Dendroecological investigations at Venner Moor (northwest Germany) document climate-driven woodland dynamics and mire development in the period 2450—2050 BC

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.

Dendrochronologically dated pine stumps document phase-wise bog expansion at a northwest German site between ca. 6700 and ca. 3400 BC

“…As mentioned before, the dying-off phases observed at TOMO_south show much synchronism with dying-off phases observed at other mire sites in northwest Germany (Eckstein et al, 2009(Eckstein et al, , 2010(Eckstein et al, , 2011, which emphasizes their climatic context. In addition to the temporal placing of the TOMO_south record (terminating 3400 BC) and its location in the northwest German lowland, where settlements were established later than on the richer soils of the adjacent hills, this synchrony makes it appear unlikely that anthropogenic clearance activity in the catchment area might have had a detectable influence on the bog's hydrology.…”

“…This has been found evident on base of abundant upward growing roots late in the tree's life, the drastically narrowing rings near the bark, peat stratigraphical context and the state of conservation at various comparable sites investigated in northwest Germany and southern Sweden (Eckstein et al, 2009(Eckstein et al, , 2010Edvardsson et al, 2012bEdvardsson et al, , 2014Leuschner et al, 2002Leuschner et al, , 2007. Therefore the death of the trees and their conservation under Sphagnum peat dates raised bog expansion, and using in situ stumps adds location to the event.…”

“…They were able to show that the tree dying-off is mostly due to hydrological changes, while fire and storm have been found to be of little relevance on the investigated sites. Particularly the observed synchrony of pine establishment and dying-off phases in different mires shows that the local mire development documented by the trees is strongly linked to climatic variations (Eckstein et al, 2009(Eckstein et al, , 2010(Eckstein et al, , 2011Edvardsson et al, 2012a, b).…”

“…The trees dated absolutely by dendrochronology deliver the finest temporal reconstruction of bog expansion and local bog formation, particularly by their dying-off dates (Eckstein et al, 2009(Eckstein et al, , 2010Edvardsson et al, 2014Edvardsson et al, , 2016Krąpiec et al, 2016;Leuschner et al, 2002Leuschner et al, , 2007. Eckstein et al (2009Eckstein et al ( , 2010Eckstein et al ( , 2011 have shown that the tree dying-off occurs in phases, which are often synchronous in different mires. They were able to show that the tree dying-off is mostly due to hydrological changes, while fire and storm have been found to be of little relevance on the investigated sites.…”

“…Eckstein et al (2010Eckstein et al ( , 2012 and Leuschner et al (2007) have referred to this as germinationdying-off phases (GDOs). Even though a spatial pilot study (Stenzel, 2013) did not find the newly established trees directly in the area shaded by those trees which had just died off, the mechanism causing the coherence is still suspected to be driven by competition for light.…”