Zelkova carpinifolia (Pallas) K. Koch in Holocene sediments of Georgia—an indicator of climatic optima

Abstract

Zelkova pollen has been found in Oligocene- to Pleistocene-aged deposits from many parts of Europe and northern Africa, but became extinct in mainland Europe prior to the last glacial maximum. This paper presents some observations on the ecology, pollen productivity and Holocene history of Zelkova carpinifolia to further understanding of Quaternary climatic trends. Georgia is one of the last refuges of this Tertiary relict tree. Based on palynological data from 20 Holocene sediment profiles in Georgia, we have established that Zelkova pollen is almost always accompanied by elevated proportions of thermophilous taxa ( Castanea sativa, Quercus hartwissiana, Quercus iberica, and Pterocarya pterocarpa) in pollen spectra. These spectra are associated with phases of climatic amelioration and humidification. Zelkova carpinifolia is characterised by low pollen productivity and is underrepresented in pollen spectra by four-to-five orders of magnitude. Because of this, even single grains of Zelkova pollen may play a significant role in pollen-based climatic reconstructions. Six major climatic optima occurred in Georgia through the course of the Holocene, the longest and warmest of which occurred in the mid-Holocene and reached its maximum between 6000 and 5500 Cal. yr BP. During that period, Zelkova and Castanea forests were widespread. In Western Georgia, the upper tree line was elevated by as much as 300 m above its present-day level. In semiarid Eastern Georgia, the tree line may have been 500–600 m higher. Other climatic optima are seen in late-Holocene pollen spectra dating to 3800–2500 Cal. yr BP and 1350–800 Cal. yr BP.