5 Discussion
Salvia natans
next sectionA recent overview of Salvinia finds from archaeobotanical literature by Out (2010), which also includes the finds in the present study, shows that Schokkerhaven is the most northern location of mid-Holocene Salvinia finds. It is the only location outside the Rhine/Meuse river area. Formerly, finds of Salvinia were dated to the Atlantic (early-Neolithic) mainly based on Zandstra (1966). The overview by Out provides more finds that are dated in the transition from Atlantic to Sub-Boreal and shows the occurrence of Salvinia during the Sub-Boreal (late-Neolithic) period. This fits with the occurrence of an other thermophilous aquatic species, namely Trapa natans elsewhere in the Netherlands. Trapa was frequently recorded in the early Subboreal (archaeobotanical database RADAR; Van Haaster & Brinkkemper 1995; Behre 1970).
Reconstruction of vegetation development
The presence of Alnus, Ulmus and Tilia pollen in the sandy soil points to an Atlantic (approximately late Mesolithic) age (Van Geel et al. 1981) of the base of the studied profile. Before peat growth started (Zone Ia), the presence of spores and macrosporangia of Salvinia indicate local aquatic conditions. The presence of Salvinia is followed by Riccia (Zone Ib), probably indicating alternating wet and dry conditions. On the higher parts of the dune a Tilia forest was present with some open areas where Artemisia, Asteraceae and Brassicaceae were growing. As a result of a rise in the groundwater level, the Tilia forest disappeared and a wetland forest with Alnus and other marsh- and wetland species could develop.
The variety of herbaceous taxa increased when the first cereal cultivation started (Zone Ib). The landscape became more open and richer in nutrients, favouring plants such as Solanum dulcamara, Valeriana and Succisa (Zone Ib and Zone II).
Later, the Alnus forest declined and a Salix carr could develop in most wet areas. Furthermore, the crops and the species associated with meadows declined when conditions became wetter (Zone III). Lythrum, Alisma, Mentha and Typha were found in the nutrient-rich marshes (Zone IVa). Open water was present in the lower parts down slope, but whether there was permanent open water or seasonal inundations is not clear.
On the dry parts of the dune Quercus trees and meadow plants remained present. When Salix declined, many other trees became less important (Zone IVb). Poaceae, filices and Cyperaceae increased and the landscape became an open wetland. Corylus and Pteridium probably inhabited the slopes of the dunes and forest edges.
During the upper part of the studied profile Myrica shrubs developed in the area (Zone V). Myrica possibly was growing on the drained, decaying peat surface and there may have also been some cover of wet forest of Alnus and Salix Chenopodiaceae were linked to increased brackish or marine influence in the landscape.
Human impact
Of special interest are the first attempts of prehistoric farming. The change from hunter-gatherers to organized farming communities is considered a key process in the history of humankind. This change would have been a long process of progress and setbacks. The first occurrence and location of agricultural practice in northwest Europe is disputed. Concluding whether an archeological site contains evidence for agriculture is rather subjective. Findings of single pollen grains of the Cerealia-type have sometimes been used to reconstruct an entire Mesolithic agricultural society, an approach that was strongly criticized by Behre (2007). Herbaceous species linked to human occupation or disturbance (light; open soils) are indicated as apophytes (sensu Behre 1981). The natural habitat of apophytic species was in places disturbed by wind, fire or other natural causes (Wittig 2004). With an increasing amount of human settlements there were more disturbances, which was advantageous for the apophytes as they became associated with disturbance related to human settlements.
The palynological data from site Schokkerhaven-E170 show some close links between the occurrence of pollen of Cerealia-type and apophytes. This link could not be established in Zone Ia (Late Atlantic period). Both Asteraceae (both tubuliflorae and liguliflorae) and Caryophyllaceae occur in this zone but no cereals were found. In Zone Ib (radiocarbon dated: ca. 5000 BP; corresponding to ca. 3950 to 3700 cal BC) relatively high values of Cerealia, Rumex acetosella-type, Brassicaceae, Melampyrum, Jasione-type and Caryophyllaceae occur. Single finds of Succisa, Polygonum persicaria and Fallopia are also present in Zone Ib, while low amounts of pollen of Urtica dioica occur at the transition from Zones Ib to Zone II. Some apophytes follow the Cerealia-type curve more closely than others. The recorded plant taxa indicate the presence of various plant communities, Urtica points to nitrogen rich areas. Other taxa (Fallopia, Polygonum persicaria, Asteraceae, Rumex acetosella) are typical for arable fields while Succisa and Plantago lanceolata indicate meadows. The occurrence of Cerealia-type pollen alone cannot be taken as proof for agriculture (Behre 2007), but the presence of apophytes supports this interpretation.
The record of pollen of Cerealia at site Schokkerhaven-E170 is supported by a variety of other indicators for agriculture and apophytes, charcoal particles and the records of coprophilous fungi provide further support for human impact. The spores may have developed on human feces, but dung of wild and/or domesticated animals may have been the most important substrate for these fungi (Van Geel et al. 2003). With regard to the fungal evidence for grazing, it is not possible to distinguish between domesticated and wild animals.
In zones I and II the combined presence of cereals, apophytic species, coprophilous fungi and charcoal clearly indicate the presence of humans and agricultural activity. The three radiocarbon dates place the recorded human impact in the late-Swifterbant culture. The dates complement the radiocarbon dates obtained from charred hazelnut shells, charcoal and charred food remains from a pottery sherd (Lanting & Van der Plicht 2002) sampled at the site in the late 1980s by J.W.H. Hogestijn (see above). The Swifterbant culture in this region is succeeded by the Funnel-beaker culture. Possible indications for later human presence in the diagram are the peak of Cercophora spores in zone IVa and the small peak of Cerealia-type in zone V. Although the indications are not as strong, both could suggest local Funnel Beaker culture agricultural activity, corresponding to the radiocarbon dates obtained on some wooden posts (Lanting & Van der Plicht 2002), also sampled by Hogestijn.