Steven Waldren (Botanic Garden Curator/Administrator), Trinity College Botanic Garden, Dublin, who spent three months on Henderson Island in 1991 (and who is returning to the island in June 1997), has sent the following information on the flora of Henderson Island:
The flora of Henderson Island contains some 63 native higher plants. Perhaps 20 bryophyte species and 30 lichen taxa occur. Of the higher plants, nine occur nowhere else. The flora of Henderson is derived largely from the west, species having mainly island hopped from the Indo-Malaysian basin. Some of the species are characteristic of eastern Polynesia, there are strong affinities with the Austral Islands and the Marquesas. The occurrence of tall, woody members of the daisy family (Bidens hendersonensis and Senecio stokesii) show some affinities with an essentially southern relict flora.
Henderson has three main beaches, the strandline species which occur there are all widespread Pacific or Indo-Pacific species, which probably arrived by flotation. In the embayments behind the beachfronts, a beach swale forest occurs, but the most diverse forests are found on the top of the raised coral plateau. The typical closed forest is dominated by the widespread Pisonia grandis, with several other species such as Celtis pacifica, Nesoluma st-johnianum (endemic to Henderson) and a wide variety of additional species. The shrub layer is sometime sparse, but dominated by the Psydrax odoratum and the endemics Ixora fragrans and Geniostoma hendersonense. The ground layer is dominated by ferns and the endemic Peperomia hendersonensis. Senna glanduligera, Bidens hendersonensis and Senecio stokesii seem to favour gaps in the canopy. Many of the larger trees are semi-decumbent, with trunks often running horizontally- this means that walking on the plateau involves passing through tree crowns; the canopy generally only reaches 10 m. Add to this the occurrence of lianas, and your have a fairly impenetrable thicket to get through. The best developed soils occur in this species-rich Pisonia forest.
Towards the island centre the remains of the former shallow lagoon can be found. The vegetation is often more shrubby and open here, with poorly developed soils over coral sticks.
Where closed forest occurs, it is more species poor than the typical Pisonia forest, and is co-dominated by Xylosma suaveolens- freshly cut wood of this species smelled wonderfully of salami!
The far southern part of the island is extremely exposed to salt spray and wind- the vegetation is short and stunted, and consists mainly of salt-tolerant species.
In addition to the native species, Polynesian settlers introduced a variety of species. These include plants of religious and medicinal value, in addition to food plants. Some of the latter, such as the swamp taro, no longer occur, but are known from sub-fossils found in areas that were cleared by Polynesians for garden areas- now long since revegetated.
The vegetation of Henderson is almost unique for a raised atoll in that it shows very little sign of human disturbance- many raised atolls have been devastated by mineral exploitation. Indeed, it was a geologist who made the first comprehensive collection of plants from Henderson during survey work: thankfully no phosphate exploitation has occurred on Henderson- long may its special flora and fauna survive undisturbed!
Further information on the flora of Henderson can be found in the following:
Brooke M. de L., Jones P.J., Vickery J.A. & Waldren S. (1996). Seasonal patterns of leaf growth and loss, flowering and fruiting on a subtropical central Pacific island. Biotropica, 28: 164-179.
Florence J., Waldren S. & Chepstow-Lusty A.J. (1995). The flora of the Pitcairn Islands: a review. Biological Journal of the Linnean Society, 56: 79-119.
Fosberg F.R., Paulay G., Spencer T. & Oliver R. (1989). New collections and notes on the plants of Henderson, Pitcairn, Oeno and Ducie islands. Atoll Research Bulletin, 329: 1-18.
Fosberg F.R., Sachet M.-H. & Stoddart D.R. (1983). Henderson Island (South eastern Polynesia): summary of current knowledge. Atoll Research Bulletin, 272: 1-47.
Paulay G. & Spencer T. (1989). Vegetation of Henderson Island. Atoll Research Bulletin, 328: 1-13.
St. John H. & Philipson W.R. (1962). An account of the flora of Henderson Island, South Pacific Ocean. Transactions of the Royal Society of New Zealand, Botany, 1: 179-194.
Waldren S., Florence J. & Chepstow-Lusty A.J. (1995a). A comparison of the vegetation communities from the islands of the Pitcairn Group. Biological Journal of the Linnean Society, 56: 121-144.
Waldren S., Florence J. & Chepstow-Lusty A.J. (1995b). Rare and endemic vascular plants of the Pitcairn group, south-central Pacific Ocean: a conservation appraisal. Biological Conservation, 74: 83-98.