Abstract
Abstract 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 . 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 years, and 95% is respired in 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 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.
Highlights
The terrestrial biosphere photosynthesizes annually about 120 ± 7 PgC/year, a flux that is largely driven by productivity in the tropics (Beer et al, 2010; Jung et al, 2020) where gross primary production (GPP) is often larger than 30 MgC ha − 1 year − 1 (Fu et al, 2018)
It is likely that between 50% and 70% of the GPP flux in tropical ecosystems is lost as autotrophic respiration (Chambers et al, 2004; DeLucia et al, 2007; Doughty et al, 2018; Gifford, 2003; Waring et al, 1998); it is uncertain if the respiratory flux is composed mostly of recent photosynthates or of carbon that spends years to decades stored in the ecosystem
Our results indicate that carbon fixed during photosynthesis in a tropical forest returns back to the atmosphere at a wide range of time-scales, a property that is captured by the transit time distribution
Summary
The terrestrial biosphere photosynthesizes annually about 120 ± 7 PgC/year, a flux that is largely driven by productivity in the tropics (Beer et al, 2010; Jung et al, 2020) where gross primary production (GPP) is often larger than 30 MgC ha − 1 year − 1 (Fu et al, 2018) Most of this photosynthetically fixed carbon is assumed to return quickly to the atmosphere, with ecosystem respiration (Re) being often as large as the GPP flux (Chambers et al, 2004; Fu et al., 2018; Luyssaert et al, 2007). This lapse of time when carbon is removed from the atmosphere is relevant for tropical ecosystems given their dominance in the global GPP flux
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