A compositional meta-analysis of pollen data reveals climate influence on vegetation trends in the Central Mediterranean region during the late Holocene

The Mediterranean area represents a transitional climatic zone between the temperate conditions of continental Europe and the subtropical regime of the African continent. This ecotonal feature determines a high climatic heterogeneity within the Basin: different atmospheric mechanisms and their interplays influence the amount, distribution and seasonality of precipitations, and eventually determine changes in the distribution of plant communities on millennial/centennial time scales. In this study, we present a meta-analysis of Italian Palynological Data from 32 well-dated and detailed lacustrine and marine records, including more than 90,000 observations for the last 3200 years. All the data were interpolated and homogenized at a secular resolution, to allow comparison of the different time-series. The compositional nature of pollen data was considered statistically in a unique holistic system using the R package “compositions”. Pollen taxa were grouped in Plant Functional Types, to ecologically characterize the vegetational variations found along the Italian Peninsula through time. Concordant/discordant vegetation patterns and synchronic/diachronic fluctuations at different latitudes and in different phytogeographic and ecoregional contexts were found. The role of human impact in determining environmental changes and in blurring the natural climatic oscillations during the last thousands of years was considered. A clear 2.8 ka cal BP event is statistically recognized, involving the Italian vegetation with contrasting latitudinal trends: it appears as an arid event affecting southern Italy and leading to a forest decline south of ca. 43°N, while no drought effect on forest cover nor changes in overall composition are statistically recorded north of this latitude below the timberline ecotone. By contrast, a considerable increase in forest cover was found during the Roman Classical Period (RCP) all over the central Mediterranean. Despite increasing human impact in the last part of the Holocene, the forest development during the Roman Period is in line with the so called “Roman Humid Period”. In this case, the compositional-PCA suggests the dominance of natural climatic variability over the anthropogenic influence in determining vegetational landscape changes. Considering our data in a wider geographical context, it was possible to find evidence for a longitudinal variability characterized by opposite vegetational trends in response to precipitation distribution patterns. Thus, studying the Central Mediterranean region provides an important contribution for understanding the so called “see-saw pattern” between the western and eastern regions of the Mediterranean Basin.