Adaptation to semi-arid environments, namely in the Mediterranean, may be used as a paradigm for the range of plant traits adaptive to water scarcity. In Figure 6.3a, plants of group I have drought-avoiding behaviour without photosynthetic active parts during dry periods but survive in a resistant form. These are a majority in the flora of most semi-arid and arid environments (e.g. annuals, chamaephytes). Another extreme is plants of group II, which are water spenders without tolerance of dehydration, exploiting specific habitats that permit access to water during most of the year. The other groups in Figure 6.3 consist of 'drought persistent' (i.e. perennial plants that maintain some photosynthesis during the dry periods) according to Noy-Meir (1973). Some of these are true xerophytes, but others may be very vulnerable to climate change such as the lauroid schlerophyllous (group V), which are relicts from the Tertiary, such as Arbutus and Myrtus, that may be eradicated if rainfall becomes more irregular than in the present period (Figure 6.3b). Groups III and IV succeed either by avoiding dehydration through stomatal closure (group III) or by some dehydration avoidance (e.g. deep rooting) and a variable degree of tolerance to dehydration (Valladares et al., 2004b).
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