Margins And Climate Change

Despite varying concepts as to what constitutes a margin, boundaries provide an opportunity for observing

Fig. 1.2 Limes convergens as seen in two natural treelines in Patagonia at the frontier between Chile and Argentina (40° 30' S; 70° 50' W). Below the snow-covered peaks can be seen an upper limit to tree survival with the deciduous southern beech (Nothofagus pumilio). Below is the upper limit for the evergreen Nothofagus dombeyi.

Fig. 1.2 Limes convergens as seen in two natural treelines in Patagonia at the frontier between Chile and Argentina (40° 30' S; 70° 50' W). Below the snow-covered peaks can be seen an upper limit to tree survival with the deciduous southern beech (Nothofagus pumilio). Below is the upper limit for the evergreen Nothofagus dombeyi.

limits to plant survival. Consequently, it is the condition of plants at the approach to a boundary and how they may respond to environmental changes that provides much of the subject matter of this book.

Comparisons between geographically different areas need to be made with caution. Although some boundaries such as treelines and scrub zones may appear similar, and contain species with comparable functional types, they may have geographically different evolutionary and ecological histories and be controlled by diverse environmental factors. Isomorphism - the occurrence of similar forms in unrelated taxonomic groups - is common in plants. It can therefore be dangerous to make global generalizations without careful examination. Even within a single biome, boundary positions may differ in their response to climate change.

In the interface between the arctic tundra and the boreal forest (see Chapter 5), the intuitive prediction would suggest that the tundra will retreat and the boreal forest should expand in response to climatic warming. However, in Alaska, the treeline is currently at its most northerly Holocene extent, while in north-eastern Canada there has been a retreat since the mid Holocene (Edwards & Barker, 1994). A forest retreat southwards (see Section 5.3.1) has been observed in the Siberian Lowlands (Kremenetski et al., 1998). In Finland an inland rather than a northward migration of the bulrush (Typha latifolia; Fig. 1.4) has been attributed to reduced ice cover rather than just warmer temperatures at higher latitudes (Erkamo, 1956).

Some well-defined communities contain groups of species sharing a common boundary (e.g. salt marshes and wetlands; Figs. 1.5-1.6). In these cases it is possible to define such specific plant communities in terms of their present-day species composition. Nevertheless, in terms of geological time these assemblages are only temporary, and species and population aggregations have no biological permanence. The late-glacial floras of Great Britain and Denmark and Russia consisted of communities which contained species which are now dispersed as plants of tundra, steppe, mountains and dunes. In the same way, the northern European forests during the Holocene had an ever-changing species composition (Huntley, 1990).

Fig. 1.4 Eastward spread of Typha latifolia in Finland 1900—50. Symbols: red, at or before 1900; blue, new records 1926—50. (Adapted from Erkamo, 1956.)

Species survival in peripheral regions is not controlled merely by the impact of average probability of adverse climatic conditions. Survival is also profoundly affected by competition from other species and by the frequency of extreme events, such as drought, flooding, freezing and disturbance. These periodic events, sometimes measured in terms of decades or centuries, can create a complicated pattern for limits to plant existence both geographically and at a microsite level. Marked differences can also be found between populations of the same species. One of the most widespread and ancient of arctic flowering plants is the early flowering purple saxifrage (Saxífraga oppositifolia; see Section 6.4.2). Plants of this species from the High Arctic (Spitsbergen) can be deprived of oxygen for months when encased in ice and yet survive. Such a well-developed tolerance of anoxia is not found in more southerly populations ofthis species (Crawford et al., 1994).

Fig. 1.5 Community limits imposed by a regular flooding regime. A very distinct boundary is visible between the non-flooded dune vegetation and the upper limit of flooding in a salt marsh marked by a zone of Suaeda vera near Romney, south-east England.

Boundaries can also be found at a local level, particularly in regions with marked seasonal variations in climate. Many local factors modify the degree of exposure of plants to the adverse season with the result that boundaries can be found in relation to topography, geology, and soil type. Changes in bedrock, particularly at outcrops of chemically basic rocks such as limestone can produce an abrupt boundary that extends over considerable distances (Fig. 1.7). Flooding frequency and depth is also a powerful discriminator between plant communities and creates easily visible boundaries (Figs. 1.5-1.7).

0 0

Post a comment