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 (Hulten & Fries, 1986; Meusel & Jager, 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. Environmental change is likely to create diffuse boundaries as one community gradually replaces another but this can also result from changes in land use. Many alpine pastures are no longer grazed in summer with their former intensity and in many areas this has allowed a gradual uphill advance of tree cover even on steep mountain sides (Fig. 1.1).

In terms of plant distribution, living at the edge of any habitat or community poses the question: why do these plants grow there and no further? Numerous biological disciplines have applied their respective methods in the investigations of limits. Biogeography, demography, reproductive biology, physiology and genetics all provide cogent explanations as to why limits occur where they do. Each discipline is correct in its own particular way and can provide adequate answers to the questions that are asked. Whether or not the different disciplines provide an answer as to a prime cause for any particular limit depends entirely on human perceptions of the problem. Where there are no geographical obstacles, physiological failure might be expected to account for the inability of a species to survive. On the other hand a geneticist might explain the boundary as due to a lack of variation and failure to adapt (see Section 2.2), or merely dismiss the importance of boundaries with the comment that 'plants are static and it is their genes that migrate'. By contrast a demographer would claim that boundaries are no more than the place where recruitment finally fails to balance mortality. The recognition of boundaries can depend therefore in large measure on what the observer is capable of seeing.

Recognizing a margin and explaining the reasons for its existence are interconnected processes. What is observed as a margin can depend on the manner of observation. Life-forms present obvious boundaries that are immediately visible, as with treelines, or when rising water tables cause meadows to become marshes and bogs or changes in coastal topography cause dunes to give way to flood-prone dune slacks. Other boundaries may be discernible only when species or populations are examined in detail for morphological, demographic or genetic characteristics. Understanding the causes of these wide-ranging limits requires an equally comprehensive approach in recording the nature of variation in plant populations. The negative effect of warm winters on the carbon balance of certain arctic species can explain their southern limits, while their northern boundary is more likely to depend on the time available for growth and resource utilization rather than resource acquisition (see Section 5.6.2). Plants with adequate reserves and low resource demands may be able to survive a negative balance for many years and thus ensure continuance of peripheral populations during temporary episodes of climatic deterioration. The study of marginal areas therefore requires perceptive recording of relevant biogeographical data for matching with possible causes of limits to distribution from what is known about demography, physiological requirements and genetic variation.

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