclosures in the field), for up to 5 months during enforced dispersal (Clarke 1993). None of these studies, however, sought to determine the maximum longevities of propagules, which are essential for assessing long-distance dispersal capability.

The aim of this experiment was to determine maximum longevities of propagules from two common Pacific mangrove species, Rhizophora mucronata Lamk. and Rhizophora apiculata Bl. Longevities of both species were then used to compare their long-distance dispersal capabilities.

Materials and Methods

The island of Kosrae, Federated States of Micronesia (5° N, 163° E) was chosen for propagule collection because of the large, productive, and intact stands of both species that grow interspersed in mangrove forests in that location (Ewel et al. 1998). On 6-7 October 1997, mature R. apiculata and R. mucronata propagules were collected from trees within riverine populations on the northern and western parts of the island. Kosrae is a high volcanic island approximately 112 km2 in size that contains 1562 ha of mangroves represented by seven species (Whitesell et al. 1986). The watersheds sampled are relatively pristine, with only minimal impact from agricultural activities and logging. Average air temperature on Kosrae is approximately 27.4°C (Mueller-Dombois and Fosberg 1998), seawater temperature is approximately 28-30°C, and salinity outside the fringing reef is between 33.6 and 34.5 ppt (unpubl. data).

After collection, propagules damaged from boring insects were discarded, so that maximum propagule longevities could be determined without confounding effects from herbivory. In total, 82 R. apiculata and 104 R. mucronata propagules were wrapped in moist newspaper, sealed in plastic bags, placed in sturdy cardboard boxes, and shipped overnight to Honolulu, Hawai'i, by air (this method has been used frequently for transporting mangrove propagules [pers. comm., Edward Proffitt, U.S. Geological Survey, National Wetlands Research Center, Lafayette, Louisiana]). On 8 October, each propagule was weighed, measured, and labeled. On the following day propagules were taken...