Trade-offs in performance between contrasting environments represent potential limits to species' geographic distributions. A common pattern in annual plants is the occurrence of populations at arid margins of a species' range that are early-flowering, self-pollinating, and vegetatively small. This pattern occurs, for example, in the Californian winter annual Clarkia xantiana (Onagraceae), in which ongoing experiments are testing the hypothesis that early flowering evolved via selection to avoid early summer drought, while small size and other traits evolved as correlated characters. The present study investigates whether C. xantiana subspecies with contrasting life histories exhibit a trade-off between vegetative size and competitive ability. Across several measures of performance, competition with the introduced grass Bromus tectorum reduced individual performance of the small-statured, self-pollinating subspecies (parviflora) significantly more than that of the larger-statured, outcrossing subspecies (xantiana). Studies of seedling competition show that parviflora individuals' smaller initial size helps explain their poorer competitive performance, while other unexplained differences between the subspecies also contribute. Field studies confirmed that parviflora generally occupies more arid regions than xantiana and that where they occur sympatrically, parviflora occupies less-productive sites than xantiana. Differences in competitive performance between the subspecies may reflect past selection on life history and competitive ability, and they also may limit current geographic distributions at local and regional scales.

Key words: breeding systems, Clarkia xantiana, competition, life-history evolution, Onagraceae