The challenge of comparing pollen-based quantitative vegetation reconstructions with outputs from vegetation models – a European perspective
Abstract. We compare Holocene tree-cover changes in Europe derived from a transient MPI-ESM1.2 simulation with high spatial resolution LPJ-GUESS time-slice simulations and pollen-based quantitative reconstructions of tree cover based on the REVEALS model. The dynamic vegetation models and REVEALS agree with respect to the general temporal trends in tree cover for most parts of Europe, with a large tree cover during the mid-Holocene and a substantially smaller tree cover closer to the present time. However, the decrease in tree cover in REVEALS starts much earlier than in the models indicating much earlier anthropogenic deforestation than the prescribed land-use in the models. While LPJ-GUESS generally overestimates tree cover compared to the reconstructions, MPI-ESM indicates lower percentages of tree cover than REVEALS, particularly in Central Europe and the British Isles. A comparison of the simulated climate with chironomid-based climate reconstructions reveals that model-data mismatches in tree cover are in most cases not driven by biases in the climate. Instead, sensitivity experiments indicate that the model results strongly depend on the tuning of the models regarding natural disturbance regimes (e.g. fire and wind throw). The frequency and strength of disturbances are – like most of the parameters in the vegetation models – static and calibrated to modern conditions. However, these parameter values may not be valid during climate and vegetation states totally different from todays. In particular, the mid-Holocene natural forests were probably more stable and less sensitive to disturbances than present day forests that are heavily altered by human interventions. Our analysis highlights the fact that such model settings are inappropriate for palaeo-simulations and complicate model-data comparisons with additional challenges. Moreover, our study suggests that land-use is the main driver of forest decline in Europe during the mid- and late-Holocene.
Anne Dallmeyer et al.
Status: final response (author comments only)
- RC1: 'Comment on cp-2023-16', Anonymous Referee #1, 02 May 2023
- RC2: 'Reviewer's comments on cp-2023-16', Anonymous Referee #2, 17 May 2023
Anne Dallmeyer et al.
Anne Dallmeyer et al.
Viewed (geographical distribution)
The authors compare pollen-based reconstructions of Holocene vegetation (REVEALS) with simulated vegetation from an Earth System model (ESM) and a dynamic global vegetation model (DGVM). They find that the simulated decline in Holocene forest cover is too abrupt and late in both models and attribute this to errors in the land-use history applied in the models. The general underestimation of forest fraction in the ESM is attributed to the parameterisation of disturbance.
This is interesting study with an important finding which is very relevant for this journal and which is strengthened by the use of two very different modelling approaches. This and the level of detail is a strength. My main comment (see below) is that the paper is overly long and the authors should aim to substantially shorten the text in several places in order to make it more accessible.
1) I appreciate that this is a technical paper probably mostly aimed at those within the field, but I found the text in places hard to digest. In the methods I would recommend that you include some summary paragraphs at the start of the section 2 to very briefly explain the differences between MPI-ESM, JSBACH, LPJ-GUESS and REVEALS. More generally I believe that you can make the text more concise, especially section 2 (currently ~ 6 pages), section 3.1 (2 1/2 pages), 4.1 (3 pages) and section 4.5 (2 pages). Some of the detail in section 2 could also optionally be moved to an appendix.
2) Can the authors comment on whether other parameterisations besides the ones explored (disturbance and windthrow) could contribute the biases in the forest cover simulations?
Line 19: specify that LPJ-GUESS is the dynamic vegetation model.
Line 297: I could not see a figure with these results, please can you check this?
Figure 5: could you add a key to this figure to make it easier to read?
Lines 588-600: It is probably worth clarifying here that the KK10 reconstruction has substantially earlier and more widespread land-use than LUH2.
Line 615: I cannot comment on how topography is treated in REVEALS. As written this sounds a bit uncertain. Can you confirm that topography is not accounted for and how this would influence the model results. At the moment it sounds as if you’re unsure whether it does or not.
Line 705: This seems to contradict the sensitivity test with increased disturbance you present earlier, can you comment?
Line 717: This seems to echo results by Kaplan et al 2017 (Land). Is that correct? If so, can comment on this here and earlier in the text?