Variation in germination functional traits within and across species suggests climate vulnerability in a clade of native wildflowers

Background/Question/Methods

Germination is a critical step in the life cycle of plants, making quantification of the germination niche pivotal for linking climate change to fitness, population dynamics, and species distributions. Important aspects of the germination niche include cardinal temperatures, i.e., minimum (Tb), optimal (To), and maximum (Tc) temperatures, which describe conditions under which germination can occur (i.e., the germination niche). Determination of cardinal temperatures is uncommon for wild plants and scarcely done at the population-level among closely related species. However, with this information we can identify drivers of population- and species-level variation in germination niches and assess the impacts of increasing temperature from climate change on germination proportions. Using 11 species across the Streptanthus clade of Brassicaceae and its allies, we ask: 1) How do cardinal temperatures compare within and among species?, 2) What drives variation in these temperatures?, and 3) How will future temperature increases alter germination proportions? To address these questions, a thermal germination experiment was conducted and thermal-time models were built to estimate cardinal germination temperatures. Using these, we evaluated how well genetic distance, geographical distance, and climatic variables explained variation in cardinal temperatures. From there, we explored germination patterns expected under future climate change.

Results/Conclusions

Within populations and species, minimum (Tb) and optimal (To) temperature values were found to be more conserved than maximum (Tc) values. Downscaled climate data of germination months (Sep-Jan) explained the greatest variation in cardinal temperatures. Within S. tortuosus, variables that explained maximum amounts of variation among populations differed for each cardinal temperature; Tb: mean temperature plus geographical distance, To: mean temperature plus genetic distance, and Tc: total precipitation plus genetic distance. Preliminary results also suggest that different sets of variables best explain cardinal temperatures among species. Future climates are projected to alter the conditions under which germination occurs, reducing germination proportions in the absence of adaptation. By 2030, projected mean temperatures will already be greater than To for at least one species, suggesting the possibility of population decline, barring rapid adaptation to warmer temperatures. This research shows the ecological and evolutionary relevance of quantifying cardinal temperatures and highlights that closely related species and even populations within species may react differently to changing climate. Further, these patterns may lead to mismatches between optimal germination timing and formerly adaptive temperature cues, contributing to negative fitness consequences in the future.