Demography of herbivorous mammal populations may be affected by changes in predation, population density, harvesting, and climate. Whereas numerous studies have focused on the effect of single environmental variables on individual demographic processes, attempts to integrate the consequences of several environmental variables on numerous functional traits and demographic rates are rare. Over a 32-year period, we examined how forage availability (vegetation assessed through NDVI) and population density affected the functional traits and demographic rates of a population of Columbian ground squirrels (Urocitellus columbianus), an herbivorous hibernating rodent. We focused on mean population phenology, body mass, breeding success and survival. We found a negative effect of population density on demographic rates, including on breeding success and pup and adult survival to the next year. We found diverging effects of vegetation phenology on demographic rates: positive effects of earlier start to growing season on adult female and juvenile survival, but no clear effect on male survival. Interestingly, neither population density nor vegetation affected population phenology or body condit