Effect of spring green-up dynamics on body mass and growth of bighorn sheep

Abstract Climate change is altering phenological traits across many taxa. While northern temperate species often adjust migration or breeding to track these shifts, such adjustments may be insufficient, leading to mismatches between trophic levels. Most studies focus on changes in the timing of spring onset, but few assess how climate change affects the speed of seasonal changes. Rapid spring green-up can shorten the period of high-quality forage, potentially constraining herbivore nutrition. We investigated how both the timing and duration of vegetation green-up influence body mass in bighorn sheep (Ovis canadensis). Using 23 years of satellite-derived Enhanced Vegetation Index (EVI) data, we quantified the onset, date of maximum green-up rate, and end of green-up, and assessed their effects on lamb weaning mass and female autumn mass. Green-up occurred increasingly early over time, but its duration showed no consistent trend. Green-up date weakly affected female mass, whereas neither green-up date nor duration influenced lamb weaning mass. These findings suggest that while shifts in timing are detectable, their demographic effects are limited. Future research should consider not only the onset of vegetation growth but also its rate and the timing of browning down, as both can shape forage availability and herbivore performance.