Effects on Plant Collections due to High Tides and Flooding

During the month of September we experienced one our annual highest tides, sometimes called the King Tide. These are normally occurring tides throughout the months of September, October, and November during the full moon phase. This year, it was coupled with low-pressure systems bringing strong northeast winds that impacted the flow of the Gulf Stream, causing it to back-up water in the Keys and Florida Straits. There have been numerous flood warnings this fall and there has been plenty of footage in the news of the flooding in Ft. Lauderdale and Miami Beach.

Due to the unique, porous geology of the limestone in south Florida, and especially in the Keys, which have formed on an ancient coral reef, we cannot shore up our waterfronts with sea walls and levees; the water will eventually come up from below. We also have very flat topography so once the sea rises, it will not matter how far inland you are. What do botanic gardens do in this situation? How do we effectively preserve our collections in the face of sea level rise?

Currently, we have many native and exotic salt-tolerant plants on our Florida Bay waterfront. Aside from sea level rise and extreme high tides, this environment is tough for plants in general with salt spray during winter storms that blow in from the north and full, hot sun throughout the summer. Salt-tolerant plants, however, are just that, ‘tolerant’, and some of our plants may soon reach their thresholds. One potential casualty from last month’s high water is our Argusia gnaphalodes, sea-lavender or beach heliotrope.

Our sea-lavender after this summer's high tides

This evergreen shrub is highly salt-tolerant and extremely drought-tolerant once established. Our specimen is over 13 years old and has a 5-foot, gnarled trunk with branches sweeping down to the ground. Apparently, beach heliotrope is difficult to establish so I feel lucky to have such a mature specimen in our collection. Unfortunately, all the leaves fell off after the high water and we are waiting to see if it will regenerate. A similar situation occurred in 2011 and our BG-Base records indicate that it “came back well after storm damage” so my fingers are crossed.

Sea-lavender regrowth after 2011 storm damage

 Other salt-tolerant species we have along our shoreline are Coccoloba uvifera, sea-grape; Hyophorbe verschaffeltii, spindle palm; Jacquinia keyensis, joewood; Pandanus utilis, screw-pine; Serenoa repens, saw palmetto; Sesuvium portulacastrum, sea purslane; Uniola paniculata, sea oats; and all four native mangrove species: Avicennia germinans, black mangrove; Conocarpus erectus, buttonwood; Rhizophora mangle, red mangrove; Laguncularia racemosa, white mangrove.

Not only are we concerned with protecting our botanical collections, but more importantly, we need to protect the rare plant populations in the wild that are being affected by sea level rise. The inhospitable conditions produced by salt-water make it impossible for many species to survive in their present locations. Even common plants are unable to deal with excess salt, their seeds unable to germinate, they will eventually be displaced by mangroves or other salt-tolerant species. In this case, botanic gardens and nurseries are their only hope for continued propagation.

In the Florida Keys, we have many rare species that have been propagated for years by local conservation institutions such as Fairchild Tropical Botanic Garden and a handful of local nurseries. Regional watchdogs like the Institute for Regional Conservation have performed baseline inventories for monitoring rare plant populations so we know how they are doing over time. Without human intervention, these plants and their habitats will disappear and we do not fully understand the implications of this on a local level, and much less on a global scale.

After each storm and high tide, as I notice the multitude of mangrove propagules deposited along the shoreline, I wonder how long it would take for the trees to return our developed shoreline back into mangrove forest. I guess we’ll see over the next 30 years! Hopefully, we will be able to change the way we live and use our resources more sustainably so life in coastal areas may continue and we will not have to relocate our garden to the Lake Wales ridge – the previous shoreline of Florida.

Emily B. Magnaghi

Associate Director