The success of Japanese stiltgrass as an invader may be due to its ability to respond to stochastic events (e.g., by sexual reproduction via chasmogamous [CH] flowers) and to maintain a beneficial genetic make-up (e.g., by self-fertilizing via cleistogamous [CL] flowers) when conditions are stable. This paper evaluates the importance of Japanese stiltgrass seed type (chasmogamous seeds, cleistogamous seeds, and seeds originating from forest-interior [F-I] plants) in terms of seed mass, viability, and germination across variable moisture regimes (three regions in West Virginia) and at two light levels (roadside and forest interior). Seeds from nine populations were sampled in three site types in 2005 and 2008 and stored at 5 C until testing in April 2009. Seeds were tested for viability using a dye test. Seeds were germinated under both constant and fluctuating day/night temperatures. Additional samples of CH and CL seeds collected in 2008 were tested for viability again in September 2010 for a measure of seed longevity. CL and F-I seeds were smaller in mass than CH seeds. Seeds from the drier sites were smaller in mass than seeds from the more mesic sites. CL seeds, followed by F-I seeds, were less viable than CH seeds in 2005 and 2008. CL and F-I seeds had lower germination rates than CH seeds for each site type in 2005, but germination rates of the seed types did not differ in 2008. Differences in seed longevity for 2008 seeds were lower for CL compared to CH seeds, but only in the drier sites. Japanese stiltgrass' longer-lived and larger CH seeds from the roadsides may ensure population survival over the long term. Younger CL and F-I seeds differ less from CH seeds in terms of germination than older seeds, which may help Japanese stiltgrass to maintain populations under relatively stable conditions in the short term.
Nomenclature: Japanese stiltgrass; Microstegium vimineum (Trin.) A. Camus.
Interpretive Summary: The differences in viability of the three seed types (CH, CL, and F-I) revealed in this study add support for focusing management efforts on Japanese stiltgrass populations in which CH seed production is highest, such as those along roadsides or in open areas exposed to higher light levels. The relative importance of the CH seeds suggests that Japanese stiltgrass has adapted to less-stable conditions (i.e., seeds that are a product of out-crossing are of more use than seeds that are product of self-fertilization). Nonetheless, Japanese stiltgrass produces large amounts of the less-viable CL seeds, which may enable it to maintain populations during stable conditions in the short term. Control treatments such as mowing (prior to flowering) stiltgrass populations in higher-light areas, such as roadsides, is a way to minimize CH seed output. Although any remaining upper CL flowers may transform into CH inflorescences, the amount of CH seeds produced should still be reduced. Treatment with herbicide of the roadside Japanese stiltgrass populations may better ensure that the forest interiors are not receiving a steady supply of CH seeds with comparatively high viability. The influence of regional moisture variation on Japanese stiltgrass seeds is less defined than the effects of light, but may be related to drought-induced seed dormancy of CH seeds. Seed longevity of both CL and F-I seeds is shorter in drier environments compared to seed longevity of CH seeds, which indicates that removal of Japanese stiltgrass populations that produce the most viable CH seeds may be even more effective in drier environments than in mesic environments. It is conceivable that after removing roadside populations of Japanese stiltgrass, small forest populations could eventually go extinct, especially under drier conditions.