Moss and Fern Getting Along Famously

Moss (Octoblepharum albidum) and Shoestring Fern  (Vittaria lineata)

(Octoblepharum means 8 eyelids in reference to the spore release system with that many teeth;  albidum means white.   Vittaria comes from Latin for ribbon, noting long flat shoestring linear (lineata) leaves.

John and I worked indoors this week, but no worries, we can talk about a notable treetrunk partnership.   Many species grow on Cabbage Palms:  Boston Ferns,  Golden Polypody Ferns,  Grapes,  Hand Ferns,  Laurel Figs,  Lichens,   Liverworts, Peppervines,  Poison Ivy vines,  Smilax tangles,  Strangler Figs,  Tuberous Swordferns,  Umbrellatrees, Virginia Creeper,s  Wax Myrtles and more.

Lichen on the palm trunk.

Two Cabbage Palm trunk residents have a special relationship:

A beautiful moss Octoblepharum albidum often graces the bases of Cabbage Palms trunks with fibrous decaying leaf bases.   The moss is white-toned, thick, pillowy, absorbent, and abundant.  It may occur alone or in the company of lichens (often I think Cladonia subradiata), or with Shoestring Fern, our second species of interest.

The classic relationship.   The white moss immediately above a young clump of Shoestring Fern.

The moss often grows without the fern, but not vice versa.  The young fern rarely holds forth apart from physical contact with the moss.  (Stats listed below.)  Witness to the dependence surfaces sporadically in the literature.  For instance (from A.F.W. Schimper, Plant Geography on a Physiological Basis, 1903):

That the moss occurs happily without the fern while the fern is rare moss-free suggests the moss has something the fern wants.  Alternative explanations are possible*, but we’re going to run with “the fern loves the moss.”

To ponder these things, we need to know Shoestring Fern better.  It lives on tree trunks, predominantly Cabbage Palms.   Ferns pass through a vulnerable early life stage called the gametophyte (gam-EAT-oh-fight) which is tiny, rootless, veinless, and delicate..  In most ferns the gametophyte is short-lived, although generally less true in epiphytic ferns, and the genus Vittaria is especially known for long-lived gametophytes.   In some regions self-perpetuating Vittaria gametophytes are the only form of the fern,  reproducing babies sans adults.    The little gametophytes make pups (gemmae) that break off and grow.  The main central gametophyte reportedly tends to be female, with its spinoff progeny tending to become male.   All this takes time on a tree trunk where water and nutrients can be hard to come by.  Maybe the moss offers support and protection.

Gametophytes (glossy green) to the left and adjacent moss buddies (narrow leaves) on the right.

The trouble with mosses and ferns is inadequate study.     In 2018 we know mosses sometimes benefit associated plants, but the mechanisms remain murky, with some good ideas afoot.

First of all, Octoblepharum is no ordinary moss.   Its leaves are thick like a mini-succulent, and they are tightly clustered.  In short, it is a little reminiscent of Sphagnum used to keep garden plants moist, holding water internally and externally on that windy sunny palm trunk.  Maybe the moss is a nice “growing bed” for the fern.   Yes, but not that simple either.   The position of the fern relative to the moss matters.    Ninety percent of the young ferns  are in physical contact with the moss, often at the edge of the moss patch,  but only four percent of the contacts are at the top (uphill on the trunk). By contrast,  24 percent of the young ferns reside at the bottom edge and 19 percent alongside the moss or surrounded by it.   Looks like something good washes downward or a bit sideways from the moss.  Nutrients?

Mosses are becomingly increasingly known for nitrogen fixation (capture from the atmosphere) with the help of microbes.    Moreover, recent research shows some mosses to take up nutrients from the substrate, unsurprising until I remind you they have no roots or veins,   traditionally regarded as reliant on rain, dew, and mist.   Octoblepharum sits anchored to the decaying palm leaf bases.   Nutrients could plausibly move from the decaying palm into the pseudo-rooted moss, and then to the fern before or after the moss dies.

If nothing else, the moss is a spongy reservoir for rainwater and  dissolved nutrients,  presumably releasing them slowly by stem dribble to the fern usually alongside, below, or surrounded.

To venture out into shameless speculation,  the moss has a conceivably relevant super-power.   Many tropical plants possess a form of photosynthesis called Crassulacean Acid Metabolism (CAM).   No need for a physiology lesson, the important thing is that CAM allows a plant to thrive under tropical conditions unsuitable for “normal” plants.   Our moss remarkably can toggle between “normal” photosynthesis and CAM, allowing it to stay lively under a wide range of conditions.   That extended vigor may help its needy fern buddy.

To summarize a highly speculative scenario,  somehow the moss seems to support the fern’s younger stages.   In addition to serving as a moist growing bed and retaining and rationing stemwash, the moss could perhaps also provide nutrients from its own decay,  or fixed nitrogen,  or nutrients taken up from its substantial attachment.

My son Evan tried growing essentially all the local ferns from spores in an indoor lab.  The only flop was Shoestring Fern.  Did it miss its moss?

Does the moss return the favor?  Not that I know of.   My theory is, “no good deed goes unpunished.”   Large mature clumps of the fern smother the mosses they come to dwarf.

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*A few more details on the friendship for the interested.

Could the near-restriction of the fern to intimacy with the moss (90%) just be a reflection of a mutual need for decaying palm leaf bases, with the fern then having an additional narrower need within the shared habitat?  Maybe, but that seems a stretch, and if so, that would be a remarkable story in its own right.

Alternatively,  could the moss be so pervasive on mutually suitable habitats that the fern has to tolerate it to have a place to live?   Again, possible, but I doubt it, since there is plenty of non-moss-covered apparently suitable habitat, and the position of the fern relative to the moss clump matters.

Fifty trees with fern clumps were checked sequentially walking along paths in Riverbend Park, Jupiter, Florida.  One young fern per tree was recorded.   When more than one fern occupied a tree, the clump closest to my eye level was the one recorded.   Massive old fern clumps were not counted.  Here are the results in descending order of frequency:

• Fern immediately below and touching or overlapping Octoblepharum moss clump 48% of trees examined
• Fern surrounded by and touching Octoblepharum moss clump 22%
• Fern immediately alongside and touching or overlapping Octoblepharum moss clump 16%
• Fern above and touching Octoblepharum moss clump 4%
• Fern alone, not touching a moss clump 8%
• Fern surrounded by a different species of moss 2%

Summary:

Fern touching Octoblepharum 90% of fern-bearing trees examined

Fern touching Octoblepharum at the top (upstream) of the moss pad 4%

Fern not touching a moss 8%