A pollen rain-vegetation study along a 3600Â m mountain-desert transect in the Irano-Turanian region; implications for the reliability of some pollen ratios as moisture indicators

- The alpine and montane zones are characterized by predominance of grass pollen.
- The xerophytic steppes are highly represented by Artemisia and Chenopod pollen.
- The halophytic zone is characterized by absolute dominance of Chenopod pollen.
- P/A and P/C pollen ratios better differentiate mesic from arid steppes.
- C/A and (A + C)/P ratios better distinguish halophyte and xerophyte desert vegetation.

A set of 42 modern pollen samples has been investigated to determine the relationship between pollen percentages and vegetation composition along a 3600 m elevational mountain-desert transect in central Iran. The studied transect shows three main vegetation groups including a “high altitude zone” (embracing subnival, alpine and montane subzones), a “xerophytic desert steppe zone”, and a “halophytic zone”, correlated with the groups defined in Correspondence Analysis (CA) of vegetation dataset and Principal Component Analysis (PCA) of pollen dataset. The subnival subzone is characterized by high values of Asteraceae, Brassicaceae and Cyperaceae pollen, while alpine and montane subzones are characterized by the highest pollen diversity with a predominance of grass pollen along the whole transect. The halophytic zone is dominated by Chenopodiaceae pollen while xerophytic desert steppe shows a high occurrence of Artemisia pollen. The comparison of pollen percentages with the corresponding vegetation plots shows a high congruency between pollen and vegetation compositions of alpine subzone and undisturbed xerophytic desert steppe but a weak correlation between those of the subnival and montane subzones and human affected xerophytic desert steppe. In addition, pollen representation of frequently encountered or important plant taxa in the Irano-Turanian region is provided. The widely used Chenopodiaceae/Artemisia = C/A pollen ratio, as an aridity index, is shown to be unreliable in the Irano-Turanian steppes. Our results suggest that a combined graph of all four indices (C/A pollen ratio, Poaceae/Artemisia = P/A, Poaceae/Chenopodiaceae = P/C and (A + C)/P ratios) can represent the vegetation and climate relationships more accurately. In conclusion, surface pollen composition can reflect the actual vegetation zones/subzones in Irano-Turanian steppes. Together, P/A and P/C ratios are more confident to differentiate mesic from arid steppes, while C/A and (A + C)/P ratios provide a useful tool to differentiate halophytic desert vegetation developed in endorheic depressions with saline soils from xerophytic desert steppe developed in well-drained soils.