Mangroves: Coastal Problem Solvers

Yellow-crowned Night Herons in Mangroves by Kaia Thomson

Yellow-crowned Night Herons in Mangroves by Kaia Thomson

by Bryan Jauregui, Todos Santos Eco Adventures

This article first appeared in Janice Kinne’s Journal del Pacifico

There are four mangrove species that dominate in Baja California, and in 2010 they were all reclassified from “special protection / Protección especial” to “endangered / Amenazadas” species by SEMARNAT, Mexico’s environmental protection agency. These include: Red mangrove/ Mangle rojo (Rhizophora mangle), Black mangrove / Mangle negro (Avicennia germinans), White mangrove / Mangle blanco (Laguncularia racemosa), and Buttonwood mangrove / Mangle botoncillo (Conocarpus erectus).

Red mangroves are the most prevalent in Baja California Sur and they are easily identified by their gangly, aboveground “prop roots”, which have earned them the nickname “walking mangroves”. Like all mangrove species, red mangroves had to solve the problem of breathing and collecting nutrients in anaerobic, or oxygen-free soil, and the prop roots are a brilliant solution. These roots grow down from branches above and in the water, and eventually anchor in the mud, transporting air and nutrients. This nutrient-rich environment attracts a huge array of organisms, from protozoa to sea turtles to dolphins -creating a magnificent and complex food web. In the tropics, red mangroves can shoot past 80 feet, but in the arid climate of Baja California they rarely get past 15. On Isla Espiritu Santo, in San Gabriel Bay, the Magnificent Frigatebird uses a red mangrove canopy as a huge, elongated nursery, just a few feet off the water. It’s a sight worth seeing. Of all the mangrove species, red mangroves in Baja extend into the deepest water, the seaward zone, where conditions are harshest. Many species spend their entire life cycle among the red mangrove’s prop roots.

Black mangroves, like all mangroves, must deal with the troublesome issue of being a tree growing in seawater with a high concentration of salt – which is toxic to trees.  Black mangroves solved this problem by exuding surplus salt through their leaves, where it appears as salt crystals. And, in contrast to red mangroves, they solved the problem of breathing in anaerobic soil by growing aerial roots up from a subterranean main root growing in the mud. When the tide is low these aerial roots are exposed to air and they breathe, and when the roots are covered in seawater, they take in all the nutrients they need.  The black mangroves grow in the shallowest water of all the Baja California species, the landward zone.  Like red mangroves, the aerial roots of the black mangroves form a natural living room, dining room, bedroom and nursery for a vast array of species along a very long food chain.

White mangroves, like their other Baja mangrove counterparts, have solved the problem of how to reproduce in such harsh, watery conditions by adopting a reproductive strategy that is more commonly found in mammals than in plants: vivipary. That is to say, they bring forth live young. Unlike most plants, the mangrove seeds germinate while still safely attached to the parent plant, producing “mangroves to go” or “propagules” – complete, miniature, folded plants that have already completed almost half a year’s growth while still attached to the parent tree.  When mature, these baby plants fall into the water and drift with the sea currents. When a propagule reaches land, it immediately produces roots and soon sends up a seedling. At 40 days, the drift period is longest for red mangroves and at 5 days, it is shortest for white mangroves. Uniquely, the drift period of the white mangrove also includes germination, making it the only semi-viviparious of the Baja California mangrove species. White mangroves reach maximum density between the red and black mangroves, and the three species together form the bulk of the mangrove forests of Baja California.

Buttonwood mangroves, like all mangrove species, are working to solve the problem of global carbon emissions by burying enormous amounts of carbon in their peat soils. In fact, mangroves sequester 15 times more carbon, 50 times faster, than inland forests.  Buttonwood mangroves are very sparsely distributed across Baja California, and generally as individual plants of 7 to 9 feet. Only one stand of buttonwood mangrove has been documented in BCS and that is at Isla Espiritu Santo.

So what about sweet mangroves / mangle dulce / Maytenus phyllanthoides? In Baja California sweet mangroves often form part of the salt-scrub vegetation associated with mangroves on the landward side. Yet, because they grow inland, sometimes even a good distance from the water’s edge, many more people of the Baja peninsula are familiar with this species and have a great fondness for it. As sweet mangroves are typically not in watery conditions, they take a much more traditional approach to reproduction, putting their seeds in a bright red aril, or seed covering, that develops from the seed stalk. The bright color attracts birds who act as seed dispersers. Happily, sweet mangroves do not appear on SEMARNAT’s list of protected species in Mexico.

Also check out our article, Mangroves: The Trees of Life

© Copyright Sergio and Bryan Jauregui, Casa Payaso S de RL de CV, 2015

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