Technical Abstract: • Plant productivity is predicted to increase in northern latitudes due to climate warming; however, this may depend on whether biological N-fixation also increases. We evaluated how variation in temperature and light affect N-fixation by two boreal feather mosses, Pleurozium schreberi and Hylocomium splendens, which are the primary source of N-fixation in most boreal environments. • We measured N-fixation rates two and four weeks after exposure to a factorial combination of normal, intermediate, and high temperature (16.3, 22.0, and 30.3 °C) and light (148.0, 295.7, and 517.3 µmol m-2 s-1) environments. • Our results showed that P. schreberi achieved higher N-fixation rates relative to H. splendens in response to warming treatments but that the highest warming treatment eventually caused N-fixation to decline for both species. Light strongly interacted with warming treatments, having positive effects at low or intermediate temperatures and damaging effects at high temperatures. • These results suggest that climate warming may increase N-fixation in boreal forests, but that increased shading by the forest canopy or occurrence of extreme temperature events could limit increases. They also suggest that P. schreberi may become a larger source of N in boreal forests relative to H. splendens as climate warming progresses