Modelling late Quaternary changes in plant distribution, vegetation and climate using pollen data from Georgia, Caucasus

Abstract

Aim : To use pollen data, numerical analysis and modelling to reconstruct late Quaternary vegetation and climate in a complex, mountainous environment. Location : Georgia (Caucasus region). Methods : Pollen data were assembled from various sources and used to map: (1) changing frequencies of individual taxa; (2) vegetation changes; and (3) reconstructed climatic parameters for the past 14,000 years. Numerical analyses were performed using two-way indicator species analysis (twinspan), detrended correspondence analysis (DCA), the modern analogue technique (MAT) and weighted averaging (WA). Results : Mapping of pollen taxa showed that Chenopodiaceae, Artemisia and Ephedra were most abundant in the study area during the late-glacial. Betula and Corylus expanded during the early Holocene, yielding to Abies, Carpinus, Fagus, Quercus and Castanea. Picea, Pinus, Juglans and Ostrya-type expanded during the late Holocene. Mixed forests grew in the moist, Black Sea refugium throughout the late Quaternary. Elsewhere in Georgia, the Pleistocene-Holocene transition is recorded as a shift from desert-steppes to oak-xerophyte communities and mixed forests. This kind of vegetation remained relatively stable until the mid-late Holocene, when coniferous forests and mountain grasslands advanced. DCA showed that rainfall was most strongly correlated with pollen composition in the study area (r 2 = 0.55). No temperature signal was detected. A weighted-averaging transfer function linking pollen percentages to annual precipitation was selected over a MAT model as it performed better when applied to a validation data set. Rainfall reconstructions indicate widespread aridity at the terminal Pleistocene, followed by a gradual increase in precipitation, peaking during the mid Holocene (7000-4000 cal. yr bp) and generally decreasing thereafter. Main conclusions : On a regional scale, the results confirm those from previous studies of palaeovegetation and palaeoclimate in Western Asia. On a local scale, reconstructions from individual sites often diverge from the regional trend because of edaphic changes, ecological succession, human impacts and other disturbances. Some of these factors are probably responsible for the increasing heterogeneity of Georgia's vegetation in the latter half of the Holocene. Show more