Abstract
Excellently preserved subfossil pine and oak tree remains from the bottom layer of raised bog peat were dendroecologically investigated at Venner Moor (northwest Germany). Tree-ring analyses were combined with observations of stem and root morphology, preservation state, mineral soil relief, peat stratigraphy and pollen analysis to reconstruct in great detail environmental changes leading to the start of the raised bog formation. Hydrology was identified as the main determinant influencing tree growth and population dynamics at Venner Moor, as documented by different growth patterns and dying-off dates in relation to the mineral soil elevation. The woodland phase has been dendrochronological dated to the period from 2421—2077 BC (4371—4027 cal BP). In this period, a general change from more or less open landscape with dominating heath to wet pine forest and eventually to open raised bog occurred at the site. Comparisons with pine population dynamics at the nearby Voerdener Moor and with the independent Lower Saxony Bog Oak Chronology (LSBOC) indicate that the reconstructed ecological changes at Venner Moor are mainly triggered by climate variations, in particular wet shifts on the decadal timescale. This example shows the value of subfossil pine layers from northwest German bogs as a high resolution proxy archive of Holocene humidity fluctuations.
Highlights
Subfossil wood buried under peat, at the bottom of lakes or covered by soil or glacial deposits, is one of the best archives of Holocene environmental changes due to the precise dating opportunity and annual resolution
Tree-ring analyses were combined with observations of stem and root morphology, preservation state, mineral soil relief, peat stratigraphy and pollen analysis to reconstruct in great detail environmental changes leading to the start of the raised bog formation
Hydrology was identified as the main determinant influencing tree growth and population dynamics at Venner Moor, as documented by different growth patterns and dying-off dates in relation to the mineral soil elevation
Summary
Subfossil wood buried under peat, at the bottom of lakes or covered by soil or glacial deposits, is one of the best archives of Holocene environmental changes due to the precise dating opportunity and annual resolution. In northwest Germany, subfossil oaks from mire habitats were extensively studied dendrochronologically during the last few decades (Leuschner et al, 1985, 1987, 2002) and subfossil pines have become the focus of more recent investigations (Bauerochse et al, 2006; Leuschner et al, 2007; Eckstein et al, 2009). As a result some 1700 oaks and 800 pines were dated. We hypothesise that hydrology is the main factor influencing tree growth and population dynamics, since the subfossil trees originate from mire habitats. A detailed reconstruction of past site conditions and changes were in most cases impossible, since the great majority of the samples were found ex situ, i.e. relations to the peat stratigraphy and differences between individual trees were unknown
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