HUMAN SETTLEMENT IN PERIPHERAL AREAS

Adaptability is a feature of mankind, with the result that even the most marginal habitats are seldom devoid of human settlements. Furthermore, once having populated a peripheral area the community often appears reluctant to abandon what they have come to consider as their home territory, even when deteriorating environmental conditions have brought prolonged periods of hardship. Ecologically, the range of habitats in which prehistoric human settlements developed knew few boundaries. There is increasing evidence that the first human beings to cross into North America used coastal migration routes even before the late Pleistocene ice had retreated from the Beringian shores (Shreeve, 2006). It is therefore not surprising that many early human communities can be found on offshore islands, marshes, and flood-prone deltas. Archaeological records from the poles to the tropics have revealed the rapidity with which our early ancestors spread through an astonishing range of marginal environments.

Peripheral areas can present both opportunities and risks for their human inhabitants. On one hand, historical and archaeological records provide numerous examples where fluctuating climatic conditions opened up areas of opportunity, which attracted migration and settlement. On the other hand, fluctuating conditions could lead to crop failure or the collapse of other resources such as hunting or fishing and force the abandonment of areas that were formerly attractive. Such events are taking place today at an ever-increasing rate, particularly in regions where large human populations are having their agricultural and pastoral activities destroyed by prolonged droughts, possibly connected with global warming.

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.

ENVIRONMENTAL LIMITS TO REPRODUCTION

Limits to plant distribution can arise either from a failure to grow or an inability to reproduce. In many cases a failure to reproduce may be a more common response to environmental limitations than a failure to grow, probably because reproductive success requires more than just the development of viable seed. Reproduction is accomplished only when there is successful establishment of a new generation of reproductive individuals. The continued existence of viable populations in marginal areas is therefore dependent on accomplishment of flowering, fertilization, viable seed production, germination, and the establishment of new individuals in regions where the environment is uncertain and variable. These basic requirements for completion of the reproductive cycle illustrate the appropriateness of measuring genetic fitness in the Darwinian sense as the ability of an individual to contribute genetically to the next generation.

Fortunately, at least for some perennial plants, the arrival of the next generation is not as urgent as it is in animals with their genetically fixed and discrete lifespans. For plants in marginal situations failure to reproduce sexually due to climatic deterioration does not necessarily result in abandonment of the habitat.

‘Adapt or migrate’ is sometimes presented as the only possible choice open to organisms that live in fluctuating environments where adversity may make sexual reproduction not possible for long periods. Reasonably regular sexual reproduction may be necessary to perpetuate populations of most animals, but many plant species are able to survive for centuries and even millennia without reproducing sexually.

Margins have long provided key questions for ecological investigation. Today with climatic warming becoming ever more apparent margins as regions of ecological change invite an assessment of their responses to environmental alteration. The purpose of this book is therefore to examine how marginal plant communities in different parts of the world are responding to climate change. Practically every aspect of modern biological enquiry can be used to address the nature of margins. Biogeography, demography, reproductive biology, physiology and genetics all provide cogent explanations as to why limits occur where they do. The aim of this book is to bring together, wherever possible, different avenues of enquiry in relation to explaining the existence of limits to plant distribution. Each of these disciplines can contribute to our understanding of the biological consequences of climatic warming.

Marginal areas have a number of features in common. These can be seen in demographic limits to population renewal, in adaptations to shortness of the growing season, in problems of access to resources, and impediments to reproduction. To avoid repetition an attempt is made therefore to discuss these common features before moving on to individual case studies.

Part I examines the nature of margins and their effects on biodiversity. Part II is functional, and explores how plants in marginal areas overcome the shortness of the growing season and other physical limitations in acquiring resources and reproducing. The remaining chapters look at individual examples of marginal areas which have been selected on the supposition that they may be sensitive to climatic change.

THE TUNDRA–TAIGA INTERFACE

Will climatic warming allow the boreal forest to advance onto the treeless tundra? This is one of the most tantalizing questions that can be asked in any discussion in relation to vegetation margins. The zone between the northern limit of the boreal forest (taiga) and the southern extent of the arctic tundra is the world's only circumpolar vegetation boundary and stretches for 13,400 km around the northern hemisphere and across three continents (Figs. 5.1–5.3). It is probably more exact to refer to this boundary as a zone, for in many places, and particularly in Russia, there is a well-developed interface region. This variable interface ecotone is best developed in Siberia where it is referred to by Russian ecologists as lesotundra (Russian les, forest). The term forest–tundra has also been adopted in describing similar ecotones or zones in eastern Canada created over the last 3000 years by deforestation as a result of the combined action of forest fires and climatic cooling (Asselin & Payette, 2005).

Such is the length of this boundary that it cannot be expected to have the same appearance or behave uniformly throughout its entire length. In some places it can be seen as a relatively abrupt change from forest to open tundra while in others, and particularly in Siberia, there is a lesotundra transition zone that can be several hundred kilometres deep.