the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 09 Oct 2024
Evaluation of long-term carbon dynamics in afforested drained peatlands: Insights from using the ForSAFE-Peat Model
Abstract. Afforested drained peatlands have significant implications for greenhouse gas (GHG) budgets, with contrasting views on their effects on climate. This study utilized the dynamic ecosystem model ForSAFE-Peat to simulate biogeochemical dynamics over two full forest rotations (1951–2088) in a nutrient-rich drained peatland afforested with Norway spruce (Picea abies) in southwest Sweden. Model simulations aligned well with observed groundwater levels (R² = 0.71) and soil temperatures (R² ≥ 0.78), and captured seasonal and annual net ecosystem production patterns, although daily variability was not always well represented. Model outputs were analysed under different system boundaries (soil, ecosystem, and ecosystem plus the fate of harvested wood products named ecosystem+HWP) to assess carbon exchanges using the net carbon balance (NCB) and the integrated carbon storage (ICS) metrics. Results indicated negative NCB and ICS across all system boundaries, except for a positive NCB calculated by the end of the simulation at the ecosystem+HWP level. The soil exhibited persistent carbon losses primarily driven by peat decomposition. At the ecosystem level, net carbon losses were reduced as forest growth partially offset soil losses until harvesting. NCB was positive (1015 gC m-2soil) at the ecosystem+HWP level due to the slow decay of harvested wood products, but a negative ICS (-7.0×105 gC yr m-2soil) due to initial carbon losses. This study highlights the importance of system boundary selection and temporal dynamics in assessing the carbon balance of afforested drained peatlands.
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RC1: 'Comment on egusphere-2024-2754', Anonymous Referee #1, 11 Nov 2024
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Escobar et al presented a peat module into a biogeochemical model ForSAFE and applied it to a forest on drained agricultural peatland. After benchmarked the model with a few years of water table, soil temperature and with three years of NEE data, they analyzed the simulated C flows over two rotation periods with different system boundaries and concluded that incorporating the fate of the harvested wood products would shift the results from sink to source for forest peatland carbon accounting. While many model development works have been done, this paper, in my view has many flaws.
First, a full forest rotation approach has been undertaken earlier in He et al. 2016 Biogeosicences study for the same site, although use another model but the same conclusion was already made there. Moreover, Kasimir et al 2018 GCB further presented full rotational GHG balance (not only CO2 but also CH4, N2O) of their studied site for several land use scenarios, including final harvest of the forest. Thus, I would argue their findings on the system boundaries are already known. The advance of ForSAFE has a few interesting developments in it (e.g. simple dynamic volume for peat, although they did not discuss much about it), but put it in the community of peatland models, the novelty is rather minor.
Second, I have many concerns on their model evaluation, specifically: do three years of NEE data (one year under mature forest, two years after clear cutting) enough for constraining (or support) their full carbon budget analysis for the two-rotation periods? One-year of 2008 (note even a dry year thus not a normal climate year) NEE data to evaluate, or precisely speaking benchmark, 80 year (first rotation) of modelled C results, with a very detailed processed-based approach (multiple factors control the C flows) like ForSAFE. Not saying many of its simulated C flows did not benchmark with any field data at all. Their model data comparison (Fig. 5) shows large derivates between the measured and model daily data, suggesting these controls of the CO2 exchange are poorly captured by current model structure. The uncertainties in these (at least partly) also reflected by their simulated striking increase (double) growth over the second rotation period compared to the first rotation period (Fig. 6c). So, what cause these mismatches? if ForSAFE can not adequately simulate the underlaying process control and how these controls respond to clear cutting, as current results suggest. How can we trust the long-term predictions made by the model. This is a crucial question for the authors to think it over.
Third, the authors commented the first rotation period was non-conventional management, however, still design their second rotation periods with the same management. Why not have a simulation design with conventional management that additionally evaluate the sensitivity of the results to varying and more realistically forest management, because the chance of having a late 72% thinning and minor storm harvest two rotation in a row, is just way too low if not impossible. A model simulation with more realistic management would make more sense for forest management implications.
To summarize, the current version of the paper suffers from many drawbacks, e.g. data do not support their long-term analysis, and revealed fact that the model so far can not capture the underlying processes (Fig. 5). I would thus recommend rejecting current version. I am sorry that I can not be more positive at this stage. However, I do wish those reviews will be helpful in future revision of the ms. One suggestion for the authors is to investigate the response of the underlying processes to clear cutting using the measured water table depth, and soil temperature as a first step to understanding the C dynamics over this transitions period before a meaningful upscale to the rotation period to be made.
Please also note there are important issues concerning your data file shared on Zenodo: 1) the measured GWL data only contains the first two years; 2) the measured soil temperature data at 0.05 and 0.15m differ with what reported in your paper and clearly are wrong.
Minor comments:
Line 16 negative meaning uptake?
Line 37 gC
Line 115 section 2.2 site description. I generally lack the clear cutting description, is there any rewetting effects for the site, like blocking drainage ditches, do they replanting spruce trees or they left small trees to regrow? How is the hydrological state of the clear cutting site, also I am lacking the descriptions of the eddy tower data, since the measured EC data is not published before so a brief description of the set up and measure and process processing is of need.
Line 120 The site is agricultural used before tree planting how is the initial conditions for the soil consider those land use, does it make any difference or not. He et al 2016 clearly show how the initial conditions influence the results of the simulated GHG balance.
Line 140 Site description for the clear-cut site will also help the reader to understand the relevance of the understory vegetation in the C exchange for the forest few years of clear cutting
Line 173 why only these two parameters calibrated, even though we know the limKsat quite well. Plus, how these parameter uncertainties contribute to the uncertainties in the short-term model-data mismatch and long-term upscaling results?
Figure 2, why not show the simulated fluxes with the width of the arrows.
Figure 3, there are clearly more than measured WTD time series, need to add that in the figure captions
I do have many concerns on how ForSAFE-peat handles hydrology which is arguably the most important variable for peatlands development and regulates C dynamics over this transitional clear-cutting period. The hydrological changes (including ET, Runoff etc) over the clear-cutting period and compare to before would be interesting to test the model in detail.
Figure 5 what is cold and warm period, define it and also why not show the daily measured vs daily modelled numbers
Citation: https://doi.org/10.5194/egusphere-2024-2754-RC1
Data sets
Skogaryd data used for the paper: Evaluation of long-term carbon dynamics in afforested drained peatlands: Insights from using the ForSAFE-Peat Model. Daniel Escobar et al. https://zenodo.org/records/13626717
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