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Fig. 7.2 Recent sea level rise as recorded from 23 annual tide gauge records and satellite altimetry. (Reproduced with permission from www.globalwarmingart.)

The potential impact on vegetation of the expected large rises in sea level will vary depending on the geological nature of the coastline and its vegetation cover. Coasts can be divided into soft coasts, such as sand dunes and salt marshes, and hard coasts where the interface between land and sea is made up of hard rocky shores or cliffs. It is in the soft coasts that the stability of the shoreline is determined by the resilience of the maritime plant communities to the onslaught of oceanic tides and winds. Soft shores have the greatest ability to accrete and respond to rising sea levels provided there is a sufficient supply of sediment. However, they are also the most vulnerable when physical erosion outstrips their ability to consolidate fresh sediment.

When examined in detail, the vegetation of coastal regions reveals not one but a series of margins depending on the relative tolerance of different plant species in relation to proximity to the sea. As discussed in Chapter 1, aerial photographs, coupled with ground inspection on cliff tops, sand dunes and salt marshes, frequently show a gradient of marginality in the zon-ation of plant communities in relation to distance from the sea (Figs. 1.5, 7.12).

On a worldwide basis, plant forms that are able to survive near the sea range from diminutive herbs to substantial forest trees (Fig. 7.3). A paradox of the maritime environment is that as well as being hazardous for plant survival it also provides ecological opportunities for species that are not found in more densely vegetated and competitive inland communities. It is the coastal environment that now provides the only refuge for some of the coniferous trees that once had a worldwide distribution. The coastal Californian redwood (Sequoia sempervirens) in common with several species of Chamaecyparis and Taxodium all had transcontinental distributions during the Tertiary period but now are found only in very restricted coastal habitats (Fig. 7.4). Apparently, the oceanic niche with its diminution of climatic extremes, together with a reduction in competition from more recently evolved tree species, provides many relict tree species with an environment in which they are still viable (Laderman, 1998a).

The coastal habitat, although often marginal in terms of physiological stress, is not usually limiting in terms of geographic distribution. Shorelines provide an unequalled highway for plant migration. In this modern age where human impact on the landscape has reduced many inland plant communities to isolated pockets or

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