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Middle Holocene vegetation and woodland exploitation of the lower Scheldt valley

Koen Deforce (UGent)
(2014)
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Abstract
The Lower Scheldt valley is a key area for studying the Mesolithic-Neolithic transition in northern Belgium. To get more insight in the environmental context of this change in subsistence strategy, several aspects of the middle Holocene vegetation in this area have been studied. Also the way local populations used this vegetation was assessed. This is done by the combination of archaeobotanical analyses of samples from archaeological sites and palaeoecological analyses of contemporary natural deposits. The main methods used are pollen, wood and charcoal analysis but also results from other analyses such as loss-on-ignition, the study of seeds and fruits, mosses, diatoms, animal bones and molluscs are used in this study. The results show that in the lower parts of the Lower Scheldt valley, eutrophic alder (Alnus)-dominated carr vegetation developed between c. 5700 and 4630 cal BC as a consequence of the postglacial relative sea level (RSL) rise, resulting in the deposition of wood peat. From c. 4600 cal BC onwards, (peaty) clay was deposited on top of this peat. This clay contains many marine and brackish microfossils suggesting tidal activity and an increase of salinity. This estuarine influence ceases at most sites before c. 3800 cal BC and peat growth resumed. Sedges (Cyperaceae), ferns (Filicales) and birch (Betula) have now become important in the wetland vegetation indicating more mesotrophic conditions. Between c. 3840 and 1830 cal BC also yew (Taxus baccata) occurs in this carr vegetation. Autogenic succession of the mire vegetation and the combined effects of a decline in the rate of RSL rise and a shift to a wetter, more oceanic climate eventually resulted in the development of oligotrophic bogs. The occurrence of yew (Taxus baccata) in the fen carr vegetation in the Lower Scheldt valley during the middle and late Holocene is remarkable seen its current distribution and habitat preferences. The current natural distribution of yew in Belgium is limited to a few localities in the southern part of the country where it grows on steep, calcareous slopes, which is also believed to be its natural habitat in this part of the world. In northern Belgium, yew is considered not to be native and Taxus stands are interpreted there as being planted by humans or as garden escapes. A review of all postglacial palaeobotanical and archaeobotanical records of yew from Belgium and neighbouring regions however shows a completely different image. During the middle Holocene, yew occurred in the coastal plain, the Lower Scheldt valley, and other coastal lowlands bordering the southern North Sea, where it was part of the carr vegetation on peat. Yew thus seems to have had a completely different distribution and habitat preferences during this period. By c. 1830 cal BC, yew has disappeared from this region, most probably caused by the transition of the carr vegetation to oligotrophic bogs. In contrast to this wetland vegetation, the vegetation on sand dunes and natural levees was for most of the middle Holocene period dominated by oak (Quercus), lime (Tilia), elm (Ulmus) and ash (Fraxinus) and was further characterized by a very large diversity of shrubs. The identified taxa together with their geomorphological position and the presence of contemporary alluvial sediments point towards an alluvial hardwood forest (Querco-Ulmetum minoris Issler 1924) on these sand dunes and levees. Several of these sand ridges, representing the only relatively permanent dry places within the Lower Scheldt valley, where occupied by local Swifterbant populations between c. 4600 and 4000 cal BC. The analysis of charcoal and charred seeds from two of these Swifterbant sites shows that these forests were exploited for firewood and for the collection of plant food. High numbers of mistletoe (Viscum album) charcoal and ivy (Hedera helix) seeds suggest the use of these plants as leaf fodder. This would imply the presence of domestic animals at these sites from c. 4540 cal BC onwards, a period for which there is still much doubt on possible animal husbandry by local populations in this region. Finally, a multi-proxy study of plant and animal remains (pollen, seeds, fruits, mosses, diatoms, vertebrate remains, mollusks) from middle Holocene alluvial deposits and peat at the foot of a levee resulted in a detailed reconstruction of one of these alluvial forest and the associated aquatic environment. The use of multiple proxies not only resulted in a taxonomically more detailed and environmentally more comprehensive reconstruction of terrestrial as well as aquatic habitats. The results also demonstrate possible biases in palaeoecological reconstructions of alluvial and estuarine environments from single proxies. Many locally occurring woody taxa are underrepresented or remain undetected by pollen analyses. Seeds and fruits also proved to be inadequate proxies for the detection of several locally important taxa such as elm (Ulmus) and ivy (Hedera helix). Diatoms, pollen and other microfossils indicate brackish conditions where molluscs, fish bones and botanical macro remains provide proof of a strictly freshwater environment. This is explained by the deposition of sediments during spring tides or storm surges, when estuarine waters penetrated further landward than normal. Suspended sediment trapped in the tidal node at the upstream limit of tides following net upstream advection, was deposited even further upstream. This resulted in the deposition of marine and brackish microfossils and pollen from halophyte vegetation in a freshwater environment.

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Citation

Please use this url to cite or link to this publication:

Chicago
Deforce, Koen. 2014. “Middle Holocene Vegetation and Woodland Exploitation of the Lower Scheldt Valley”. Ghent, Belgium: Ghent University. Faculty of Arts and Philosophy.
APA
Deforce, K. (2014). Middle Holocene vegetation and woodland exploitation of the lower Scheldt valley. Ghent University. Faculty of Arts and Philosophy, Ghent, Belgium.
Vancouver
1.
Deforce K. Middle Holocene vegetation and woodland exploitation of the lower Scheldt valley. [Ghent, Belgium]: Ghent University. Faculty of Arts and Philosophy; 2014.
MLA
Deforce, Koen. “Middle Holocene Vegetation and Woodland Exploitation of the Lower Scheldt Valley.” 2014 : n. pag. Print.
@phdthesis{5686688,
  abstract     = {The Lower Scheldt valley is a key area for studying the Mesolithic-Neolithic transition in northern Belgium. To get more insight in the environmental context of this change in subsistence strategy, several aspects of the middle Holocene vegetation in this area have been studied. Also the way local populations used this vegetation was assessed. This is done by the combination of archaeobotanical analyses of samples from archaeological sites and palaeoecological analyses of contemporary natural deposits. The main methods used are pollen, wood and charcoal analysis but also results from other analyses such as loss-on-ignition, the study of seeds and fruits, mosses, diatoms, animal bones and molluscs are used in this study. 
The results show that in the lower parts of the Lower Scheldt valley, eutrophic alder (Alnus)-dominated carr vegetation developed between c. 5700 and 4630 cal BC as a consequence of the postglacial relative sea level (RSL) rise, resulting in the deposition of wood peat. From c. 4600 cal BC onwards, (peaty) clay was deposited on top of this peat. This clay contains many marine and brackish microfossils suggesting tidal activity and an increase of salinity. This estuarine influence ceases at most sites before c. 3800 cal BC and peat growth resumed. Sedges (Cyperaceae), ferns (Filicales) and birch (Betula) have now become important in the wetland vegetation indicating more mesotrophic conditions. Between c. 3840 and 1830 cal BC also yew (Taxus baccata) occurs in this carr vegetation. Autogenic succession of the mire vegetation and the combined effects of a decline in the rate of RSL rise and a shift to a wetter, more oceanic climate eventually resulted in the development of oligotrophic bogs. 
The occurrence of yew (Taxus baccata) in the fen carr vegetation in the Lower Scheldt valley during the middle and late Holocene is remarkable seen its current distribution and habitat preferences. The current natural distribution of yew in Belgium is limited to a few localities in the southern part of the country where it grows on steep, calcareous slopes, which is also believed to be its natural habitat in this part of the world. In northern Belgium, yew is considered not to be native and Taxus stands are interpreted there as being planted by humans or as garden escapes. A review of all postglacial palaeobotanical and archaeobotanical records of yew from Belgium and neighbouring regions however shows a completely different image. During the middle Holocene, yew occurred in the coastal plain, the Lower Scheldt valley, and other coastal lowlands bordering the southern North Sea, where it was part of the carr vegetation on peat. Yew thus seems to have had a completely different distribution and habitat preferences during this period. By c. 1830 cal BC, yew has disappeared from this region, most probably caused by the transition of the carr vegetation to oligotrophic bogs.
In contrast to this wetland vegetation, the vegetation on sand dunes and natural levees was for most of the middle Holocene period dominated by oak (Quercus), lime (Tilia), elm (Ulmus) and ash (Fraxinus) and was further characterized by a very large diversity of shrubs. The identified taxa together with their geomorphological position and the presence of contemporary alluvial sediments point towards an alluvial hardwood forest (Querco-Ulmetum minoris Issler 1924) on these sand dunes and levees. 
Several of these sand ridges, representing the only relatively permanent dry places within the Lower Scheldt valley, where occupied by local Swifterbant populations between c. 4600 and 4000 cal BC. The analysis of charcoal and charred seeds from two of these Swifterbant sites shows that these forests were exploited for firewood and for the collection of plant food. High numbers of mistletoe (Viscum album) charcoal and ivy (Hedera helix) seeds suggest the use of these plants as leaf fodder. This would imply the presence of domestic animals at these sites from c. 4540 cal BC onwards, a period for which there is still much doubt on possible animal husbandry by local populations in this region.  
Finally, a multi-proxy study of plant and animal remains (pollen, seeds, fruits, mosses, diatoms, vertebrate remains, mollusks) from middle Holocene alluvial deposits and peat at the foot of a levee resulted in a detailed reconstruction of one of these alluvial forest and the associated aquatic environment. The use of multiple proxies not only resulted in a taxonomically more detailed and environmentally more comprehensive reconstruction of terrestrial as well as aquatic habitats. The results also demonstrate possible biases in palaeoecological reconstructions of alluvial and estuarine environments from single proxies. Many locally occurring woody taxa are underrepresented or remain undetected by pollen analyses. Seeds and fruits also proved to be inadequate proxies for the detection of several locally important taxa such as elm (Ulmus) and ivy (Hedera helix). Diatoms, pollen and other microfossils indicate brackish conditions where molluscs, fish bones and botanical macro remains provide proof of a strictly freshwater environment. This is explained by the deposition of sediments during spring tides or storm surges, when estuarine waters penetrated further landward than normal. Suspended sediment trapped in the tidal node at the upstream limit of tides following net upstream advection, was deposited even further upstream. This resulted in the deposition of marine and brackish microfossils and pollen from halophyte vegetation in a freshwater environment.},
  author       = {Deforce, Koen},
  language     = {eng},
  pages        = {XIV, 192},
  publisher    = {Ghent University. Faculty of Arts and Philosophy},
  school       = {Ghent University},
  title        = {Middle Holocene vegetation and woodland exploitation of the lower Scheldt valley},
  year         = {2014},
}