| Successful establishment of shrub and tree legumes in the southwestern United States is dependent on rainfall amount, frequency and distribution, seedbed temperatures and available water conditions affecting seed germination and seedling establishment. The line source sprinkler irrigation gradient system (LSSIGS) was used to test effects of different seedbed available water and temperature conditions on seedling emergence and establishment, and morphological and physiological characteristics of catclaw (Acacia greggii), jojoba (Simmondsia chinensis), velvet mesquite (Prosopis juliflora var. velutina) and blue paloverde (Cercidium floridum) during the summers of 1992 and 1993 at the Tucson Plant Materials Center. Irrigation plus precipitation in July and August added 356.7, 344.6, 265.1, 209.8, 106.9 and 76.2 mm of water in 1992 and 285.2, 252.6, 207.7, 186.6, 106.6 and 68.1 mm in 1993 for distances of 1.5, 4.5, 7.5, 10.5, 13.5 (lowest irrigation) and 16.5 (no irrigation) respectively, from the line-source sprinkler. Irrigation created available soil moisture differences between irrigated and unirrigated soils during rainless periods in July but generally created little difference in soil water availability within the irrigation system different distances from the line-source sprinkler. Lack of large soil water availability differences resulted from an initially wet soil profile and natural rainfall in August of both years.; Jojoba seedling emergence lagged about 2-3 weeks behind that of the other species which emerged within a week of irrigation. Seedlings emerged on unirrigated soils after August rains on both years. Although seedling establishment and plant growth was generally highest for the highest levels of irrigation, establishment was acceptable (at least 2 plants per meter of row) at all irrigation levels and on unirrigated soils for all species for both years, except for unirrigated jojoba in 1992. Lack of large differences in available soil water precluded precise determination of the significance of physiological and morphological characteristics for drought tolerance during seedling establishment of these species. However, rapid and high seedling emergence and root growth of paloverde, mesquite, and acacia make these species good selections for abandoned farmland revegetation.; A possible establishment strategy for these 3 species on similar soils in the Sonoran desert would be to irrigate either pre or post-sowing in July to bring the upper 40-60 cm of soil profile to field capacity, then irrigate daily for about a week until seedlings emerge. Irrigation could then be discontinued and seedlings should survive on subsurface soil moisture. Establishment of jojoba might require longer irrigation periods. A more rigorous comparison of the drought tolerance of these species with the LSSIGS would be to sow and irrigate them on an initially dry soil profile. |
