The timing of life history events, such as germination and reproduction, influences ecological and selective environments throughout the life cycle. Often the fitness consequences of each life history transition depend on previous and subsequent events in the life cycle. For species with such complex, contingent life histories, shifts in environmental conditions can create cascading effects throughout the life cycle, which can influence fitness, selection on life history traits, and population viability. Many organisms evolve responses to environmental cues, such as temperature, day length, or precipitation, in order to time these key life history events with favorable conditions. However, in variable or changing environments, such cues can become less informative or cause mismatches between life history timing and optimal conditions. We examined variation in cue responses for the timing of germination and flowering in Streptanthus tortuous, which displays remarkable life history variation in timing of first reproduction (annual vs. biennial/perennial) and number of reproductive episodes (semelparous vs. iteroparous). We then manipulated seasonal germination timing and winter chilling to evaluate the fitness consequences of contingent life history trajectories for a low elevation population at the warm, trailing edge of the species range.
Results/Conclusions
Our results reveal population differences in life history cueing, which create contingency in life history expression across populations. Populations from higher elevations, which germinate after spring snowmelt, had stronger chilling requirements for germination and flowering and were more likely to perennate after flowering than low elevation populations. Germination timing also affected life history expression within populations. Within a low elevation population, fall-germinating plants were more likely to flower in the first spring and less likely to perennate and flower the next year than spring germinants. This seasonal life history contingency, mediated by a winter chilling requirement for flowering, may maintain annual, biennial, and iteroparous perennial life histories within low elevation populations. By manipulating the chilling cue, we discovered that the chilling requirement for first year flowering can reduce lifetime fitness for the low elevation population, and is therefore maladaptive. Warming winters and increasing mortality from summer drought stress will favor annual life histories, selecting for genotypes with lower chilling requirements and earlier germination to maximize first year fitness.