Rooting and vegetative propagation in Laguncularia racemosa
Introduction
Vegetative propagation in mangroves is considered insignificant for spreading and colonisation (Rabinowitz, 1978; Tomlinson, 1986). In general, mangrove species are supposed to have little or no capacity for vegetative regeneration, and no natural capacity for vegetative dispersal (Tomlinson, 1986). However, reports on vegetative propagation can be found: some mangrove genera (e.g., Avicennia, Rhizophora, and Sonneratia) have a limited ability to spread vegetatively, because their lower branches may recline under their own weight and root distally (Tomlinson, 1986). A vegetative seaward expansion through clone formation can be observed in Sonneratia alba J. Smith and in an analogous manner in Rhizophora sp. (Holbrook and Putz, 1982; Crewz and Lewis, 1991). Propagation through shoot cuttings and air-layering is known for Sonneratia apetala B. Ham. and Xylocarpus granatum Koen. (Kathiresan and Ravikumar, 1995). Many authors wrote about air-layering of Rhizophora mangle L., Avicennia germinans (L.) Stearn, and Laguncularia racemosa (L.) Gaertn. f. (Pulver, 1976; Carlton and Moffler, 1978; Crewz and Moffler, 1984; González and Rivas, 1993). Although most of these experiments showed promising results during the layering phase and all species can take roots, only layers of L. racemosa, S. apetala and X. granatum were reported to survive transplantation into the field.
Reproduction and dispersal of mangroves occurs mainly through propagules. As a result, most mangrove restoration efforts have relied upon planting of propagules and seedlings, although seedling mortality is often extremely high. In some cases older, already established plants have been transplanted (Davis, 1940; Pulver, 1976; Elster, 1997; Elster, 1998). Limited work has been carried out on vegetative propagation, but it could be a very fast and efficient way of regeneration. Therefore, we investigated the feasibility of propagating mangroves by the planting of shoot cuttings. Two species were promising due to their characteristic properties:
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R. mangle, because the tips of the aerial roots develop a normal root system in contact with water and these roots can then establish themselves in the soil firmly;
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L. racemosa, because adventitious roots arise on the lower part of the stems (Jenı́k, 1970), shoots start rooting above ground when flooded and partly inundated branches can develop an extensive root system (Fig. 1(a) and (b)). The occurrence of lower branches with secondary roots is a common phenomenon in L. racemosa in the study area due to prolonged inundations. Many lower branches are continuously flooded for several months during the rainy season (Elster, 1997). Most of the flooded branches develop roots during this time. In addition to these observations local fishermen report that L. racemosa branches used as piles for their stilthouses start to grow.
Section snippets
Study sites
The present study was undertaken at two highly disturbed sites in the north of the Ciénaga Grande de Santa Marta, a brackish lagoon system on the Caribbean coast of Colombia (Fig. 2). Human induced changes in the hydrological system of the area, which have obstructed freshwater entering the connecting channels, has led to a severe increase in salinity during the past 35 years. As a result, ca. 30 000 ha of the original 51 150 ha of mangrove forest died (González, 1991). The surviving mangrove
L. racemosa
During the main rainy season 1995 (August to November), young shoots of L. racemosa were cut and planted under different conditions to observe their rooting and growth response. The cuttings varied in diameter (0.3–2.4 cm) and length (20–105 cm) and had at least four to five buds in accordance with Flinta (1960), who recommended at least three to four buds and a diameter of up to 2.5 cm for cuttings of Populus. The shoots used were from healthy, mature trees growing throughout the study area and
Results
None of the R. mangle cuttings survived the experiments. However, the results in L. racemosa were promising; shoot cuttings in one of the experimental conditions had a 95% survival rate after the first six months (see Table 1, Table 2, and Fig. 4). In addition, a capacity for natural vegetative propagation and localized spread was encountered. Lower branches that recline, both root extensively in contact with water (Fig. 1(a) and (b)), and in the moist ground. Several `young plants' were
Discussion
Secondary root growth is very abundant in L. racemosa when sufficient water is available. The rooting of lower branches in moist soils permits a natural vegetative propagation and local spread of L. racemosa, similarly as reported by Tomlinson (1986)for the genera Avicennia, Rhizophora, and Sonneratia, and by Holbrook and Putz (1982)and Crewz and Lewis (1991)for Rhizophora sp. and Sonneratia alba. It is often difficult to distinguish between L. racemosa plants that established themselves in
Acknowledgements
We thank the INVEMAR and CORPAMAG/PRO-CIÉNAGA (both Santa Marta, Colombia) for collaboration and access to laboratory and boat facilities as well as C. Carbonó and J. Döring for technical assistance. We further thank E. Ashton for improvements on the manuscript. The research was supported by the GTZ (TÖB, “Flanking Program for Tropical Ecology”, PN: 90.2136.1), Eschborn/Germany.
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