Preston, Nicholas James: Geomorphic Response to Environmental Change : The Imprint of Deforestation and Agricultural Land Use on the Contemporary Landscape of the Pleiser Hügelland, Bonn, Germany. - Bonn, 2001. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Nicholas James Preston}},
title = {Geomorphic Response to Environmental Change : The Imprint of Deforestation and Agricultural Land Use on the Contemporary Landscape of the Pleiser Hügelland, Bonn, Germany},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2001,
note = {Central Europe has in many areas been subjected to continuous agricultural land use for several millennia. Deforestation and the introduction of agriculture to a previously undisturbed environment was not only a perturbation to the physical landscape system, but a shift to a qualitatively different landscape system. Within this landscape the production and redistribution of sediment has had consequences for the morphological development of the landscape. Furthermore, it has important implications for the sustainability of land use, for the design of engineering infrastructure, for contamination of waters and aquatic environments through sediment itself and as a result of associated particulate fluxes, for landform development, and for prognoses of all of these phenomena under changing climate and land use scenarios. Of particular interest, therefore, is a greater understanding of the relative significance of climatic and anthropogenic influences on the behaviour of geomorphic systems. This is important as we attempt to manage our activities so as to avoid, or at least minimise, adverse impacts on the landscape. This study deals with issues relating to geomorphic responses to environmental change.
Investigations into late Holocene colluviation took place in the Pleiser Hügelland, a region of predominantly agricultural loess-covered hill country to the east of Bonn. Widespread deforestation occurred in this area in the early to middle Medieval period, and on isolated sites probably during the Iron Age or earlier. The agricultural land use which followed this deforestation introduced a new mechanism for the generation and redistribution of sediment throughout the landscape, i.e. tillage. Colluvial sediments are principally derived from loess and loess soils, but to an extent also from basalt, trachyt and Pleistocene alluvium. This investigation has sought to characterise the geomorphic response to this environmental change. Investigation took place in three separate study areas that represent landform elements of different hierarchical magnitude: zero-, first- and second-order drainage basins.
Rates of sediment redistribution in the Auf dem Scheid catchment for the last 45 years have been modelled on the basis of 120 samples of 137Cs concentration. This indicates that for this period greater rates of sediment redistribution have occurred on arable land than on pasture. The two arable zones are net sediment export areas, with high sediment delivery ratios (88%, 77%), while the pasture zone is a net accumulation area (-23%). Furthermore, modelling the effects of two different process types highlights the way in which their interaction contributes to the sediment flux. High frequency/low magnitude tillage prepares sediment for further transport by water erosion processes. There is thus a constant supply of material for sediment redistribution. Water erosion is also the mechanism by which sediment is exported beyond the boundaries of arable areas.
An attempt was made to develop a high resolution late Holocene sedimentation chronology in the Auf dem Scheid catchment using Optically Stimulated Luminescence (OSL) dating. Because of the mineral composition of the sediments in Auf dem Scheid, only limited success was achieved, and a high resolution chronology was not possible. Nevertheless, a considerable body of chronological information for colluvial sedimentation in a small area has been acquired. This and the stratigraphic information revealed during sample recovery indicates a rather complex history for the small (~5.4 ha) Auf dem Scheid catchment. At least four different phases of erosion-deposition can be inferred on the basis of OSL ages. The earliest of these is from sediments dated at ~4,400 a. The greatest rates of accumulation are associated with a Medieval event, which may be correlated with widely reported episode of catastrophic gullying. A long term sediment budget gives an estimated sediment delivery ratio of 74%. However, when it is recognised that many of the sediments associated with earlier erosional phases have been themselves eroded and exported in the meantime, this sediment delivery ratio can be considered to be an overestimate. In the absence of high magnitude events, further reworking of colluvial sediments stored in the Auf dem Scheid catchment is unlikely. Thus, although there is a reasonably efficient mid- to long term coupling of source and sink within the Auf dem Scheid, the external coupling is weak and Auf dem Scheid can be considered to be a closed system.
OSL ages were also acquired from the fill of two fossil gullies in a single slope study area in the nearby Forstbach catchment. They have been filled with sediments eroded from the formerly agricultural slopes. The ages of gully fill indicate that these features are ~1,300 years old. In general, the ages of colluvia in the both Auf dem Scheid and Forstbach give evidence of considerable erosional activity occurring at a considerably earlier period than had previously been thought. An inference is that deforestation and the introduction of agriculture may have occurred much earlier than has previously been thought in at least this part of the Pleiser Hügelland. Further, both study sites are dominated by the evidence of agricultural processes. Although gullying associated with large scale rainfall-runoff has occurred in both, there is no evidence of this in the contemporary landscape. Thus, expressed in terms of transient form ratios, both are considered insensitive to gullying, but sensitive to low magnitude diffusive erosional processes.
Stratigraphic evidence of process behaviour was derived on the basis of a series of core transects in the Heidersiefen basin (2nd order). Large volumes of agriculturally initiated sediment are also present in this area. A long term sediment budget (FEISE 1999) indicated relatively low sediment delivery ratios (~30%) for first and second order drainage basins. However, construction of infrastructure in more recent times has caused a decoupling of sediment source areas from channel deposition sites, and sediment delivery within the Heidersiefen catchment has been reduced. This has influenced the geomorphic effectiveness of rainfall-runoff, which is capable of eroding in-channel sediment storage in even relatively low magnitude events. The presence of a perennial channel means that the sediments in storage in Heidersiefen are more susceptible to reworking than those in Auf dem Scheid or the Forstbach study site. Thus, more than the other study sites, Heidersiefen does not exhibit the same sensitivity to agricultural land use.
The two principal processes of sediment generation and redistribution that operate within this landscape (tillage and rainfall-runoff) are markedly different with respect to their frequency/magnitude/effectiveness. Tillage occurs with high frequency, i.e. on a regular basis, 2-3 times per year. Tillage is a very effective transport mechanism in terms of the volume of material it moves over longer periods, but its effects remain localised as transport is only over very small distances. More significantly, tillage increases the susceptibility of soils to water erosion processes. There is greater variability in the frequency/magnitude/effectiveness of rainfall-runoff events. Those with sufficient energy to erode and transport some small amount of sediment (i.e. through overland flow and rilling) probably occur at least as frequently as tillage. These events are able to transport small amounts of material over small distances. But those with sufficient energy to cause gullying - and thus potentially the erosion and transport of volumes of material comparable with a single ploughing operation - are considerably less frequent. Not only does the magnitude of rainfall-runoff influence the amount of material moved, it also determines the distance of transport. The effects of small magnitude events remain relatively localised, just as with ploughing. Sediment may simply remain within the field and be reincorporated into the plough horizon with the next ploughing operation; or it may be transported to a local depositional zone. At the other extreme, gullies generated by high magnitude rainfall-runoff events are potentially capable of transporting sediment out of the system entirely. The relative frequency of rainfall-runoff events of varying magnitude, and their temporal sequence, is thus an important aspect in determining the pattern of sediment generation and redistribution. There is evidence that high magnitude rainfall-runoff events have formed gullies. However, the age of these features - as revealed by the luminescence-derived ages of their fills - indicates that this has not happened very frequently. In the intervening period between the recurrence of high magnitude events, their morphological effects have been removed by the sediment generated by smaller events.
Of equal importance are configurational aspects of the landscape system. These relate to the spatial distribution of the individual process domains, both relative to each other and within the landscape system. It also relates to the spatial distribution of sediment sources/sinks and of the different components of the system hierarchy, and importantly the extent to which (a) sources and sinks and (b) systematic elements are coupled. This is influenced by both the magnitude of each of these elements, and by the energy available for transporting material from one element to the next.
The principal geomorphic response to environmental change has been the generation of large amounts of sediment through (a) initial deforestation and (b) the subsequent effects of tillage and rainfall-runoff processes. However, the low levels of energy available for transport of this sediment - due to both low relief and a low frequency of high energy rainfall events - and sometimes weak coupling between components of the sediment flux have restricted the amount of sediment entering the regional sediment flux. The post-deforestation, agricultural landscape can thus be characterised with a transport limited sediment flux. This contrasts with a generally assumed stable pre-deforestation landscape which exhibited low rates of process behaviour and therefore a supply limited sediment flux.},

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