Population Differentiation ofImpatiens capensis(Balsaminaceae) at the Range Limit

A plant species that occupies a large geographic range must survive in a variety of environments.Impatiens capensis, an herbaceous annual that grows contiguously from the east coast of North America to the Colorado Rocky Mountains, prefers moist environments and is easily susceptible to drought.Impatienspopulations at the species’ western range limit (Colorado) must endure a dramatically different environment than eastern populations do (Rhode Island and Illinois). This study investigated the stress response of a weedy annual at its range limit by examining how regional populations ofI. capensismorphologically, physiologically, and phenologically adjust to drought. The results suggest that each population has evolved along a different evolutionary axis; each population exhibits a unique strategy for responding to drought stress. Rhode Island populations have evolved along a temporal axis to avoid stress with an accelerated phenology. An Illinois population has evolved along a physiological axis and exhibits a drought-tolerance strategy. Colorado populations have evolved along a morphological axis and tolerate drought. Thus, dealing with drought stress may involve population differentiation along different axes of functional plasticity.