A Two-dimensional Model for Water Uptake by Desert Succulents: Implications of Root Distribution

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Abstract
Water uptake by Agave deserti and Ferocatus acanthodes was predicted using a two-dimensional simulation model in which the soil around a plant was divided into a series of layers and concentric cylindrical shells. Root lengths in 0.05 m thick soil layers were determined for both species in the field, where mean root depths were only 0.11 m for A. deserti and 0.10 m for F. acanthodes. For a year with average precipitation (159 mm), 42 per cent of the annual precipitation could be taken up by A. deserti and 25 per cent by F. acanthodes. Predicted water uptake by both species was greater from the upper soil layers (above 0.15 m) for average and dry years, but was greater from the deeper layers for a wet year. The actual root distribution for both species led to more water uptake than when all of the roots were in a single layer. The large number of days per year when the soil temperatures exceeded 57 °C (the temperature for 50 per cent inhibition of uptake of a vital stain by root cells) may exclude roots from the 0.00–0.05 m soil layer, even though water uptake when all roots were located there was predicted to be maximal. Therefore, the observed root distribution of A. deserti and F. acanthodes may be limited near the soil surface by high temperatures and at maximum depths by water availability for all but wet years.