The production and consumption of cereals: a question of scale

In their paper Understanding The British Iron Age: An Agenda for Action, Haselgrove et al. (2001, iv) identify regionality and the nature of socio-economic changes as two of the five key areas of future research on the British Iron Age. As farming formed the basis of all societies in this period, and as most settlements were farmsteads and most people were farmers (ibid., 10), any assessment of regional differences and socio-economic change will have to include an assessment of farming practices. At a basic level this concerns an assessment of the scale of agricultural production (e.g. ability to produce a surplus, intensive/extensive cultivation regimes) and of the level of specialisation (e.g. crops versus animals, farming versus non-farming settlements). It goes without saying that the success of such assessments hinges on choosing the right methodology and the right scale of analysis and interpretation. To date, much discussion of intra- and inter-regional variation in Iron Age crop production has focussed on the level of specialisation, namely the identification of producer and consumer sites. A model developed by M. Jones (1985) and applied to sites in the upper Thames valley was the first apparently successful attempt to identify settlements which produced their own crops (arable or producer sites) and those which received crops that had been grown elsewhere (pastoral or consumer sites). This pioneering work brought archaeobotanical data into the forefront of mainstream archaeological debate and has stimulated much of the more recent research in this area. The model aimed to facilitate easy comparison between sites and monitor the movement of arable produce across the landscape, and the results allowed M. Jones (1996, 35) to suggest the existence of ‘neighbourhood groups of agrarian sites engaged in a common network of plant production and consumption’. While the main assumptions underlying the model and the method of constructing the diagrams were criticised early on (G. Jones 1987; Van der Veen 1987; 1991; 1992, chapter 8), the model, and the conclusions drawn from it, are still widely used. In this paper we argue that the problems associated with M. Jones’ model are such that it cannot be used to distinguish between producer and consumer sites, and that recent explanations of differences between archaeobotanical assemblages at sites in the upper Thames valley (Campbell 2000; Stevens 2003) are also flawed. Here we briefly summarise M. Jones’ model, and the criticisms it has received, and review the more recent interpretations of the observed site differences. We then approach the problem from a different angle, proposing levels of analysis and interpretation more appropriate to the data available and the questions posed. Finally, we put forward our own interpretation of the patterning observed. As the model is based on the interpretation of charred plant remains, our arguments inevitably involve detailed consideration of the formation processes at work. In this paper we try to put our case without recourse to complex archaeobotanical jargon, to keep the paper accessible to a wider readership. Some basic features of cereals and relevant terminology do, however, need to be explained first

Impact of a defoliation treatment on plant survival, size and yield.

<p>Impact of defoliation treatment on (a) survival (%), (b) plant height, (c) number of tillers, (d) number of seeds, and (e) potential yield in crop progenitors and wild species. The defoliation treatment is shown by the white bar and the control treatment (no defoliation) is shown by the black. Data are means + SE of 8 replicates.</p

Relationship between size standardised RGR (sRGR) and seed mass.

<p>Regression slope for the relationship between sRGR and seed mass (F_2,6 = 6.186, <i>P</i><.05, R² = 0.6734) for the three crop progenitors (closed symbols) and six wild species (open symbols).</p

Initial seed mass in the three crop progenitors and six wild species.

<p>The black bars show the mean seed mass of accessions used in experiment 1 and the white bars show those used in experiment 2 (+SE). Standard errors are not shown for experiment 1 because seeds were not weighed individually.</p

Relationship between seed germination, seedling mass and seed mass.

<p>Regression slopes for the relationship between (a) time to 50% of seeds germinated and seed mass (F_1,7  = 5.55, <i>P</i><.05, R² = 0.44); and (b) seedling mass and seed mass, [experiment 1: circles (F_1,7 = 120.156, <i>P</i><.001, R² = 0.9756), experiment 2: squares (F_2,6 = 75.78, <i>P</i><.001, R² 0.9619)] for the three crop progenitors (closed symbols) and six wild species (open symbols).</p

Relationship between sNAR, sSLA and sLMR and sRGR.

<p>Regression slopes for the relationships between (a) sNAR and sRGR (F_2,6 = 31.98, <i>P</i><.001, R² = 0.914); (b) sSLA and sRGR (F_1,7 = 6.781, <i>P</i><.05, R² = 0.492); and (c) sLMR and sRGR for the three crop progenitors (closed symbols) and six wild species (open symbols).</p

∆¹³C results.

<p>The ∆¹³C values for wheat grain (105 samples from 8 sites) are mostly between 16.2‰ and 17.7‰ (mean ±1σ). In terms of the water status framework based on stable isotope analysis of present-day crops [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127085#pone.0127085.ref015" target="_blank">15</a>], this range encompasses moderately watered crops (>c.16‰) and well-watered crops (>c.17‰).</p><p>∆¹³C results.</p

∆¹³C results for pulse seeds.

<p>Dashed lines indicate the suggested 'boundaries' between ∆¹³C ranges indicative of lentils grown under poorly (low ∆¹³C), moderately, and well (high ∆¹³C) watered conditions, based on the analysis of present-day crops [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127085#pone.0127085.ref015" target="_blank">15</a>]. ● = lentil (Lens culinaris), ◆ = pea (Pisum sativum), ▲ = bitter vetch (Vicia ervilia).</p

∆¹³C results for cereals grains from Tell Brak samples grouped by chronological period.

<p>Bars indicate means and standard deviations. <b>○</b> = glume wheat and <b>◇</b> = barley.</p

Difference between mean ∆¹³C for barley grain and mean ∆¹³C for wheat grain at each site.

<p>Dashed lines indicate the ∆¹³C difference predicted if two-row barley (at +1‰) or six-row barley (at +2‰) were grown with the same water availability as wheat, based on the analysis of present-day crops [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127085#pone.0127085.ref015" target="_blank">15</a>].</p