Populations of woodland caribou (Rangifer tarandus caribou) are declining throughout their range and many are at risk of extirpation, yet the role of nutrition in these declines remains poorly understood, in part owing to a lack of information about available nutritional resources during summer. We quantified rates of in-take of digestible protein and digestible energy by tame caribou foraging in temporary enclosures in the predominant plant communities of northeastern British Columbia, Canada, during summer–autumn and compared intake rates to daily requirements for protein and energy during lactation. We tested hypotheses related to the nutritional adequacy of the environment to support nutritional requirements during lactation (with and without replenishment of body reserves) and simulated scenarios of foraging by caribou in these plant communities to better understand how wild caribou could meet nutritional demands on these landscapes. Nutritional resources varied among plant communities across seasonal, ecological, and successional gradients; digestible energy intake per minute and per day were significantly greater in younger than older forests; dietary digestible energy and per‐minute and daily intake of digestible protein were greater, though not significantly so, in younger than older forests; and dietary digestible protein was greater in older than younger forests, though differences were not significant. Tame caribou were unable to satisfy protein and energy requirements during lactation, even without replenishment of body reserves, at most sites sampled. Further, foraging simulations suggested widespread nutritional inadequacies on ranges of wild caribou. Selection for habitats offering the best nutrition may mitigate some nutritional inadequacies, but given low availability of vegetation communities with high nutritional value, performance (e.g., calf production, growth, replenishment of body fat and protein) of caribou may be depressed at levels of nutrition documented herein. Our results, coupled with recent measurements of body fat of wild caribou in northeastern British Columbia, refute the hypothesis that the nutritional environment available to caribou during summer in northeastern British Columbia is adequate to fully support nutritional demands of lactating caribou, which has implications to productivity of caribou populations, recovery, and conservation.