Common name: Tree Fern
Scientific name:Dicksonia arborescens
IUCN status: Abundant
The tree ferns were once numerous on Diana’s peak, it forms a thicket covering of St Helena’s highest peaks, reaching up to 823 metres above sea level. It forms the key species in the vegetation type of the highest altitudes on St Helena.
In 1502, the tree fern thicket extended along the central ridge of the Island both on the southern and eastern sides from Green Hill to High Peak with a few scattered patches growing at the depot and from Green Hill back around to the Cabbage Tree Road.
Small pockets and isolated tree ferns can still be seen along the road side particular along the ridges of Sandy Bay and Cason’s area as an indication of their extent.
This ‘tree fern thicket’ has a deep humus layer beneath it. Fallen tree fern trunk can sprout along their length to form new growth. Black Cabbage tree seed germinate freely on the trunk and can often be seen growing out from the tree fern. On the lower slopes around the tree fern thicket grew a woodland of endemic cabbage trees. Naturally the tree ferns would have grown amongst black cabbage trees, dogwoods, whitewoods, he cabbage and other tree species.
Now found growing amongst the tree ferns are flax, bilberry, fuchsia and buddleia. They are invasive species and continue to reduce teh area of surviving tree fern thicket.
The tree fern is very easy to recognise as it is the only fern on the Island with a distinct ‘trunk’ from the top of which grows fronds giving a rather palm-like appearance. The trunk, which is covered with hair-like adventitious roots, is between 15-20cm in diameter and grows to 4-5m in height. The fronds are dark green and are over 1, in length, at a young stage it is hairy and looks like a monkey’s tail.
In 1995 it was estimated that the tree fern thicket was being lost at a rate of 5m a year because of the invasion of mainly flax, wild fuchsia and bilberry with the latter two being confined to isolated areas acroos the Peaks. Flax is a threat because tree ferns cannot survive on soil invaded by flax because of the dense growth and probably by the flax causing the soil to become too acid.
A five year Management Plan has been put together to stop this loss to help rehabilitate the tree fern thicket growing within the National Park.
NUMBER OF SURVIVING POPULATIONS
The tree fern is the most abundant species growing amongst the endemic trees on the wetter part of the central ridge, Cuckolds to Mt. Actaeon also occasionally in other places on the central ridge.
CONSERVATION RECOVERY PROGRAMMES
There’s no real threat to this species on the Peaks, it usually generates naturally on its own, although it has been invaded by flax, wild fuchsia and bilberry. Currently the Habitat Management team on the Peaks are removing fuchsia from the tree fern thicket from a compartment below Mt. Actaeon above Deep Valley and will remove to other priority areas as outlined in the Management Plan.