Bryophyte diversity and terrestrial plant ecology in the subantarctic: Heard Island, a case study

This study examines various aspects of terrestrial vegetation on subantarctic Heard Island. A major phytosociological study was conducted focussing on the bryophyte component of the flora. A revised annotated checklist for the Heard Island bryoflora, comprising 42 moss taxa and 12 liverwort taxa is presented. Heard Island's bryophyte and vascular plant floras are small compared with those of other subantarctic islands. General environmental conditions that affect plants on Heard Island are outlined and ecological amplitude of major taxa are described. Hypotheses concerning ecological strategies of major taxa are proposed. Many taxa on Heard Island such as Azorella selago and Dicranaweisia spp. exhibit wide ecological amplitude. Patterning of vegetation in a variety of feldmarks was examined and a model concerning bryophyte establishment and maintenance of bare areas is presented. This study has identified five major vegetation types on Heard Island, based on floristic diversity as well as the relationship between vegetation and environmental characteristics. The dominant vegetation type is open Azorella cushion vegetation. A highly significant feature is that bryophytes are major components of most vegetation on Heard Island and in many cases are the dominant component, such as in closed bryophyte vegetation on shallow stable ground. Taxa in the most abundant vegetation types (Azorella cushion and other vegetation; feldmark vegetation on shallow stable ground, feldmark vegetation on labile ground) are generally effective tolerators of stress or disturbance. More competitive taxa are absent, allowing tolerators, such as Azorella, Pringlea antiscorbutica and mosses such as Dicranoweisia spp. and Ditrichum immersum to be more significant elements of the vegetation on Heard Island than on other subantarctic islands such as Marion and Prince Edward Islands. Heard Island's position within the subantarctic is reviewed. It is the island within the Kerguelen Province on which the effects of the last glacial maximum are persisting longest. It is still emerging from extensive glaciation. With the current significant ice-retreat, new areas for colonization are being revealed. At present the island can be interpreted as an 'archipelago' with small islands of icefree land separated by glaciers, each requiring separate colonization events in order for vegetation to become established.