The Conservation of Biological Diversity in the Coastal Lowlands of Western Samoa

 

1992

 

Department of Conservation, New Zealand

 

 

 

2.0 A'OPO-LETUI-SASINA COASTAL FOREST (Site S-9)

 

2.1 Introduction

 

A primary aim of conservation is to ensure that the ecosystems and landscapes that best typify the nature of a country, and are unique to it are protected. Western Samoa is made up of a diversity of environments. No single environment can be said to typify the country. But the one that perhaps best captures the essence of the natural ecology of Western Samoa's coastal lowlands, and illustrate how the country's distinctive rainforests develop, is the succession of vegetation of different ages associated with recent volcanic activity.

 

No single locality contains a better assemblage of these ecosystems than the mosaic of lava plains on northern Savai'i between the coast and the villages of A'opo, Letui and Sasina. A conservation area spanning the lowlands between A'opo and Sasina villages, bounded by the sea on one side and the main road on the other, would contain an excellent representation of the mosaic of the younger kinds of lava-flow forest vegetation and have excellent potential for long-term conservation of a very important component of Western Samoa's biological diversity. Geologically, Western Samoa is a young country. Its two main islands are shield volcanoes consisting of basalt characteristic of the Pacific basin and derived from the magma material beneath the earth's crust. The entire coastal lowlands are formed, almost exclusively, by successive lava flows.

 

In a tropical environment such as this, soil weathering and forest development proceeds rapidly on the young lava flows. But their flat and undulating topography renders them readily accessible to human activity. Consequently, although the succession of young forests that develop on these land forms are probably the most characteristic of all Western Samoa's lowland forests, only a few of them are represented in the existing protected areas; in the national park, O le Pupu Pu'e and the new forest preserves at Falealupo and Tafua where conservation agreements with villages have been negotiated. But beyond these areas little opportunity to safeguard a greater range of lava flow ecosystems now remains.

 

Once common and widespread throughout the country, high quality stands of lava plain forest ecosystems are today few and far between. The modern network of roads and the forest clearance that follows it, has led to the situation where, although examples of most of the different kinds of forest still occur, not one intact, contiguous sequence of the mosaic of lava flow ecosystems from the coast to the interior mountains still survives.

 

There is nowhere in Western Samoa where the full range of land surfaces, and the different stages of vegetation development, from the most recent lava flow to the oldest of the advanced stages of land dissection, and soil and forest development can be conserved in one geographic area. Instead a wide geographic range of several protected areas is necessary to fulfill this objective.

 

The vegetation of the coastal lowland between A'opo and Sasina villages strongly reflects the fact that this part of Savai'i is in the area of Western Samoa that has the most pronounced dry season. Nonetheless, the fact that i t has the most representative range of the younger series of lava flow ecosystems of any single area in the country means it is of great conservation significance.

 

Most of the area proposed for conservation would not conflict with potential land development for agriculture. The land is simply not suitable for that purpose. However the area has potential for forestry plantations of dry-zone timber species that could dramatically alter the natural ecological qualities for which it is identified as a conservation priority. Furthermore, at a time of impending national shortfall in indigenous timber supplies, it contains accessible stands of valuable timber species. I f they were logged the conservation value of the area would be seriously compromised.

 

The area proposed for conservation has two main components: The western portion, mainly belonging to A'opo village, is a broad, and only very slightly weathered recent lava flow of open vegetation; rockfields, scrub, vine thickets and low forest. with a few islands, or kipuka, of older, more weathered lava not overrun by it. The younger lava flow is derived from the historic A'opo Volcanics that erupted about 1760. Although now well covered with young forest and scrub, the lava has an almost undissected surface. The soils are classified as "A'opo loamy sand", but there is in fact very little soil development, nor is there a water supply adequate to support any kind of settlement or permanent agriculture. The native vegetation is thus largely free from active human modification and thus from the exotic weeds that commonly occur elsewhere in these lava plain forests. It continues to develop, in excellent condition, with its natural state virtually intact.

 

Access to the area is via A'opo village's fishing track to the sea coast at a former village site, Anini, and encounters the full range of vegetation. The track passes through two kipuka of forest vegetation on another young lava surface of Pu'apu'a Volcanics. This dates from considerably earlier than 1760.

 

No detailed work has been done on the chronology of forest development in Western Samoa, but the young age of most of the ecosystems in the A'opo-Sasina locality can be determined from the historical evidence that one flow occurred about 1760 and the fact that adjacent, older lava flow surfaces are still thinly clothed in forest of only small stature trees. Before the 1760 eruption, the A'opo coast west and east of Anini was known as "the coast of one hundred villages." It was apparently fertile and densely populated before the lava overran it. Traces of only a few of these abandoned villages remain. They are said to have been small hamlets rather than villages; a, very different pattern of settlement to the villages that has prevailed in post-Christian times.

 

The eastern area, mainly belonging to Sasina and Letui villages is dominated by the older Pu'apu'a Volcanics. It is mainly in young lowland forest, some of it very impressive, including some of the last stands in Western Samoa of the tree traditionally most revered for its hard timber, ifilele (Intsia bijuga). These stands are readily accessible from the fishing track from Letui to the sea coast at Avaevai and the dry bed of the Foa River near Sasina. They continue to be sought by milling interests for their timber.

 

Birds play a vital role in the successional development of the rain forest on these lava flows, and any conservation proposal for these ecosystems must depend on the extent to which their habitats within the area and beyond i t can be safeguarded. At present the youth and generally dry and scrubby nature of much of the lava flow vegetation does not allow for the levels of bird diversity that can occur in more well-developed forest. Nevertheless, it is largely by birds dispersing seed, the larger fruit-eating species especially, that future succession to well-developed rainforest will occur. It is important to note that some of the rarer forest birds, notably the tooth-billed pigeon, were seen flying inland over the lava flows during this survey. Furthermore, the upper lowland forests inland of Letui and A'opo, to which these birds were presumably flying, contained the some of the highest counts of bird diversity and abundance of any area sample in Western Samoa. It suggests that the lava flow vegetation, despite its youth and the relative absence of the larger fruit-bearing trees, is quality food habitat for some species. It also indicates that the greatest diversity and abundance of birds occurs where there is a wide range of feeding habitat. This could prove to be crucial for the conservation of some of the forest birds like the tooth-billed pigeon whose future is causing international concern.

 

The forests on the lava flow are hunting areas for pigeons, and tracks through the area provide the only access to the coast for the gathering of sea foods by A'opo and Letui villages. While the lava flow ecosystems contain little potential for agriculture, they are nonetheless important economically, in their natural state, for the local villages. Any conservation agreement will need to integrate this with the objective of conserving an area for its natural ecological diversity. The area also contains a range of medicinal plants and continues to be used as a source of traditional medicines.

 

The whole area's size, its splendid and accessible lowland forest, the extent to which its ecological integrity is unmodified by destructive human activities and its distinctive Samoan character provide it with considerable potential for nature tourism if the area was nationally and internationally promoted. The villages to which it belongs could co-ordinate such a venture with other areas in Western Samoa that have similar potential. Similar lava flow landscapes in the Hawaiian national parks have considerable visitor appeal for extensive nature walks and the interpretation of volcanism.

 

The villages of Sasina and A'opo are aware of the conservation values of their land that has been surveyed by this study. Matai of Sasina have discussed means of protection with government conservation officers. The supply of water to these villages would give considerable incentive to the prospect of a decision to conserve their forest for the future rather than seek the short-term gains from logging it.

 

There are apparently long-standing boundary disputes between the villages that will need to be resolved before any conservation agreement or nature tourism venture could be established. These disputes have had their advantage in that they have served to prevent timber extraction to date. Timber extraction is still the main danger to the high conservation value of this area, because of the ease of access to the main timber stands from the main road. Overall, the area of forest concerned, the tall closed rainforest in the eastern portion of the A'opo - Letui - Sasina forest, is probably the best area of lowland forest remaining on Savai'i, including the stands in the conservation agreement areas at Falealupo and Tafua.

 

2.1 Vegetation

 

At least three vegetation types are present: (1) littoral forest, with associated herbaceous stand immediately on the coast; (2) lowland forest, with at least two types; and (3) lowland lava flow scrub.

 

2.1.1 Littoral Forest

 

Only a small area of the littoral forest was seen, and this was at an area marked on the map as Le Pu, located at the end of the trail from A'opo. At this point, there a is Hibiscus tiliaceus (fau) thicket just inland from the beach, but this may occur only here and be associated with human activities (an old village?).

 

The littoral forest along the coast formed by the A'opo lava flow appears to be a thicket that is kept low by the salty sea winds. It is probably a mixture of littoral trees and shrubs, but with scattered large individuals of Pisonia grandis (pu'avai). There is a zone of mostly barren rock between these thickets and the sea, where the sparse, low vegetation is dominated by herbaceous strand species such as Portulaca samoensis (tamole), Cyperus stoloniferus, and Lepturus repens, and littoral shrubs such as Capparis spinosa, Clerodendrum inerme (aloalo tai), Scaevola sericea (to'ito'i), and Wollastonia biflora (ateate). On the Sasina portion of the coast, which was formed from the older Puapua volcanics, the dominant littoral forest trees are probably Barringtonia asiatica (futu) and/or Calophyllum inophyllum (fetau), but this area was not studied. The beach area of Le Pu is covered with littoral vines, the most common of which are Canavalia rosea (fue fai va'a) and Ipomoea pes-caprae (fue moa).

 

Of interest here is Heritiera ornithocephala, a tree found in littoral forest and sometimes in lowland forest near the shore. It is very uncommon in Samoa, and on Savai'i is known only from here and Asau. Several large individuals were seen near the coast at Le Pu.

 

2.1.2 Lowland Forest

 

The lowland forest northwest of Letui is typical lowland forest in stature, but Pometia pinnata (tava), which is the dominant tree around most of the island at this elevation, is virtually absent. This is probably due to the aridity of this area of Savai'i that is caused by a rainshadow effect. Instead of Pometia, the dominant species is Syzygium samarangense (nonu vao), a tree that is apparently of recent introduction to Samoa. In one sample of vegetation Table 7), this tree had an estimated relative dominance of 25%, and was the most numerous tree there as well.

 

This forest is very diverse, however, with 25 recorded in the sample. Other trees high in relative dominance include Garuga floribunda (magaui), Intsia bijuga (ifilele), and Ficus prolixa (aoa). The relative dominance of the latter species, a giant banyan, could only be guessed at since it has numerous hanging roots that surround one or more other trees. Other common trees include Terminalia catappa (talie), Planchonella torricellensis (mamalava), Dysoxylum maota (maota), Mammea glauca (manapau), Syzygium inophylloides (asi toa), and Calophyllum neo-ebudicum (tanlanu). Common, but of little relative dominance is Aglaia samoensis (laga'ali), which is an understorey tree.

 

The canopy is probably over 30 m in height, and is mostly closed, which allows relatively little light penetration. The forest floor is mostly barren rock, covered with a layer of dead leaves. Because of the aridity, very few ferns are present, and consequently ground cover is minimal. Only two typically terrestrial ferns were noted, Arthropteris repens, which climbs the lower parts of tree trunks, and Asplenium polyodon, which was mostly withered in the dry climate. However, there are numerous saplings of the dominant trees and shrubs such as Aidia cochinchinensis (ola mea). Vines are also common, particularly Rourea minor, Gynochtodes epiphytica, and Alyxia bracteolosau (lau maile). Epiphytes are also uncommon, except for Dendrobium dactyodes, which was found on the upper portions of most of the tall trees.

 

Lowland forest is also present in two kipuka on the A'opo flow. The forest in these "islands" is basically similar to that found in the Letui forest, but there are some species differences. The dominant tree species in the kipukas are Arytera brackenridgei (taputo'i), Intsia bijuga (ifilele), Garuga floribunda (magani), Terminalia catappa (talie), Guettarda speciosa (puapita), Mammea glauca (pau), Syzygium inophylloides (asi toa), and Ficus prolha (aoa). Overall, however, the dominant was probably Garuga. Pometia pinnata (tava) was almost entirely absent, except for a few saplings seen in one kipuka.

 

Common understorey trees include Meryta macrophylla (ma'ulu'ulu), Diospyros samoenis ('au'auli), and Syzygium clusiifolium (asi vai). In disturbed areas, particularly along the trail, Macaranga harveyana (laupata) is common, as are various species of weeds.

 

The differences between this forest and the scrub are not always distinct, and the boundary between them was often an ecotone with the species of the two vegetation types mixing. One particularly interesting tree found here is Chionanthus vitiensis, which was previously known from only a single record in Samoa (Nu'utele Island, Aleipata). Only a single individual was found on the A'opo flow.

 

Table III.4. Relative dominance of trees in the Letui lowland forest.

 

 

Species

# trees

Relative
dominance

1

Syzygium samarangense

28

25

2

Garuga floribuunda

9

18

3

Ficus prolixa

4

13

4

Intsia bijuga

10

12

5

Terminalia catappa

2

5

6

Diospyros samoensis

5

5

7

Planchonella torricellensis

12

4

8

Mammea glauca

3

4

9

Syzygium inophylloides

3

3

10

Callophyllum neo-ebudicum

3

2

11

Dysoxylum samoense

6

2

12

Canarium harveyi

4

1

13

Guttarda speciosa

1

1

14

Dysoxylum samoense

1

1

15

Buchanania merrillii

2

1

16

Myristica fatua

7

1

17

Pisonia sp. nov.

1

1

18

Harpullea arborea

2

+

19

Planchonella garberi

1

+

20

Neonauclea forsteri

1

+

21

Aglaia samoensis

5

+

22

Canaga odorata

1

+

23

Pometia pinnata

1

+

24

Canthium merrillii

1

+

25

Anacolosa insularis

1

+

 

 

Unlike the lava flow scrub, the understory is mostly devoid of ferns. It is largely barren rock, but numerous shrubs and the saplings of the forest trees are common. The most common shrubs are Ixora samoensis (filofiloa), Aidia cocochinchinensis (ola mea), and Micromeliim minutum (tamafalu). Many areas had an accumulation of dead leaves on the surface. Epiphytes were not very common but were more common than on the lava flow scrub.

 

2.1.3 Lowland Lavaflow Scrub

 

This vegetation occurs on the A'opo lava flow that covers much of the area between the village of A'opo and the ocean, and dates to a high elevation eruption in about 1760. Much of the area is chunky and ropy pahoehoe, over which grows a dense scrub forest. The dominant trees are mostly 4-6 m in height, but scattered through i t are emergent trees up to 12 m or more. In some areas, particularly near the coast, there are large patches of herbaceous vegetation and barren lava, and this may be been a result of past human activity (the area was the site for a "village" known as Faleselau) or to the density of the lava at that point.

 

The dominant woody species are Arytera brackenridgei (tapnto'i), Fagraea berteroana (pualulu), Rapanea myricifolia (tagovao), Rhus taitensis (tavai), and Homalium whitmeeanum. The latter tree is particularly common closer to the coast, and Rhus is particularly common on the upper portion of the flow. A number of other tree and shrub species were recorded from the site, but many of these are rare or uncommon, and some were seen only once.

 

The ground cover is dominated by xerophytic ferns, most commonly Nephrolepis hirsutula (vao tuanui), Phymatosorus scolopendria (lau auta), and Davallia solida. Also common is the scandent shrub Alyxia stellata (gau, lau maile). Epiphytes are uncommon, except for Dendrobium dactylodes, which was often the only species present on trees bearing epiphytes.

 

2.1.4 Conservation Significance of Vegetation

 

The Letui forest at this site is one of the best ones visited during the study, because of the relatively small amount of hurricane damage, and because of the species diversity of forest. Twenty-five tree species were recorded in the small sample taken along the trail from Letui to the coast, and many others are undoubtedly present. This is one of the few lowland forests on Savai'i that is not dominated by Pometia pinnata (tava), probably because of the dry climate, and it is significant for this reason. It is practically the only area of quality lowland forest left on the whole north coast of Savai'i. Lowland forest once covered almost the whole north coast (except for the areas of lava flow), but nearly all of i t has been destroyed for agricultural purposes, and recently for timber for the 'Asau mill. The A'opo lava flow scrub vegetation is also very significant, since it is a unique type of vegetation. It has high ecotourism potential, since the low-stature vegetation is very visible to visitors. It probably is not in any imminent danger, since it is useless for timber and the soil is too rocky for agriculture.

 

2.2 Wildlife

 

2.2.1 Introduction

 

The history of Western Samoa as wildlife habitat is dominated by natural disturbance, particularly by cyclones and volcanic activity. This has resulted in a mosaic of communities ranging from young bare lava to tall rainforest on well developed soils. The development of forest vegetation depends upon the presence of suitable agents for seed dispersal. In Samoa, pigeons, doves, honeyeaters, starlings and flying fox are the major seed dispersers and, to some extent, the long-term survival of tall forest depends on the continued survival of those species. Each group of animals eats a range of fruit, though each fruiting tree species is probably best dispersed by a particular species. Where forest is evolving on a lava flow, as at A'opo, nearby intact forest which acts as a reservoir for both the tree species and their dispersers is an important part of the ecosystem as a whole. In Western Samoa the greatest forest reservoir remains in the central uplands of each island, particularly on Savai'i.

 

While that high altitude forest may not be available as a timber resource, or cover land suitable for agriculture, its continued existence is the reason for the survival of a number of Samoan bird species. Rare species such as the tooth-billed pigeon, mao and Samoan silvereye certainly owe their continued existence to the presence of this large tract of forest. The situation for the more abundant species is less clear, but there is little doubt that a large forest reservoir contributes to the survival of species such as the other pigeons and the blue-crowned lory in the remnant forest areas of the lowlands.

 

While the effects of cyclones such as Ofa and Val are devastating, it is clear that the larger the forest area, the more likely that there will remain some areas which have escaped disturbance. It is from these plant and animal "refugia" that forest will eventually re-establish and repair itself.

 

The lowland forest remnants of A'opo, Sasina and Letui are relatively close to the high altitude forests and are strongly likely to be influenced by them ecologically. Since one of the purposes of the survey was to assess the "external" factors which might influence the long-term survival of the priority lowland sites being surveyed, it was important to assess the bird populations of the high altitude forest. As important in a broader, Pacific context is some estimate of the intrinsic importance to birds of those high altitude forests, particularly to the species only rarely encountered elsewhere.

 

Counts were undertaken in young scrub on the A'opo lava field, in a remnant of mature forest bypassed by the most recent lava flow and at two sites in high altitude forest, above A'opo village and above Letui village.

 

2.2.2 Results

 

During the survey period it was possible to undertake a total of 81 counts in the area (Table III.5). This total was sufficient to give a picture of a large area of developing forest with a bird species richness and abundance which was not great in comparison with that of the more mature lowland forest remnants elsewhere in Western Samoa. Contained within that are, however, is a mature forest patch with amongst the highest densities of birds in the lowlands. More crimson-crowned and many-coloured fruit doves were recorded there than anywhere else during the survey, while wattled honeyeaters were at their greatest density on the mainland and Pacific where also in high abundance.

 

While it was not possible to measure directly the influence of the upland forests as a reservoir for the lowland ones, larger birds were observed moving between them. One of the most significant sightings was a pair of tooth-billed pigeons flying high over A'opo village from one of the coastal forest remnants towards the mountains.

 

The high altitude forests were the only places in Western Samoa where toothbilled pigeons and mao were counted in any abundance. They also contained amongst the highest densities of Pacific pigeons, many-coloured fruit doves, white-rumped swiftlets, Samona fantails, wattled honeyeaters, cardinal honeyeaters, Polynesian starlings and Samoan starlings. In addition, the Letui forest contained the highest density of crimson-crowned lories recorded. Overall, these sites exhibited the greatest bird abundance and species richness of any of the areas surveyed during the study. They were the only places to reach the levels of bird abundance recorded by Lovegrove in 1982 at Elietoga, before the logging there (see Lovegrove 1984).

 

2.2.3 Discussion and Recommendations

 

Though the lowland A'opo, Letui and Sasina forests as a whole did not exhibit extremely high densities of birds and richness of species, the tall forest remnants contained in them illustrated the potential for the areas to become a major centre of bird diversity over time. The presence of high densities of harvested pigeons was of particular significance. Part of the value of these forests lies in their proximity to those of the uplands. The movement of fruit-eating birds between them is an important ecological process.

 

The results obtained from the high altitude forests further confirms their significance internationally and as a source of the continuing development of the younger lowland counterparts.

  1. Protection of the areas which were the focus of this survey should be accompanied by efforts to ensure the long-germ survival of the high altitude forest adjacent.

  2. As an important precursor to that protection, and in the wake of Cyclone Val, there is an urgent need to assess in more detail the upland forests so that those areas of greatest diversity and abundance of wildlife can be delineated.

 

The SPREP survey (Pearsall and Whistler 1991) identified the A'opo Lowland Ecosystems and Lava Flows as priorities for conservation, although not ranking them as highly as this study. The boundary of the SPREP proposal included only the younger successional lava flow vegetation and excluded the important, and potentially merchantable, mature lowland rainforests of the eastern portion.

 

Table III.5. Summary of results of bird counts in young forest on the A'opo Iava field compared with those from older remnant lowland forest and from high altitude sites inland from A'opo and Letui villages.

 

Banded Rail

White-browed Rail

Sooty Rail

Purple Swamphen

White-throated Pigeon

Pacific Pigeon

Tooth-billed Pigeon

Crimson-crowned Fruit Dove

Many-coloured Fruit Dove

Friendly Ground Dove

Blue-crowned Lory

Barn Owl

White-rumped Swiftlet

Long-tailed Cuckoo

Flat-billed Kingfisher

Polynesian Triller

Samoan Triller

Bulbul

lsland Thrush

Samoan Fantail

Samoan Broadbill

Scarlet Robin

Samoan Whistler

Samoan Whiteeye

Mao

Wattled Honeyeater

Cardinal Honeyeater

Red-headed Parrot Finch

Polynesian Starling

Samoan Starling

Jungle Myna

Jungle Fowl

Grey Duck