Population histories of variable reproductive success and low winter precipitation correlate with risk-averse seed germination in a mediterranean-climate winter annual

Premise

Seed germination involves risk; post-germination conditions might not allow survival and reproduction. Variable, stressful environments favor seeds with germination that avoids risk (e.g., germination in conditions predicting success), spreads risk (e.g., dormancy), or escapes risk (e.g., rapid germination). Germination studies often investigate trait correlations with climate features linked to variation in post-germination reproductive success. Rarely are long-term records of population reproductive success available.

Methods

Supported by demographic and climate monitoring, we analyzed germination in the California winter-annual Clarkia xantiana subsp. xantiana. Sowing seeds of 10 populations across controlled levels of water potential and temperature, we estimated temperature-specific base water potential for 20% germination, germination time weighted by water potential above base (hydrotime), and a dormancy index (frequency of viable, ungerminated seeds). Mixed-effects models analyzed responses to (1) temperature, (2) discrete variation in reproductive success (presence or absence of years with zero seed production by a population), and (3) climate covariates, mean winter precipitation and coefficient of variation (CV) of spring precipitation. For six populations, records enabled analysis with a continuous metric of variable reproduction, the CV of per-capita reproductive success.

Results

Populations with more variable reproductive success had higher base water potential and dormancy. Higher base water potential and faster germination occurred at warmer experimental temperatures and in seeds of populations with wetter winters.

Conclusions

Geographic variation in seed germination in this species suggests local adaptation to demographic risk and rainfall. High base water potential and dormancy may concentrate germination in years likely to allow reproduction, while spreading risk among years.