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

Winter annual legumes in the genus Medicago (medics) are native to arid 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 US receiving between 100-200 mm winter precipitation (November-May). Five accessions from four species M. laciniata, M. littoralis, M. polymorpha, and M. truncatula that could avoid drought were identified in a preliminary screening nursery. 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 one accession (M. truncatula "Cyprus'). New plant re-establishment and seed production was observed in 1990-91 for all five accessions from seed produced in 1989-90 with supplemental irrigation (300 mm) in addition to precipitation. Rapid maturing medics that exhibit breakdown of hardseededness by autumn appear to be well adapted to S Arizona sites receiving as little as 110 mm winter precipitation. If such introductions are to be successful, initial seeding rates >115 pure live seeds/m² may be necessary to develop a large soil seed bank. -from Authors