Red Mangrove (Rhizophora mangle)

The most common species of mangrove in Puerto Rico is the red mangrove. It is usually found in direct contact with the sea, thus becoming the first mangrove species encountered on the coastline, going from the sea towards the land (Figure 6).

Main characteristics:

Leaves: Red mangrove leaves are large (8-10 cm in length, 4-5 cm in width), thick and with a waxy texture, which prevents water loss and slows decomposition. The leaves are simple, opposite (Figure 8) and are usually grouped at the tips of the branches.

Roots: The red mangrove is the only mangrove presenting aerial prop roots, which branch out of the trunk or from the lateral branches and droop towards the ground like silts (Figures 7 and 9). These roots provide the tree with more stability, allowing it to become established in finely-sedimented, poorly consolidated soils. Furthermore, the aerial roots fulfill vital functions such as nutrition, aeration and salt filtration. The roots filter the salt through osmosis so that the tree can absorb the water necessary to survive. If the mangrove absorbed salt, it would dehydrate.

Bark: The bark covering this mangrove’s roots (Figure 10) contains tannins, organic substances which protect it against bacterial decomposition and give it a characteristic reddish coloring (tannins are also present in red wine). Also, the bark on the roots and trunk feature pore-like structures called lenticels (Figure 11), which aid in the mangrove’s gas exchange (breathing). This is an adaptive response, since the trees grow in poorly-oxygenated or anoxic soils. Red mangrove bark is complete and uniform, differing from other mangrove species.

Flowers: Red mangrove flowers are small, approximately 2.5 cm in diameter, with four thick, lanceolate sepals (Figure 12). The flowers have four white petals which turn dark and fuzzy inside, along with eight stamen. Flowers are often present year-round.

The fruit and the propagule

The red mangrove’s fruit germinates while still attached to the tree (Figure 13a). It matures in about two to three months, and develops into an embryo, known as a propagule (Figure 13b). The propagule grows while attached to the tree for 11 or 12 months, and by the time it detaches, it is already a small plant, with two leaves at the top ready to photosynthesize and establish itself as a new red mangrove tree (Figure 14).

The propagules growing among the established mangrove roots tend to develop as small shrubs of about 1.5 meters in height due to the limited amount of sunlight they receive. These young, small trees are called the dormant forest, and serve as a reserve population. When a full-grown mangrove tree dies, a space opens up and allows sunlight to reach the substrate. The small, dormant trees, usually living in the shadow of their larger counterparts, readily take advantage of these new and favorable sunlight and space conditions and quickly grow to their full size. Thus, the dormant forest eventually replaces the main one through a cyclical process. This is a crucially important process to mangrove forest survival, especially after suffering the effects of strong weather events like hurricanes or tsunamis. On the other hand, if the propagule falls into the water, it floats to the surface and is transported by currents, since its small trunk is corklike. Initially, it floats horizontally, but as it absorbs water, its bottom part becomes heavier and sinks, orienting the propagule vertically. Upon reaching a flat surface, the propagule has the chance of establishing itself as a new mangrove tree. This reproductive mechanism allows red mangroves to disperse and colonize areas distant from the usual coastal mangrove zones (such as cays).

Dormant forest - group of small mangrove trees inhabiting the shadows from the main forest.

Lenticels - projections from the tree’s trunk and root surfaces, having a pore to aid in gas exchange.

Osmosis - movement of water molecules through a semipermeable membrane, from an area of higher water concentration to one with less water concentration.

Sepals - each of the harder, greenish leaves which form the flower’s calyx.

Stamen - the flower’s male reproductive organ, containing pollen.

Tannins - organic substances present in the red mangrove’s vegetal tissue and which provide its characteristic reddish hue; tannins also offer protection against bacterial decomposition.