Influences of root distribution and growth on predicted water uptake and interspecific competition.

Root distribution and growth measured in the field were incorporated into a water uptake model for the CAM succulent Agave deserti and its nurse plant Hilaria rigida, a common desert bunchgrass. Agave deserti responds to the infrequent rainfalls of the Sonoran Desert by extending its existing established roots and by producing new roots. Most of such root growth was completed within one month after soil rewetting, total root length of A. deserti increasing by 84% for a seedling and by 58% for a mediumsized plant in the summer. Root growth in the winter with its lower soil temperatures was approximately half as much as in the summer. For a 15-year period, predicted annual root growth of A. deserti varied more than 18-fold because of annual variations in rainfall amount and pattern as well as seasonal variation in soil temperature. Predicted annual water uptake varied 47-fold over the same period. The nurse plant, which is crucial for establishment of A. deserti seedlings, reduced seedling water uptake by 38% during an average rainfall year. Lowering the location of the root system of a medium-sized A. deserti by 0.24 m reduced its simulated annual water uptake by about 25%, reflecting the importance of shallow roots for this desert succulent. Lowering the root system of a medium-sized H. rigida by 0.28 m increased the simulated annual water uptake of an associated A. deserti seedling by 17%, further indicating the influence of root overlap on competition for water.