November 2010

ONE GOOD FERN DESERVES ANOTHER

Ferns are the most common under-story plants along our RE-OPENED AND IMPROVED CYPRESS SWAMP BOARDWALK. Come see for yourself or join one of our interpreters on a guided walk (see 'events' page).

In this column I will discuss the species, the life cycle, the microhabitats, and the ecology of our ferns.

Our ferns are not woody and all have leaves, called fronds, with distinctive patterns of spore cases, sori, usually on the underside of fertile fronds.

Diagram of the parts of a fern frond showing
rachis, blade, rhizome, and roots

Fern Life Cycle

As shown in this diagram, the visible part of the life cycle is the large diploid (two of each chromosome) sporophyte with multiple fronds. It has the spores that germinate to make the tiny haploid (one of each chromosome) gametophyte which develops only in a damp microhabitat. Each gametophyte has both a sperm-producing male and egg-producing female part. The swimming sperm fertilize the eggs. The fertilized egg is now diploid and is called a zygote. It germinates to grow into a sporophyte; this completes the life cycle.

While guiding folks on our cypress boardwalk, the late Jay Brenner was fond of describing the alternation of generations in a fern's life, sporophyte to gametophyte to sporophyte to gametophyte...

Jay Brenner was fond of fronds

Except on the internet, I have never seen a fern gametophyte or the immature gametophyte called a prothallus. Why? One answer is that they are tiny, one cell thick and 5mm across. Another is that most are eaten since, unlike the sporophyte, the gametophyte has no chemical defenses. And another is that the right conditions for spore germination are exceedingly rare. I would love to have someone find one and show it to me!

The title for my column on ferns is from a poem by John Burns in his book Biograffiti: A Natural Selection. Norton, 1975

One Good Fern Deserves Another

• Prothallia of ferns are always haploid
• Producing sperms and eggs that seize the procreative role
• When, of a dampness, they unite to from a diploid.
• Up springs the frondly sporophyte, with rhizome, root, and rachis
• And a meristem that's apical and tight.
• It uncoils; but on a leaf that is preparing for meiosis
• Sporangia in clusters make a very sori sight.

Close-up of sori, each with sporangia, on part of a frond

An apical meristem that is uncoiling is called a fiddlehead

Only ostrich, bracken, and cinnamon fern fiddleheads are non-poisonous. Other species should probably be cooked before eating to inactivate possible toxins. I have nibbled several species of fern fiddleheads. One tasted a bit like walnuts but to me most have no taste or are bitter. My rationale for not eating them raw is that fiddleheads might be poisonous because plants protect new leaves more than old leaves. I hesitate to use goat's method of eating some and waiting for bad effects before eating a lot.

Ferns are spore plants, not seed plants

Most of our plants, including cypress and pine (Gymnosperms) or palms, red maple, and pond apple (Angiosperms) reproduce by seeds [see my column on TREES]. Seeds, especially of Angiosperms, have nutrients that help the young seedling plant get started; think of peanuts or coconuts. In addition, seeds, especially of Angiosperms, often have accessory tissues to help ensure their dispersal to a suitable microhabitat for germination [see my column PLANT-ANIMAL MUTUALISMS II: DISPERSAL]. Spores are very tiny with essentially no nutrients for the young plants. And, spores have no special aids for dispersal. Without extra nutrients or specializations for dispersal, plants that reproduce by spores must have many trillions of spores to be sure that at least one gets to a suitable microhabitat.

Why are ferns in our cypress swamp so common?

More ferns are in our small cypress swamp than in several kinds of marsh habitat that make up over 90% of our Refuge area. The total number of species in our swamp so far is 11. Only 0 to 3 are in any other habitat.

I have considered at least four hypotheses (H1, H2, H3, and H4) as to why our cypress swamp boardwalk has so many species of ferns. The same hypotheses may explain why ferns are so abundant in our swamp.

H1. Ferns have especially good chemical defenses against herbivores. I believed this since I never saw ferns eaten by caterpillars. But a careful 1983 study in Biotropica showed that three species of tropical ferns had the same 6 - 10% leaf area loss as three Angiosperms in the same area. Since reading this study I have looked more carefully for damage to our ferns and have seen evidence that they do have enemies. So I reject H1.

Damage to ferns in our cypress swamp

H2. Our swamp has especially good microhabitat conditions for ferns to establish. This still seems reasonable to me. Based on tree ring counts of cypress we cut for safety reasons, our swamp cypress were apparently clear-cut 50 - 70 years ago. This clear-cutting left many stumps that are now rotting and are perfect sites for ferns to establish. They apparently cannot establish on the swamp bottom because it is covered by water many months of the year. Virtually all of our cypress swamp ferns, and most other plants, grow above the high-water line on old stumps or broken logs or the spreading buttresses of living cypress. Old-growth cypress swamps have never been cut and so have few places for ferns to establish.

A colony of strap ferns on a broken cypress log

H3. The filtered and patchy light levels of summer and only slightly higher light levels in winter are especially suitable for fern growth. Remember that our dominant over-story tree is the pond cypress. It is deciduous [see previous TREE column]; its loss of leaves in winter allows more light to filter down. Also remember that hurricanes regularly snap off many cypress and that they either die or regenerate only slowly. This has opened up some small areas with no big cypress and these have especially dense shrubs and ferns. In addition the leaves of our pond cypress are very fine and cast very little shade.

Even in summer light is patchy and locally bright in our cypress swamp

Our pond cypress do not form a closed canopy and the fine leaves do not cast dense shade

H4. High humidity, protection from wind, and absence of fires allow ferns to survive and grow. Our cypress swamp has few or no flammable plants like grasses, rushes, sedges, or cattail. With standing water for six to eight months of the year any fires that start cannot spread. And the high humidity with lack of wind stops any fires that start from getting very hot or big.

How can 11 species of fern co-exist in our cypress swamp?

This is a bit of a mystery of the swamp about which I have considered only two hypotheses (H1 and H2).

H1. The 11 species have different growth habits with different microhabitats. Only four species have distinct microhabitats. One species Salvinia minima the water spangle, floats on water and is found even in the shadiest parts of the swamp. Two relatively rare ferns are epiphytes. One, Pleopeltis polypodioides the resurrection fern, grows on rough bark of a few shrub species. Another, the golden polypody Phlebodium aureum, grows on newly fallen logs and buttresses of live cypress.

Water spangles

Resurrection fern dried and after six hours of rehydration

H2. The other eight species are only weak competitors. Even the four largest and most abundant species - the giant leather, the giant sword, the boston, and the swamp -- have only slightly different growth patterns and so must compete to some extent. But this competition must be weak since I see most of the species in all of ten different boardwalk sites where I stand and count.

In a number of cases I saw 3 or 4 species growing next to and even among each other so they must not be strong competitors. All of these species have tall fronds. Most commonly our largest species, the giant leather Acrostichum danaeifolium, grows with other species. The giant leather fern has as many as many as 25 huge 3 meter long and half meter wide fronds in a dense clump. Our second largest species, giant sword fern Nephrolepis biserrata, has dozens of densely packed 1 - 2 meter long and quarter meter wide fronds. Our boston fern Nephrolepis exaltata has a half dozen half meter long and a tenth meter wide fronds. Finally our hottentot fern Thelypteris interrupta has the fewest number of fronds that are up to a half meter long and one third meter wide.

Questions you can help me answer

1. Do our ferns grow new fronds only at a particular season? I have only recently started to look carefully. As I walked the boardwalk in early October all species had fiddleheads and fresh new fronds. And I think that there was not much if any fresh growth last winter before the boardwalk was closed for reconstruction. But I am not sure since I was not taking notes.
2. Do our ferns disperse spores at a particular time of year? Even when there are fertile fronds with well-developed sori I have rarely been able to shake spores onto a piece of white paper.
3. How does our swamp fern tolerate seasonal submergence of its roots? It does not have air tissue in its frond rachis since it is thin and hard.
4. Why is our giant sword fern, Nephrolepis biserrata, so dominant around the edges of our cypress swamp? It is not the only one of our ferns to spread by rhizomes.
5. Do you have a question about our ferns that I have not considered?

REVIEW QUESTIONS - Answers follow the questions

1. Ferns differ from Flowering seed plants (Angiosperms) in their life cycle and mode of reproduction. True? False?
2. In ferns the gametophyte phase of the life history is dominant in frequency of occurrence and size. True? False?
3. The fern species in our cypress swamp are on old rotting tree trunks and stumps or on buttresses of live cypress. True? False?
4. Our cypress swamp is an especially favorable habitat for ferns. True? False?

REVIEW QUESTION ANSWERS

1. The answer is TRUE. Ferns reproduce by spores, not seeds, and havean alternation of generations with the sexual phase, the gametophyte, separate from the non-sexual phase, the sporophyte.
2. The answer is FALSE. The sporophyte of ferns, with its fronds, is much much larger. We do not really know if the gametophyte is much less frequent because it is so tiny and hard to find.
3. The answer is FALSE. Resurrection fern and golden polypody are epiphytes on bark of live shrubs and buttresses of cypress trees. Water spangle fern is aquatic on the water surface. And swamp fern often has fronds that emerge from water along a shore.
4. The answer is TRUE. Our cypress swamp has many more fern species with greater abundance than any other habitat at our Refuge. The favorable conditions include partial shade, high humidity, low wind, and no fire.