the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Apr 2024
LegacyVegetation 1.0: Global reconstruction of vegetation composition and forest cover from pollen archives of the last 50 ka
Abstract. With rapid anthropogenic climate change future vegetation trajectories are uncertain. Climate-vegetation models can be useful for predictions but need extensive data on past vegetation for validation and improving systemic understanding. Even though pollen data provide a great source of this information, the data is compositionally biased due to differences in taxon-specific relative pollen productivity (RPP) and dispersal.
Here we reconstructed quantitative regional vegetation cover from a global sedimentary pollen data set for the last 50 ka using the REVEALS model to correct for taxon- and basin-specific biases. In a first reconstruction, we used previously published, continental RPP values. For a second reconstruction, we statistically optimized RPP values for common taxa with the goal of improving the fit of reconstructed forest cover from modern pollen samples with remote sensing forest cover.
The data sets include taxonomic compositions as well as reconstructed forest cover for each original pollen sample. Relative pollen sources areas were also calculated and are included in the data set of the original REVEALS run. Additional metadata includes modeled ages, age model sources, basin locations, types and sizes.
The improvements in forest cover reconstructions with the REVEALS reconstruction using original/optimized parameters range from 1/0 % (Australia and Oceania/Australia and Oceania) to 58/65 % (Europe/North America) relative to the mean absolute error (MAE) in the pollen-based reconstruction. Optimizations were considerably more successful in reducing MAE when more records and RPP estimates were available. The optimizations were purely statistical and only partly ecologically informed and should, therefore, be used with caution depending on the study matter.
This improved quantitative reconstruction of vegetation cover is invaluable for the investigation of past vegetation dynamics and modern model validation. By collecting more RPP estimates for taxa in the Southern Hemisphere and adding more records to existing pollen data syntheses, reconstructions may be improved even further. Both reconstructions are freely available on PANGAEA (see Data availability section).
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Status: open (until 28 May 2024)
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CC1: 'Comment on essd-2023-486', Marie-Jose Gaillard, 19 Apr 2024
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Please see the entire comment in the attached pdf file
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CC2: 'Comment on essd-2023-486: consider rejection', Michela Mariani, 22 Apr 2024
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The manuscript claims to adapt the REVEALS model for global climate-vegetation reconstructions. However, several critical issues compromise the validity of its findings, please see below for further details.
While quantitative palynologists globally aspire to achieve global-scale reconstructions of vegetation, the reality is that this goal is probably decades away. The manuscript presents a methodologically flawed approach that fails to meet the scientific standards required for reliable global vegetation reconstruction using the REVEALS model. The assumption that RPPEs (relative pollen productivities) are constant continentally, hemispherically and temporally undermines the meticulous and dedicated empirical work conducted by the scientific community across recent decades in creating RPPEs. Particularly, this applies to regions across the Southern Hemisphere and tropical/sub-tropical, where RPPEs are being developed for the first time.
We recommend rejection (or extensive revisions) to be undertaken to address the issues outlined below, ensuring that the model is applied appropriately and effectively within its intended ecological, temporal and geographical constraints. If a revised version is produced, this should only be geographically constrained (probably just the Northern Hemisphere might be feasible in fact).
Major issues:
Inadequate regional calibrations: The generalization of RPPEs across broad geographical scales (hemispheres) ignores crucial ecological and bioclimatic regional variations. This approach most likely leads to significant inaccuracies in the vegetation reconstructions, in spite of what the presumed ‘validation’ approach suggests (see below).
Questionable data assumptions and methodological gaps: The use of northern hemisphere RPPE values for taxa not natively present in the southern hemisphere, such as Alnus in Australia, introduces substantial and confusing biases. Presumably, the authors have not consulted the relevant scholars who worked within this field and the geographical areas mentioned. Similarly, defaulting RPP to 1 for taxa without specific data oversimplifies pollen-vegetation relationships. The paper does not adequately address the absence of data for the Southern Hemisphere, leading to a misleading portrayal of global vegetation.
It is suggested >50% of RPPEs are missing for Australia and Oceanic pollen records. So, in this work a decision was made to run these records using the Northern Hemispheric RPPEs, despite very different bioclimatic and ecological contexts. This extrapolation of Northern Hemisphere RPPEs to southern locations missing PPEs without considering ecological or bioclimatic differences is particularly problematic. RPPEs empirically produced using ground truthing work (field surveys and surface pollen collection) were ignored, especially across the Southern Hemisphere (see some references below).
Duffin, K. I., & Bunting, M. J. (2008). Relative pollen productivity and fall speed estimates for southern African savanna taxa. Vegetation History and Archaeobotany, 17, 507-525.
Mariani, M., Connor, S. E., Theuerkauf, M., Kuneš, P., & Fletcher, M. S. (2016). Testing quantitative pollen dispersal models in animal-pollinated vegetation mosaics: An example from temperate Tasmania, Australia. Quaternary Science Reviews, 154, 214-225.
Mariani, M., Connor, S. E., Fletcher, M. S., Theuerkauf, M., Kuneš, P., Jacobsen, G., ... & Zawadzki, A. (2017). How old is the Tasmanian cultural landscape? A test of landscape openness using quantitative land‐cover reconstructions. Journal of Biogeography, 44(10), 2410-2420.
Mariani, M., Connor, S. E., Theuerkauf, M., Herbert, A., Kuneš, P., Bowman, D., ... & Briles, C. (2022). Disruption of cultural burning promotes shrub encroachment and unprecedented wildfires. Frontiers in Ecology and the Environment, 20(5), 292-300.
Oversimplified and incorrect spatial and temporal settings: The inclusion of incorrect basin types in the model without appropriate adjustments is very concerning. Why are marine records included for a model explicitly designed to work for large lakes of closed basins with wind dispersal as the only mechanism for pollen deposition?
The manuscripts states that ‘all sites that were not classified as lakes were run with peatland settings’ = can we consider the ocean a peatland? REVEALS cannot work with marine records and it definitely does not make sense to apply the ‘peatland’ settings for marine records with some random arbitrary basin radius (100m?). Further, using a deep temporal scope (50ka) without any consideration for massive climatic shifts (likely larger than the effect of regional RPPEs values vs regional bioclimate variations) are concerning oversights, making any pre-Holocene glacial REVEALS reconstructions unrealistic with current interglacial PPEs.
Dubious optimization and validation: Optimizing PPEs to match remote sensing data risks validating the model based on its own assumptions rather than providing an unbiased estimation of past vegetation, which REVEALS is designed to do. This circular reasoning undermines the scientific integrity of the model's outputs. While an interesting concept this needs to be validated separately on a much smaller spatially and higher resolution scale before such a widespread application. This cannot really be called a ‘validation’.
The reconstructed forest cover for the past 500 years was compared to modern remote sensed cover. Why not a smaller and more recent age bin was considered? In the past 500 years many areas of the world have been colonised by Europeans and have experienced major shifts in vegetation structure, as management transferred from Indigenous to colonial regimes (e.g. the Americas and Australia). This means that forest cover over the whole 500 years bin is not comparable to modern remote sensing data. This highlights a Eurocentric view of the global vegetation patterns.
An example of validation of RPPEs using modern vegetation data (with surveys) has been done in the following papers:
Mariani, M., Connor, S. E., Fletcher, M. S., Theuerkauf, M., Kuneš, P., Jacobsen, G., ... & Zawadzki, A. (2017). How old is the Tasmanian cultural landscape? A test of landscape openness using quantitative land‐cover reconstructions. Journal of Biogeography, 44(10), 2410-2420.
Mariani, M., Connor, S. E., Theuerkauf, M., Herbert, A., Kuneš, P., Bowman, D., ... & Briles, C. (2022). Disruption of cultural burning promotes shrub encroachment and unprecedented wildfires. Frontiers in Ecology and the Environment, 20(5), 292-300.
Citation: https://doi.org/10.5194/essd-2023-486-CC2
Data sets
Global REVEALS reconstruction of past vegetation cover for taxonomically harmonized pollen data sets Ulrike Herzschuh et al. https://doi.org/10.1594/PANGAEA.961588
Optimized REVEALS reconstruction of past vegetation cover for taxonomically harmonized pollen data sets Laura Schild et al. https://doi.pangaea.de/10.1594/PANGAEA.961699
Model code and software
[Analysis code] LegacyVegetation 1.0: Global reconstruction of vegetation compositions and forest cover from pollen archives of the last 50 ka Laura Schild and Peter Ewald https://doi.org/10.5281/zenodo.10191859
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