Palynostratigraphy of the last centuries in Switzerland based on 23 lake and mire deposits: chronostratigraphic pollen markers, regional patterns, and local histories

A total of 23 pollen diagrams [stored in the Alpine Palynological Data-Base (ALPADABA), Geobotanical Institute, Bern] cover the last 100 to over 1000 years. The sites include 15 lakes, seven mires, and one soil profile distri buted in the Jura Mts (three sites), Swiss Plateau (two sites), northern Pr e-Alps and Alps (six sites), central Alps (five sites), southern Alps (thre e sites), and southern Pre-Alps (four sites) in the western and southern pa rt of Switzerland or just outside the national borders. The pollen diagrams have both a high taxonomic resolution and a high temporal resolution, with sampling distances of 0.5-3 cm, equivalent to 1 to 11 years for the last 1 00 years and 8 to 130 years for earlier periods. The chronology is based on absolute dating (14 sites: Pb-210 11 sites; C-14 six sites; varve counting two sites) or on biostratigraphic correlation am ong pollen diagrams. The latter relies mainly on trends in Cannabis sativa, Ambrosia, Mercurialis annua, and Ostrya-type pollen. Individual pollen stratigraphies are discussed and sites are compared withi n each region. The principle of designating local, extra-local, and regiona l pollen signals and vegetation is exemplified by two pairs of sites lying close together. Trends in biostratigraphies shared by a major part of the p ollen diagrams allow the following generalisations. Forest declined in phas es since medieval times up to the late 19th century. Abies and Fagus declin ed consistently, whereas the behaviour of short-lived trees and trees of mo ist habitats differed among sites (Alnus glutinosa-type, Alnus viridis, Bet ula, Corylus avellana). In the present century, however, Picea and Pinus in creased, followed by Fraxinus excelsior in the second half of this century. Grassland (traced by Gramineae and Plantago lanceolata-type pollen) increa sed, replacing much of the forest, and declined again in the second half of this century. Nitrate enrichment of the vegetation (traced by Urtica) took place in the first half of this century. These trends reflect the intensif ication of forest use and the expansion of grassland from medieval times up to the end of the last century, whereas subsequently parts of the grasslan d became used more intensively and the marginal parts were abandoned for fo rest regrowth. In most pollen diagrams human impact is the dominant factor in explaining inferred changes in vegetation, but climatic change plays a r ole at three sites. (C) 2000 Elsevier Science B.V. All rights reserved.