Natural disturbance shaped forest communities for millennia, but fire suppression and timber harvest declines have altered forest structure across the western United States, reducing the abundance of forage for ungulates. We evaluated quality and quantity of forage resources for lactating elk (Cervus canadensis) and their calves in relation to season, succession, and biogeoclimate, the latter indexed by potential vegetation (PV) zones, across 36,500 km² in Idaho's Clearwater and St. Joe River basins. In 0.2-ha macroplots (n = 359), we measured characteristics of forest overstory, biomass of current annual growth of undergrowth vegetation (kg ha^-1), and nutritional content of these plants. Using biomass, digestible energy (kJ g^-1), digestible protein (DP, g per100 g forage), and prior knowledge of elk diet selection and nutritional constraints, we developed 8 forage resource metrics. The greatest abundance of undergrowth vegetation (500 to 1,000 kg ha^-1) occurred during the first 20 years after stand-replacing disturbance and declined as the overstory closed in wetter PV zones. Digestible energy decreased, whereas DP increased as stands aged. Evidence of nutritional limitations for lactating elk increased markedly after mid-summer; early-seral, high-elevation spruce–fir forests on productive soils provided the best opportunity for lactating elk to satisfy their requirements in late summer. Our findings demonstrate the importance of disturbance regimes that maintain early-seral communities in mosaics with mid- and late-seral stages, and suggest that implementing stand-replacing disturbance in relatively moist forest zones at mid and high elevations provides the greatest improvement in forage resources for lactating elk and their calves in summer.