Assessing spatial patterns of soil erosion in a high-latitude rangeland

Thanks to NSF grant 1202692 for support to RS. NC was supported by a grant from Churchill College, Cambridge. Thanks to Jim Woollet for advice and support. DEMs provided by the Polar Geospatial Center under NSF-OPP awards 1043681, 1559691, and 1542736.High‐latitude areas are experiencing rapid change: we therefore need a better understanding of the processes controlling soil erosion in these environments. We used a spatiotemporal approach to investigate soil erosion in Svalbarðstunga, Iceland (66°N, 15°W), a degraded rangeland. We used three complementary datasets: (a) high‐resolution unmanned‐aerial vehicle imagery collected from 12 sites (total area ~0.75 km2); (b) historical imagery of the same sites; and (c) a simple, spatially‐explicit cellular automata model. Sites were located along a gradient of increasing altitude and distance from the sea, and varied in erosion severity (5–47% eroded). We found that there was no simple relationship between location along the environmental gradient and the spatial characteristics of erosion. Patch‐size frequency distributions lacked a characteristic scale of variation, but followed a power‐law distribution on five of the 12 sites. Present total eroded area is poorly related to current, site‐scale levels of environmental stress, but the number of small erosion patches did reflect site‐level stress. Small (<25 m2) erosion patches clustered near large patches. The model results suggested that the large‐scale patterns observed likely arise from strong, local interactions, which mean that erosion spreads from degraded areas. Our findings suggest that contemporary erosion patterns reflect historical stresses, as well as current environmental conditions. The importance of abiotic processes to the growth of large erosion patches and their relative insensitivity to current environmental conditions makes it likely that the total eroded area will continue to increase, despite a warming climate and reducing levels of grazing pressure.PostprintPeer reviewe

Arctic tundra shrubification can obscure increasing levels of soil erosion in NDVI assessments of land cover derived from satellite imagery

This research was supported by the St Andrews World Leading Scholarship.Monitoring soil erosion in the Arctic tundra is complicated by the highly fragmentated nature of the landscape and the limited spatial resolution of even high-resolution satellite data. The expansion of shrubs across the Arctic has led to substantial changes in vegetation composition that alter the spectral reflectance and directly affect vegetation indices such as the normalized difference vegetation index (NDVI), which is widely applied for environmental monitoring. This change can mask soil erosion if datasets with too coarse spatial resolutions are used, as increases in NDVI driven by shrub expansion can obscure concurrent increases in barren land cover. Here we created land cover maps from a multispectral uncrewed aerial vehicle (UAV) and land cover survey and assessed satellite imagery from PlanetScope, Sentinel-2 and Landsat-8 for several areas in north-eastern Iceland. Additionally, we used a novel application of the Shannon evenness index (SHEI) to evaluate levels of pixel mixing. Our results show that shrub expansion can lead to spectral confusion, which can obscure soil erosion processes and emphasize the importance of considering spatial resolution when monitoring highly fragmented landscapes. We demonstrate that remote sensing data with a resolution < 3 m greatly improves the amount of information captured in an Icelandic tundra environment. The spatial resolution of Landsat data (30 m) is inadequate for environmental monitoring in our study area. We found that the best platform for monitoring tundra land cover is Sentinel-2 when used in combination with multispectral UAV acquisitions for validation. Our study has the potential to improve environmental monitoring capabilities by introducing the use of SHEI to assess pixel mixing and determine optimal spatial resolutions. This approach combined with comparing remote sensing imagery of different spatial and time scales significantly advances our comprehension of land cover changes, including greening and soil degradation, in the Arctic tundra.Publisher PDFPeer reviewe

The influence of burial rate on variability in tephra thickness and grain size distribution in Iceland

This work was supported by the NERC Doctoral Training Partnership Ph.D. studentship: NE/ L002558/1 to PT.We explore whether the rate at which a tephra deposit is buried influences the variability (thickness and grain size distribution) within the tephra layer subsequently preserved within the stratigraphic record. This has important implications for understanding how processes of soil formation interact with the creation of a volcanic record. To assess the relationship between soil formation and the preservation of tephra layers, the thickness and grain size distribution of the Katla 1918 tephra in Iceland and the rate at which it was buried (inferred from the thickness of the overlying soil) was measured 1620 times at six locations. Tephra layer thickness does not correlate with rate of burial, but the proportion of original deposit retained does, and variations in grain size distribution are correlated with burial rate. Our results indicate that whilst medium term (i.e. years-decades) burial processes may contribute less to tephra layer variability than environmental processes operating immediately after deposition, rapid burial facilitates better preservation of the original fallout characteristics with important implications for the accurate reconstruction of past volcanic eruptions based on tephra layer characteristics. There are two key implications: firstly, sites need to be chosen where surface characteristics minimise the initial alterations of tephra deposits, and secondly sites with rapid burial will produce the best quality data, although workable data can be gathered elsewhere if areas of uncertainty are acknowledged.Publisher PDFPeer reviewe

The spatial organization and microbial community structure of an epilithic biofilm.

Microbial biofilms are common on lithic surfaces, including stone buildings. However, the ecology of these communities is poorly understood. Few studies have focused on the spatial characteristics of lithobiontic biofilms, despite the fact that spatial structure has been demonstrated to influence ecosystem function (and hence biodegradation) and community diversity. Furthermore, relatively few studies have utilized molecular techniques to characterize these communities, even though molecular methods have revealed unexpected microbial diversity in other habitats. This study investigated (1) the spatial structure and (2) the taxonomic composition of an epilithic biofilm using molecular techniques, namely amplicon pyrosequencing and terminal restriction fragment length polymorphism. Dispersion indices and Mantel correlograms were used to test for the presence of spatial structure in the biofilm. Diversity metrics and rank-abundance distributions (RADs) were also generated. The study revealed spatial structure on a centimetre scale in eukaryotic microbes (fungi and algae), but not the bacteria. Fungal and bacterial communities were highly diverse; algal communities much less so. The RADs were characterized by a distinctive 'hollow' (concave up) profile and long tails of rare taxa. These findings have implications for understanding the ecology of epilithic biofilms and the spatial heterogeneity of stone biodeterioration.This work was supported by the Engineering and Physical Sciences Research Council (grant no. EP/G011338/1).This is the author accepted manuscript. The final version is available from OUP at http://femsec.oxfordjournals.org/content/91/3/fiu027

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

Tephra transformations: variable preservation of tephra layers from two well-studied eruptions

Financial support was provided by the National Science Foundation of America through grant 1202692 ‘Comparative Island Ecodynamics in the North Atlantic’, and grant 1249313 ‘Tephra layers and early warning signals for critical transitions’ (both to AJD).Volcanologists often use terrestrial tephra layers to reconstruct volcanic eruptions. However, the conversion of fresh tephra deposits into tephra layers is poorly understood. To address this knowledge gap, we surveyed tephra layers emplaced by the 1980 eruption of Mount St Helens, USA (MSH1980) and the 1947 eruption of Hekla, Iceland (H1947). We compared our measurements with observations made shortly after the 1947 and 1980 eruptions, to calibrate the subsequent transformation of the tephra deposit. We expected the tephra layers to retain the broad characteristics of the original deposits, but hypothesized a) changes in thickness and mass loading due to re-working, and b) positive correlations between thickness and vegetation density. We observed some systematic changes in tephra layer properties with distance from the vent and the main plume axis. However, the preservation of the layers varied both between and within our survey locations. Closed coniferous forest appeared to provide good conditions for the preservation of the MSH1980 tephra, as expected; preservation of the H1947 deposit in sparsely vegetated parts of Iceland was much more variable. However, preservation of the MSH1980 deposit in sparsely vegetated areas of eastern Washington State was also excellent, possibly due to biocrust formation. We concluded that the preservation of tephra layers is sensitive to surface conditions at the time of the eruption. These findings have implications for the reconstruction of past eruptions where eruption plumes span regions of variable surface cover.PostprintPeer reviewe

Global ecological predictors of the soil priming effect.

Identifying the global drivers of soil priming is essential to understanding C cycling in terrestrial ecosystems. We conducted a survey of soils across 86 globally-distributed locations, spanning a wide range of climates, biotic communities, and soil conditions, and evaluated the apparent soil priming effect using 13C-glucose labeling. Here we show that the magnitude of the positive apparent priming effect (increase in CO2 release through accelerated microbial biomass turnover) was negatively associated with SOC content and microbial respiration rates. Our statistical modeling suggests that apparent priming effects tend to be negative in more mesic sites associated with higher SOC contents. In contrast, a single-input of labile C causes positive apparent priming effects in more arid locations with low SOC contents. Our results provide solid evidence that SOC content plays a critical role in regulating apparent priming effects, with important implications for the improvement of C cycling models under global change scenarios