Germination responses to changing rainfall timing reveal potential climate vulnerability in a clade of wildflowers

Abstract

1. The timing of germination, driven by seasonal cues, is critical for the life cycle of plants. Variation among species in germination responses can reflect evolutionary processes and adaptation to local climate and can reveal vulnerability to changing conditions. Climate change is altering the timing of precipitation and associated temperatures, which may interact with germination cueing to affect the timing, quantity, and speed of germination. Germination responses to climate change can then have consequences for individual fitness, population dynamics, and species distributions. 2. Here we assessed responses to the timing of germination-triggering rains and corresponding temperatures for 11 species across the Streptanthus (s.l.) clade of Brassicaceae. To do so, we experimentally manipulated the onset date of rainfall events and measured effects on germination fraction and rate. We also evaluated how responses varied with phylogeny. We then explored the possible consequences of these responses to contemporary shifts in precipitation timing. 3. Later onset rains and cooler temperatures significantly decreased germination rates for all species. Germination fractions decreased with later rains and cooler temperatures for all species except three Caulanthus species. We found that six species are likely already experiencing significant decreases in germination fractions and/or germination rates with observed climate change, which has shifted the timing of rainfall towards the cooler, winter months in California. Species germination responses to the timing of rainfall and seasonal temperatures were phylogenetically constrained, with Caulanthus species appearing less sensitive. 4. Synthesis. Across the Streptanthus clade, later onset of seasonal rains during cooler temperatures decreases germination fractions and rates. Contemporary shifts toward later rainfall offset the effects of ongoing climate warming and may already be negatively affecting germination in several species. Species germination responses may have evolved along the phylogeny, with Caulanthus species, from drier and more variable climates, found to be less sensitive to the timing of rainfall events or their associated temperatures than species from wetter, less variable environments.