Soil moisture, stressed vegetation and the spatial structure of soil erosion in a high latitude rangeland

Funding: Research was supported by a NERC PhD studentship (ref: NE/L002558/) to Polly Thompson and a World-Leading Scholarship, funded by St Leonard’s Postgraduate College, University of St Andrews, to Georg Kodl.Soil erosion has been a persistent problem in high-latitude regions and may worsen as climate change unfolds and encourages increased anthropogenic exploitation. We propose that soil moisture is likely to shape future erosion trends, as moisture stress reduces the capacity of vegetation cover to retard erosive processes. However, the spatial variability of soil moisture in high-latitude soils—and the ways in which this variability drives the spatial distribution of erosion features—is poorly understood. We addressed this knowledge gap with a study of andosol erosion in southern Iceland. Our study used a combination of high-resolution (10 m from eroded terrain. We found lower moisture availability close to existing erosion features: mean volumetric soil moisture content varied from 17% (proximal to erosion patch) to 36% (distal to erosion patch). We also found that variability in soil moisture decreased with distance from eroded areas: the coefficient of variation (CV) in soil moisture varied from 0.33 (proximal to erosion patch) to 0.13 (distal to erosion). Our findings indicate that the margins of erosion patches have a stressful soil environment due to exposure to the atmosphere. The vegetation in these locations grows less vigorously, and the exposed soil becomes more vulnerable to erosion, leading to erosion patch expansion and coalescence. If these conditions hold more generally, they may represent a feedback mechanism that facilitates the lateral propagation of soil erosion in high-latitude regions.Publisher PDFPeer reviewe

Impact of small-scale vegetation structure on tephra layer preservation.

The factors that influence tephra layer taphonomy are poorly understood, but vegetation cover is likely to play a role in the preservation of terrestrial tephra deposits. The impact of vegetation on tephra layer preservation is important because: 1) the morphology of tephra layers could record key characteristics of past land surfaces and 2) vegetation-driven variability in tephra thickness could affect attempts to infer eruption and dispersion parameters. We investigated small- (metre-) scale interactions between vegetation and a thin (<10 cm), recent tephra layer. We conducted surveys of vegetation structure and tephra thickness at two locations which received a similar tephra deposit, but had contrasting vegetation cover (moss vs shrub). The tephra layer was thicker and less variable under shrub cover. Vegetation structure and layer thickness were correlated on the moss site but not under shrub cover, where the canopy reduced the influence of understory vegetation on layer morphology. Our results show that vegetation structure can influence tephra layer thickness on both small and medium (site) scales. These findings suggest that some tephra layers may carry a signal of past vegetation cover. They also have implications for the sampling effort required to reliably estimate the parameters of initial deposits

Variations in tephra stratigraphy created by small-scale surface features in sub-polar landscapes

Financial support for this work was provided by NERC Doctoral Training Partnership Ph.D. studentship NE/L002558/1 to Polly I. J. Thompson.We explore the effect small-scale surface features have on influencing the morphology and grain-size distribution (GSD) of tephra layers within the Quaternary stratigraphy of sub-polar landscapes. Icelandic thúfur, small cryogenic earth mounds, are used to assess how and why the morphology and GSD of tephra layers vary over such formations. Through measurement of tephra layer thickness and GSD, Hekla 1947 and Grímsvötn 2011 tephra layers are analysed. Results indicate that such microtopographic features do indeed alter the form of tephra deposits and therefore the tephra layer that is preserved in the stratigraphy. Tephra thickness is significantly greater in hollows than on the thúfur crests. There is greater variation in tephra thickness measurements from thúfur in comparison to control measurements from a surface where thúfur are absent. Thúfur crests contain larger grain sizes than hollows, for both H1947 and G2011 tephras; however this was only statistically significant for the G2011 tephra. Such morphological patterns are thought to arise from an interplay of tephra characteristics, altered topography from the thúfur formations and earth surface processes operating at the sites. This study provides insight into the potential of tephra layer morphology and internal structures as indicators of Quaternary landforms and processes. Additionally, it provides important context for the appropriate sampling of tephra layers to infer volcanological processes, as the characteristics of preserved layers do not necessarily reflect those of the original fall-out.Publisher PDFPeer reviewe

Degrees of success : evaluating the environmental impacts of long term settlement in South Iceland.

This paper focuses on the occupation and landscape history of Dalur and Mörk, two areas of long-term settlement in the Eyjafjallahreppur district of southern Iceland. The aim is to illustrate the importance of evaluating not only farm occupation and abandonment, but also to assess the complexities of the environmental interactions of long-term settlements. Environmental records are assessed using data from 50 sediment profiles, constrained by tephrochronology, located in the farm infields (tún) and outfields (hagi). This record indicates that despite similar outward appearances today, the environs of Dalur and Mörk have experienced different histories of environmental change over the last 1000 years. At least 14 subsidiary settlements were at one time or another established within Dalur, or were dependent on the Church farm there. Ten of these settlements were subsequently abandoned and sediment accumulation rates, a proxy indicator of erosion, remained low, indicating restricted local human impact. We conclude that this illustrates the importance of access and rights to additional resources out with the principal farm. In addition, much of the immediate environs of the main farm site was probably un-wooded at the time of settlement, so the total degree of vegetative change was limited. In contrast, palaeoenvironmental data indicates that the environs of Mörk were extensively wooded at the time of Landnám, but this woodland was rapidly cleared and this was followed by several centuries of landscape instability. In a cultural contrast, the landholdings of Mörk experienced less subdivision with a total of only five dependent farms established across the land belonging to three main farms (all of which had chapels). From the early 10th to 14th centuries there was significantly enhanced erosion within the Mörk landholdings, but this stabilised and the principal farms endured and to form sites of long-term settlement. The ownership of additional resource rights, including woodland further up-valley, may have made the crucial contribution to long term endurance

Settlement history, land holdings and landscape change, Eyjafjallahreppur, Iceland

In this paper we focus on a region in south Iceland and assess the confidence with which it is possible to determine the changing patterns of settlement history for a region and relate it to contemporaneous land boundaries. For a geographically coherent group of up to 38 possible farm sites in the northern and western part of Eyjafjallahreppur, south Iceland, the timing of occupation and abandonment is assessed. In addition, boundaries of landholdings, probable status and inter-site relationships are identified. Currently only 10 sites are occupied. There is some uncertainty over the location of the earliest settlements, but after the 11th century, successful principle farm sites such as Dalur, Seljaland and Mörk remain in the same locations. As a result, recent data on landholdings combined with a knowledge of the form of landholding sub-division, offers insights into pre-modern times, with some data relevant to medieval times. An ‘abandoned frontier’ exists in the inland area of Þórsmörk, but abandonment is not restricted to the uplands; it has occurred throughout the region. Most desertion occurred before the cold phases of the Little Ice Age in the 18th century. Overall, much farm desertion can be attributed to landscape destruction as a result of river migration, but it is also biased towards small dependent farms often established for social reasons (such as the property requirement for marriage) and evidently insufficiently resourced for long term success in a changing physical environment

An over-optimistic pioneer fringe ? environmental perspectives on medieval settlement abandonment in Þórsmörk, South Iceland.

We assess environmental factors that may have contributed to farm abandonment in Þórsmörk, south Iceland. Here farms were established during the initial Norse colonisation of Iceland and abandoned by the 13th century AD. Soil erosion has been identified as a possible factor in this settlement change. This hypothesis is assessed using sediment profiles constrained by tephrochronology in Þórsmörk and the nearby area of Stakkholt. In Þórsmörk, there is evidence for episodes of landscape instability between the 10th and 13th centuries and localised episodes of soil erosion to bedrock that ended before 1300 AD and the onset of the Little Ice Age (LIA) climate changes. This early instability is absent from Stakkholt. Later LIA stability in Þórsmörk contrasts with episodes of instability in Stakkholt. The implication is that Þórsmörk was sensitive to early settlement impacts that lead to extensive erosion. After farm abandonment in Þórsmörk, the surviving woodland was successfully conserved as a valuable source of wood and charcoal for lowland farms where woodland resources had been depleted. Mounting pressure on woodland resources in the 11th -12th centuries could have been an important factor in determining the precise timing of abandonment

Medieval Iceland, Greenland, and the New Human Condition: A case study in integrated environmental humanities

This paper contributes to recent studies exploring the longue durée of human impacts on island landscapes, the impacts of climate and other environmental changes on human communities, and the interaction of human societies and their environments at different spatial and temporal scales. In particular, the paper addresses Iceland during the medieval period (with a secondary, comparative focus on Norse Greenland) and discusses episodes where environmental and climatic changes have appeared to cross key thresholds for agricultural productivity. The paper draws upon international, interdisciplinary research in the North Atlantic region led by the North Atlantic Biocultural Organization (NABO) and the Nordic Network for Interdisciplinary Environmental Studies (NIES) in the Circumpolar Networks program of the Integrated History and Future of People on Earth (IHOPE). By interlinking analyses of historically grounded literature with archaeological studies and environmental science, valuable new perspectives can emerge on how these past societies may have understood and coped with such impacts. As climate and other environmental changes do not operate in isolation, vulnerabilities created by socioeconomic factors also beg consideration. The paper illustrates the benefits of an integrated environmental-studies approach that draws on data, methodologies and analytical tools of environmental humanities, social sciences, and geosciences to better understand long-term human ecodynamics and changing human-landscape-environment interactions through time. One key goal is to apply previously unused data and concerted expertise to illuminate human responses to past changes; a secondary aim is to consider how lessons derived from these cases may be applicable to environmental threats and socioecological risks in the future, especially as understood in light of the New Human Condition, the concept transposed from Hannah Arendt's influential framing of the human condition that is foregrounded in the present special issue. This conception admits human agency's role in altering the conditions for life on earth, in large measure negatively, while acknowledging the potential of this self-same agency, if effectively harnessed and properly directed, to sustain essential planetary conditions through a salutary transformation of human perception, understanding and remedial action. The paper concludes that more long-term historical analyses of cultures and environments need to be undertaken at various scales. Past cases do not offer perfect analogues for the future, but they can contribute to a better understanding of how resilience and vulnerability occur, as well as how they may be compromised or mitigated

How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit

Tephra layers are frequently used to reconstruct past volcanic activity. Inferences made from tephra layers rely on the assumption that the preserved tephra layer is representative of the initial deposit. However, a great deal can happen to tephra after it is deposited; thus, tephra layer taphonomy is a crucial but poorly understood process. The overall goal of this research was to gain greater insight into the taphonomy of terrestrial tephra layers. We approached this by a) conducting a new survey of the tephra layer from the recent, well-studied eruption of Mount St Helens on May 18th, 1980 (MSH1980); b) modelling the tephra layer thickness using an objective mathematical technique and c) comparing our results with an equivalent model based on measurements taken immediately after the eruption. In this way, we aimed to quantify any losses and transformations that have occurred. During our study, we collected measurements of tephra layer thickness from 86 locations ranging from 600 km from the vent. Geochemical analysis was used to verify the identity of tephra of uncertain origin. Our results indicated that the extant tephra layer at undisturbed sites was representative of the original deposit: overall, preservation in these locations (in terms of thickness, stratigraphy and geochemistry) had been remarkably good. However, isopach maps generated from our measurements diverged from isopachs derived from the original survey data. Furthermore, our estimate of the quantity of tephra produced during eruption greatly exceeded previous estimates of the fallout volume. In this case, inaccuracies in the modelled fallout arose from issues of sampling strategy, rather than taphonomy. Our results demonstrate the sensitivity of volcanological reconstructions to measurement location, and the great importance of reliably measured low/zero values in reconstructing tephra deposits

Bioarchaeological and Climatological Evidence for the Fate of Norse Farmers in Medieval Greenland

Greenland, far north land of the Atlantic, has often been beyond the limit of European farming settlement. One of its Norse settlements, colonized just before AD 1000, is — astonishingly — not even at the southern tip, but a way up the west coast, the \u27Western Settlement\u27. Environmental studies show why its occupation came to an end within five centuries, leaving Greenland once more a place of Arctic-adapted hunters