The human race depends on forests, trees and other vegetation for its survival and well-being. Women, men and children are attracted and attached to trees, shrubs, herbs and other vegetation for various reasons and purposes. Some trees are culturally valuable and some others are important in terms of social norms and beliefs as well as traditional systems but many of them are essential to satisfy basic human needs such as food, shelter, clothing and employment. They also play an important role in safeguarding environmental integrity. In an atoll environment like the Maldives, they are also important for reasons such as stabilization of sand and protection against salt spray. Trees and shrubs also play a critical role in reducing the impact of natural calamities, such as tidal waves and tsunamis on human lives and properties (Danielsen et al., 2005; Selvam, 2005).
As in many small islands, vegetation in the Maldives has changed both quantitatively and qualitatively over time due to overexploitation by increasing human populations, unsound land use practices, poor land tenure policies and intentional and unintentional introduction of exotics and commercial species (Wills and Gardiner, 1901; Zuhair, 1997). Such changes have made the islands of the Maldives, their ecosystems and human populations more vulnerable to natural calamities such as cyclones, tidal waves and tsunami and man-made calamities such as rising sea levels.
Taking these facts into consideration, this book on "Trees and Shrubs of the Maldives" aims to improve awareness of the trees and shrubs of the Maldives and their ecological importance; provide an overview of their local uses and potential role in increasing the economic security of Maldivian communities; and outline propagation and management techniques for their cultivation.
The Maldives is a large archipelago of 1190 coral islands, spreading over 860 km in a north-south direction in the Indian Ocean and covering an area of 90,000 sq km. Only 202 of these islands are inhabited. The islands are grouped into 26 natural atolls and 19 atolls for administrative purposes (Fig. 1). These atolls are situated atop a 1600 km long undersea mountain range called as Laccadive-Chagos Ridge, which extends into the Central Indian Ocean from the south-west coast of the Indian subcontinent. Most of the atolls consist of a ring-shaped live coral reef supporting numerous islands. Most of the islands are small and vary in size between 0.5 and 5 sq km. They are flat and without hills or rivers. Nearly 80% of the land area is less than 1 m above mean high tide level (MHAHE, 1999).
The climate of the Maldives is equatorial, warm and humid with two pronounced monsoon seasons, the south-west and the north-east monsoon seasons. The temperature is fairly constant throughout the year with a mean annual temperature of 28°C. The average summer temperature ranges between 26.3 and 31.8°C and winter temperature between 25.1 and 30°C. The diurnal variation is very small, rarely exceeding 6°C. April is the hottest month with an average temperature of 30.8°C and October is the coolest with an average of 25°C. Relative humidity is high throughout the year, ranging from 73 to 85%.
The annual average rainfall in Maldives is 1890 mm. The rainfall in the southern atolls is greater with an annual average of 3050 mm, whereas it is only 1520 mm in the northern atolls. The south-west monsoon, which extends from the end of April to the end of September, brings heavy rain to the entire archipelago. The rainfall decreases considerably during the north-east monsoon season that prevails from December to March and during this season periods of drought may be experienced, particularly in the northern group of islands. However, the weather patterns of the Maldives do not always confirm to the monsoon patterns of South Asia. For example, heavy rain over the whole country has been known to occur continuously for up to one week even during the midst of the dry season.
The Maldives is outside the main area of tropical cyclones and therefore gales are uncommon and cyclones are very rare. However, during the south-west monsoon season strong winds and storms may hit the archipelago and can cause severe damage. On average, it is reported that thunder storms hit the Maldives on 23 days per year and strong winds on 12 days per year. In May 1991 tidal waves, created by violent monsoon winds, caused damage to thousands of houses, jetties and piers and flooded arable land with seawater. The damage caused was estimated at US $30 million.
Fig. 1. Map of the Maldives
The soils of the Maldives are geologically young and consist of substantial quantities of the unweathered coral parent material, coral rock and sand. In most of the places, soils are coarse in texture and shallow in depth with a top layer of brown soil (0 to 40 cm in depth) followed by a transition zone on top of the underlying parent material of coral reef limestone (MFAMR, 1995). In some low-lying areas and areas subjected to significant mechanical breakdown from human activity, fine deep soils are found with accumulated deposits of clay. In a lagoon environment (locally called kulhi) the depth of the clay may be substantial due to the accumulation of material from marine and biological sources over a long period of time (MEEW, 2006). In many places, top layers of the soils have a weakly developed structure and at times a 30 cm thick hard-pan layer cemented with calcium carbonate is present, preventing penetration of the roots of most plants except large trees. The water-holding capacity of the soil is very poor due high porosity and very high infiltration rates.
The soils of the Maldives are generally alkaline with pH values between 8.0 and 8.8. This is mainly due to the presence of excess calcium and, soils containing higher levels of humus, as in depressions and lagoons, are less alkaline. The soils are generally poor and deficient in nitrogenous nutrients, potassium and several micronutrients particularly iron, manganese and zinc. Though the phosphorus content of the soils is high it is present mostly in the form of calcium phosphate and, thus, remains unavailable to plants.
Though the climate of the Maldives provides ideal conditions for luxuriant growth of tropical trees and shrubs, other factors such as salinity, the highly calcareous nature of soils and the salt-laden winds create harsh environmental conditions. This is one of the main reasons why the number of species in the Maldives, either native or naturalized, is limited.
The islands of the Maldives can, in general, be divided physiographically into three zones namely, i) the foreshore or lower beach, ii) the beach crest (beach top) and iii) the inner island. The foreshore can be further divided into high tide and high-storm levels. The high tide level is normally located at an elevation of 0.5 m above mean sea level and high storm level, which is beyond the reach of normal tides, is located at about 0.8 to 0.9 m. The storm level is affected by storm waves and is composed of gravel or shingle. The average elevation of the beach crest is about 1.2 m and the inner islands are at about 1.45 m above mean sea level (Morner et al., 2003). Each of these zones provides relatively uniform environment with its own associated plant community. Plant community found in different physiographic zones of the Maldives is more or less similar to plant association reported in Nukunonu Atoll of Western Samoa (Parham, 1971).
i) Plant communities of the foreshore
The foreshore or lower beach zone, which includes the beach area between the high tide line and the beach crest, is totally exposed to wave action, wind and salt spray. It is unstable and composed mainly of coarse coral sand in the lower portion and shingle. As a result of the harsh environmental condition, this zone supports no vegetation except occasional creeping sand-binders such as Ipomoea littoralis and I. biloba along with a few individuals of Launaea pinnatifida and Portulaca alata in the upper portion.
ii) Plant communities of the beach crest
The beach crest or beach top rises gradually and sometimes abruptly to a height of 0.8 to 1 m above the high tide line and includes a stable beach frontage composed of coral sand and rubble. Like the foreshore environment, it is also exposed to winds and salt spray and its lower margin is occasionally or, in the case of an eroding beach, regularly inundated by seawater during spring tides. The beach crest may extend 5 to 20 m inland and provides a suitable environment for strand plant communities including a distinct association of trees and shrubs and a few sand-binding creepers and herbaceous plants. These strand plant communities include:
the Scaevola taccada scrub community, which forms an effective windbreak of about 3 to 4 m height on the seaward side of the islands immediately above spring tide level. It is normally found on sandy soils or soils dominated by coral rubble. It is the most common scrub community found on beach crests of both northern and southern islands of the Maldives.
the Pemphis acidula scrub community, which is commonly found on elevated reef rock, coral conglomerate beach rock or hard pan coral in open sites at or above the high tide level. Pure stands of closely growing Pemphis acidula trees, which are impenetrable, can be seen in these areas and it is usual for the roots of these trees to be regularly wetted by seawater during high tide. In sandy areas Pemphis acidula can also be seen growing in association with a similar looking plant, Suriana maritime. These areas may have coral rock at very shallow depths.
the Tournefortia argentea community is found as a dominant strand community of the beach crest particularly in drier places in some of the northern islands. It is located very close to or just above the high tide line and may not form an effective windbreak as the trees do not grow closely together. It is sometimes associated with Pandanus tectorius and Scaevola taccada.
the Guettarda speciosa community is normally found only on highly elevated beach crests and is characterized by the presence of other species such as Scaevola taccada, Pandanus tectorius and a scattering of Pisonia grandis and Cordia subcordata trees.
iii) Plant communities of the inner island
The microclimate of the inner islands, protected by the beach-crest communities, supports the growth of a number of trees and shrubs, which occur either in pure stands or as a mixed forest (Forsberg, 1957). In many islands coconut plantations are present immediately adjacent to beach-crest vegetation and in moist areas the shelter provided by a complete coconut tree canopy supports the growth of under story tree species such as Morinda citrifolia and Guettarda speciosa. In some places, Pandanus odoratissimus, Calophyllum inophyllum and Hibiscus tiliaceus are also found in low numbers within coconut groves (Forsberg, 1957). In some other, particularly moist, areas small pure stands of Hernandia nymphaeifolia, Cordia subcordata and Barringtonia asiatica are present. In drier places including the northern group of islands, pure stands of Hisbiscus tiliaceus and Premna serratifolia are also seen. Where extensive coconut plantations are not present mixed species forest is the most common vegetation type found next to beach-crest scrub community. The principal tree species in these forests are Pandanus, Hibiscus tiliaceus, Cordia subcordata, Hernandia nymphaeifolia, Calophyllum inophyllum, Barringtonia asiatica, Ochrosia oppositifolia, Guettarda speciosa, Adenanthera pavonina and Terminalia catappa. These mixed forests also support good growth of under story species such as Allophylus cobbe, Morinda citrifolia etc. No regular features in terms of the dominance, frequency or density of tree and shrub species are prominent in the mixed forests. In many islands the original distribution of trees and shrubs has been greatly disturbed by the establishment of extensive coconut plantations. As a result, beach-crest scrub communities and mixed forests are only found up to a short distance from the shoreline in many of the islands before merging into coconut plantations. As described in the species fact sheets, most of the trees and shrubs present in the beach scrub community and mixed forests are tolerant of salt-laden winds, salt spray, soil salinity and shallow nutrient-poor soils.
The above description of the plant communities of the Maldives islands and the overview of the ecology, propagation, management and economic uses of different species given in the following fact sheets provide a background to the opportunity that exists for the establishment of multi-tiered multispecies coastal bioshields or green belts. Such bioshields are essential for the ecological security of the Maldives islands and the economic security of the Maldivian people in light of future coastal hazards and predicted increases in sea levels.