Rooting and vegetative propagation in Laguncularia racemosa

Abstract

Reproduction and dispersal in mangrove species occurs mainly through propagules. Vegetative propagation is considered to be insignificant by many researchers, however, for restoration projects, it could be an important advantage. Therefore, we carried out experiments with shoot cuttings of Laguncularia racemosa and Rhizophora mangle under controlled conditions as well as in the field to prove the feasibility of vegetative propagation in these mangrove species.

None of 110 planted R. mangle cuttings survived the experiments, but the results with L. racemosa were promising. We observed that the single most important factor for survival of L. racemosa cuttings was rooting of the shoots prior to cutting from the parent trees. This is a common feature in the study area, the Ciénaga Grande de Santa Marta, Colombia. Seasonal floodings of lower branches induce secondary root growth and can even lead to natural vegetative propagation. Other factors that have favorable effects on the survival of the cuttings are a large shoot diameter, manual defoliation to reduce transpiration, and direct sun irradiation. Additionally, burying a small part of the roots in a way that allows some roots to freely float in the surface water, improves the survival rate to 95%. However, the watering with rooting agents shows slightly negative effects. The surviving shoots grow rapidly and fruit set can be observed within six months after planting in the field.

In restoration attempts with L. racemosa it is, therefore, possible to bypass the very sensitive seedling phase by planting shoot cuttings. The value of this method is linked to quicker establishment and reproduction of the cuttings as well as the possibility to plant them in flooded areas, where establishment of propagules and survival of seedlings of L. racemosa is very difficult to achieve.

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:

• 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;

• 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.