Water limitation is a primary driver of plant geographic distributions and individual plant fitness.  Drought resistance is the ability to survive and reproduce despite limited water, and numerous studies have explored its physiological basis in plants. However, it is unclear how drought resistance and trade-offs associated with drought resistance evolve within plant clades. We quantified the relationship between water availability and fitness for 13 short-lived plant taxa in the Streptanthus clade that vary in their phenology and the availability of water in the environments where they occur. We derived two parameters from these relationships: plant fitness when water is not limiting, and the water inflection point (WIF), the watering level at which additional water is most efficiently turned into fitness.  We used phylogenetic comparative methods to explore trade-offs related to drought resistance and trait plasticity, and the degree to which water relationship parameters are conserved.

Taxa from drier climates produced fruits at the lowest water levels, had a lower WIF, flowered earlier, had shorter lifespans, more plastic water use efficiency (WUE), and lower fitness at non-limiting water.  In contrast, later-flowering Streptanthus taxa from less xeric climates experienced high fitness at non-limiting water but had no fitness at the lowest water levels. Across the clade, we found a trade-off between drought resistance and fitness at high water, though a single ruderal species was an outlier in this relationship. Our results suggest that drought escape trades off with maximal fitness under non-limiting water, and both are tied to phenology. We also found that variation in trait plasticity determines how different plant species produce fitness over a water gradient.