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Mature columnar cardon (Pachycereus pringlei) and saguaro (Carnegiea gigantea) cacti sometimes grow in very dense stands without apparent effect on growth; their seedlings and young plants are commonly found in clusters under legume nurse trees. The potential preference between space and water of young cardon was quantitatively measured under controlled environments for 30 months. The assessment used two types of experiments, one with different plant densities and the other of two plant densities combined with different irrigation regimes. Increases in population density reduced height and dry weight of the plants, but increased their volume and hydration; the water potential of the plants and the soil was less negative for denser populations. Addition of water above the minimum moisture required for growth made water potential less negative in plants and soil. The denser the population in a pot, the less soil surface was exposed to hot air. We conclude that water evaporation from soil surface from exposure to hot air during cultivation of this cactus was significantly reduced when the population density increased. Since loss of soil water from transpiration by cacti is very limited, evaporation directly from soil surface becomes dominant. With less evaporation under high plant density (shading), more water remains in the plant-soil system to be available for storage in the plant tissue. (C) 2010 Elsevier B.V. All rights reserved.