Monday, 8 September 2014

Did Mangroves Evolve from Beach Vegetation?

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.
Hibiscus tiliaceus, Terminalia catappa on foredune
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.

Abnormal cyclone induced high tide
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.

Fiddle leaf fig leaf litter after brief salt water incursion
A park tree (Ficus lyrata) dropped its leaves due to a short period of saltwater inundation.
Paperbark swamp affected by seawater
Seawater gets into some freshwater swamps but can't get out and kills all the vegetation.
Sesuvium carpet around saline pool
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.

Terminalia catappa
A beach almond dropping leaves after saltwater inundation.
Carnavalia rosea after a cyclone
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.  
Cardwell cabbage affected by salt
Beach Cabbage (Scaevola taccada) with signs of salinity stress
White spider lily affected by salt water
Crinum lily damaged by exposure to seawater
Severe wilting of Cordia subcordata seeding on a sand bar in a mangrove swamp.
However not all strand trees are impacted as severely by salinity.  Some species hardly seem to notice.  Beach casuarinas are a bit special.  They drip concentrated saltwater from their leaves and are perhaps the only sandy soil specialist to easily cope with high loads of salt.  Most other species which cope with seawater inundation can also survive on riverbanks with dense silty soils and clays so I think there is a link between soil type preference and reaction to salinity.  Beach hibiscus (H. tiliaceus) , Portia trees (Thespesia populena) and Pongamia (Milletia pinnata) keep on growing until the sea physically washes them away. In one small section of swamp, I counted more than twenty living Portia trees suspended by mangrove roots.  They can almost teach mangroves tricks about living in salt water.
Beach casuarina beside sea
Beach casuarina growing at approximately neap high tide level.
A Pongamia tree survives where other trees have perished.
Thespesia populnea
A leaning Portia tree perched in the mangroves well below spring high tide level.
Thespesia populnea
Portia trees continue to grow and fruit despite exposure to the sea. 
In summary, salinity has dire effects on most vegetation.  However some species seem to cope easily with high levels of salinity and have seeds dispersed by sea, yet are not mangroves.  My conclusion is that most species that are specialised for growing in clean sand are likely to have adaptations that seem to make them susceptible to salinity. Mangroves are unlikely to have evolved from strand species.  A few of the species present in strand vegetation also grow around the terrestrial margins of salt flats or adjacent to the landward zones of mangrove swamps where soils are heavy silts and clays.  These species handle seawater inundation better.  I suspect that mangroves evolved in river deltas, rather that from beach species that crossed over the high tide line.

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