Summary/Abstract
Forest ecosystems play a central role in climate policy because their growth removes nearly a third of the anthropogenic carbon emissions to the atmosphere. Yet, warming and drying with climate change threaten the capacity of forests to absorb carbon in some regions. Measuring the carbon sink of forests and its climatic control is thus essential for ecosystem management and climate policy, but current measurements are overwhelmingly concentrated on the Northern Hemisphere leaving a gap in the Southern Hemisphere. Here, the authors present the first measurements of carbon fluxes of an adult 300-year-old temperate rainforest of the ultra-long-lived conifer Fitzroya cupressoides in southern Chile. This conifer is a paleoendemic tree confined to a few cool and very wet climate refugia. The study’s results show that the forest can photosynthesize year-round because it is particularly adapted to cold and rainy conditions. Photosynthesis consistently exceeds ecosystem carbon losses by respiration between July and January, resulting in a moderate carbon sink compared with temperate forests in the Northern Hemisphere. Summer drying and warming enhance carbon losses by respiration and thus weaken the carbon sink capacity, implying that the ongoing and future climate drying and warming in the region threaten the carbon sink of this unique rainforest.