An objective method based on assemblages of subfossil plant macro-remains to reconstruct past natural vegetation: a case study at Swifterbant, The Netherlands

Mans Schepers 0 1 2 3 4 J. F. Scheepens 0 1 2 3 4 Rene T. J. Cappers 0 1 2 3 4 Onno F. R. van Tongeren 0 1 2 3 4 Daan C. M. Raemaekers 0 1 2 3 4 Renee M. Bekker 0 1 2 3 4 0 J. F. Scheepens Section of Ecology, Department of Biology, University of Turku , 20014 Turku, Finland 1 M. Schepers (&) R. T. J. Cappers D. C. M. Raemaekers Groningen Institute of Archaeology, University of Groningen , Poststraat 6, 9712 ER Groningen, The Netherlands 2 Communicated by C. C. Bakels 3 R. M. Bekker Community and Conservation Ecology Group, Centre for Ecological and Evolutionary Studies, University of Groningen , PO Box 11103, 9700 CC Groningen, The Netherlands 4 O. F. R. van Tongeren Data-Analyse Ecologie, Vrij Nederlandstraat 57, 6826 AW Arnhem, The Netherlands We present a new method of identifying past plant communities based on a palaeobotanical dataset. The dataset used as a case study consists of plant macroremains retrieved from the Neolithic settlement Swifterbant S4, The Netherlands. Taxa were grouped based on their present-day concurrence values. Subsequently, phytosociological analysis was performed on the subfossil taxon groups using the software package PALAEOASSOCIA, adjusted for this type of research. Results show that syntaxonomic knowledge on the concurrence of plant species can be used to reconstruct parts of the past vegetation. We further discuss the theory behind the reconstruction of syntaxa, with special emphasis on actualism. - The reconstruction of past vegetation in the vicinity of archaeological sites has always been one of the key goals in archaeobotany, giving insight into the conditions and exploitation possibilities of the area for its former inhabitants. In the present study, a new objective method is introduced for identifying past vegetation through phytosociology, the study of plant communities. For an introduction to phytosociology, see Braun-Blanquet (1964). The method applies to natural vegetation and the samples analyzed here were not, strictly speaking, from an archaeological feature. We will therefore refer to the samples as palaeobotanical instead of archaeobotanical. In the case study presented in this paper, focus lies on the reconstruction of the regional vegetation around the site for the relatively brief period from 4300 to 4000 cal. B.C., and is based on the analysis of plant macro-remains. The methods presented, however, can also be applied to pollen, wood or, and perhaps preferably, a combination of all data available from the site under study. The methodology presented in this paper shows that reliable vegetation reconstruction based on phytosociology can be achieved, even with palaeobotanical samples representing a mixture of plants from different syntaxa (plant communities defined by phytosociology). Once palaeobotanical data have been gathered, there are two established approaches for their interpretation towards a reconstruction of past vegetation: the individualistic approach and the assemblage approach, which have both been defined by Birks and Birks (2005, p. 343) and used for climate reconstruction (see below). These methods heavily rely on the uniformitarian assumption, also called actualism. Actualism can only fully be falsified if pure and complete samples are found, providing insight into the composition of a specific past vegetation type. However, pure and complete samples are rare for both macro-remains and pollen samples. Therefore it is necessary to find ways to divide and characterize taxon sets that clearly show a mixture of several vegetation types, as well as to define missing taxa. Individualistic approach The individualistic approach is based on information on the environmental optima and tolerances of a particular taxon. Abiotic values can be derived, for example, from Ellenberg et al. (1991) or Runhaar et al. (2004). These individual, taxon-bound values may be used to reconstruct specific abiotic conditions of the environment, like salinity or moisture availability (Behre 1991; Cappers 1995a). By combining different abiotic values, a taxon list can be divided into subsets probably sharing the same habitat. Thus, the individual approach is used as an indirect way to establish an assemblage (see below) as well as an indication of the variability of habitats in the landscape. This approach is suitable, assuming that the response of the taxa to environmental factors did not change and that the combinations of environmental conditions are comparable between the past and nowadays (actualism), so that most probably the composition of vegetation did not change very much over time. A disadvantage of using abiotic values is that these are based on field observations of growth locations, but insight in which factors influence the occurrence of a taxon is lacking (Bogaard 2004, p. 7; Charles et al. 1997, p. 1152). Therefore, Charles et al. (1997) and Bogaard (2004) propose using functional attributes (biotic factors) such as leaf life span and root length to reconstruct vegetation types for which one might assume that a modern analogy of a combination of factors influencing the chances of a taxon occurring is lacking: a prime example is arable weed vegetation. Recent studies on historical changes in synanthropic vegetation (affected by human activities) confirm that changing land use and lifestyle considerably alter such vegetation (Lososova and Simonova 2008). The community and assemblage approach explores the interspecific relationships (plant sociology) of plant taxa occurring together (concurring) at a site. The interspecific relationships of plants can be expressed in two different ways. The first is by means of ecological grouping of taxa. Ecological taxon groups can be adopted directly from the literature (Arnolds and Van der Maarel 1979; Ellenberg et al. 1991, pp. 7175; Runhaar et al. 2004, pp. 2426), by adjusting adopted taxon groups to palaeobotanical datasets (Kreuz 2005, p. 85, after Ellenberg et al. 1991; Out 2012, after Arnolds and Van der Maarel 1979), or they can be constructed manually. Manually means here that the groups are formed by the individual researcher, based on expert knowledge, for example of the taxons past or current environment. The ordering of the data in ecological taxon groups is particularly useful in archaeological contexts, where the relationships between human impact and ecology are an important research goal. Ecological taxon groups like arable weeds and plants of trampled places may be better suited to archaeological interpretations than possibly related synanthropic vegetation units like the syntaxa Veronico-Lamietum hybridi or Plantagini-Lolietum perennis. In contrast to syntaxonomy, where concurrence is based on many actual vegetation descriptions of taxa occurring together, ecological groups have been artificially created by combining plant taxa and environmental characteristics. Concurrence of the taxa in these groups needs not to have been actually witnessed in a re (...truncated)