This study was undertaken in the mangrove forests of Bunaken National Park which is located on the coast of North Sulawesi, Indonesia (between 1°35'41" and 1°16'44" N; 124°32'22" and 124°50'50"E). This Park includes mangroves growing along the north coast of the mainland as well as those on several off-shore islands. The largest of these is Mantehage Island.
The mangrove forests of Bunaken National Park are among the most distinctive and unusual in Southeast Asia, not only because of the species that the forests contain but also because of the large size of individual trees and changes to the ecological processes that are now taking place. In some places this has led to senescence and death of the larger trees (subsequently referred to as "die-back"). The dynamic changes to the mangrove communities in the study area are driven by two main factors: forest
maturation and human induced change through cutting mangroves. The first factor is because the climate and intertidal habitats have been stable for a long time allowing ample time for stands to be mature. The second factor varies from small to large scale, and includes both temporal and spatial changes. Unlike the first factor this human factor can be controlled by Park managers. Even so the effect on the mangrove ecosystems can be serious, ranging from the loss of particular species through to massive structural changes that can affect whole ecosystems if the disturbance is too frequent and too severe.
In this study all key factors which have been hypothesised to significantly affect the dynamic process occurring in the mangrove forests of Bunaken National Park were investigated. The level of investigation varied from a study of literature to long term experiments depending upon the level of detail needed, types of data required, as well as time and funds availability.
The three main objectives of the study were to: 1) assess the relationship between the distribution of mangrove communities and environmental conditions in Bunaken National Park, 2) examine patterns of "die-back" in relation to patterns of regeneration and growth, and 3) evaluate patterns of human use and the effect of these on the forest.
Mangroves within the National Park have developed under several specific climatic regimes. These include a high rainfall varying from 2,500 to 4,000 mm per annum and mean monthly temperature regime which does not significantly vary outside a range between 25.5 and 26.5°C.
At least 30 species have been collected from the mangrove forests of Bunaken National Park. This is more than the number of morphological types recognized by villagers and more than the number previously believed to be present. These are distributed across the intertidal environments of the National Park where there are at least
ten habitat types characterised by dominant physical processes of tidal inundation, water circulation with sediment transport and freshwater inflow, edaphic attributes including differences in soil surface texture, salinity, and field moisture content. On the segment of the Park located on Mantehage Island a some hundred years of physiographical change has resulted in the formation of at least three different mangrove habitats: 1) physically stable coral sand berm habitat, 2) habitat with deep and poorly drained substrate, and 3) habitat with less steep and shallow substrate. In the southern section of the Park on the mainland the process of physiographical change has resulted in the formation of an intertidal environment with a wide but deeply indented fringing reef, and a broadly scalloped landscape of the coast with regular shallow embayment between outcropping points.
Mangrove forests demonstrate some obvious changes over small spatial scales in the intertidal
environment of the Bunaken National Park. Because mangrove species have different physiological attributes it is not surprising that there is considerable variation in the identity of the dominant species and in the structural attributes of these forests across these environmental gradients. The life forms differ, and the structural formations range from tall-closed-forest with heights in excess of 30 m to low open-scrub that are less than 2 m tall. Eleven distinct mangrove associations were recognised within the extensive mangrove forests present on Mantehage Island and in the southern section of the Park. The impact of habitat conditions, tree cutting, and die-back is especially significant in several of these associations.
Over much the mangrove forests in the National Park, trees of S. alba were emergent if not dominant in the canopy. They were hypothesised to be the earliest mangrove communities in these mangrove forests. The fact that trees of
this species were subjected to die-back, that young trees are very rare, and that seedlings are absent, suggests the collapse of these early populations. This stage may be indicative of a forest successional process where the first generation is replaced by another that is, in this case, apparently different in species composition (dominated by trees of Rhizophora spp.). A different sequence is more likely to occur in the areas where B. gymnorrhiza and Rhizophora spp. trees are currently subjected to die-back. In these areas seedlings and saplings of these species are still present, so there will be no change in the species composition of the community.
People have a significant impact on the mangrove ecosystems of Bunaken National Park. Within the National Park mangroves are exploited for fuel wood, stakes for seaweed cultivation and drying floor construction, construction materials, temporary elevated shelters
(daseng), fish traps (sero) and other minor purposes. Conversion of the mangrove forest for shrimp ponds (locally called 'tambak') and tourism infrastructure has also been significant during the last two decades.
Several important changes are underway in the manner in which these mangroves are used. On Mantehage Island there has been a decline in the need for timber for cultivating seaweed and a decline in the collection of timber for firewood (caused by an increase in the use of kerosene). However, large amounts of timber are being sold in the nearby city of Manado. In the southern section of the Park timber are still harvested for seaweed cultivation and large Rhizophora spp. Bruguiera gymnorrhiza are ringbarked and large S. alba are logged for boat making. Significant areas of mangroves are also been cleared for tourism or shrimp ponds. Firewood collection
is carried out, most commonly in landward section of mangroves where access is easier.
The extent of forest disturbance was studied and the results suggest that: 1) many areas of mangroves have been subjected to human exploitation and this has affected the structural characteristics of the mangrove forests, 2) the intensity of tree removal varied between sites but has been greatest near the village of Rap-Rap, Sondaken, and Pungkol where people usually cut trees for seaweed cultivation stakes, 3) the evidence of natural tree death caused by suppression/competition is common in R. apiculata and R. mucronata; and the sizes of these trees varies from 19 - 25 mm, 4) the size of trees removal varies from small (normally Rhizophora spp.) to large trees (S. alba), 5) soil erosion has been extensive where tree harvesting has been most intensive, 6) the extent to which gaps created by logging are refilled
by seedlings or re-growth varies between sites, and 7) the recovery of gaps is slow in areas where felling is continuous and the habitat conditions do not support fast growth and the colonisation of new recruits; by contrast this can be fast in areas where Rhizophora spp. are dominant.
The process of regeneration in the mangrove forests of Bunaken National Park in relation to seed production and propagules availability and seedling establishment and growth was studied. The results suggest that: 1) all species of mangrove surveyed had periods of flowering and fruiting during the period of observation, although there was considerable variation in the periodicity and duration of both processes between the various species, 2) predation rates are significantly different among propagules of the four species tested with the highest rate being on the propagules of A. marina and B. gymnorrhiza; and grapsid crabs seems to be
responsible for these differences, 3) insect attack on propagules of Ceriops spp. and fruits of S. alba are significant, but insignificant for other species, 4) seedlings in most communities are representative of the canopy overstorey except for Sonneratia alba which are absent in all site conditions, 5) seedlings of Rhizophora are widely distributed from the seaward sites to the middle part of the mangrove forests, 6) seedlings of B. gymnorrhiza are rarely abundant but can also occur temporarily or permanently under canopies of different species, 7) seedling survival varies greatly between species and forest types, and 8) seedlings of Ceriops can be re-established in the usually saline habitats (landward areas) even after the site has been heavily exploited (Site 1 in planting trial), and the best way to do planting in this area is using new germinated or young
Finally, results in this study are also important for the improvement of the Park management strategy. Three key aspects should be taken into account if the conservation objectives for the maintenance of species diversity and other ecological values are to be achieved. The first relates to the fact that the rare species such as Camptostemon philippensis and A. rumphiana are present in the physically unstable habitats. These habitats have also subjected to heavier exploitation. Another fact is that several locations in the southern section of the Park such as on Tatapaan Island and areas near Tanjung Pasir Putih still contain old trees which are ecologically important. Ways must be found to protect these unusual forests. The second relates to the ecological processes underway in these mangrove communities. Careful monitoring needs to be carried out to follow the processes of development and regeneration, and habitat change.
The third is the need for resource substitution to reduce the intensity of mangrove exploitation, and the need to develop a proper and limited method for harvesting mangroves which will allow for the establishment of Ceriops species in the disturbed landward areas.