Can grass phenology in the western U.S. track climate change?
Grasslands are important sources of biodiversity and productivity, but may be vulnerable to climate change because they contain many shallow-rooted, short-lived species. The capacity of grass species to alter life cycle events in response to changes in their environment (phenological plasticity) can be an important indicator of their ability to persist under climate change. We used long-term records (1878 – present) of grass reproductive phenology for over 100 species in the western U.S. to assess which species had the highest phenological plasticity, where plasticity was most pronounced, and which aspects of climate were associated with phenological shifts. We also explored how phenological plasticity varied by grass photosynthetic pathway, longevity, and other traits.
Our results demonstrate high variability in the degree to which perennial grasses in the western U.S. shifted their phenology in response to climate. Increasing temperatures accelerated reproduction of C3 grasses by as much as 7 days/°C but generally delayed reproduction of C4 grasses. Seasonality of precipitation influenced phenology, as increases in summer precipitation delayed reproduction of C4 grasses and had little effect on C3 grasses. The timing of reproduction in many annual grasses was over 2x as plastic as perennial grasses. We also found that phenological shifts and associated precipitation and temperature drivers were not uniform across ecoregions and were largely influenced by climatic regime. Our results suggest that grasses have different capacities to track climate, which has important implications for future grassland composition and resilience.