Abstract
Fossil testate amoeba assemblages have been used to reconstruct peatland palaeohydrology for more than two decades. While transfer function training sets are typically of local-to regional-scale in extent, combining those data to cover broad ecohydrological gradients, from the regional-to continental- and hemispheric-scales, is useful to assess if ecological optima of species vary geographically and therefore may have also varied over time. Continental-scale transfer functions can also maximise modern analogue quality without losing reconstructive skill, providing the opportunity to contextualise understanding of purely statistical outputs with greater insight into the biogeography of organisms. Here, we compiled, at moderate taxonomic resolution, a dataset of nearly 2000 modern surface peatland testate amoeba samples from 137 peatlands throughout North America. We developed transfer functions using four model types, tested them statistically and applied them to independent palaeoenvironmental data. By subdividing the dataset into eco-regions, we examined biogeographical patterns of hydrological optima and species distribution across North America. We combined our new dataset with data from Europe to create a combined transfer function. The performance of our North-American transfer function was equivalent to published models and reconstructions were comparable to those developed using regional training sets. The new model can therefore be used as an effective tool to reconstruct peatland palaeohydrology throughout the North American continent. Some eco-regions exhibited lower taxonomic diversity and some key indicator taxa had restricted ranges. However, these patterns occurred against a background of general cosmopolitanism, at the moderate taxonomic resolution used. Likely biogeographical patterns at higher taxonomic resolution therefore do not affect transfer function performance. Output from the combined North American and European model suggested that any geographical limit of scale beyond which further compilation of peatland testate amoeba data would not be valid has not yet been reached, therefore advocating the potential for a Holarctic synthesis of peatland testate amoeba data. Extending data synthesis to the tropics and the Southern Hemisphere would be more challenging due to higher regional endemism in those areas.
Highlights
Testate amoebae are a polyphyletic group of microscopic, unicellular protists that occur globally in a range of soils, rivers, lakes and wetlands, including ombrotrophic peatlands – environments which are frequently used to reconstruct Holocene palaeohydrological change (Mitchell et al, 2008)
We compared our reconstructions with output from the most geographically extensive published transfer function in North America (Booth, 2008) and tested the significance of all new reconstructions (Telford and Birks, 2011b) using the ‘randomTF’ function in the R package palaeoSig (Telford, 2015; Payne et al, 2016). 330 To test for regional variability within the continental dataset, we developed regional models based on United States Environmental Protection Agency (EPA) Level 1 eco-region sub-divisions, using the pruned WA-Tol model as the basis for regional model development (n = 1696)
Even values at 2500 km were comparable to 100 km. These results suggest that, despite the initial decline in r2, the WA-based models are more robust to spatial autocorrelation than WMAT K5 or maximum likelihood (ML). 413 Taxon-specific values for WTD optima and tolerance showed taxa ordered along a WTD gradient both 414 comparable to the diagonal WTD vector along axis 1 of the non-metric multidimensional scaling (NMDS) (Figure 3) and to other studies
Summary
Testate amoebae are a polyphyletic group of microscopic, unicellular protists that occur globally in a range of soils, rivers, lakes and wetlands, including ombrotrophic peatlands – environments which are frequently used to reconstruct Holocene palaeohydrological change (Mitchell et al, 2008). Statistical analyses Considering the key aim of this study to produce a transfer function for palaeohydrological reconstruction, non-metric multidimensional scaling (NMDS) was used sensu Amesbury et al (2016) to objectively examine the dataset and potentially exclude outlying sites and/or samples that were non297 typical for the ombrotrophic peatlands commonly used in palaeoclimate research. 318 319 As part of the testing process, models were applied to independent (i.e. no modern samples from the sites were included in the transfer function) palaeo-data from two Sphagnum-dominated ombrotrophic peatlands: Framboise bog, Nova Scotia
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