Most water loss is from transpiration through leaf surfaces, so dropping leaves conserves water in the stems. The roots of drought tolerant shrubs and trees are extensive compared to those of plants in wetter climates, covering an area up to twice the diameter of the canopy. Rooting depth controls opportunities for growth cycles. In contrast to the succulents' shallow-rooted strategy, a substantial rain is required to wet the deeper root zone of shrubs and trees. The tradeoff between this strategy and that of succulents is that once the deeper soil is wetted by several rains it stays moist much longer than the surface layer, supporting several weeks of growth.
Succulents can absorb water only when the soil is nearly saturated.
In contrast drought tolerant plants can absorb water from soil that is much drier. Similarly these plants can photosynthesize with low leaf moisture contents that would prove fatal to most plants. Drought Avoidance Annual plants escape unfavorable conditions by not existing. They mature in a single season, then die after channeling all of their life energy into producing seeds instead of reserving some for continued survival. Most Sonoran Desert annuals will germinate only during a narrow window in the fall, after summer heat has waned and before winter cold arrives.