|Review of Yousefpour et al.|
Title: Simulating growth-based harvest adaptive to future climate change
Authors: Rasoul Yousefpour1,2*, Julia E.M.S. Nabel1, Julia Pongratz1,3
The authors represent a modelling study on the effect of wood harvest in managed forests and ground-based harvesting (sustainable yield by harvesting only the increment in biomass) worldwide using the JSBACH land-surface model. The authors apply the model to a range of climate scenarios (RCPs) to compare the global harvest potential in terms in carbon units and to put into context of climate mitigation options. Despite being an important topic to address and thus addressing an important topic for a paper to be published in Biogeosciences, it has several conceptual problems that appear from the design of this study. It implies that any forest worldwide is regarded as a potential wood harvest, no biodiversity hotspot, conservation areas or last wilderness areas are excluded from the potential area, thus carbon potential for wood harvest. The only restriction applied refers to accessibility. Thus, by design this study has a very global view on forest conditions, neglecting that natural forests, i.e. woods, are not managed and should not be managed, i.e. harvested. It seems to be triggered by the image of European forestry of what forests are worldwide that I find highly disputable, it ignores biome- or ecoregion-specific conditions, above all biodiversity, ecosystem functions and habitat for a wide range of organism groups. It builds on the general finding that climate, and environmental or ecological conditions affect forest growth, thus carbon storages and fluxes which is largely ignored in IAMs. The study mixes many concepts (carbon cycle, carbon-climate interaction, forestry; ESM, DGVM and IAMs that are not always directly comparable or transferable in terms of findings. Here, studies are combined to support a certain argument of the study or finding which do not mean the same thing. Interdisciplinary research demands a clear definition of terminology and concepts up-front in order to address the wide community of readership such studies aim to address. Additionally, the methods and experimental setting are not adequately described so that it is difficult follow and interpret the results to evaluate if the methods applied substantiate the results and conclusions drawn from them.
In the following I will describe in more detail, which several major and substantial improvements are required before this manuscript can be published.
Line 46-51: The text follows on with the role of fire and harvest in the carbon cycle – better refer to decent global carbon cycle quantification of all carbon sink and sources as done with the LeQuere et al. 2018 citation. This and earlier publications under the heading of the global carbon project provide a state-of-the-art knowledge on the global carbon cycle.
Line 108-111: This assumption is flawed because it ignores carbon emissions from industry, fossil fuels and land-use change. It is later explained again in the methods section but no further details on these carbon fluxes are considered in the model are provided. Such an assumption implies that only GPP and respiration fluxes are exchanged between the ocean and terrestrial vegetation, and that carbon release from wood products is the only additional source of carbon. It is thus an incomplete picture of the full carbon budget and means that the mitigation potential from wood products are overestimated. This assumption needs to be revised as it is central to the entire study.
An overview on the methodological approach of computing wood harvest in IAMs has to be provided in order to allow the reader to compare and follow on the carbon estimates of wood products quantified by ESM (your study) and IAMs (the approach you suggest to improve). There are several IAMs being used in the scenario-production work, but you do not cite any of them to substantiate your argument that the climate-dependent simulation of wood harvest is considered.
The introduction section should end with a clear explanation of the modelling concept of this study (the abstraction of representing respective carbon fluxes and pools) and its objectives to provide the reader with an overview of what to expect and to cross-check later on what to conclude from this study.
Managed forests differ in their plant traits from natural forests, the latter is described by PFTs to represent average conditions of a particular biome. This is not directly comparable to the varieties planted in managed forests. So, why are there no managed forest PFT for each biome parameterized closer to trees species used by forestry in each biome?
The Life-cycle analysis is limited to the decay rates of the 3 reason why wood is harvested, bioenergy, paper and wood products.
Define what you mean with reference period; is that the same as present period (line 151)?
Line 154: which silvicultural interventions and disturbances are implemented in JSBACH? Only then the approach to relate changes in the wood carbon pool to these processes makes sense. You describe the modelling approach in JSBACH, which means that processes abstracted in a model need to be clearly separated from processes influencing wood carbon pools in the real world. This section leaves the reader unclear about what is really implemented in the model.
Equation 1: disturbance term not specified
Equation 2: all terms and variables in this equation need to be properly explained
Lines 189-190 put equations and relation to equation 2 unclear.
Lines 196-198: modelling assumption and approach (in terms of equation) unclear. How are adapted harvest decisions implemented in the model and how do they influence mortality simulation in the model?
Line 222: Change E to E(t)
Lines 223-224: equation numbering has to continue, you have now 2 times equations 1 and 2, referencing them later on leads to confusion. Variables and meaning of terms not explained. Modelling approach remains unclear. What is (b), (p) and (c) referring to?
Swap equation 2 and 3, because 3 explains one term of equation 2 (line 233-234). Again the use of these equations is an incomplete representation of the full carbon budged (see my comment above) which questions the validity of this approach. It has to be added by a term for industrial, fossil fuel and land-use emissions in order to be able to integrate all these carbon fluxes (as carbon sources to the atmosphere) and have the amount of carbon remaining in the atmosphere correctly quantified. This is absolutely essential because from this method you indirectly quantify your carbon mitigation potential of wood harvest (cf. your statement in lines 243-244).
You get very high wood carbon potentials for harvest because you do not exclude any protected areas (IUCN categories) or natural vegetation areas which are ecologically important as biodiversity hotspots and/or last wilderness areas. Such an approach (GB for any wood increment across all forests worldwide) is highly debatable. Can you really declare every forest as harvestable even though you constrain it to the annual increment, i.e. NPP, (cf. lines 270-273)? I doubt it. To constraint such potentials only via accessibility of roads does not represent the state-of-the-art in this field. A very critical view on the role of tropical and boreal forests to harvest potentials is required (lines 270-278). Here, a very unbalanced view or description of results is presented.
I was wondering why differences across RCPs, but not SSPs were selected. With the latter you could have tested several managed forest assumptions even though you keep LU constant over time. Have you checked the narratives implemented for MF in IAMS, you criticize them, but testing their sensitivity in your model would have been much more informative. Testing climate sensitivity of terrestrial vegetation adds to the already known climate impacts on carbon pools of the terrestrial vegetation.
Lines 265-267: statement not substantiated by methods.
Lines 308-310: statement not substantiated by methods and clear explanation of the criticized IAM approach, no citation to IAM publication provided.
Line 312: reference scenario name definition
The discussion lacks a clear structure of items to be discussed, which is also because no objectives were presented at the end of the introduction. Sub-headings have to be inserted and the text re-arranged accordingly. New concepts and terminology comes in that was not used before (e.g. line 331), so it remains unclear which concept/framework you follow.
The argumentation of the pros and cons of the findings/results of this study lack a clear structuring and clear overview if studies/publications can really support your argument. For example, the statement in lines 336-338 cannot relate to your findings because you do not have a managed forest PFT for the temperate biome. The same applies to lines 342-345, your model results cannot substantiate such a statement and it is not shown in Figure 3.
Line 322-323: statement not substantiated by explanation in the introduction (IAMs approach) to allow relating the findings of this approach.
Line 355: mis-conception of how DGVMs use IAM input on managed forest
Line 357-359 Connection to this study unclear
Line 361-367: unclear separation of your findings and the connection to the cited study
Line 374: what is the maximum soil expectation value and how can this be related to simulated processes in models like JSBACH?
Line 391-392: with an approach of MF in JSBACH we are still very far away from providing information for “realistic decision-making”. This question is questionable.
Lines 405-407: experiments and setting not explained in methods
Lines 453-456: argumentation and reference unclear how it connects to this study. Explanation of the context needs to be improved.
Lines 469-473: misconception of the turnover rates quantified by Carvalhais et al., these numbers cannot be used to justify the length of the lifetime of wood products. These things are not comparable.
Lines 474-475: validation has to move to methods and results
Lines 487-493: unclear to me why discount rates are a topic of this manuscript
Conclusions additionally contain sentences that belong to the discussion. This needs to be revised.