Abstract

Robinson Crusoe Island of the Juan Fernandez Archipelago, as is the case with many oceanic islands, has experienced strong human disturbances through exploitation of resources and introduction of alien biota. To understand these impacts and for purposes of diversity and resource management, an accurate assessment of the composition and structure of plant communities was made. We analyzed the vegetation with 106 releves (vegetation records) and subsequent Twinspan ordination and produced a detailed colored map at 1: 30,000. The resultant map units are (1) endemic upper montane forest, (2) endemic lower montane forest, (3) Ugni molinae shrubland, (4) Rubus ulmifoliusAristotelia chilensis shrubland, (5) fern assemblages, (6) Libertia chilensis assemblage, (7) Acaena argentea assemblage, (8) native grassland, (9) weed assemblages, (10) tall ruderals, and (11) cultivated Eucalyptus, Cupressus, and Pinus. Mosaic patterns consisting of several communities are recognized as mixed units: (12) combined upper and lower montane endemic forest with aliens, (13) scattered native vegetation among rocks at higher elevations, (14) scattered grassland and weeds among rocks at lower elevations, and (15) grassland with Acaena argentea. Two categories are included that are not vegetation units: (16) rocks and eroded areas, and (17) settlement and airfield. Endemic forests at lower elevations and in drier zones of the island are under strong pressure from three woody species, Aristotelia chilensis, Rubus ulmifolius, and Ugni molinae. The latter invades native forests by ascending dry slopes and ridges. It successfully outcompetes endemic taxa, including its congener Ugni selkirkii. The aggressive herb Acaena argentea severely threatens to overtake native grassland