ABSTRACT
To understand carbon flows through terrestrial ecosystems, it is very important to use metrics to quantify the time carbon spends in the entire system and in particular compartments. In this chapter, we introduce the concepts of age and transit time as two fundamental metrics that characterize the speed at which carbon flows through ecosystems. Age is defined as the time carbon atoms spend in an ecosystem, from when they enter through photosynthesis until they are observed in a particular compartment. Transit time is defined as the time carbon atoms require to pass through the entire ecosystem, from the time they enter through photosynthesis until they are lost in gas, liquid, or solid form. We review here mathematical formulas for computing age and transit time in compartmental systems, distinguishing between formulas for autonomous systems in equilibrium and nonautonomous systems moving along a known trajectory.
