Hemispheric-scale reconstructions of surface temperature fluctuations over the last millennium have identified the 20th century as uniquely warm. Boulouf Lugo et al. monitor timings of wood formation in black spruce, Picea mariana, on a weekly timescale over 9 years in the boreal forest of Quebec, Canada, and use the results in conjunction with temperature records to model timings between 1950 and 2010. They find that warmer temperatures are associated with a lengthening in the duration of xylogenesis, with the greatest changes being observed in spring and at higher latitudes. In the long run, such changes could modify the short time window for growth of the boreal species and dramatically affect the growth dynamics and productivity of trees in these temperature-limited ecosystems.