Abstract
Abstract. The Holocene Peatland Model (HPM) is a widely applied model for understanding and predicting long-term peat accumulation, but it is difficult to test due to its complexity, measurement errors, and lack of data. Instead of testing the complete model, tests of individual modules may avoid some of these problems. In particular, the HPM decomposition module can be tested with litterbag data, but no such test has been conducted yet. Here, we estimate parameter values of the HPM decomposition module from available Sphagnum litterbag experiments included in the Peatland Decomposition Database and with a litterbag decomposition model that considers initial leaching losses. Using either these estimates or the standard parameter values, we test whether the HPM decomposition module fits decomposition rates (k0) in Sphagnum litterbag experiments along a gradient from oxic to anoxic conditions. Both litterbag data and model versions where HPM decomposition module parameters were estimated suggest a less steep gradient of decomposition rates from oxic to anoxic conditions and larger anaerobic decomposition rates for several species than the standard parameter values. This discrepancy may be caused by ignoring the effects of water table fluctuations on aerobic and anaerobic decomposition rates. Moreover, our analysis suggests that maximum possible decomposition rates of individual species (k0,i) vary more than suggested by the standard parameter values of the HPM plant functional types. Based on previous sensitivity analyses of the HPM, the estimated differences from the standard parameter values can cause differences in predicted 5000-year carbon (C) accumulation up to 100 kg m−2. The HPM decomposition module with standard parameter values fits k0 estimated from Sphagnum litterbag data, but model versions where HPM decomposition module parameters were estimated and differ significantly have an equivalent fit. The reason why models with different parameter values have equivalent fit is that errors in remaining masses and the design of available litterbag experiments support a range of initial leaching loss and k0 estimates. Consequently, applications of the HPM and any other peatland model should consider that a broad range of decomposition module parameter values is compatible with available litterbag experiments. Improved litterbag experiments are needed for more accurate tests of any peatland decomposition module and for obtaining parameter estimates accurate enough to allow even only approximate predictions of long-term peat accumulation. The modeling approach used here can be combined with different data sources (for example, measured degree of saturation) and decomposition modules. In light of the large differences in long-term peat accumulation suggested by the parameter estimates, we conclude that it is worth conducting such experiments, not only to improve the decomposition module of the HPM, but also to improve peatland models in general.
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Claims 10
Claim · model-written summary
Litterbag data and estimated model parameters suggest a less steep oxic-to-anoxic decomposition-rate gradient than standard parameters.
- Outcome
- gradient of decomposition rates from oxic to anoxic conditionse0
- Direction
- less steepe0
- Comparator
- standard parameter valuese0
SphagnumOrganisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Both litterbag data and model versions where HPM decomposition module parameters were estimated suggest a less steep gradient of decomposition rates from oxic to anoxic conditions and larger anaerobic decomposition rates for several species than the standard parameter values”
e1“Sphagnum decomposition rates”
Claim · model-written summary
Estimated parameters imply larger anaerobic decomposition rates for several species than standard parameters.
- Outcome
- anaerobic decomposition ratese0
- Direction
- largere0
- Qualifier
- several speciese0
- Comparator
- standard parameter valuese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“larger anaerobic decomposition rates for several species than the standard parameter values”
e1“Sphagnum decomposition rates”
Claim · model-written summary
Maximum possible species decomposition rates vary more than standard plant-functional-type parameters suggest.
- Outcome
- maximum possible decomposition rates of individual species (k0,i)e0
- Direction
- vary moree0
- Comparator
- standard parameter values of the HPM plant functional typese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“maximum possible decomposition rates of individual species (k0,i) vary more than suggested by the standard parameter values of the HPM plant functional types”
e1“Sphagnum decomposition rates”
Claim · model-written summary
Estimated parameter differences can yield up to 100 kg m−2 difference in predicted 5000-year carbon accumulation.
- Outcome
- predicted 5000-year carbon (C) accumulatione0
- Value
- 100 kg m−2e0
- Comparator
- standard parameter valuese0
Not reportedDirection
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“estimated differences from the standard parameter values can cause differences in predicted 5000-year carbon (C) accumulation up to 100 kg m−2”
e1“Sphagnum decomposition rates”
Claim · model-written summary
The standard-parameter HPM decomposition module fits litterbag-derived Sphagnum decomposition rates.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The HPM decomposition module with standard parameter values fits k0 estimated from Sphagnum litterbag data”
Claim · model-written summary
Estimated-parameter model versions had equivalent fit to the standard-parameter version.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“model versions where HPM decomposition module parameters were estimated and differ significantly have an equivalent fit”
e1“Sphagnum litterbag data”
Claim · model-written summary
Errors in remaining mass and experiment design support a range of leaching-loss and decomposition-rate estimates.
- Outcome
- a range of initial leaching loss and k0 estimatese0
- Qualifier
- errors in remaining masses and the design of available litterbag experimentse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“errors in remaining masses and the design of available litterbag experiments support a range of initial leaching loss and k0 estimates”
e1“Sphagnum decomposition rates”
Claim · model-written summary
Peatland model applications should consider the broad parameter range compatible with litterbag experiments.
- Outcome
- should consider that a broad range of decomposition module parameter values is compatible with available litterbag experimentse0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“applications of the HPM and any other peatland model should consider that a broad range of decomposition module parameter values is compatible with available litterbag experiments”
e1“Sphagnum decomposition rates”
Claim · model-written summary
Improved litterbag experiments are needed for more accurate model tests and parameter estimates.
- Outcome
- Improved litterbag experiments are needed for more accurate tests of any peatland decomposition module and for obtaining parameter estimates accurate enough to allow even only approximate predictions of long-term peat accumulatione0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“Improved litterbag experiments are needed for more accurate tests of any peatland decomposition module and for obtaining parameter estimates accurate enough to allow even only approximate predictions of long-term peat accumulation”
e1“Sphagnum decomposition rates”
Claim · model-written summary
The modeling approach can combine other data sources and decomposition modules.
- Outcome
- can be combined with different data sources (for example, measured degree of saturation) and decomposition modulese0
- Comparator
- different data sourcese0
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
e0“The modeling approach used here can be combined with different data sources (for example, measured degree of saturation) and decomposition modules.”
e1“Sphagnum decomposition rates”