A venerable oak tree stands behind the ranger station at Cayo Costa State Park, a barrier island along the Gulf coast of Florida. It is festooned with Spanish moss and other plants collectively known as epiphytes – the ones that live in the trees without ever touching the ground. Among them are bromeliads, ferns, and on this island, only one species of orchid.
Bromeliads, otherwise known as the pineapple family, are more abundant than the others in that tree and elsewhere on the island, both in terms of sheer mass as well as species diversity. All the other epiphytes together are dwarfed by the sheer presence of one species of bromeliad – Spanish moss. It is one of seven species of bromeliads on Cayo Costa. The others, collectively known as air plants, are all species of a single genus in the bromeliad family, sort of like brothers and sisters. They are Spanish moss, Medusa head, giant air plant, twisted air plant, wild pine, ball moss, southern needle leaf, and spreading air plant. They range in size from 2 inches to 2 feet tall. Most grow as a basal rosette of leaves, like the top of a pineapple fruit. They have flowers ranging from red, purple, green and yellow but the flowers of most are rather small, and some, like Spanish moss, are inconspicuous.
Spanish moss is a bromeliad, (related to pineapple), not a true moss. Of the over 2000 bromeliad species radiating from South America, Spanish moss looks least like a pineapple plant. Of all the bromeliads on Cayo Costa it is the smallest. Individual Spanish moss plants are on the order of 2 inches across, but they reproduce vegetatively as one plant becomes two by division (not from seed), forming a daisy chain with one generation attached to the next, and the next, and the next, ultimately creating those luxuriant tresses that cascade from the grotesquely twisted branches of its host, the live oak tree. To some they evoke the southern gothic charm of the leisurely south, as Gordon Lightfoot captured in his song, Spanish Moss:
Spanish moss, a-hanging down
sweeter than the southern love we’ve found.
Spanish moss, keeps on following my thoughts around…
Like many epiphytes, Spanish moss has no roots, unlike terrestrial (ground) plants which are tethered to the ground by their root systems that serve to deliver water and nutrients upward to the rest of the plant.
Imagine that you are an epiphyte. When you live in a tree in a hot climate, staying hydrated and healthy is a serious problem especially if you have no roots. Without any roots, it’s something like living in a desert. How do you get enough water and nutrients to live in the first place, and how do you conserve the water that you already have to keep from drying out? Epiphytic bromeliads have evolved several strategies to enable them to cope with life in the canopy. They can absorb water, either as rain or as humidity, from tiny gray scales on the leaves. When water becomes scarce during the heat of summer, Spanish moss and some other epiphytes have evolved a remarkable way to avoid drying out that involves staying up all night, so to speak.
The usual way plants manufacture sugar from CO2 and sunlight, through the process of photosynthesis. They take up CO2 through tiny pores called stomata on the underside of the leaf. Stomata are essentially valves on a day-night timer, which typically open during the day and close at night. Open stomata during the day let CO2 in, but that comes at a cost, because when the stomata valves are open, water vapor escapes (by evaporation) from the inside of the leaf, which can result excessive in drying, especially if the plant has no roots to replace it.
Instead, Spanish moss and many other epiphytes (and some desert terrestrials as well) avoid losing water vapor during the heat of the day by keeping their stomata closed, and only open them for CO2 uptake at night when the air is cooler, and evaporation is minimal. Of course, plants with this reverse diurnal strategy still need sunlight to drive photosynthesis, so the plant holds onto the CO2 they have acquired during the night and use it to make sugar after the sun comes up.
In addition to photosynthesis, all plants need nutrients like nitrogen, phosphorous, and others mineral nutrients to survive. Most plants acquire these from the soil via their roots. Spanish moss and other rootless epiphytes absorb nitrogen and other nutrients directly through their leaves. One source of these nutrients is from decaying leaves and other organic debris that accumulate in the nooks and crannies of the tree branches.
But there is another remarkable way in which Spanish moss and other rootless bromeliads acquire nitrogen. Although nitrogen gas is abundant in the air (much more so than oxygen) most plants cannot use it, but there are certain bacteria that can convert atmospheric nitrogen into ammonia that plants can use. Nitrogen fixation by bacterial (Rhizobium) nodules on the roots of legumes (beans, alfalfa, and some trees) is a good example. In the case of some epiphytes including Spanish moss which have no roots and therefor no nodules, “free living” N-fixing bacteria live on the surface of leaves. These bacteria are just some of the many kinds of bacteria that live in the leafy realm called the phylloplane.
So, next time you see Spanish moss either right outside your door, or a thousand miles away if you live in New York like I do, remember, it’s not moss. Think pineapple.
Ken Mudge is a recently retired Associate professor emeritus from the Department of Horticulture at Cornell University. He has taught courses in Plant Propagation and the Practicum in Forest Farming (agroforestry). He is coauthor of the book Farming the Woods by Ken Mudge and Steve Gabriel (2014, Chelsea Green Publishers) and currently writing a book called Island in the Sun, Reflections on the Natural History of Cayo Costa.
Photo of Spanish Moss by the author