Comparative achene germination requirements of the rockhouse endemic ageratina luciae-brauniae and its widespread close relative A. altissima (asteraceae)

Ageratina luciae-brauniae is endemic to sandstone rockhouses (semicircular recesses under cliff overhangs) in Tennessee and Kentucky, and is closely related to the geographically widespread species A. altissima. At the time of normal maturity in September, 45-85% of A. luciae-brauniae achenes germinated in light and 0-22% germinated in darkness at 12/12 h daily thermoperiods of 15/6, 20/10, 25/15, 30/15 and 35/20 C. In contrast, at the time of normal maturity in October, 0-53% of A. altissima achenes germinated in light, and 0% germinated in darkness over the same range of thermoperiods. After 12 wk of cold stratification in light at 5 C, 96-100% and 97-100% of the achenes of A. luciae-brauniae germinated in light and darkness, respectively, over the range of the thermoperiods, and after stratification in darkness, 73-100% and 34-92% germinated in light and darkness, respectively. After 12 wk of cold stratification in light at 5 C, 64-99% and 22-73% of the achenes of A. altissima germinated in light and darkness, respectively, over the range of the thermoperiods, and after stratification in darkness, 55-99% and 1-13% germinated in light and darkness, respectively. Peak germination of A. luciae-brauniae achenes sown in a greenhouse at near-natural temperatures in early October 1987 occurred in late October 1987, whereas peak germination of A. altissima achenes sown in late October 1994 occurred in mid-March 1995. Germination characteristics do not explain why A. luciae-brauniae is an endemic or why A. altissima is widespread. However, they do suggest why achenes of A. luciae-brauniae could germinate in rockhouses in autumn and why those of A. altissima do not germinate until spring.