Figure 1. Moisture, total net productivity and plant species diversity of selected vegetation communities, along an elevation gradient from Santa Catalina Mountains, Arizona (after Whittaker and Niering, (1975)). The elevation gradient ranges from 1000 to 3000 m above sea level. The moisture index relates to precipitation ranges of 190 mm per annum (moisture index 8) and 850 mm per annum (moisture index 1).

Figure 2. Schematic summary of the processes that influence the relationship between plants and soil water.

Figure 3. Soil water availability and soil aeration availability for two representative sandy (solid line) and clayey (broken line) soils. Soil water contents on volume basis is shown against soil water potential (suction) and against air-filled pore space (volume of pore space not occupied by water).

Figure 4. Distributions of eight sedge species showing differentiation in niche space defined by hydrological axes on a fine-scale gradient. The x-axis depicts increasing soil drying stress, while the y-axis shows increasing flooding (i.e. aeration) stress (see the section on water stress and plants for explanation). The vertically hatched area in each graph shows the range of possible hydrological regimes and the solid area indicates the zone in which the species occurs significantly more frequently than by chance. Data are cumulated across 18 different meadow sites (after Gowing et al., (2002)). Reproduced with permission from Her Majesty's Stationery Office.