Precipitation and temperature are associated with advanced flowering phenology in a semi-arid grassland

Advances in spring blooming of wildflowers have been associated with climate change; however, the majority of long-term community-level studies have been conducted in humid-temperate regions. Less is known about phenological changes in herbaceous, semi-arid plant communities. We recorded first-bloom date of common spring wildflowers in a semi-arid grassland in the Rocky Mountains, USA from 1995 through 2008 and analyzed these data along with mean monthly temperature and precipitation. Advanced flowering predominated; 75% of the 32 species displayed a negative linear regression slope, and this trend was strong for nine species (slope < 1.0, p < 0.10). Only one species showed a strong trend for later flowering. Mean advance for all 32 species was 0.61 days/year and mean advance for the nine species displaying a strong tendency to flower earlier was 1.6 days/year. Species flowering early showed stronger trends toward advanced flowering than later species. Average March temperature and January plus December precipitation, falling mainly as snow, were explanatory variables in the best multiple linear regression model for mean first-flowering date of strongly advanced species. Our results suggest that flowering phenology may be changing faster and precipitation may play a more important role in semi-arid grasslands than in humid-temperate systems.