In water-controlled ecosystems, water demand by plants is generally higher than water availability. To efficiently cope with water stress, plants have thus developed different strategies, which become more sophisticated the more intense and unpredictable the water deficit is. As we have already seen in the previous chapters, low soil moisture periods bring about an abatement of transpiration and cell turgor which with time may initiate a chain of damages of increasing seriousness. On the other hand, given the external conditions in terms of soil and climate, plants have the possibility to act on some of the components of the water balance to reduce their water stress level. Many species combine a number of complementary measures to do so, the most extreme of which include permanent forms of adaptation, such as changes in resource allocation, specialized growth of roots (e.g., cactuses build a dense network of roots to capture the light rainfall events, while some desert shrubs – the so-called phreatotypes – develop deep roots to tap the water table when present), specialized photosynthetic pathways (e.g., the CAM pathway), short and intense life cycles during favorable periods, dormancy, drought deciduousness, specialized metabolism and leaf structure to reduce water losses (high cuticular resistance, protection and changes in the dimension and density of the stomata), etc. (Jones, 1992; Larcher, 1995).

We will not deal with all this gamut of strategies, but rather focus on some of the less extreme ways of adaptation that are more commonly adopted in semi-arid climates.