Greater Thermal Plasticity Toward Heterogeneous Range-Edge Environments of Three Hypericum Species

Intraspecific variation in phenotypic plasticity can affect the ability of populations, and thus species, to respond to environmental changes. However, the prevalence and drivers of such variation are not well known. Most proposed explanations for intraspecific variation in phenotypic plasticity involve mechanisms associated with a population's position within the species' geographic range or the environmental heterogeneity experienced by the population. To assess the effect of these two drivers, and their potential interaction, we use a combination of germination and greenhouse experiments to measure thermal phenotypic plasticity in traits ranging from germination probability to flower abundance in populations of three Hypericum species sampled across their European ranges. We then relate thermal plasticity to each population's position within the species' range and to the environmental heterogeneity of the sampling site. Our results revealed that, while average thermal plasticity in several traits was similar among the three tested Hypericum species, it varied among the conspecific populations. Specifically, populations closer to the range edge tended to be more plastic in germination probability and plant height, while populations from more heterogeneous environments tended to be more plastic in flowering phenology, plant height, and flower abundance. Interestingly, for plasticity in germination phenology, plant height, and flower abundance, we found a substantial interactive effect with accentuated plasticity in heterogeneous sites near the range edge. This suggests that populations in heterogeneous environments at range edges may adjust to environmental change via phenotypic plasticity more effectively than do other conspecific populations. These results support both tested drivers and reveal important interactive patterns for some of the tested traits. Furthermore, they encourage further research on plasticity that considers both range position and environmental heterogeneity.