John and I worked today in the Haney Creek Trail natural area near Jensen Beach, Florida, although the scope of the discussion extends to multiple local wild places. An intriguing habitat type has no particular name: Let’s call them freshwater mudflats.
In these places seasonal inundation alternates with the pond bottom mud exposed bare naked anew each dry season. Sort of like Bill Murray’s Groundhog Day…ecological succession set back to “go” in an endless loop.
I don’t know to what extent various mudflats around here are the result of human activity. Either way, they are an exotic universe to explore
On the flat I’m using to base today’s account approximately six species dominate, accompanied by several subordinate interlopers. Different mudflats or even different portions of the same flat have different species mixes. The list below excludes the truly aquatic species persisting where the water has not receded. We’re talking about “land plants” on drying mud.
Here’s the dominant species list…
- Dog Fennel (Eupatorium capillifolium) in the Aster Family. This smelly weed invades just about any disturbed site where the sun shines, from cow pastures to today’s habitat, where it seizes upon the extreme disturbance of wet bare mud. Ideal! The seedlings are probably the most abundant babies on the site.
2. Sweetscent (Pluchea odorata), likewise smelly, is in the Aster Family. Its pollinators are diverse, yet in my experience mostly abundant enthusiastic wasps. The tiny individual flowers are clustered, with each cluster swaddled in small purple leaves (phyllaries) giving each cluster the look of a single urn-shaped flower, resembling flowers in the Blueberry Family. Wasps poke their faces into the opening of the urn.
Here is a brief clip of a wasp “working” the urn-shaped flower clusters…
- Mock Bishopweed, aka Herbwilliam (Ptilimnium capillaceum) in the Carrot Family has its delicate white flowers in umbels reminiscent to northeners of Queen Anne’s Lace.
4. Lax Hornpod (Mitreola petiolata) in the Logania Family was the topic of last week’s blog.
5. Bighead Rush (Juncus megacephalus) in the Rush Family loves mud. Its spherical seedheads host a moth larva whose identity I’m not sure.
6. Fragrant beaksedge (Rhynchospora odorata) in the Sedge Family is scattered throughout.
A mix of unrelated plants, and surprisingly small species count. The surrounding woody-swampy areas house probably 100 different species, so how does a seasonal mudflat filter the community to a dominant half-dozen plus a handful of others?
Let’s look at the environmental difficulties.
Difficulty 1. SEASONAL WATER EXTREMES. The plants have to cope with flooding half the year, then saturated soggy soil, then sun-baked drying mixed with intermittent deluges. How many species can transition from flood to desert? Few of today’s species actually live in standing water. Rather, they recolonize the mud each year by arriving anew via wind-borne seeds and/or persisting in the mud seedbank.
Dog Fennel and Sweetscent seeds (achenes) fly on parachutes. Although they may also survive in the soil seedbank, their seedlings seem scattered relatively randomly on the mud as opposed to showing a strong relationship to the receding waters. I vote for “new rearrival annual” as important for the parachute species.
All the others, by contrast, have small, hard, durable seeds or achenes easily dispersed by water but not by air, and presumably surviving in the pond bottom mud from year to year. Published research documents this ability for Mock Bishopweed’s seedlike fruits, which float.
This species seems to prefer germination in the natural “mulch” formed by dead aquatic species…out with the old, in with the new. Its tiny seedlings flower as toddlers, making sure of reproduction before the next flood. The flowers can self-pollinate, further assuring seed production before Noah time.
Lax Hornpod has the most conspicuous relationship to receding waters. On the wet mud within a foot or two of the water line its seedlings rise in abundance. With increasing distance from the falling water the older Hornpods diminish in abundance and increase in size. The pattern suggests that the seeds wait in the mud ready to sprout the moment the water recedes. It too is self-pollinated.
Difficulty 2. BAKING IN THE SUN Brutal sun exposure goes with the territory. Except for Sweetscent and Hornpod, all six species have narrow leaves or finely divided leaf segments adapted to minimizing sun exposure while maximizing wind-cooling. The filamentous leaves of Mock Bishopweed and Dog Fennel are almost identical:
Sweetscent has protective hairs and secretions. Hornpod has the most-“normal” leaf blades and may be the first to succumb to drying and heat.
Difficulty 3. HORRIBLE SOIL The plants live on sand mixed with fossil seashells. Submersion half the year interferes with “normal” soil microbes, with nutrient cycling, and with symbiotic mycorrhizae. Nutrients wash away during inundation. Wet soils generally have only a thin organic layer. Oxygen would be scarce much of the year. At some points the sandy soil is suffocating wet, and at other times bone dry.
Flooding, suffocation, radiation…No wonder only elite species cope!
The mud-dwellers have one apparent advantage…periphyton. Periphyton is the mat of algae, cyanobacteria, fungi, rotting detritus, sticky goo, and associated microfauna that floats in the shallow waters during the wet season and settles as a crust on the mud as the waters recede.
How does the mat impact the dry-season plants living on it? This is poorly studied, although a 2018 investigation by biologist Lu Haiying and collaborators considered this with respect to paddies. Among their observations, the periphyton crust:
- Is involved in nutrient cycling between the water and soil, retaining and releasing nutrients, sort of a green liver
- Improves the soil structure
- Serves as a slow-release bio-fertilizer
The periphyton may help overcome dormancy in seeds of some species. Just a guess, but an experiment to try, especially with Lax Hornpod. Periphyton probably blocks soil drying, and retains rainwater. Further, it dampens out temperature fluctuations.
All the nutrients captured, modified, or created by the algae and cyanobacteria during the wet months eventually become available slow release to the mud-dwellers.
Periphyton is loaded with Cyanobacteria, and many of these are nitrogen-fixers. The brief clip below shows the Cyanobacterium Oscillatoria wiggling in the stranded periphyton crust after rewetting (action speeded up a bit).
CLICK to see the dry periphyton resurrect.