The effect of plant architecture on drought resistance: implications for the evolution of semelparity in Erysimum capitatum

1. Constraints of resource allocation between reproduction and adult survival have been implicated in much life-history variation, yet physiological or functional trade-offs with juvenile survival may be just as important. Here, we examined selection on a juvenile trait that is a key determinant of semelparous (monocarpic) vs. iteroparous (polycarpic) life-history expression. 2. In Erysimum capitatum, iteroparous plants produce more rosettes at the juvenile stage than do semelparous plants; those rosettes perennate, enabling subsequent reproductive episodes. Thus, the number of rosettes produced before reproduction is a strong determinant of iteroparity. We tested whether increased rosette production compromised juvenile survival under conditions similar to those in which semelparity predominates over iteroparity. 3. Using plants from six natural populations, we tested the association between rosette production and juvenile survival under drought conditions typical of the field sites of semelparous E. capitatum populations. We also manipulated rosette number by physically removing rosettes and examined the effect of rosette removal on drought resistance. 4. Under drought conditions, plants with fewer rosettes had higher survival, and the physical excision of rosettes improved survival (significantly or marginally) under drought stress in five of six natural populations. 5. The lower production of rosettes, typical of semelparous E. capitatum, was associated with increased juvenile survival under drought stress. The results suggest adaptive differentiation of rosette production, at least partially in response to drought stress. Given the role of apical dominance in multiple rosette development, natural selection seems to favour stronger apical dominance under drought conditions. Drought stress is predicted to be more common at high elevation as a result of climate change, and the novel drought stress could increase juvenile mortality of alpine E. capitatum. Because rosette production at the juvenile stage is necessary for iteroparity, these results demonstrate that drought-induced selection on traits that determine early survival has significant potential to influence the evolution of adult life-history expression.