In temperate regions climate change has led to advances in plant phenology which may disrupt the synchrony between food availability and reproductive requirements of higher trophic levels. Because leaf quality generally drops with leaf maturation, for herbivorous animals a stoichiometric effect of climate change may be of greater importance than a shift in food quantity. We hypothesized that such a climate-related stoichiometric effect caused the low reproductive success of a beaver Castor fiber population that was translocated westward in Europe at the time of a rapid increase in spring temperatures. The staple food of these beavers was willow Salix spp. In 2003 willow bud burst occurred approximately one month earlier at the release site than at the site of origin. After bud burst, leaf quality, measured as nitrogen and phosphorus concentration, decreased in the course of spring in a predictable manner with accumulated degree-days. Nitrogen and phosphorus concentrations calculated from accumulated degree-days were, at a given date, around 20% lower at the release site during the 15 years after the translocation than at the site of origin during the 15 years prior to the translocation. Previous work had identified phosphorus as a likely limiting resource for beaver reproduction, and in most post-translocation years phosphorus concentrations at the release site were below the required level towards the end of gestation. In addition to an increase over time, annual reproductive rate of the translocated beavers was highest in cold springs. We conclude that it was initially low due to a combination of low food quality and factors not related to climate.