Quantitative landscape dynamics in Denmark through the last three millennia based on the Landscape Reconstruction Algorithm approach
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- Nielsen, A.B. & Odgaard, B.V. Veget Hist Archaeobot (2010) 19: 375. doi:10.1007/s00334-010-0249-z
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This paper explores the spatial and temporal land-cover variability within the main cultural landscape units in Denmark during the last 3,000 years. Quantitative estimates of the cover of trees, grasses, Cerealia and Calluna around nine Danish lakes were obtained using the recently developed Landscape Reconstruction Algorithm (LRA) (Sugita 2007a, b). The performance of the approach was evaluated by comparing reconstructed vegetation based on a.d. 1800 pollen spectra to land cover from historical maps of the same period. Although the model tended to overestimate grassland cover by 10–20%, the reconstructed vegetation was much more similar to the observed than the uncorrected pollen proportions. The LRA was then applied to 3,000 year long pollen records to reconstruct the vegetation development around each of the nine sites. The results support earlier conclusions regarding the relative stability of woodland, agrarian and heathland dominated landscapes in Denmark (Odgaard and Rasmussen 2000), with the distribution of the main landscape types determined by topography and soil characteristics. The present study indicates that the transition zones between agricultural and forest dominated landscapes were the most dynamic, acting as buffer zones where most of the expansions and contractions of agricultural activities took place. The quantitative vegetation reconstructions underline the importance of farming and especially pastoral activities in shaping the Danish landscapes throughout the study period.
KeywordsPollen analysisLake sedimentsQuantitative analysisLandscape Reconstruction AlgorithmLate Holocene
Evidence from fossil pollen data indicates that Europe has undergone substantial changes in land-cover since the mid-Holocene introduction of agriculture (e.g. Behre 1988; Lang 1994; Gaillard 2007). Through impacts on processes such as evapo-transpiration, changes in albedo, soil stabilization and acidification, mineralization and (de)nitrification, these changes have had strong affects on hydrology, terrestrial and fluvial erosion rates, nutrient cycles, climate and biodiversity (Bonan 2002). Quantitative estimates of past land cover changes are therefore not only of primary interest in terms of vegetation dynamics but also a potential link to the causes of associated environmental changes.
The pollen composition in sediments is a reflection of the vegetation in the surrounding landscape at the time of deposition. However, deriving a quantitative measure of vegetation cover from pollen percentage data is not straightforward. Some of the complicating factors include differential pollen production and pollen dispersal among plant species (Nielsen and Odgaard 2004).
Over the last decades, methods have been developed that allow us to distance weight vegetation data according to models of pollen dispersal and deposition (Prentice 1985; Sugita 1993); to estimate the relative pollen productivity of different plant taxa from pollen/vegetation calibration datasets (Parsons and Prentice 1981; Prentice and Parsons 1983; Sugita 1994) and to assess the source area of pollen for different basin types and sizes (Sugita 1994).
These methods have been applied in Denmark, using a historical calibration dataset of ca. a.d. 1800 pollen assemblages from 29 lakes and land cover from historical maps from the same period (Nielsen 2003). Pollen assemblages predicted by the Sugita (1993) pollen dispersal and deposition models were shown to resemble those observed in the lake sediments (Nielsen 2004). The relevant source area of the lakes was estimated to have a radius of 1,800–2,000 m, and was shown to depend mostly on the spatial structure of the surrounding vegetation (Nielsen and Sugita 2005). Also the relative pollen productivity of four plant groups: trees, cereals, grasses and Calluna, was determined, and these were shown to be useful for quantitative land cover reconstructions in the a.d. 1800 landscape (Nielsen and Odgaard 2005; Broström et al. 2008). These four plant groups can be considered some of the most important for describing the overall composition of the Danish cultural landscape, as they are the main constituents of woodlands, arable fields, grazing areas and heathlands, respectively.
In this study, we aim to apply the knowledge gained about the pollen productivity, dispersal and source area to subfossil pollen records from nine small lakes in Denmark using the Landscape Reconstruction Algorithm (LRA) proposed by Sugita (2007a, b), in order to obtain quantitative estimates of the past composition of Danish cultural landscapes during the last 3,000 years.
For the Danish area, analyses of mid- and late Holocene pollen data suggest that the macro-scale structure of the classical cultural landscape had come into existence by 3000 b.p. (Odgaard and Rasmussen 2000). This meant a differentiation into (i) hilly districts with relatively frequent woodlands, (ii) landscapes on poor soils with extensive heathlands and (iii) flat terrain on fertile soils with dominant grasslands and arable fields. These three landscape units seem to have been relatively stable over the last three millennia and land-use was apparently strongly related to two important landscapes variables, topography and soil fertility (Odgaard and Rasmussen 2000).
In this paper we investigate the spatial and temporal variability within each of the three landscape units in the classical Danish cultural landscape. Specifically, we address the following questions: Were rapid land-cover changes characteristic for some landscape types while other types showed slow or little changes? What was the landscape position of buffer zones for expansion/contraction of agricultural activities?
Sites and data
Lake site characteristics
Max depth (m)
Rasmussen and Olsen (2009)
Rasmussen et al. (1999)
The chronological control on the pollen records differs among sites. For Store Gribsø (Løvberg, unpublished), the age-depth model is based on four 14C dates in the period after a.d. 1, and 5 from the period between 1 and 4000 b.c. There has been no 14C dating done on material from the Avnsø sediments (Nielsen 1999). Instead, the series is dated by correlation to other, well dated pollen diagrams. In particular, many features of the Avnsø diagram are also found in a well dated pollen diagram from Holmegårds mose (Aaby 1986), a large raised bog around 20 km from Avnsø. These features include the sharp rise in Fagus between 1000 and 900 b.c., the first occurrence of Secale around 100 b.c. and the first occurrence of Centaurea cyanus around a.d. 1250. There have been several 14C datings made on plant remains from the core from Store Økssø (Odgaard 1999a), but most of them showed erroneous ages due to the influence of re-deposited charcoal in the samples. Thus, the age depth model for Store Økssø is based on few dates, the youngest from a.d. 690, and correlation to other, dated pollen diagrams from the same region. The time scale for Dallerup Sø is based on six 14C dates throughout the last 2,500 years. The youngest reliable data is from a.d. 1420, while the sediments from the 20th century were dated by the occurrence of spherical soot particles originating from the burning of coal and oil (Odgaard 1993). The number of 14C dates from Gudme Sø is six within the last 3,000 years, of which three are from 1000 to 1 b.c. The youngest reliable date is from a.d. 890. The sediment of Gudme Sø is slightly calcareous, but although the dates were carried out on bulk samples, the chronology seems reliable within ca. 100 years (Rasmussen and Olsen 2009). Despite repeated efforts it has proved impossible to obtain reliable 14C dates from the sediments of Gundsømagle Sø, and the age depth model is therefore based on correlation with other pollen diagrams, using the same markers as mentioned for Avnsø (Rasmussen and Anderson 2005). The top 94 cm of sediment was dated using an assay of 210Pb and 137Cs (Rasmussen and Anderson 2005). The age depth model for Skaansø for the period a.d. 1–2000 is based on eight 14C dates, the youngest of which was dated to a.d. 1166 (Odgaard 1994). From Navnsø, there are no reliable 14C dates younger than a.d. 130, due to the lack of datable terrestrial plant material in the cores. This series is thus mainly dated by correlation to dated pollen diagrams in the same region. The age depth model for the Kragsø core is also, due to a lack of datable material from the last 2,000 years based on only four 14C dates, the youngest of which is from a.d. 308, together with correlation to other pollen diagrams from Jutland and on the rise in spherical soot particles (Odgaard 1994).
The Landscape Reconstruction Algorithm
The LRA is a method proposed by Sugita (2007a, b) to reconstruct past vegetation composition using pollen percentage data from a network of sites. The present study is the first attempt to apply the LRA to fossil pollen records from Denmark.
Simulation experiments (Sugita 2007a) have shown that the most reliable estimate of the regional vegetation composition is obtained by using pollen assemblages from large lakes, typically >100–500 ha, because the site to site variation in pollen assemblages from such sites is expected to be small. However, an average estimate based on multiple smaller sites within a region can also be used to reconstruct regional plant abundance, albeit with larger error estimates. Here, we applied REVEALS to pollen data from the nine study sites. From each site, the pollen proportions were averaged over 500 year intervals before the application of REVEALS. The nine study sites were split into two groups, i.e. the sites located east (five sites) and west (four sites) of the East Jutland stationary line (Fig. 1), as regional vegetation is highly affected by differences in soil types. For each subset, the regional plant abundance estimates and their variance and covariance were calculated in 500 year intervals from 1000 b.c. to a.d. 2000, using the software REVEALS.v4.1.8 (Sugita, unpublished).
Fall speed of pollen, pollen productivity estimates relative to Cerealia and standard deviation of pollen productivity estimates (Nielsen and Odgaard 2005)
Thus the necessary input parameters to the LOVE model include the pollen dispersal model and estimates of relative pollen productivity as for REVEALS and the size of the relevant source area for the site under consideration. Here we assumed a relevant source area with a radius of 1,800 m, which is the size estimated for the lakes in the Danish calibration dataset (Nielsen and Sugita 2005). The calculations were carried out using the software LRA.LOVE.v3.1.3 (Sugita, unpublished). In instances where the estimated plant abundance of one or more taxa was negative, the relevant source area for the LOVE calculations was increased until the estimates of all taxa were ≥0, considering a 0.1 SD threshold. The program uses a hybrid method of error propagation and Monte Carlo simulations to estimate standard error on the vegetation composition estimates (Sugita 2007b).
To test the performance of the LRA for reconstructing vegetation, LOVE reconstructed vegetation composition for a.d. 1800 is compared to distance weighted vegetation composition in the pollen source area of the nine study sites derived from historical maps from the same period (Nielsen and Odgaard 2005). The performance of REVEALS for reconstructing regional vegetation is more difficult to assess, because historical land cover is not available for the whole region in the same detail as for the local areas around the lakes. The performance of REVEALS has, however, been tested using modern vegetation data and surface pollen samples from large lakes in southern Sweden (Hellman 2005; Hellman et al. 2008a, b).
Overall, the reconstruction corresponds much better with the observed vegetation than the raw pollen proportions do, but the tendency for overestimation of Poaceae cover and underestimation of tree cover at tree rich sites should be kept in mind when interpreting the vegetation reconstructions further back in time.
Regional plant abundance
In eastern Denmark, the area of cereal cultivation increased gradually through the 3,000 year period. In contrast, tree covered area generally decreased, but increased slightly in the period a.d. 500–1000. Grassland increased around 500 b.c., then decreased around a.d. 500 and has increased since. The area covered by Calluna was always low in this region where the species is mostly restricted to bogs and poor fens.
On the poorer sandy soils in western Denmark the area with cereals was low until around a.d. 500 but increased gradually. Regional tree cover decreased through the whole period, and was throughout lower than in eastern Denmark. The area of grass cover decreased slightly during the first 1,000 years and has remained stable since. The Calluna covered area expanded until a.d. 1500 and on average decreased a bit in the most recent 500 year period reflecting the cultivation of many heathland areas during the last 200 years.
Local development in land cover from 1000 b.c. to a.d. 2000
In the beginning of the last millennium b.c. woodland dominated the landscape, but around 500 b.c. an extensive change of the landscape took place, whereby the woodlands almost disappeared from the surroundings of the lake. Instead, grass-dominated areas spread, and the landscape must have been heavily impacted by grazing livestock. From at least 200 b.c. arable fields also featured in the landscape, although they only made up a small part of the area. From a.d. 600 to 700 there was a strong expansion of woodland, probably partly in the very close surroundings of the lake itself. It was first Alnus, then Ulmus, Fagus and Quercus, which reacted. The fact that both Alnus and Fagus became more abundant shows that tree cover increased both on the wet soils and on the drier ones. This was most likely due to a decrease in grazing pressure or perhaps a concentration of the grazing animals in smaller, limited areas. The area with cereal cultivation also declined in this period. From a.d. 700–1000 a new woodland decline is seen which is again replaced by woodland expansion in the period a.d. 1100–1200. During the latter phase, there seems to be little, if any, decline of arable fields. Immediately following a.d. 1200, grassland areas expanded and became dominant in the landscape once again. The cultivated area increased from the early 17th century until the end of the 19th century, after which there seems to be a decline, accompanied by an increase in grassland.
The results of this study corroborate earlier conclusions regarding the relative stability of woodland, agrarian and heathland landscapes throughout the last 3,000 years (Odgaard and Rasmussen 2000). The new results are, however, based on quantitative reconstructions with a theoretically sounder basis as vegetation proxies than raw pollen percentages. Also, the quantitative estimates allow more secure identification of landscapes with rapid versus slow change rates. Store Gribsø, Avnsø and Store Økssø all had fairly large amounts of woodland in their surroundings throughout the 3,000 year period. The areas around Dallerup Sø, Gudme Sø and Gundsømagle Sø had little woodland cover for most or all of the period, but were instead dominated by grassland vegetation with some arable fields with cereals. Also the Skaansø, Kragsø and Navnsø areas had little woodland, but were dominated by a mixture of grass and Calluna heathlands. The proportion of the areas used for cereal cultivation was smaller than on the more fertile soils in the agricultural landscapes.
Those areas of Denmark which have retained a relatively high woodland cover throughout the last 3,000 years, such as north-eastern Zealand (Store Gribsø), central Zealand (Avnsø) and eastern Himmerland (Store Økssø), are characterised by hilly terrains, which can only be cultivated with difficulty. While some of these regions have relatively sandy soils the area around Avnsø is characterised by fairly fertile soils underlining the importance of topography for the choice of areas for cultivation by prehistoric and historic farmers.
The woodland areas also provided important resources, such as timber, firewood and leaf fodder, and provided areas for forest grazing, hunting and at least in later periods, areas for feeding pigs on mast. For agriculture, these areas were generally marginal and they were less influenced by variations in agricultural intensity than certain other landscape types. However, even at Store Gribsø and Avnsø the increased woodland cover during the period a.d. 600–1000 was evident. Also the medieval expansion of non-forested areas was pronounced although it seems to have been delayed until the 14th–15th century in these areas. At Store Økssø these variations are not evident.
A characteristic pattern in the woodland landscapes was the early and rapid expansion of Fagus, in many places on Zealand and Funen already becoming the dominant woodland tree during the Bronze Age. Here, the limited cultural impact on the woodlands seemed to favour Fagus and allowed for rapid expansion. One exception is the landscape around Store Økssø, where Fagus did not really expand until the medieval period. This medieval Fagus expansion was perhaps a result of a decrease in landscape impact by animal husbandry following decimation of the human population during the Black Death.
In contrast to the hilly woodland landscapes, flatter landscapes with fertile soils were dominated by farming activities throughout the 3,000 year period. Typical examples include areas of central Zealand (Gundsømagle Sø), northern Funen (Rasmussen 2005) and those parts of Thy in north-western Jutland which were not affected by sand drift (Andersen 1993). In these areas the cultural impact was very strong, and often began vigorously as early as around 2600 b.c., in the middle Neolithic. The land-cover development in these areas can be described as a steady trend towards less woodland but more pasture and, especially later, more arable land.
Less typical were those areas situated in the border zone between hilly terrain and flat, fertile areas, as found around Gudme Sø and Dallerup Sø. The landscape development in these border areas was more dynamic, with large variations in cultivation and grazing intensity. It appears that these border zones formed the buffers into which farming could be extended during periods of economic expansion, while contracting during periods of unrest, population decline or weaker economies. These dynamics caused marked variations in woodland area. At Gudme Sø and Dallerup Sø there was a large expansion of woodland after a.d. 600 and an equally large expansion of agricultural areas during the medieval period, especially around a.d. 1200–1400 (a.d. 900–1300 at Gudme Sø). However, even in these border zone areas there was a general trend towards more cereal cultivation during the last 2,000 years, similar to the more typical and central agricultural areas referred to above.
The areas with very sandy and nutrient-poor soils found especially in western Jutland did not offer the same possibilities for agriculture as did the eastern Danish landscapes. However, farming practices involving fire management and winter grazing were adapted to the conditions, and the intensity of the cultural impact on the landscape was not necessarily lower than elsewhere. In some places, the first destruction of the original rather open woodlands began as early as in the fifth millennium b.p., when heathlands began to spread (Odgaard 1994). This heathland expansion was not primarily driven by leaching of soil nutrients due to agriculture but mainly by controlled burning of vegetation to provide heathland for grazing purposes. Studies of pollen and charcoal in sediment cores point to a very close relationship between burning and the amount of heathland (Odgaard 1992, 1994). At the same time, results show that heathlands were kept in young Calluna-dominated stages while not being allowed to develop into the Empetrum heathlands with invasion of trees and shrubs so often seen today (Riis-Nielsen et al. 2005). The purpose of the fire management was probably to achieve a mix of grasses and young Calluna plants, providing both summer and winter grazing. Heathlands were an important resource of the cultural landscape.
The heathland landscapes were not strongly affected by the fluctuations seen in the border zones between agricultural and woodland landscapes. In contrast, during the period a.d. 600–1000 when woodland expanded in the border zone and most woodland landscapes, the heathland sites showed expansions of heathland and reductions in woodland, in other words an intensification of farming impact. The heathland landscapes supported some sparse woodland until the beginning of the medieval period, and the results from Kragsø indicate that woodland persisted longest in the most nutrient-poor, sandy areas with indications of forest remnants until after the end of the medieval period.
In such a soil/topography diagram (Fig. 13) woodland landscapes of the period 1000 b.c.–a.d. 1800 occur in the top part, relatively independent of soils. The agricultural landscapes are in the right lower part while the heathland landscapes occupy the left part. While it is true that the three landscape types were relatively stable in their characteristics, quite a number of changes did occur. First, heathlands tended to expand into woodland and agricultural landscapes as a result of changes in land-use and perhaps also progressive soil leaching. Secondly, a period of expanding forest cover can be identified during the period a.d. 400–800. In contrast, forest decreased during a.d. 800–1200. These dynamics were seen in woodland as well as in agricultural landscapes but most strongly expressed in the border zone between the two. However, is seems to have affected only landscapes on relatively fertile soils, as it is not seen in the Store Økssø area or any of the heathland sites. Heathland landscapes showed a slow but progressive forest reduction throughout the period 1000 b.c.–a.d. 1800.
This first application of the LRA in Denmark has provided new insights into the overall composition and the degree of openness of the cultural landscape and its development over the past 3,000 year period, throughout which all Danish landscapes have been heavily impacted by agricultural and especially pastoral activities, resulting in a dominance of grassland on fertile soils and a mixture of grass and Calluna heathlands on sandy soils.
Despite a tendency for the LRA-model to underestimate tree cover and overestimate grass cover, it is now clear that the degree of openness of the landscapes during the last 3,000 years was much higher than it appears from uncorrected percentage pollen diagrams, and that much of the Danish landscape has been very poor in forest cover throughout this period.
We thank Peter Rasmussen and Tina Løvberg for access to raw pollen data. We are also very thankful to Shinya Sugita for use of his unpublished software and to all members of the POLLANDCAL network for useful and inspiring discussions during the numerous network workshops held between 2002 and 2009. This work was done as part of the multidisciplinary project “AGRAR 2000. Agrarian landscapes from the Birth of Christ to the 21st century.” AGRAR 2000 was supported by the previous four Danish national foundations for Sciences, for Humanities, for Agricultural Sciences and for Social Sciences. Kari Hjelle and an anonymous reviewer are thanked for their constructive and useful comments to the manuscript. We dedicate this contribution to Sheila Hicks for her life-long devoted involvement in palaeoecology during which she has been an extraordinary role model for generations of young researchers.
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