Palaeoenvironmental changes and vegetation history of the northern Upper Rhine Graben (southwestern Germany) since the Lateglacial

Abstract

The vegetation of the northern Upper Rhine Graben (southwestern Germany) is reconstructed for the end of the Lateglacial and the Holocene by means of palynological analyses in combination with AMS 14c dating. Analogous to adjacent lowland areas, the Younger Dryas climatic deterioration did not result in a complete deforestation of the area and open pine woodlands with locally birch stands and shrubs persisted. A subdivision of the Younger Dryas period, into a humid first phase, followed by a dry second phase was also reflected in our records. For the Holocene, the pollen diagrams show two regionally different vegetation developments, related to substrate and variations in annual precipitation: in the south the ‘classical’ succession of pine then hazel is followed by other deciduous trees, whereas in the northern part, pine kept its dominance far into the Subboreal.

Fluvial terrace formation in the northern Upper Rhine Graben during the last 20000 years as a result of allogenic controls and autogenic evolution

“…The relative coarsening of the overbank deposits is most likely the result of human-induced soil erosion in the loess areas upstream (as found elsewhere, e.g., Knox, 2006). Large-scale deforestation during the Bronze Age, Iron Age, and Roman times in the hinterland (e.g., Friedmann, 2000;Singer, 2004;Lechner, 2005;Bos et al, 2008) resulted in significant changes in sediment availability. Initially, this led to increased deposition on hillslopes as colluvium (e.g., Lang and Hönscheidt, 1999;Lang, 2003) and in smaller catchments upstream as alluvium, which is quantified by several studies (e.g., Lang and Nolte, 1999;Houben, 2003;Rommens et al, 2006;De Moor, 2007;Seidel and Mäckel, 2007).…”

“…8A), the pollen assemblage at the base of the infill with abundant deciduous trees is dominated by Corylus (Hazel). Bos et al (2008) dated the maximum distribution of Hazel in this area to the Boreal and the occurrence of Tilia (Lime) indicates that this pollen assemblage resembles the middle Boreal (~9.0 cal ka BP). The onset of the infill of core PALY2 (Fig.…”

“…Pollen subsamples were prepared following standard pollen preparation methods (Faegri and Iversen, 1989). Pollen assemblages present in basal abandoned channel fills were compared with pollen assemblages from absolutely dated pollen sequences in the northern Upper Rhine Graben and adjacent areas (e.g., Bos, 2001;Dambeck and Bos, 2002;Bos et al, 2008). In this way, the minimum age of infilling and thus the timing of the abandonment of the palaeomeander was reconstructed.…”

“…An allogenic explanation for the lack of a distinctively formed Younger Dryas terrace may be that changes in vegetation cover in the Rhine drainage basin upstream of the study area were too minimal to change discharge regimes significantly -and thus not changed the mode of ongoing complex response (see Section 6.1). Indeed, the persistent presence of pine forest cover over the southern half of the Rhine catchment throughout the Younger Dryas (Dambeck and Bos, 2002;Bos et al, 2008) is in contrast to the downstream, northerly parts that are almost completely deforested (e.g., Rheinische Schiefergebirge; Litt and Stebich, 1999). Andres et al (2001) pointed out that only the highest elevated (N400 m above MSL) tributary valleys in the southern half of the Rhine catchment were deforest during the Younger Dryas but that, nevertheless, the upper and lower courses of the valleys showed a transition to a braided system.…”

“…Seemingly, fluvial response to human influence in terms of terrace formation and overbank sedimentation is less complex and at least quicker than the response to Late Glacial climate changes, although in the Late Holocene the system merely changed its suspended load composition. Dambeck and Thiemeyer (2002), Dambeck (2005), and Bos et al (2008) concluded that increasing human impact during the Subboreal and especially Subatlantic was superimposed on climate change and resulted in a change in the fluvial morphodynamics (flow concentration and locally decreasing meander sinuosity, possibly by increased bank resistance -see further discussion below). We support this viewpoint, but given the considerable change in overbank composition, it seems unlikely that the subtle cooling and increase in precipitation at the Subboreal-Subatlantic transition (e.g., Van Geel et al, 1996) is equally important as the strong and widespread Bronze and Iron Age human land cultivation.…”