Abstract

The dynamics of carbon allocation in trees affect carbon storage of forest ecosystems and atmospheric carbon dioxide concentrations on Earth. Here, using carbon fluxes and xylem phenology from 84 conifer forests across the Northern Hemisphere, we quantify the phenology of carbon sources (photosynthesis) and sinks (stem growth) along a thermal gradient from −4.4 to 18.2 °C in mean annual temperature. The onset of stem growth advances by 2.3 days per degree Celsius with rising temperatures, 2 times slower than photosynthesis. Warmer sites accumulate less chilling than colder sites, thus trees require more heat to reactivate. The ending of photosynthesis and wood formation is delayed by 2.0 days per degree Celsius. Overall, the photosynthetic season lengthens by one month more than the growing season towards the warmest sites. Climate warming tends to intensify the mismatch between the phenology of carbon sources and sinks, potentially affecting the carbon sequestration in conifer forests.