A new method based on surface- sample pollen data for reconstructing palaeovegetation patterns.

Abstract

Aim: Amongst the various techniques available to reconstruct past vegetation at regional to continental scales, biomisation has been the most widely used because it does not require an extensive modern pollen data set. However, it has well well-known limitations including its dependence on expert judgement for the assignment of pollen taxa to plant functional types (PFTs) and PFTs to biomes. Here we present a new method that combines the strengths of biomisation with those of the alternative dissimilarity-based techniques. This new technique quantifies the likelihood that a sample belongs to a given biome, and allows discrimination of non-analogue vegetation types. Location: The Eastern Mediterranean-Black Sea Caspian Corridor (EMBSeCBIO) region, 28°-49°N, 20°- 62°E. Methods: Modern pollen samples assigned to biomes based on potential natural vegetation data, are used to characterize biomes according to the within-biome means and standard deviations of the abundances of each taxon. These are used to calculate a dissimilarity index between any given pollen sample and every biome, and thus assign a pollen sample to the most likely biome. We also calculate a threshold value for each biome which identifies samples that fall outside the acceptable range of likelihoods for biome assignment and hence can be used to distinguish non-analogue vegetation. We have applied the new technique to the EMBSeCBIO region to compare the performance of the new method with existing reconstructions. Results: The technique captured changes in the importance of individual taxa along environmental gradients. The balanced accuracy obtained for the EMBSeCBIO region using the new method was better than that obtained using biomisation (77% versus 65%). When the method was applied to high resolution fossil records, 70% of the evaluated entities showed more temporally stable biome assignments than obtained with the biomisation method. The technique also identifies likely non analogue assemblages in a synthetic modern data set and in fossil records. Main conclusions: The new method yields more accurate and stable reconstructions of vegetation than the biomisation method. It requires an extensive modern pollen dataset, but is conceptually simple, and avoids subjective choices about taxon allocations to PFTs and PFTs to biomes.