May 2009

Plant Adaptations
Epiphytic Lichens and Bromeliads

In and around the Cypress Boardwalk, the two most common epiphytes, plants that depend on another plant for mechanical support but not for nutrients, are lichens and bromeliads.

Spectacular green-on-black collage of
Tillandsia Bromeliads with some in bloom

Four species of lichens on a cypress tree trunk

 

LICHENS

The cypress tree trunks in our swamp often have dense populations of several epiphytic lichens. The most striking species, the Baton Rouge lichen, Cryptothecia rubrocincta, is often bright red as this photo shows.

The Baton Rouge lichen's red color is unusual since most lichens are green or grayish green with no accessory pigments. The red pigment may protect against damage from high light levels in winter when the cypress shed their leaves.

 

All lichens are a mutualistic partnership of a green alga and a fungus. The shorthand for this interaction, where both species benefit, is + / +. The fungus, with its very fine root-like hyphae, takes up water and mineral nutrients from the air and rain that runs down the tree trunk. The algal cells produce sugars by photosynthesis (see February, 2009 column). This cross section micro-photograph of a lichen shows a layer of green algal cells. The rest of the material is fungal tissue.

So, what did one algal cell say to the other? "There's a fungus among us." To which the fungus said to the algae, "I lichen my relationship with you very much!"

 

The diagram below shows the life cycle of a lichen. Note that there are two kinds of asexual reproduction and one kind of sexual reproduction. All three modes allow the lichen to disperse to new places where the colonies grow very, very slowly.

Polluted air kills lichens; even tiny amounts of sulfur dioxide kills them. Happily we have no major sources of this pollutant in south Florida because we have no coal-fired or oil-fired power plants. In addition, periodic strong winds carry away air pollution that may come from the industrial areas of Louisiana and Texas. And the Refuge cypress swamp is far enough away from major highways that the sulfur dioxide in car exhaust is not a problem. The many lichen species on trees in south Florida are "liking" our unpolluted air very much.


EXTREMES OF GROWTH HABIT IN THE PLANT FAMILY BROMELIACEAE (Bromeliads)

At one extreme is the domesticated pineapple. It is a generalized bromeliad with roots. It has broad green spiny leaves. And it has a conspicuous flower spike that turns into the pineapple fruit that we eat.

 

 

 

At the other extreme is the wild Spanish moss. It is a specialized epiphyte without roots. It has scaly grey-green strands instead of leaves. And it has tiny inconspicuous flowers that mature into a tiny fruit with a few tiny air blown propagules.

 

BROMELIAD EPIPHYTES AT THE REFUGE

Bromeliads are the most frequent, abundant, and diverse epiphytes at The Refuge. The sign about Bromeliads on our Cypress Boardwalk, labeled "living off the air," is not completely accurate. They are air plants only in the sense that they have no absorptive roots. The leaf bases of the larger species trap nutrients as dust or dead plant parts are blown in the air or fall from above. And, all the species get their water and most of their mineral nutrients from rain or mist. They are also air plants. In our dry season, they have to survive in pretty dry air without rain.

Do bromeliad epiphytes help, hurt, or have no effect on trees on which they grow? What is your hypothesis about the biological interaction of epiphytes with trees on which they grow? What is your thinking that leads to your hypothesis? Some of my students have hypothesized that the interaction is mutualism (+ / +) where each species benefits. Many have hypothesized parasitism (+ / -) where the epiphyte benefits and the tree is hurt. Still others have hypothesized competition (- / -) where each species is hurt. And a few students have hypothesized commensalism (+ / 0) where the epiphyte benefits and the tree is not affected.

What would you look for in nature that would support or reject any of these hypotheses? Look where the air plant grows on the tree. Note whether the tree or part of the tree where the epiphyte grows does well, does poorly, or is not affected compared to where the epiphyte is not present. The easiest observations to make are with Spanish moss and ball moss since they are common. Here are three relevant photos.

Spanish moss hangs down from a cypress branch;
the cypress sun leaves project upward

 
Ball moss grows on a telephone line

Ball moss grows on dead
lower branches of an oak tree

 

EVOLUTIONARY CHANGES FROM GENERALIZED TO SPECIALIZED
BROMELIAD SPECIES IN THE GENUS Tillandsia

An example at the Refuge of a generalized Tillandsia is the cardinal wild pine, Tillandsia fasciculata. It is our largest but least common species. As shown in the picture, it has a cluster of broad fleshy leaves that collect debris and water around the leaf bases anchoring the plant to the cypress trunk. This may be the main way it gets water and nutrients since the trichomes, hairlike growths, are less dense on these leaves than on the leaves and stems of ball moss and Spanish moss.

 

Like all the Tillandsia species, Cardinal wild pine has no real roots. Notice its conspicuous flowering spike with the bright red bracts. Each bract will eventually have a less conspicuous but pretty purple flower that is pollinated by long-tongued bees.

 

 

 

In comparison, here are the extremely reduced, wind-pollinated flowers of Spanish moss at the ends of thread-like branching stems.

 

 

 

A close-up of the flowers and stems reveals that the stems have a fuzzy covering of trichomes.  ►


 

 

 

 

Our two most common specialized Bromeliad epiphytes are the ball moss Tillandsia recurvata and Spanish moss Tillandsia usneoides. Both lack roots and both have specialized trichomes that are scale-like and act like a sponge to absorb rain-water with its low concentrations of mineral nutrients. Mineral nutrients include nitrogen as nitrates and phosphorus as phosphates. The images here show the trichomes on the surface of the rudimentary stems of Spanish moss at two magnifications.

Trichomes at 10x magnification

At 100x: Origin and development of trichomes

Tillandsia trichomes act like trap doors and help the plant resist drought. Tillandsia in the sun and wind at the top of trees have denser trichomes. As trichomes dry they open up and reflect bright light that might damage the underlying chlorophyll. Rain or mist wets the trichomes. When wet they flatten and become translucent, and so you can see the green of the chlorophyll. When the plant is sufficiently wet the trichomes erect again to retain moisture.

 

DROUGHT RESISTANCE OF Tillandsia IS HELPED BY A PHOTOSYNTHETIC PATHWAY LIKE THAT IN MORE SUCCULENT PLANTS

All Tillandsia air plants have a CAM (Crassulacean Acid Metabolism) type of photosynthesis that enables them to be epiphytes that not only survive but also slowly grow during a yearly dry season. This is the same type of photosynthesis seen in desert succulents like cacti and agaves. In climates that are seasonally dry, like south Florida, the main advantage of plants with CAM photosynthesis is that they open their stomates and absorb carbon dioxide only at night when the temperature is low, and thus very little water is lost from the stomates. During the day they photosynthesize without opening their stomates. They use stored carbon dioxide and even recycle the carbon dioxide produced during respiration.

Another advantage of CAM photosynthesis is that, during dry times, the plant can do the analogue of an idling car engine. When moisture conditions improve after drought these CAM plants can start to photosynthesize immediately with no need for "warm-up."

Tillandsia epiphytes have the disadvantage of very slow growth rates for two reasons. First, CAM photosynthesis is efficient but very slow. CAM plants can take up only a limited amount of carbon dioxide at night and store it for photosynthesis the next day. Regular plants continue to take up carbon dioxide all day and so they can photosynthesize longer and faster. The second reason for slow growth is absence of roots and reliance on rain for water and nutrients.

 

WHY IS OUR MOST SPECIALIZED EPIPHYTE CALLED SPANISH MOSS?

One story is that Santa Claus arrived in the south on a hot and sticky night and hung up his sopping wet beard on a tree to dry while he delivered his presents.

Here is part of a poem by C.V. and L. M. Bush, illustrated by Dr. Tom with a Spanish moss beard.

And now when we see that silvery hair,
Hanging from the tree so long.
They tell us it is Spanish Moss,
But we know they are dead wrong!
It's only Santa's whiskers
He shed one warm, warm night
Just before he started north
From his southern Christmas flight.

 

REVIEW QUESTIONS

a. Epiphytes like Spanish moss and ball moss are parasites (a + / - interaction).

b. The most specialized epiphytes, like ball moss and Spanish moss, get their water and mineral nutrients from rain and have no roots.

c. The specialized epiphytic Spanish moss has a large red flowering stalk with multiple pretty purple flowers pollinated by bees.

d. The high frequency and density of lichens on cypress trunks in our swamp is an indicator of excellent air quality.