The World Atlas of Mangroves second edition is a publication compiled in joint support from ITTO, ISME, FAO, UNESCO-MAB, UNU-INWEH, UNEP-WCMC and The Nature Conservancy that brings together up-to-date mangrove information from all around the world with updated mangrove distribution maps and data. One of the most exciting section of this publication is chapter one which I have briefly summarized so those new to mangroves like me can understand what makes mangroves unique.
Mangroves are the most spectacular colonizers of the intertidal zone: which is the space between the low and high tides that is characterized by its openness to the vagaries of both land and sea – powerful storms and heavy rains, high salinities and drought, shifting sediments, inundation and exposure but also a rich environment with many rewards. The special feature of mangroves is their ability to continuously develop adaptation strategies in morphology, physiology and reproduction so as to survive this harsh ecological niche.
Mangroves have a number of characteristic features that arise from their adaptation to this environment. First, in coping with salinity, mangroves are more salt tolerant than terrestrial plants but most exclude salt in order to survive. This is through their xylems using some degree of physical ultra-filtration at the endodermis of their roots. Some remove salt from their internal tissues by depositing them in the bark of their stems and roots while others dump them on their leaves. Others have special salt secreting glands to aid salt removal from their tissues.
Secondly, is their ability to transport oxygen to the roots in waterlogged and anaerobic soils by developing aerial roots e.g. the stilt roots, pneumatophores, knee roots and buttress roots. These roots have abundant pores to enable gaseous exchange on the surface of the roots and are connected to fine systems of aerencyma tissues through which air is able to diffuse easily. Their ability to close the pores (lenticels) when submerged (the diffused oxygen is used up) and open them when the tide recedes leads to reduced pressure within the roots hence air is sucked into the roots faster.
Lastly, during propagation mangroves invest heavily in producing seeds and fruits to increase their chances of survival in the harsh intertidal zone environment. Mangroves also use the water and the flushing of the tides to disperse their offspring. Some mangroves have developed a special adaptation strategy where rather than producing the seeds and fruits, they release an already growing plant (vivapary). Their varied ability to survive different condition is also vital in their propagation process where species that are less well adapted to inundation live at the higher edges of the tidal frame while others can only survive near the inundation. Some species can only be found in the upper estuarine locations because they cannot survive salinities equivalent to pure sea water.
The World Atlas for Mangroves second edition can be accessed on: http://www.amazon.com/World-Atlas-Mangroves-Mark-Spalding/dp/1844076571