EPOW - Ecology Picture of the Week

Rainforests in the Ocean

Old cut rainforest trees
Libreville, Gabon, West Africa

Explanation:  What is going on in this photo?  This week we are strolling along a sandy beach by the Atlantic Ocean, just a few miles north of the equator in Libreville, Gabon, in west Africa.  Littering the beach are old logs cut from rainforest trees, washing ashore, sinking into the sand, and then washing back out to sea.  

These logs have resulted from cutting, but in times past such trees likely fell and more naturally washed seaward.  

What ecological roles might such large wood play in the ocean?   

Studies in temperate rainforests of the Pacific coast of North America suggest that large trees can provide a variety of functions in the ocean, including serving as habitat for many marine invertebrates and sources of nutrients.  Large wood in the ocean is decomposed largely by a few genera of wood-boring isopod crustaceans and by bivalve mollusks including shipworms (Gonor et al. 1988).  

Wood in the ocean can be transported by currents, and can be driven ashore in piles by wind and waves where they temporarily provide wind blockage and a place where sand and invertebrates can accumulate.  

Drifting trees play a role in erosion and deposition cycles.  In a rocky intertidal zone, logs provide surfaces for sessile plants and animals.  Floating logs in the open sea can serve as rafts for many small plant and animal species, transporting them where they may not otherwise be able to disperse.  When logs sink to the ocean floor, they add to the nutrient and energy base of the benthic community.

What such roles might large logs play in tropical environments, such as here in Gabon?  

Information:
     Gonor, J. J., J. R. Sedell, and P. A. Benner. 1988. What we know about large trees in estuaries, in the sea, and on coastal beaches.  Pp. 83-113 in: C. Maser, R. F. Tarrant, J. M. Trappe, and J. F. Franklin, eds. From the forest to the sea: a story of fallen trees. General Technical Report PNW- GTR-229.  USDA Forest Service, Portland OR. 
     Gorman, D., B. D. Russell, and S. D. Connell. 2009. Land-to-sea connectivity: linking human-derived terrestrial subsidies to subtidal habitat change on open rocky coasts. Ecological Applications 19(5):1114-1126.
     Maser, C., R. F. Tarrant, J. M. Trappe, and J. F. Franklin, eds. 1988. From the forest to the sea: a story of fallen trees. USDA Forest Service, Pacific Northwest Research Station, General Technical Report PNW-GTR-229. USDA Forest Service, Portland OR.
     Sedell, J., and C. Maser. 1994. From the forest to the sea: the ecology of wood in streams, rivers, estuaries, and oceans. St. Lucia Press, Delray Beach, FL. 200 pp.