• Annual medic establishment and the potential for stand persistence in southern Arizona

      Brahim, K.; Smith, S. E. (Society for Range Management, 1993-01-01)
      Few perennial legumes have been successfully introduced into western North American rangelands receiving less than 250 mm annual precipitation. Winter annual legumes in the genus Medicago (medics) are native to add sites in North Africa and the Middle East and have been successfully introduced into arid and semiarid rangelands. The objective of this study was to evaluate the potential of establishing medics in areas of the southwestern U.S. receiving between 100 and 200 mm winter precipitation (November-May). Five medic accessions from 4 species (M. laciniata (L.) Miller, M. littoralis Rhode ex Loix. Delong., M. polymorpha L., M. truncatula Gaertn.) that could avoid drought were identified in a preliminary screening nursery in 1987-89. These accessions established and produced seed in 1989-90 in a field plot at Tucson, Ariz., with 125 mm winter precipitation. Less than 5% of all seed produced by these accessions germinated following summer precipitation. Plant re-establishment in the winter 1990-91 (181 mm precipitation) from pods produced in 1989-90 was observed for only 1 accession (M. truncatula 'Cyprus'). New plant re-establishment and seed production was observed in 1990-91 for an 5 accessions from seed produced in 1989-90 with supplemental irrigation (300 mm) in addition to precipitation. Failure to observe comparable establishment from seed produced without irrigation was attributed to the scarcity of germinable (permeable) seeds in the soil seed bank. Rapid maturing medics that exhibit breakdown of hardseededness by autumn appear to be well adapted to southern Arizona sites receiving as little as 110 mm winter precipitation. If such introductions are to be successful, initial seeding rates in excess of 115 pure live seeds/m2 may be necessary to develop a large soil seed bank.
    • Biological and physical factors influencing Acacia constricta and Prosopis velutina establishment in the Sonoran Desert

      Cox, J. R.; Alba-Avila, A.; Rice, R. W.; Cox, J. N. (Society for Range Management, 1993-01-01)
      Over the past century woody plants have increased in abundance on sites formerly occupied by grasslands in the Sonoran Desert. Woody plant invasion has been associated with a multitude of biological and physical factors. This study was conducted to determine temperature, soil, fire, rodent, and livestock effects on the germination and establishment of whitethorn acacia (Acacia constricta Benth.) and velvet mesquite (Prosopis velutina (Woot.) Sarg.). Optimum termination temperatures for both shrubs ranged from 26 to 31 degrees C, and seedling emergence was greatest from seed sown at 1 to 2 cm depths in sandy loam soil. Merriams kangaroo rats (Dipodomys merriami) fed seeds in the laboratory removed seed coats and planted embryos at 2 to 4 cm depths in a sandy loam soil. Prescribed fire killed 100% of seed placed on the soil surface but had no measurable effect on the germination of seed planted at 2 cm. After passage by sheep, about 6% of the A. constricta and 13% of the P. velutina seeds germinated while after passage by cattle, only 1% of the A. constricta and 3% of the P. velutina seed terminated. Embryo planting by rodents may improve survival efficiencies for these legunminous shrub seedlings, but seed consumption and passage by sheep and cattle appear to adversely affect seed germination. Dipodomys merriami, rather than domestic livestock, may be responsible for the spread of these shrubs in the Sonoran Desert.