Fern  (Pteris adscensionis)

Pteris adscensionis is classified as Critically Endangered (CR) on the IUCN Red List (1).

This fern consists of bright green fronds that may form large tufts reaching up to 1 metre in height (2). The frond is divided into leaflets, which are themselves composed of smaller leaflets known as ‘pinnules’. The spore-producing structures (known as ‘sori’) line both edges of the pinnule (2).

Endemic to Ascension Island in the south Atlantic, Pteris adscensionis is found mainly in Cricket Valley with a remnant population in Breackneck Valley (3). The total population is estimated to be no more than 200 plants (3).

Cricket Valley is situated on the central mountain of Ascension Island (known as Green Mountain) at a height of 460 metres above sea level (3).

Ferns are ‘primitive’ plants that spread by releasing spores rather than by producing flowers and fruits. The distinctive frond stage of the fern lifecycle is asexual; spores are released from the fronds, which then germinate into minuscule heart-shaped structures known as ‘prothalli’. It is here that the sexual stage of the lifecycle occurs; male and female organs on the prothallus produce sperm and eggs respectively. If the female eggs are fertilised successfully, a new fern plant will begin to grow and the cycle starts again (4).

In Pteris adscensionis however, the sexual stage of the lifecycle does not occur and the spore-producing plant develops directly from the prothallus (4).

Pteris adscensionis once constituted an important component of the vegetation of Ascension Island; this species has undergone a dramatic decline however, and is now absent from the summit of Green Mountain. Competition with introduced species is likely to be responsible for this decline (2). Today, populations are extremely reduced and Pteris adscensionis is thought to be the most threatened of Ascension’s flora, with less than 200 plants remaining in the wild (3).

Ascension Conservation is carrying out vital cultivation of this species and is monitoring the highly vulnerable wild population (3).