As Robert Lugiewicz alluded to in his column last week, spring is in the air and that means lots of babies are being made. On my last trip out to Charlotte Harbor from my house on Alligator Creek, I couldn’t help but notice all of the red mangrove propagules.
Propagules, for those not familiar, are the long seed pods red mangroves produce. They look a bit like string beans hanging from the tree. The other mangrove species produce propagules too, but they’re not nearly as prominent.
You might wonder why we call seed pods propagules. Well, we do so because propagules are really more than just seeds. They’re an embryo that has already germinated. Think of it as a pre-sprouted seed. This is one of the many adaptation strategies mangroves rely on to survive in a salty, tidally influenced environment.
Mangroves reproduce sexually via small flowers that are pollinated by a variety of animals including bees, butterflies and bats. The resulting offspring grow for several months attached to the parent plant. This method of reproduction is known as vivipary. You may have some viviparous plants in your landscape, such as walking iris and Agave desmettiana.
In mangroves, this strategy aids seedling success by facilitating the development of salt tolerance, nutrient storage, buoyancy, and quick rooting before release from the parent tree.
After being released from the parent tree, mangrove propagules are dispersed in the water for a few days to several weeks before developing roots and becoming established in the sediments. Dispersal of propagules depends on factors such as tides and currents, as well as the durability of propagules and their ability to remain buoyant. Propagules can remain viable for several months prior to establishment — as long as they remain in seawater.
Propagules that survive dispersal are subject to high mortality during establishment and early growth. Failure to establish, predation, and drying out are primary causes for early mortality. Survival and growth of seedlings that do establish is largely dependent on factors such as extent of shading, seedling orientation (e.g., upright vs. horizontal), soil richness, and the degree to which they are flooded by tides.
Back in 2004, when Hurricane Charley hit our area, Charlotte Harbor’s mangrove shorelines were significantly affected. It was estimated that we lost close to 100 percent of our fringing red mangroves. For the next several years, the word propagule was the buzz word at my office. My co-workers liked the way the word flowed off the tongue. They heard the word a lot because I was leading a massive effort to restore the Harbor’s mangroves with propagules.
Many of our local boating, fishing and environmental organizations were involved in the effort, which restored about 4 miles of shoreline along the west wall and several of the outer islands along the eastern shore of Charlotte Harbor. Volunteers collected, sorted, painted and counted propagules before stuffing them into bags. More volunteers planted mangroves or dispersed them in the shallow waters during an incoming tide. I monitored these areas for two years to evaluate restoration success.
I gave a lot of talks back in those days to update folks on the progress of our efforts, but every so often someone asks how we did. Well, we did great! I compared areas where we planted and areas where we dispersed to areas where we did nothing. Our study design actually had four plots of each (plant, disperse, nothing) to remove any site bias. For the restoration we dispersed twice as many propagules than we planted, since we expected greater mortality with that method.
At the end of the monitoring, the results showed a statistical difference between the treatments (planting and dispersal) and the control (do nothing), but no statistical difference between the planting and dispersal treatments. Lesson learned: In all cases, doing something was better than doing nothing. And even though we used twice as many propagules in the dispersal treatments, tossing them from a boat idling along shore during an incoming tide is far easier than trampling through dead brush to plant them.
Research studies indicate that mangrove forests do have the ability to self-restore when given sufficient time (15 to 30 years). It’s now been 16 years since Hurricane Charley, and ours looks pretty good. I’d like to think the efforts of this community restoration project helped put us on the low end of that timeframe.
Betty Staugler is the UF/IFAS Extension Charlotte County agent for the Florida Sea Grant Program. She is active in many areas relating to boating, fishing, and watershed/coastal living. Sea Grant supports research and education activities that help Florida’s shoreline communities, industries and citizens wisely use the state’s coastal and marine resources. Contact her at email@example.com or 941-764-4346.