Each biome is made up of many species of plants, and each one of these species has its own particular "distribution range'', the area in which it grows naturally. Sometimes, towards the edge of a biome a lot of plant species seem to reach their limit at just about the same point. For example, in eastern North America, quite a few species of deciduous trees die out at the southern edge of the boreal forest in Canada. This is not surprising, because there must be a point along a temperature gradient at which the strategy of being deciduous is no longer quite so viable. Thus a lot of trees that use this strategy will tend to reach their limit at about the same place.
However, most of the species of plants present in each biome have their own idiosyncratic distribution ranges that do not show much relationship to the boundaries of the biome. In many cases the species range boundaries do seem related to aspects of climate, though not necessarily the same factors that define the edges of the biome.
Often, if one plots the distribution limits of a particular species it turns out to correspond quite closely to a climatic parameter such as the mean temperature of the warmest summer month, the annual rainfall or the yearly minimum winter temperature. It is perhaps too easy to keep trying different climate parameters until one "fits", but often the correlation between a parameter and the species range limit is so striking that it is hard to believe that it could be just coincidence. It seems that beyond a certain extreme of temperature or rainfall conditions, each species of plant is physiologically unable to survive (e.g., it cannot survive the frosts, or the summer drought, etc.). But the tolerance limits vary greatly between different species of plants, according to their own anatomical and physiological peculiarities.
In parts of the temperate latitudes, many plant species have what is called an "oceanic" distribution pattern that roughly follows coastlines, even though they may extend inland several hundred kilometers away from sea shores. The oceanic distribution tends to occur because these are plants that do best under cool summers and/or mild winters, in the climates which result from the moderating influence of the ocean. One example of an oceanic species is the ivy, Hedera helix, which is concentrated along the western fringe of Europe. Another is the cross-leaved heath (Erica tetralix) (Figures 2.25a*, b) which occurs along the western edge of Europe from Norway down to Portugal.
Moving farther inland, the "oceanic" species drop out and are replaced by certain other species which seem to thrive under the hotter summers, colder winters and lower rainfall conditions. These are known as "continental" species, because they are associated with the more extreme continental climates. In England, continental species of wildflowers—whose ranges tend to extend to the steppe environments of Ukraine—are found in meadows in southeastern England, especially on warmer drier south-facing slopes and on sandy soils which tend to imitate the warm droughty conditions of the steppe grasslands. In the wetter, cooler west of England, these species are absent. An example of a continental species in Europe is the stemless thistle, Cirsium acaule, which towards the more oceanic northwestern limits of its range is confined to the warmer, drier more "continental" south-facing sides of hills.
The individual shapes of ranges cannot always be put down to climate. They also seem to be affected by soils, and in some cases chance aspects of history such as where that particular species managed to survive during glacial times and how far it managed to disperse out of these refuges before reaching topographic barriers (see Chapter 3). An example of this historical effect from Europe is the purple-flowered rhododendron, Rhododendron ponticum (Figure 2.26*). It thrives when introduced to Britain and Ireland, and has escaped to fill many woodlands there, yet its natural range was confined to the mountains of Spain, the Balkans and Turkey. The same plant turns up as fossils from an earlier warm period in Britain, so we know that it once grew there too. It seems that Rhododendron was pushed back by ice age cold and aridity and then never managed to regain its former range, largely due to bad luck by being hemmed in by nearby areas of unsuitable climate. It was only when humans helped it out by importing it as a garden shrub that Rhododendron ponticum managed to make the leap to favorable climates in northwest Europe.
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