Recruitment in variable environments: plasticity and bet hedging in the timing of seed germination in three closely...

Phenotypic plasticity and bet hedging are mechanisms that can facilitate population persistence in variable environments. In plants, the timing of seed germination can be responsive to reliable environmental cues as well as a mechanism for spreading risk when post-germination conditions are difficult to predict. The goal of this study was to quantify and compare plasticity and potential bet hedging in the timing of seed germination in three closely related species that segregate across a fine-scale gradient in hydrological variability.

We conducted a growth chamber experiment that measured variation in seed germination within and among three species of Lasthenia (Asteraceae) that occupy different microhabitats along gradients in soil moisture variability in California vernal pool grasslands. We measured the timing of germination in sibling seed groups from each species under two different moisture regimes to characterize the mean and variability in seed germination timing in response to consistent, high moisture and low, variable moisture conditions.

Germination of viable seeds was extremely high (∼98 %) across all species and treatments. All three taxa showed strong plasticity in response to water treatment by germinating faster with higher, more consistent moisture availability. The two species from higher topographic positions in vernal pool grasslands had greater within-season variance in germination timing, consistent with increased bet hedging, than the deep pool specialist. The extent of within-season bet hedging in these two species was largest in the drier and more variable water treatment, revealing phenotypic plasticity in the extent of bet hedging expressed by these species.

This work supports the hypothesis that plasticity and bet hedging in seed germination timing are favoured in variable and unpredictable environments, and that these strategies can evolve rapidly in association with habitat divergence among closely related species.