The palaeoecological history of the Praz-Rodet bog (Swiss Jura) based on pollen, plant macrofossils and testate amoebae (Protozoa)

Stratigraphy, radiocarbon dating and analyses of pollen, plant macrofossils and testate amoebae were used to reconstruct the development and ecology o f a small raised bog in a karst-dominated landscape in the Swiss Jura Mount ains. Special focus was on past vegetation and on the history of Pinus rotu ndata in relation to anthropogenic and climatic influences. Testate amoebae were used to reconstruct past local soil pH and water-table depth. The inf erred development of the Praz-Rodet bog typifies a classic hydroseral terre strialization of a small basin. Two features are specific for this site. Fi rst, the bog was much wetter than today for a long period; according to our hypothesis, this only changed as a consequence of human activities. Second , two hiatuses are present at the coring location (Younger Dryas-early Preb oreal, and 4700-2800 cal. yr BP), the former probably caused by low lake pr oductivity due to cold temperatures and the latter by the erosional activit y of the adjacent small river. The date of 2800 cal. yr BP for renewed peat accumulation may be related to climatic change (Subboreal-Subatlantic tran sition). Pollen indicators failed to show one hiatus: an apparently complet e pollen sequence is therefore no guarantee of an uninterrupted sediment ac cumulation. Evidence of early minor human impact on the vegetation in the J oux Valley dates back to c. 6850 calendar years, congruous with the early N eolithic in the Jura Mountains. The history of Pious rotundata appears to b e more complex than previously believed. Human activity is clearly responsi ble for the present abundance of this species, but the tree was naturally p resent on the bog long before the first evidence of important human disturb ance of the site (1500 cal. yr BP). It is suggested that, in karst-dominate d landscapes, dense forests growing on mineral soils around raised bogs may significantly reduce summer evapotranspiration by acting as windbreaks. Fo rest clearance results in increased evapotranspiration, causing a lowering of the water table on the bog and a modification of the vegetation cover. T his hypothesis has implications for the management of similar small raised bogs in karst-dominated landscape.