The paper is on means and quantiles of transit times of C in the Porce forest (Columbia). Furthermore, the concept of $\mathrm{NPP}/\mathrm{GPP}$ as $\mathrm{CUE}$ is questioned and an interpretation as $\mathrm{NPP}/\mathrm{GPP} = R_h/\mathrm{GPP}$ is suggested.

  • The time that C fixed as $\mathrm{GPP}$ spends in an ecosystem is relevant to understand feedbacks between ecosystems and the climate system.
  • Plot level estimates of
  • Used average of parameters from a parameter set obtained by MCMC.
    • Average okay for linear models?
    • What about nonlinear models?
  • autotrophic pools: foliage, wood, fine roots, coarse roots
  • heterotrophic pools: fine litter, CWD, soil carbon
  • Most metabolic processes operate on an intra-annual time-scale.

We obtained an average value of 0.3 for the ratio NPP:GPP for the forests at equilibrium, a ratio that is often called carbon use ef- ficiency ($\mathrm{CUE}$) (Chambers et al., 2004; DeLucia et al., 2007; Gifford, 2003; Malhi et al., 2015). According to common interpretation, this ratio would suggests that 30% of the photosynthetically fixed car- bon is used for biomass production. Similar values for $\mathrm{CUE}$ with simi- lar interpretations are also given by Chambers et al. (2004) and Malhi et al. (2013), although larger variability in $\mathrm{CUE}$ is reported in Doughty et al. (2018). However, we believe that this common interpretation of $\mathrm{CUE}$ has problems since, as our transit time distribution showed, au- totrophic respiration is composed of carbon that spends some time in biomass before being respired. The amount of time carbon stays in plant cells can vary from hours to decades, but photosynthates have to be metabolized from living cells (biomass) for CO2 production to occur. Thus, autotrophic respiration originates from biomass already produced; however, most of this metabolism occurs very quickly as the transit time distribution suggests, giving the false impression that a large proportion of carbon was not used to produce biomass. As other authors have shown (DeLucia et al., 2007; Gifford, 2003), estimates of $\mathrm{CUE}$ depend largely on whether estimates are made on short or long periods of time, and the transit time distribution pro- vides good support for avoiding an interpretation of this ratio out of the context of the time-­scales involved.

\begin{equation} \nonumber \frac{\mathrm{NPP}}{\mathrm{GPP}} = \frac{\mathrm{GPP}-R_a}{\mathrm{GPP}} = \frac{R_e}{\mathrm{GPP}} \end{equation}

  • $\mathrm{NPP}/\mathrm{GPP} = 0.3$ means that 70% of total photosynthates are respired by autoptrophs and only 30% by heterotrophs.
  • no relation to biomass production (as $\mathrm{CUE}$), but interpretation in terms of different pathways of fixed C