Studies on the ecological life cycle of the native winter annual grass Alopecurus carolinianus, with particular reference to seed germination biology in a floodplain habitat

Alopecurus carolinianus is a widely-distributed native winter annual grass that grows in the same cultivated floodplain habitat as the narrowly-endemic winter annual Lesquerella lescurii (Brassicaceae). Most seeds of A. carolinianus buried under nonflooded conditions and exposed to seasonal temperature changes had an annual dormancy/nondormancy cycle, being dormant in spring and nondormant in autumn. However, 1-16% of the seeds exhumed in spring could germinate at 15/ 6 and 20/10 degreesC. Nondormant seeds were not induced into dormancy by a 12-wk period of burial at 5 degreesC; mean monthly minimum temperatures were about 1-3 degreesC when buried seeds entered dormancy. Seeds flooded in October, November, or December under natural temperature conditions germinated to 61-99% at 15/6, 20/10, and 25/15 degreesC the following May, but those flooded in February, March, or April germinated to only 1-23% at these temperatures. Thus, flooding prior to the time that temperatures are low enough to induce dormancy could result in many seeds being capable of germinating in spring. Plants flowered under short and long days and had little or no vernalization requirement for flowering. Consequently, plants from spring-germinating seeds can complete their Life cycle if not destroyed by soil cultivation. The species has a small, relatively short-lived persistent seed bank. In contrast, buried seeds of L,. lescurii have an annual dormancy/nondormancy cycle with dormancy induced at temperatures of 20/10 and 15/6 degreesC in October and November (Baskin t al. 1992). Lesquerella lescurii also has a large, long-lived seed bank, which ensures persistence at the site if no seed production occurs in spring. Although flooding may destroy all A. carolinianus plants from autumn-germinating seeds, it also prevents many seeds from entering dormancy. Production of seeds by A. carolinianus plants from spring-germinating seeds would help ensure persistence at the site and compensate for lack of prolonged seed viability.