DEFINING MARGINS

All species have limits to their distribution, and populations that demarcate margins demonstrate an end-point in adaptation to a changing environment. Margins are therefore of particular interest as they represent limits to survival that may alter with climatic change. Plants are ideally suited for the study of peripheral situations as their sedentary nature facilitates mapping and historical recording. Many plant atlases record limits to plant distribution both past and present (Hultén & Fries, 1986; Meusel & Jäger, 1992). Boundaries can also be observed between biomes (vegetation formations characterized by distinct life-forms) as in the latitudinal and altitudinal limits to tree growth. The interface between one vegetation type and another can vary as to whether it is abrupt and easily visible even at a distance (limes convergens or ecotone), as in the natural treelines of the Nothofagus forests in the Andes (Fig. 1.2), or whether it is diffuse, as one vegetation zone gradually merges into another (limes divergens or ecocline) as at the interface between the southern limits of the boreal forest and the northern limits of the deciduous broad-leaved forest. In this latter case, a more quantitative approach is needed for monitoring change, which may be ecologically just as significant as the movement of discrete boundaries (Fig. 1.3).

Significant plant migrations are to be expected as a response to climatic change. However, care is needed in distinguishing climate-induced changes from the current effects of widespread alterations in land use.