Germination Response of Artemisia tridentata (Asteraceae) to Light and Chill: Patterns of Between-Population Variation

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Abstract
Artemisia tridentata Nutt. is the dominant shrub over much of semiarid North America. It consists of several subspecies and includes ecotypes that occur over a range of habitats from warm desert fringes to montane forest and meadow communities. Fifteen seed collections representing the three common subspecies [ssp. tridentata, ssp. vaseyana, ssp. wyomingensis] and a spectrum of habitats were subjects to a series of laboratory light and chill treatments. Recently harvested seeds were mostly nondormant at 15 C but required light for full germination. Removal of the pericarp, afterripening in dry storage, and short chill treatments all resulted in a reduction in light requirement. When examined on a by-collection basis, seed germination response variables were significantly correlated with each other and with mean January temperature at the seed collection site, an index of winter severity. Collectons from colder sites were 16%-36% dormant in the light at 15 C and nearly 100% light-requiring, while collections from warm desert fringes were nondormant in light and only 50%-70% light-requiring. Cold winter collections required up to 98 d to germinate to 50% at 1 C in light, whereas warm winter collections germinated to 50% in as little as 16 d. There was no clear relationship between germination behavior and subspecific identity. The observed climate-correlated variation in germination response to light and chill appears to be of adaptive significance, but a genetic basis for patterns of infraspecific variation in germination response in this species has not yet been demonstrated.