Throughout the world, dwarf woody plants living beyond the treeline are a significant component of the vegetation. They are found in a wide variety of habitats from coastal heaths to upland and alpine moorlands. With human disturbance of forest regeneration, beginning already in the Neolithic, their geographical range has been greatly extended. More recently, the last hundred years has seen the so-called reclamation of these peatlands with deep ploughing and reseeding. In many of the remaining moorlands overgrazing is now rapidly reducing the extent of scrub and heath communities. If left undisturbed it seems probable that woody scrub species will respond in a variety of ways should significant climatic warming continue in boreal and arctic regions. Climatic warming in northern latitudes will undoubtedly create new ecological opportunities for vegetation advance as ice sheets retreat and permanent snow cover is reduced. Changing thermal conditions will also be likely to alter the species composition of existing northern plant communities.

The nature of the migration of species into these vacated areas or changing communities is unlikely to be a mere latitudinal shift northwards of existing species assemblages. Depending on proximity to the ocean, and this includes also the Arctic Ocean, the degree ofwinter versus summer warming is likely to differ. In northern regions, the influence of oceanicity on plant distribution is very marked. The models described above suggest that among some of the commonest woody species winter warming may in some cases cause a significant retreat in areas where the climate is influenced by the ocean, while in other more continental areas the dwarf woody species may make significant advances. It appears therefore highly probable that species migration will be strongly influenced by sea-sonality of temperature change and any one species might migrate in opposite directions for the same patterns of change in different parts of its range. Attention will therefore have to be given to the roles of seasonality gradients as potential barriers to migration notwithstanding overall warming.

Fig. 10.1 Mountain isolation as seen at 1692 m on the summit of the Dedo de Deus (the Finger of God) in the Serra dos Orgaos, Teresopolis, Brazil. Plants that occur in this apparently challenging environment include bromeliads, orchids, Velloziaceae, and even Cactaceae which grow directly on the rocks or in association with moss colonies. In addition, several Ericaceae and Melastomataceae shrub species are present. In the Asteraceae, the genus Baccharis is widespread at high altitudes in south-east Brazil, and plants belonging to this genus are just visible in the photograph as a woody component ofthe scrub vegetation. (Photo Miguel D'Avila de Moraes and botanical notes from Professor F. R. Scarano.)

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