It has been said that evolution defies the laws ofphysics and that enzymatic processes liberate living organisms from the dictates of the Arrhenius equation (Barcroft, 1934). If such a statement is justified it might be claimed to be an achievement of high mountain vegetation. It is one of the wonders of nature that plants can survive on the highest mountains in the world while depending on a metabolism based on the making and breaking of the covalent bonds of carbon, which take place spontaneously only at temperatures of 60-70 °C. This achievement has to be judged not in terms of growth rates or biomass accumulation - these are only attributes of interest to potential herbivores (the human race included). From the point of view of the plant itself, growth rates and biomass are characteristics that can be modified to suit their environment. They are also characteristics that can be acted on selectively with relative ease. Twenty million years ago New Zealand was a collection of low-lying tropical islands with a forest dominated by the giant Podocarpus trees. In a space of only ten million years, as the mountains of New Zealand's southern island rose ever higher, so did the flora evolve to occupy these new mountain climates. The same genus that contains the giants of the lowland forest also has evolved its high-altitude counterparts in the diminutive snow totara (Podocarpus nivalis; see Fig. 9.25). Given liquid water and light for a few weeks in the year there is scarcely any habitat to which the flowering plants can not adapt, such is their ability to overcome the apparent obstacles of the physical world.

Fig. 11.1 The Icelandic upland farm of Hraun in Oxnadalur (northern Iceland) in June. Birthplace of the Icelandic poet and botanistJonas Hallgrimsson (1807-1845). A modern example of tenacity of occupation in a marginal environment where late springs and unpredictable growing seasons were a particular feature during the Little Ice Age.

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