Precipitation niche is a key aspect of plant habitat that is rapidly changing in vast areas with historic droughts. Many studies have explored the physiological basis of drought tolerance and avoidance in plants, but it is unclear how water relations evolve in plant clades in order to connect broad patterns of habitat use with drought tolerance. We grew 13 short-lived Streptanthus taxa in seven climate-relevant watering levels spanning half the mean 30-year minimum of precipitation the driest species to the mean 30-year precipitation of the wettest species to assess drought performance curves. We used phylogenetic comparative methods to test
key hypotheses about the evolution of drought tolerance.
We found evidence for a tradeoff between drought tolerance and maximum fecundity that was linked to plant phenology and their home environment. We found that drought tolerance/avoidance had marginal phylogenetic signal. The water level at peak performance closely mirrored the long-term precipitation trends at field sites for each population, suggesting local adaptation to water availability. We found evidence for a tradeoff between drought tolerance (fitness at low water) and maximum lifetime fitness, though this tradeoff was not apparent in the one ruderal species. Differences in drought-performance curves among Streptanthus species were largely driven trade-offs with phenology, where early-flowering species reproduced despite low water conditions but never achieved high fecundity under ample water conditions.