Adaptation to current and future climates can be constrained by trade-offs between fitness-related traits. The timing of seedling emergence strongly influences plant fitness in seasonal environments, but a trade-off between earlier within-year emergence and greater spreading of emergence among years could constrain adaptive evolution. Consequently, selection for both earlier within-year emergence and greater among-year emergence — as is expected in more arid environments — may result in combinations of trait values for which gains in either function can only be achieved with a loss in the other (i.e., Pareto optimal). We sourced seeds of two grasses, Stipa pulchra and Bromus diandrus, from multiple maternal lines within populations across an aridity gradient in California and subjected them to 5 months of watering in the greenhouse. We monitored emergence and assayed nongerminated seeds for viability to determine seed persistence, a metric of potential among-year emergence.
Results/Conclusions
In both species, maternal lines with larger fractions of persistent seeds emerged later, indicating a trade-off between earlier within-year emergence and greater among-year emergence that can constrain adaptation. Site aridity was not a strong predictor of emergence traits when each was considered individually. However, when considered jointly, in both species the source populations that were Pareto optimal for early emergence and high seed persistence fraction occupied significantly more arid sites than non-Pareto optimal populations, consistent with constrained evolutionary responses to selection for earlier within-year emergence and greater among-year emergence. Our results demonstrate how Pareto optimality can be applied to the joint responses of associated traits when evaluating adaptation across environmental gradients.