The fate and transit time of carbon in a tropical forest

Tropical forests fix large quantities of carbon from the atmosphere every year; however, the fate of this carbon as it travels through ecosystem compartments is poorly understood. In particular, there is a large degree of uncertainty regarding the time carbon spends in an ecosystem before it is respired and returns to the atmosphere as CO2. We estimated the fate of carbon (trajectory of photosynthetically fixed carbon through a network of compartments) and its transit time (time it takes carbon to pass through the entire ecosystem, from fixation to respiration) for an old-growth tropical forest located in the foothills of the Andes of Colombia. We show that on average, 50% of the carbon fixed at any given time is respired in <0.5 years, and 95% is respired in <69 years. The transit time distribution shows that carbon in ecosystems is respired on a range of time-scales that span decades, but fast metabolic processes in vegetation dominate the return of carbon to the atmosphere. Synthesis. The transit time distribution integrates multiple ecosystem processes occurring at a wide range of time-scales. It reconciles measurements of the age of respired CO2 with estimates of mean residence time in woody biomass, and provides a new approach to interpret other ecosystem level metrics such as the ratio of net primary production to gross primary production.