Where does the carbon go? A model-data intercomparison of vegetation carbon allocation and turnover processes at two temperate forest free-air CO2 enrichment sites.
Bottom Line: Observed eCO2 effects on allocation were dynamic.Allocation schemes based on constant fractions or resource limitations performed less well, with some models having unintended outcomes.Our recommendations to reduce uncertainty include: use of allocation schemes constrained by biomass fractions; careful testing of allocation schemes; and synthesis of allocation and turnover data in terms of model parameters.
Affiliation: Department of Biological Sciences, Macquarie University, Sydney, New South Wales, 2109, Australia.Show MeSH
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Mentions: We compared the CO2 effect on NPP with the CO2 effect on biomass incrementover the duration of the experiment (Fig. 5a,b). Most of themodels predicted that the effect of eCO2 on biomass increment exceeded the effect ofeCO2 on NPP. The difference between the CO2 effect on biomass increment andthat on NPP depends on how far the simulated stand is from steady state, that is, the point wheregains from NPP equal losses to turnover and mortality. In the very early stages of stand growth,before notable turnover or tree mortality commences, the simulated CO2 effect on biomassincrement will be equal to the CO2 effect on NPP. At steady state, by contrast, the rateof biomass increment (at aCO2) is zero, so any stimulation of biomass increment byeCO2 will result in a very high relative response. This stand stage effect accounts forthe large percentage increase in biomass seen in the ISAM model at both the Duke and ORNL FACEsites. A shift in allocation towards long-lived woody components will also increase the percentagebiomass increment response compared to the NPP response, because woody tissue has a long lifespan.This effect can be seen in the TECO simulations, particularly at Oak Ridge where woody allocationincreases by 10% (Fig. 3l), and as a result a36% stimulation of NPP results in a 109% increase in biomass increment over the courseof the experiment.
Affiliation: Department of Biological Sciences, Macquarie University, Sydney, New South Wales, 2109, Australia.