Climate change has important consequences for plant communities from the leaf to ecosystem scale. Shifting climatic regimes can cause changes in species composition and shifts in the onset of plant growth and reproduction. Dryland plant communities are expected to be particularly vulnerable to climatic change, especially when they are pushed beyond their historical range of variability. One source of historical data that has received considerable attention over the last several decades is herbarium specimens, which have proven to be an important resource for tracking plant phenology with changing climate. We used herbarium specimens to ask: how has the flowering and fruiting time of big sagebrush (Artemisia tridentata), a widespread and important shrub species in the western US, shifted over the last two centuries? We utilized 1,670 big sagebrush specimens from multiple herbaria dating back to 1873 to explore shifts in flowering and fruiting over the last 144 years. We also quantified more recent trends in the last 50 years. We removed redundant collections and those without day or year information. We calculated a day of year (doy) for all specimens and determined the relationship between flowering and fruiting date and year using Pearson correlations.
Results/Conclusions
Since 1873, flowering has shifted 17 days earlier on average (r = -0.09 between flowering doy and year), and the strength of that relationship increased over the last 50 years (r = -0.15). The timing of fruiting also shifted to earlier in the year (68 days earlier on average) and the relationship between fruiting doy and year was stronger than for flowering (r = -0.28 and r = -0.36 since 1911 and 1970, respectively). These results indicate smaller advances and more variability in flowering through time and more substantial and consistent shifts in fruiting to earlier in the year. The timing of fruit set can have large implications for big sagebrush regeneration, as it cannot reproduce vegetatively. Earlier fruiting may decrease regeneration success, as late summer is expected to be warmer and drier in the future, especially in low-elevation sites and in the southwestern US. However, earlier fruiting may increase regeneration success in cold, wet big sagebrush sites at high elevation, where moisture is sufficient and minimum winter temperatures limit establishment. Our results have implications for the future distribution of big sagebrush plant communities and the species that depend on them.