Variation in life history expression in response to seasonal climate drives fitness and population dynamics in a native California wildflower

Jennifer R. Gremer and Johanna Schmitt, Center for Population Biology, University of California, Davis, Davis, CA, Jennifer R. Gremer and Elena Suglia, Evolution and Ecology, University of California, Davis, Davis, CA, Alec Chiono, University of California, Davis, Davis, CA

Background/Question/Methods
In variable environments, timing is everything. Timing life history transitions with favorable conditions is critical but may be difficult in variable environments, especially as climate change shifts mean conditions and increases environmental variability. Such shifts change the environmental cues for timing life history transitions, but also affect the fitness consequences of life history timing. In plants, fitness depends on three critical transitions: 1) when to germinate, 2) when to switch from vegetative growth to reproduction, and 3) how many times to reproduce (semelparity vs. iteroparity), which are typically studied in isolation. Here we investigate how these life history schedules interact to affect fitness and population dynamics in a native California wildflower, Streptanthus tortuosus. This species inhabits variable Mediterranean climates and exhibits extraordinary variation in life history timing, both within and among populations. We used five years of phenology and demography data for five populations along an elevational and climatic gradient to compare variation in life history timing across populations and assess the fitness consequences of timing. Further, using demographic models, we explore optimal strategies and how they compare under current conditions as well as those expected with future climate change.
Results/Conclusions
Our results reveal that populations are highly sensitive to temperature and precipitation cues for the timing of life history transitions, and that there is substantial variation in life history cueing across populations along the elevational gradient. Low elevation populations germinate in fall in response to germination-triggering rain events, while high elevation populations germinate in the spring, after receiving winter chilling. Our results also reveal variation in germination timing within populations, which correspond with differences in life history timing later in the life cycle, including probability and timing of flowering, as well as fitness consequences. In a low elevation population, early cohorts were more likely to flower and had higher fitness than later cohorts, but these effects were mediated by whether individuals expressed annual versus perennial life histories. Our study reveals insights into the forces shaping life history timing, how the consequences of that timing cascade throughout the life cycle to influence fitness, and how these patterns affect population dynamics in variable environments.