The timing of life history events, such as germination and reproduction, can have profound impacts on individual fitness, population dynamics, and distribution across the landscape. Many organisms have evolved responses to seasonal environmental cues, such as temperature and precipitation, to time these critical life history functions with favorable conditions. These responses interact with local variation in seasonal conditions to define the seasonal climate niche, and may reflect local adaption. In variable or changing environments, cues can become less informative or cause mismatches between life history timing and optimal conditions. Here, we examine variation in cue responses for timing of germination and flowering in Streptanthus tortuosus, which displays remarkable life history variation in the timing of germination, first reproduction (annual vs. biennial/perennial), and number of reproductive episodes (semelparous vs. iteroparous). Further, we explore correlations among germination traits, morphological and functional traits, and reproductive traits and how those vary across populations. Finally, we consider how these patterns influence responses to climate change, particularly shifting temperature and precipitation regimes.
Results/Conclusions
Our results suggest that the seasonal niche for S. tortuosus is highly influenced by temperature and precipitation, cues which are likely to shift year-to-year and in response to climate change. Populations from higher elevations, which germinate after spring snowmelt, had stronger chilling requirements for germination and flowering. In contrast, low elevation populations did not require chilling to germinate, germinated under a broader range of temperatures, and had lower chilling requirements for flowering. These patterns are expected to interact with past and future climate change to influence life history timing, fitness, and population persistence. In the last three decades, winter temperatures have increased and summer drought conditions have intensified, with the most pronounced changes occurring at higher elevation sites. Similarly, spring snowpack levels have decreased for high elevation populations. Forecasts suggest these shifts will continue, which is likely to alter life history timing and have strong impacts on fitness, selection on traits, and the potential persistence of these populations. The critical question is whether adaptation can occur fast enough for evolutionary response, and how plastic and genetic changes in life history traits will affect the fate of populations.