Premise of study: Forecasting how species will respond phenologically to future

changes in climate is a major challenge. Many studies have focused on estimating

species- and community-wide phenological sensitivities to climate to make such

predictions, but sensitivities may vary within species, which could result in divergent

phenological responses to climate change.

Methods: We used 743 herbarium specimens of the mountain jewelflower (

Streptanthus tortuosus, Brassicaceae)collected over 112 years to investigate whether

individuals sampled from relatively warm vs. cool regions differ in their sensitivity to

climate, and whether this has resulted in divergent phenological shifts in response to

climate warming.

Key results: During the past century, individuals sampled from warm regions exhibited

a 20-day advancement in flowering date; individuals in cool regions showed no

evidence of a shift. We evaluate two potential drivers of these divergent responses:

differences between regions in (1) the degree of phenological sensitivity to climate and

(2) the magnitude of climate change experienced by plants, or (3) both. Plants sampled

from warm regions exhibit higher sensitivities to temperature-related variables and

experienced a greater degree of climate warming than those from cool regions; thus

our results suggest that the greater temporal shift in flowering date in warm regions is

driven by both of these factors.

Conclusions: Our results are among the first to demonstrate that species exhibit

intraspecific variation in sensitivity to climate and that this variation can contribute to

divergent responses to climate change. Future studies attempting to forecast temporal

shifts in phenology should consider intraspecific variation.

Keywords: Phenological shifts, flowering time, herbarium specimens, climate change