 The main environmental determinants of vegetation on subantarctic islands are wind exposure, water availability, parent soil composition, salt spray exposure, nutrient availability, disturbance by trampling (from seabirds and seals) and, possibly, altitude.At Heard Island, exposure to salt spray and the presence of breeding and moulting seabirds and seals are particularly strong influences on vegetation composition and structure in coastal areas.Evidence from microfossil records indicates that ferns and woody plants were present on Heard Island during the Tertiary (a period with a cool and moist climate).