Sand stabilising plants

Colonising vegetation on Bird Island’s northern beach (Photo: Chris Feare)

While Bird Island’s sands are highly mobile (see blog post of 23 June), within seasons some areas of beach can remain relatively unaffected by wind, tides and currents. Some plants have evolved to capitalise on this by producing fruits or seeds that survive well in seawater and are dispersed by it. These are the first colonisers of open sand that remains fairly static for some time at the top of island beaches. On Bird Island the main pioneer woody species are Scaevola taccada (Veloutye in Seychelles Creole, Beach Cabbage often used as an English name), Suriana maritima (Bwa Matlo, Bay Cedar) and Tournefortia argentea (Bwa Tabak, Tree Heliotrope), all of them tolerant of salt spray and nutrient-poor coral sands. Additional pioneers are the herbs Portulaca oleracea (Kourpye, Sea Purslane), Boerhavia repens (Patat covan, Spiderling), Cyperus pachyrhiza (a beach sedge) and an unidentified grass.  Young plants can form a scattered assemblage at the top of the beach but the entire community is vulnerable to being washed away when the seasons change.

Scaevola taccada
Tournefortia argentea
Suriana maritima
Pachyrhiza sp.
Portulaca oleracea

 

 

 

 

 

 

 

 

On Bird Island’s northern beach, however, ridges of sand have built up over several years and have so far proved immune from erosion by high tides. This has allowed the pioneer plant community to develop further into a well-established bushy region interspersed with a ground layer of some of the herbs. This region is adjacent to the Sooty Tern colony and from May to October overflying birds will have deposited a rain of droppings offering a nutrient input of marine-derived organic matter to the sand. Furthermore, last year the Sooty Tern colony spread into the new vegetation with birds nesting between and under the bushes and the trend has continued this year. The birds will undoubtedly contribute further marine nutrients although accumulation in the surface sand will be limited by leaching through the porous sediments. Nutrient enrichment will also be provided by leaf litter from the growing plants. Further vegetational developments seem likely and already a few germinating Coconut (Cocos nucifera) and other coastal plants of Seychelles islands, such as Cordia subcordata (Porse, Kerosene Wood) and Thespesia populnea (Bwa de Rose, Indian Tulip Tree), have taken hold.

More mature Scaevola, Suriana and Tournefortia bush being occupied by beach-nesting Sooty Terns (Photo: Chris Feare)

While the roots of this vegetation help to stabilise the surface sands of the upper levels of beaches, we must remember that all of the beaches on sand cays are prone to erosion at some stage and all of this could disappear in the event of a large storm surge. Rising sea levels doubtless increase the risk of this. Even if this happens, however, the development over the last few decades on Bird Island has shown that newly deposited sand forms a substrate ready for colonisation by plants whose seeds float in the sea surface and are ready to take advantage of any new opportunities.

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