Mangroves are not the only trees with floating seeds that are dispersed by sea. Plants growing in tropical sandy foredunes also have floating seeds and fruit. The most famous of these plants is the coconut. Despite growing side by side with mangroves, there seems to be a strong ecological barrier that prevents plants from the foredune from evolving into mangroves. Others have noted this barrier but there is only vague speculation on what the nature of this barrier is. In this post, I will investigate the barrier between mangroves and beach vegetation.
|Strand vegetation near Cairns with taller dark green littoral rainforest behind|
I refer to vegetation dominated by plants with seeds that are dispersed by sea as strand vegetation. Wikipedia defines strand vegetation as ‘a plant community of flowering plants that form along the shore in loose sand just above the high tide line.’ It goes on to describe the key features of this habitat. “Many plants that grow in this area are endemic to the strand. The community has low species diversity because so few plants can tolerate the harsh conditions of high winds, battering salt spray, and extreme high temperatures in the summer. Plants must also be adapted to sandy saline soils, with extremely low nutrient loads, and low water holding capacity.” Although this definition was crafted for North American strands, the term strand vegetation is also used in the same way from Africa to Asia. In Australia, the term foredune vegetation is often used, however I tend to associate this term with the high energy surf beaches and not the relatively calm tropical coasts. There are several distinct vegetation communities that occur beside the high tide line, so vegetation community names should be used with care. For example, strand forests often occur beside littoral rainforests but littoral rainforest trees are almost all bird dispersed and littoral rainforests create and grow on soils rich in humus and depend on efficient nutrient recycling.
Nature provided a natural experiment which helped me to see the barrier between mangroves and beach vegetation. Some recent weather events effectively raised the normal high tide level by twenty or thirty centimetres and the strand vegetation was subject inundation with sea water. Cyclone Dylan came to visit in January 2014, however Cairns being on the northern side of the system had offshore winds and the sea was quite calm save for a low swell. The low atmospheric pressure associated with cyclones can lift the surface of the sea by approximately one metre. In the Cairns area, the cumulative effect of the lower air pressure and high spring tides was to raise the sea by approximately 30 cm above normal (~3.6-3.8 m AHD) and the lower parts of the foredune were inundated.
|High sea levels associated with Cyclone Dylan and swells gently swashing over the foredune|
Some low lying coastal parks and some freshwater swamps were also inundated. In the parks, fig trees dropped their leaves from the shock of salt water. Freshwater swamps usually occur in basins such as old billabongs and swales. As the seawater has trouble draining back out of the basins, many of the freshwater swamps were badly or permanently damaged with large paperbark trees and groundcover vegetation being killed. Many of these swamps will change into mangrove swamps with time, especially as erosion of the barrier between the swamp and sea is often reduced by these events.
|A park tree (Ficus lyrata) dropped its leaves due to a short period of saltwater inundation.|
|Seawater gets into some freshwater swamps but can't get out and kills all the vegetation.|
|A freshwater swamp turning into a mangrove swamp, Redden Island.|
It turns out that even though strand plants live by the sea, most of them are also sensitive to saltwater inundation. Many species drop their leaves. In mangroves, salt is accumulated in old leaves before they are dropped. I am not sure that this is the case with other species.
|A beach almond dropping leaves after saltwater inundation.|
|Beach creepers were complete defoliated.|
Cyclones are not the only reason plants suddenly find themselves exposed to seawater. Sometimes erosion removes the land from around the trees. Other times, plants become established on low lying ground and grow for months before encountering very high tides and rough seas from trade winds that put swash across the beach.
|Beach Cabbage (Scaevola taccada) with signs of salinity stress|
|Crinum lily damaged by exposure to seawater|
|Severe wilting of Cordia subcordata seeding on a sand bar in a mangrove swamp.|
|Beach casuarina growing at approximately neap high tide level.|
|A Pongamia tree survives where other trees have perished.|
|A leaning Portia tree perched in the mangroves well below spring high tide level.|
|Portia trees continue to grow and fruit despite exposure to the sea.|