'East Rennell makes up the southern third of Rennell Island, the southern most island in the Solomon Island group in the western Pacific.

Rennell, 86 km long x 15 km wide, is the largest raised coral atoll in the world. The site includes approximately 37,000 ha and a marine area extending 3 nautical miles to sea.

A major feature of the island is Lake Tegano, which was the former lagoon on the atoll. The lake, the largest in the insular Pacific (15,500 ha), is brackish and contains many rugged limestone islands and endemic species. Rennell is mostly covered with dense forest, with a canopy averaging 20 metres in height. Combined with the strong climatic effects of frequent cyclones, the site is a true natural laboratory for scientific study. The site is under customary land ownership and management.'

The age of the lake as a body of almost fresh water is unknown, although it seems probable that the lagoon was cut off from the sea not long after the uplifting began in the late Pliocene. The lake is brackish with the elevated salt concentrations being maintained by a subterranean duct system which connects it with the sea. Dolines (sinkholes), caves and scattered small freshwater swamps in the lower areas occur commonly as a result of sub-surface flow and solution weathering.

The lake lies at about sea level and is slightly saline (2.8-6.2%) on average, varying from one tenth to one fifth of the salinity of sea water. The pH is slightly alkaline with recordings of 7.3 and 7.8. The water temperature is about 30˚ Celsius and 3-5 degrees higher than that in the rock pools which are the drainage from rain water. The water is faintly sulphurous through the anaerobic breakdown of organic material.

There are many residual small islands at the western end of Lake Tegano that are eroding through solution of the limestone. This is accelerated by wave action. Most of the lake shore is bordered by limestone outcrops that vary in height although there are swampy areas at the northern end of the lake. In the south west of East Rennell, there are two hills named Soaika and Kasipa which rise about 75m above sea level.

There are no streams or rivers on Rennell although there are freshwater springs around the lake edge and they emerge in various places from the cliffs at the coast. Sub-surface water flow is likely to be directed along the lower central axis of the island from the higher western end eastward until it enters the lake.

SOILS

The soils of Rennell are derived from weathered coral limestone and are present as small scattered pockets. There has been no soil enrichment by volcanic dust or sea-borne river sediment because of the great distance from the larger islands and the direction of the prevailing wind and sea currents.

CLIMATE

Rennell and Bellona experience a typical tropical climate characterized by high and rather uniform temperatures (between 22.7˚C and 32.2˚ C) and humidity. Rennell and Bellona are much wetter than Honiara on Guadalcanal and are several degrees cooler. Rennell’s annual rainfall indicates a range between 3000 and 4000mm. There are two distinct climates with West Rennell normally being drier and less humid than East Rennell which can be quite stormy.

No long term weather recording has been carried out on Rennell in the past. The island experiences an equatorial monsoonal climate. There may be a marked dry period from May to June during the middle of the trade wind season. This can extend from May to August with a very dry month in July. The southeast trade wind prevails from April to the end of November and during the rest of the year, the prevailing winds are from the west and northwest.

                                        An undercut coral limestone island in Lake Tegano, East Rennel. (P. Rayan)

CYCLONES

These are the major natural force that influences the vegetation, fauna and lifestyle of the people on Rennell. Rennell is situated within the band of known cyclone paths and is subject to cyclones at relatively frequent intervals. Analysis of cyclone risk areas, using track density per grid square since 1966, reveals a maximum (6per square) concentration of cyclones between Rennell and Makira (170 km to the east) and also in the area 50 km due north of Rennell. The two grid squares which Rennell straddles have received a high number of cyclones (5 per square) during the period. Principal directions of cyclone travel are towards the southwest and southeast.

The latest major cyclone to hit Rennell was Nina in 1993 which caused extensive damage to the forests and villages. Cyclone Nina originated in the Gulf of Carpentaria and had time to intensify before reaching Rennell. People of Rennell relate that a cyclone more intense than Nina occurred between 1898 and 1900. After this cyclone, it was said that one could see from end to end of the island. This could explain the present structure of Rennell forests, where trees have lesser stem diameters than other forests in the Solomons. Cyclone Nina also led to a dramatic fall in the number of ground feeding birds and many dead birds and bats were found at the edges of the lake and sea shore when the cyclone abated

VEGETATION

The flora of Rennell is significant and warrants World Heritage listing because the species composition is biogeographically unique. Although there are currently only ten endemic species recorded for Rennell, this is likely to increase as specialist collections are made. The endemic species found at East Rennell include an orchid found on the lake islands, three species of Pandanus and an undescribed palm.

The forest of the island interior is unusual in the composition of main canopy tree species. Many of the common canopy species on Rennell are uncommon else where in the Solomon group. In contrast, most of the species common elsewhere in the Solomon Islands appear to be absent from Rennell. Included amongst the absentees are species which are present in the Santa Fcruz group and some of those species are present on even more remote island groups in the west-central Pacific such as Fiji, Vanuatu, Tonga and Samoa.

There are three principal vegetation types on Rennell: low stature forest of the karst ridge on the island perimeter, tall forest of the island interior and the beach flora of the Lake Tegano Margin. There are also small areas of mangrove vegetation on the lake margin. The differences in species recorded and structural attributes between the karst ridge and island interior forests are the result of the different physical conditions on the exposed island margins (also with shallow soils) and the relatively sheltered island interior where deeper soils occur.

FLORA OF LAKE TEGANO

The lake flora includes diatoms and algae. There is a rich variety of diatoms: 60 genera, 312 species and 80 additional varieties or forms. Of the 456 records of species, 61 percent were collected only in the lake and 34 percent occur in the lake and in rock pools. In the lake, freshwater species, alkaline forms and forms living in the littoral region of lakes, dominate. Most species are cosmopolitan and only eight species and four varieties are endemic.

Two types of pandanus trees grow commonly around the lake. They are probably endemic species. The local people have different uses for these trees and they are highly valued. (P. Ryan)

This is a panadanus fruit : the fruit of some species can be eaten (P. Ryan)

ENDEMISM:

To date, no endemic tree species have been found on Rennell but there are ten endemic plants that have so far been recorded. There is a rare endemic orchid (Dendrobium rennellii) which occurs on the small islands of Lake Tegano. There are also two endemic species of Pandanus (P. lacustris and P. rennellensis) and one is found at the lake margin (P. lacustris). Another new Pandanus has been collected but the specimen was too incomplete to enable description. There is an endemic, erect understorey palm (Drymophloeus sp.nov.) which is yet to be described.

Other endemic vascular plants are:

Pseuderanthemum bibracteatum Fosberg (Acanthaceae); Rennell and Bellona

Dischidia melanesica Fosberg (Asclepiadaceae); Rennell

Acalypha crockeri Fosberg (Euphorbiaceae); Rennell

Hedyotis rennellensis Fosberg (Rubiaceae); Rennell

An endemic fungus, Lasiosphaeria noona-daniae

Dendrobium Rennellii Orchid

MAMMALS

The fruit bats (Family = Pteropididae) are the most diverse family of bats in the south west Pacific. They also have an extremely high degree of endemism. The diversity and wide distribution of this fruit-eating group of mammals is paralleled among birds in the fruit-eating doves and pigeons which are a major proportion of the avifauna.

Rennell has three species of large, fruit-eating bats and they are important in spreading the seeds of trees. They are; the Solomons Bare-backed Fruit-bat (Dobsonia inermis, local name = Puli); the Rennell Flying-fox and the Pacific Flying-fox (Pteropus tonganus, local name = Peka). These bats are probably the sole agents that spread the canoe trees (Palaquim amboinense and Inocarpus fagiferus, local names = Ghaimega and Isi); the timber tree (Xanthophyllum papuanum, local names = Aliupagegho), the secondary forest staging trailer / shrub (Dendrocnide rechingeri, local name = Siago) and the edible fruit tree, Eugenia aquea (local name = Ghaghaga) which has been introduced from Malesia.

The other bat species feed on insects and they are not unique to Rennell. The non-endemic bats appear to be derived from three groups; an “oceanic” group with only one species, the Pacific Flying-fox.

It is widespread and a good flyer and could have colonized naturally. These flying-foxes were carried as pets by Polynesians so it is possible that it may have been introduced to Rennell.

The second group has species derived from the Solomon Islands and, more distantly, Papua New Guinea. This includes the Solomons Bare-backed Fruit-bat; the Large-eared Sheath tail-bat; the Spurred Horseshoe-bat (Hipposideros calcaratus, local names = Lakulaku and Tagingaabaga) and the Large Melanesian Bentwing-bat (Miniopterus propitristis).