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

 Mangroves make up a group of plants that connect me deeply with my coastal SW Florida roots. Mangrove" is a collective term for a variety of unrelated species that share certain biological and structural traits which enable them to survive in highly saline and oxygen-poor soils. Three species in the U.S. are recognized as mangroves: the red mangrove, Rhizophora mangle, the black mangrove, Avicennia germinans, and the white mangrove, Laguncularia  racemosa.  Buttonwood, Conocarpus erectus, is considered a "mangrove associate" because it often grows on the land-most side of a mangrove forest. Red mangroves, with their conspicuous aerial and prop roots and often-contorted trunks, are probably the most familiar of the species. They also tend to grow the closest to the water's edge, but not always.

From the time I was 9 years old until I left for college I lived in a world bordered and somewhat defined by mangrove forests. Our house sat in a mixed palmetto-pine-scruboak habitat, but if was only about a mile by foot to the mangroves. By water they practically were next door, because it took only a few  minutes down our canal to get to the mangrove portion of the Estero River. Further upriver the mangroves gradually gave way to more terrestrial plants probably due to a rise in elevation. Boats of various sizes and types, including crude but effective homemade ones, were a huge part of our family life. We always headed downriver, toward Estero Bay, a half-world between water and land, rimmed by mangroves, dotted with mangrove islands, and rife with unmarked shoals and oyster beds.

Red Mangrove, Pen & Ink (in progress)

My father made aerial photographs of the mouth of the Estero River at low tide, and then marked the channel out into the bay with the most upright small black mangrove trunks he could find. I'm sure they've long been replaced by something more official. One woman who didn't know much about the way channels wind and bend commented once that the markers looked like they had been placed by a drunk. My father just chuckled and said that a little whisky helped when the work was cold and wet.

He got interested in canoes, and tried to make his own. They were clunky, tipsy craft made from plywood salvaged from some old signs he had. The first ones were impossible to balance, and we got lots of laughs from his capsizes in the canal, but he gradually worked out better proportions. They were still tippy, though, and he had to cajole me into going with him. Even wearing a life jacket, with him behind me, I felt my heart lurch with every paddle stroke. The boats were so short that the passenger had to wear a raincoat for protection against the steady "rain" of water thrown off from the double-bladed paddles.

Because my father wanted us back before motorboat began in earnest, we left before daylight, often in the dense predawn fog typical of the sw Florida coast. That made the experience even spookier for me. The mangroves seemed to form solid black walls above me, and the way forward was shrouded in mist.

It wasn't long before he bought 2 real canoes from Sears, Roebuck. They had wide, flat bottoms, and were difficult to capsize, and by then I had lost most of my fear of water, and had learned in Girl Scout camp how much fun capsizing canoes could be.

We scarcely were purists. My father had a genuine kayak paddle but didn't like the wrist motion. He made his own double-bladed paddles out of wood - crude things somewhere between 2x2's and 2x4's with notches for thumbs and a half-piece of soft pipe to cushion the inside of the fingers. Because he had knee trouble he sat up on the thwart, so we did too. We had 2 canoes, and sometimes my father and the three oldest of us children went out with him, exploring, racing, or just paddling. The one who drew passage with my strong, reliable father was in the catbird seat. Pairing with another sibling inevitably produced mutual accusations of slacking and ineptitude.

On lower-tide days our dog Trixie would follow us, and howl most piteously when the mangroves grew right into the water and blocked her progress. Taking her along was a dubious business, though, because she usually got seasick all over the boat. In those days there weren't many alligators around, so we didn't have to worry about her.

Mangrove Bay

When I was in high school my father and I "canoed" every morning before we had to leave for school, and went out most weekend mornings as well. Sometimes we talked; often we preferred to absorb the quiet as the skies lightened, and the dark walls formed by the mangroves flanking the river began to take form with leaves, branches and prop roots. At one point the river widened to a broad cove, and that was our turn-around place. We called it "Daybreak Cove," because our eastward turn revealed the brilliant pinks and golds of the rising sun flooding the sky and spilling over  the mangroves.

That was my first taste of a mangrove phenomenon that I truly love - the experience of coming out of a walled- in channel to emerge suddenly into pure space. It can be almost vertiginous - you feel you could just as well be floating high up in a volcanic crater instead of in a broad expanse at sea level. If it's windstill, the bay or cove's surface mirrors the sky, and the only sounds are of water quietly gurgling through the mangrove roots, or a bird's call. Even the distant drone of an airplane does not dispel the sensation of being in a primordial world, untouched by human influence.

Red Mangroves, Pencil Sketches 

Through college and the first years of grad school I returned home during breaks, and always found rejuvenation through canoe trips on the river and into the bay. Once my brother and I made it all the way to Estero Bay before the sun had risen fully. We beached on a mud flat and marveled at the rivulets formed by the outgoing tide flowing full of the dawn's rose-gold light.

Mangroves along the Estero River grow in a compacted peat substrate, and are somewhat stunted. When I first saw the majestic trees lining the Joe River in Everglades National Park years later, I didn't even realize at first that they were mangroves. Living in Miami, my husband and I loved to trailer our 16-foot motorboat to Flamingo, the southernmost tip of the park, and explore its rivers and trails. Whitewater Bay, the Joe, Roberts, and Shark Rivers were ours to discover. Often once away from the boat ramp, we would be the only boat around, especially in summer. Alligators loved to sun on the concrete ramp, so during the summer you had to take the best place to put in that was available.

Nearly 30 years later, after Hurricane Andrew blew us out of Miami and onto Florida's west coast, we still love the watery world bounded by the mangroves. By now we've acquired enough local knowledge to find our way, but we still carry a chart.  It never gets boring. Each trip reveals something new. We've gotten too old to sail, which is a blow, but we still can putt around in the backwaters. My parents moved to north Florida, along the Suwannee River, in the early 1970's. SW Florida had become too crowded. The Suwannee River has its own beauty, and my father loved it, but he never forgot the strange, flat world of the mangrove forests.