White Mangrove and Buttonwood

I have been working hard on finishing pieces for a show in January, which means I 've been neglecting sketching and blogging. I'm taking a little time now trying to catch up for a while.

White mangrove, Laguncularia racemosa, and buttonwood, Conocarpus erectus, both bloomed in late July, and well into August. They are in the family Combretaceae, which also includes the black olive, sometimes used as a shade tree. My neighborhood in Miami was lined with these stately trees until Hurricane Andrew uprooted dozens - maybe hundreds - of them along with huge slabs of sidewalks.

White mangrove, obviously, is classified as a mangrove, but buttonwood, which lacks special adaptations to cope with highly saline, saturated, and oxygen-starved soils, is considered a "mangrove associate." Typically it will be the most landward, highest and driest species in a mangrove forest habitat. It often grows in mixed stands with the white mangrove. However, white mangrove requires at least moist soils, whereas buttonwood is very drought-tolerant.

White Mangrove, Laguncularia racemosa

The presence of  2 bumps, one on each side of the petiole just beneath the leaf, is characteristic of the Combretaceae. They once were thought to excrete salt, but the general consensus now is that they are "extra-floral nectaries," glands outside the flowers that secrete nectar. It's possible that the reliable source of nectar attracts ants and wasps, which then protect the plant from leaf-eating insects. Only the new leaves of white mangrove produce nectar.(1)*

White Mangrove, Extra-floral Nectaries in Center of Illustration

Another, really cool, feature of this plant family is the presence of "domatia" on the undersides of the leaves. "Domacium" is Latin for "house, home, etc." - think "domicile." A domatium on a plant is a pit, pouch, or tuft of hairs that serves as a residence for ants or tiny arthropods like mites. There probably is a symbiotic or mutualistic relationship between creature and leaf.

Under magnification the domatia on these 2 plants look like mini volcanos, with the crater being the home. They don't go all the way through the leaf, but do make slight bumps on the other side. In buttonwoods the domatia tend to be lined up along the midvein where secondary veins branch off. They are located about a quarter of an inch inside the outer margins on the white mangrove.

Domatia, Buttonwood on Left; White Mangrove on Right;
Expanded View of Domatium on Right

The white  mangrove is primarily a "salt excretor," getting rid of salt through its leaves. Lenticels on the trunk help with air exchange in very wet situations, and it can send up "peg roots," like the pneumataphores of black mangroves, to aid in air exchange if it is inundated regularly.

White mangrove and black mangroves are the most salt-tolerant of our mangroves, able to survive in areas with salt concentrations as high as 40-50,000 ppm, but buttonwood is usually destroyed at concentrations of 40,000 ppm. (2)

At first glance, the idea that the red mangrove, which can grow directly in the salty water of estuaries,  is  the least salt-tolerant of the group seems counter-intuitive, at least to me. Actually, constant tidal flushing and freshwater runoff into these estuarine habitats reduce salinity in the red mangrove root zone. In fact, the red mangrove reaches its most luxurient growth along tidal rivers and even in freshwater swamps if it is not out-competed. White and black mangrove are flooded only intermittently, so salts accumulate in their  soils.

The white mangrove gets its common name from its light-colored bark. The leaves are opposite, succulent, yellow-green and oval, and get to about 3 inches long. They may be notched at their tops.  They can be shiny, but sometimes are dulled by salt residue. The plant can get up to 60' tall, but often remains shrubbier. It is the least cold tolerant of our mangrove species.

White Mangrove in Mixed Stand with Other Species 

Vase-or-cup-shaped greenish white flowers, which can be mildly fragrant, are produced on stalks. As is the case with other mangroves, the fruit is a propagule. Some of my sources say it germinates while still on the plant, others say it germinates only upon dispersal. It stays viable for about 35 days. The propagule is green, narrow at the base, widest in the middle, and ribbed. It gets up to about a half-inch long.

Flowers

The white mangrove has a rare means of reproduction, called androdioecy.  Trees produce either male flowers or hermaphroditic flowers. Most of the white mangroves in Florida are hermaphroditic, with the frequency of males declining as you move northward along the peninsula. In Mexico, there are proportionately many more "male" trees. The sex difference between the 2 types of flowers is reflected by differing sizes and behaviors of the pollinators that visit. (3)

Propagules

Buttonwood will have to wait for another post. All this is probably boring for normal people, but for a plant nerd like me it is utterly fascinating.




Notes and Sources:

1; 3. Beverly J. Rathcke. Mangroves: Ecology and Reproduction. Laguna Bacalar Institute Symposium, 12/28/08. I can't get a link to work, but if you search for "The Wild and Wonderful World of White Mangrove" you should find the article.

2. Frank Craighead Sr. The Trees of South Florida. Vol !: The Natural Environments and Their Succession. University of Miami Press. 1971. P.71.

Mangroves, Part II - Red Mangroves

I am incredibly lucky to live in an area dominated by mangroves. In fact, the watery mangrove landscape beloved by fishermen and naturalists alike has its own name around here, the "back country" or "back waters." This has been a good year for red mangroves. They are hanging heavy with "pencils," which any day now will drop off and float until they wash up onto a suitable growing surface.

Marco Island is considered the northernmost of the "10,000 Islands," the greatest concentration of mangroves left in the state. The development of Marco Island and my community, Isles of Capri, wreaked massive damage on the native flora and fauna, and mangroves in particular.  Fortunately there are a few places left in both areas where the mangroves  either escaped the devastation or have reestablished themselves. And both areas are now nestled within area protected by the Rookery Bay National Estuarine Sanctuary and Everglades National Park.

Undisturbed Mangroves Part of Rookery Bay Sanctuary

A warren of prop roots makes the red mangrove easy to distinguish. These roots stabilize the shallow-rooted trees, and  are vital in transporting oxygen. They possess numerous pores, lenticels, which admit oxygen and allow its transport to the feeding roots, and which close when inundated. In a dense growth of red mangroves it can be impossible to tell which roots belong to which trunk.

Prop Roots

The trees not only send down roots from the trunk, but also  long roots from large branches. The lenticels and the plant's low rate of transpiration make it possible for the plant to maintain high osmotic pressure in the roots, which allows them to take in water, but exclude the salt by a process of reverse osmosis.

Aerial Red Magrove Roots, Mixed Growth, HWY 952

Where hurricane action is rare, the trees can grow to 80 feet tall or more, with a girth of around 3 feet, but the frequency of storms in Florida keeps them much shorter and shrubbier. Some mangrove stands can show tall, straight trunks, while other trunks are contorted and can look even animate, tortured or ecstatic.

Red mangroves have shiny, slightly  yellow-green leaves arranged opposite each other on the branch. A special characteristic for the species is the long interpetiolar stipule. (Stipules in between the leaf stems, the petioles). At the very tip of the branch in this sketch the stipule has not fallen off yet.

Red Mangrove, Leaves, Stipules

The edges of the leaves are smooth. Undersides are paler than the top sides. They can get up to  a good 4 inches long, and an inch or more wide. Leaves are clustered at or near the ends of the branches, in order to harvest as much light as possible. There is not much understory in a red mangrove forest, but seedlings can get established where there are gaps in the foliage cover. They will stay in the seedling stage until enough light is created for them to grow larger.

Flowers are bell-shaped, with 4 waxy yellow sepals partly hiding 4 feathery white petals. The plant can self-pollinate. Pollen is wind-borne, but it is possible that some may be spread by insects.

Red Mangrove Flowers 

One of the criteria of inclusion in the "mangrove" group is viviparity - live birth.  The actual "seed" of mangroves germinates on the plant. The brownish-olive structures below are the fruits, with the propagules, the "pencils," beginning to develop.

When the propagules are mature they drop from the plants. Many land at the base of the plant, but others will float away, often in "rafts,"  with the next tide. Incredibly, the propagules can stay alive for a year. At first they float horizontally, but as they become waterlogged the heavier base begins to sink until the pencil is vertical. If it washes up onto a suitable site, it will send out roots and sprout leaves.

Red Mangrove Propagule

Propagule with New Roots Developing

It is impossible to overstate the importance of red mangroves, and mangroves in general, to the ecosystem. Their roots stabilize shell and mud deposits, provide anchoring places for microorganisms and mollusks, and serve as a nursury and shelter for small fish and other vertebrates. They, in turn supply food for wading birds and other predators. Roots, leaves and pencils are consumed by a variety of native animals and insects. The root region is rich in phytoplankton, the base of the marine food chain.

Mangroves absorb a huge amount of wind energy from hurricane winds and storm surges, often at their own cost. One needs only to see the path of mangrove devastation after a storm like Irma in 2017 to see the incredible force they absorb and deflect. Dense mangrove forests also provide places for birds to perch during the day and mangrove islands provide safe rookeries at night.

Mangroves are enormously effective at capturing and storing carbon. When mangrove habitats are destroyed, the environment gets a double whammy - first by the release of massive amounts of stored carbon, and second by the loss of a significant carbon-removing source.

In some regions red mangroves are used medicinally. They have proven antibacterial properties. Leaves and branches contain high levels of tannin, which is what makes the waters of estuaries and tidal rivers tea-colored. Around the world mangrove tannin is used for curing leather.

The very stuctures that allow the red mangroves to survive can also be their undoing. Lenticels can allow the entry of pathogens. If the lenticels are clogged by fine silt, oil or other clinging pollutants the plant will die. Large swaths of mangroves in Everglades National Park were killed by silt deposits by Hurricane Donna in 1960.

The plant also dies if water levels rise enough to cover the lenticels permanently. While mangroves are threatened in their native range by rising sea levels, they also are posing a threat to salt marsh habitats farther north. Previously these vital habitats were protected by the mangroves' intolerance of freezing weather, but warmer winters with fewer freezing epidsodes are allowing mangroves to invade. Red mangrove also is considered invasive in Hawaii, where it was introduced.

Red Mangrove Leaves and Stipules 

Sources: Frank C. Craighead, Sr. The Trees of South Florida. Vol I. University of Miami Press.
1971.

Ronald L. Myers and John J. Ewel, Eds. Ecosystems of Florida. University of Central Florida Press. 1990.

"Mangroves Against the Storm."

"Why Blue Carbon is REDD Hot."

Beverly J. Rathcke. "Mangroves: Ecology and Reproduction."