Geomorphic Disturbance Research Articles

Suspended‐sediment response to wildfire and a major post‐fire flood on the Colorado Front Range

AbstractWildfires, and the sediment‐rich floods that commonly follow, increasingly threaten riverine ecosystems and water infrastructure. Suspended sediment exported throughout fire–flood sequences poses particular risks due to rapid transit times and direct impacts on water quality. However, opportunities to measure suspended‐sediment transport during and after post‐fire floods, and therefore to illuminate what controls the magnitude and timing of suspended‐sediment export from burned, flooded watersheds, are rare. A ~ 100‐year flood that occurred one year into a three‐year study monitoring suspended‐sediment response to wildfire in the Colorado Front Range provides a unique opportunity to (1) quantify how suspended‐sediment concentrations and loads change throughout a fire–flood sequence, and (2) infer what controls the timescale over which suspended‐sediment dynamics recover toward pre‐fire conditions. We find that suspended‐sediment concentrations (SSCs) during summer storms declined monotonically to background conditions over 3 years. Snowmelt SSCs peaked in the second year before declining to background levels. Sediment load calculations reveal that the flood exported ~35 years' worth of suspended sediment and triggered ~1.5 years of elevated SSCs and sediment loads. SSCs and sediment loads indicate a fairly short post‐fire recovery timescale of about 3 years. We suggest that the flood accelerated recovery by (1) exporting much of the available suspended sediment from this supply‐limited landscape and (2) facilitating the export of remaining sediment by making it more accessible to subsequent flows. Our results indicate that large post‐wildfire floods, though representing major geomorphic disturbances, may hasten post‐fire suspended‐sediment recovery to background conditions, at least in supply‐limited regions.

Spatial variation of net methane uptake in Arctic and subarctic drylands of Canada and Greenland

The importance of uptake of atmospheric methane (CH4) in dry Arctic soils for the total Arctic CH4 budget is unresolved. This is partly due to lack of data on the spatial variability of net CH4 consumption and understanding of the main process drivers. We measured net CH4 consumption in Arctic and subarctic landscapes located in in Disko Bay Area and Kangerlussuaq in Western Greenland and in the St. Elias Range in the Yukon, Canada, respectively. Our aim was to characterize the in situ spatial variability of net CH4 uptake in hitherto unexplored Arctic dry upland soils to explore possible limits and environmental drivers across the Arctic geodiversity. Furthermore, we sampled soil for incubation experiments to investigate how net CH4 oxidation responded to changes in soil moisture in contrasting geomorphic settings and parent geological parent materials. We used a laser-based fast deployable chamber system for flux measurements. All studied sites were net sinks of atmospheric CH4 with an average flux −7.5 ± 5.6 μmol CH4 m-2h−1, that are in the upper range of reported net CH4 uptake fluxes in similar soils of the Arctic. The large observed spatial variability within all studied sites (coefficient of variation 50 – 120 %) highlights the need for careful research design allowing for many spatial replicates to achieve representative values. Sites with an active geomorphic environment (abrasion plateau, riverbeds, mountain tops) generally had lower than average net CH4 uptake. Our incubation studies revealed that subsurface CH4 oxidation is the main driver of net surface-atmosphere exchange and that net CH4 oxidation in these layers responded more to changes to soil moisture than in near surface layers. Our study shows surprisingly similar flux magnitudes of net CH4 uptake across widely different landscape forms and geologic parent material, but responding similarly to soil hydrology and geomorphic disturbance, indicating global controls on the net CH4 oxidation in these dry upland environments.

Primary succession and its driving variables – a sphere-spanning approach applied in proglacial areas in the upper Martell Valley (Eastern Italian Alps)

Abstract. Climate change and the associated glacier retreat lead to considerable enlargement and alterations of the proglacial systems. The colonisation of plants in this ecosystem was found to be highly dependent on terrain age, initial site conditions and geomorphic disturbances. Although the explanatory variables are generally well understood, there is little knowledge on their collinearities and resulting influence on proglacial primary succession. To develop a sphere-spanning understanding of vegetation development, a more interdisciplinary approach was adopted. In the proglacial areas of Fürkeleferner, Zufallferner and Langenferner (Martell Valley, Eastern Italian Alps), in total 65 plots of 5×2 m were installed to perform the vegetation analysis on vegetation cover, species number and species composition. For each of those, 39 potential explanatory variables were collected, selected through an extensive literature review. To analyse and further avoid multicollinearities, 33 of the explanatory variables were clustered via principal component analysis (PCA) to five components. Subsequently, generalised additive models (GAMs) were used to analyse the potential explanatory factors of primary succession. The results showed that primary succession patterns were highly related to the first component (elevation and time), the second component (solar radiation), the third component (soil chemistry), the fifth component (soil physics) and landforms. In summary, the analysis of all explanatory variables together provides an overview of the most important influencing variables and their interactions; thus it provides a basis for the debate on future vegetation development in a changing climate.

Similar vegetation‐geomorphic disturbance feedbacks shape unstable glacier forelands across mountain regions

AbstractGlacier forelands are among the most rapidly changing landscapes on Earth. Stable ground is rare as geomorphic processes move sediments across large areas of glacier forelands for decades to centuries following glacier retreat. Yet, most ecological studies sample exclusively on stable terrain to fulfill chronosequence criteria, thus missing potential feedbacks between geomorphic disturbances and vegetation colonization. By influencing vegetation and soil development, such vegetation‐geomorphic disturbance feedbacks could be crucial to understand glacier foreland ecosystem development in a changing climate. We surveyed vegetation and environmental properties, including geomorphic disturbance intensities, in 105 plots located on both stable and unstable moraine terrain in two geomorphologically active glacier forelands in New Zealand and Switzerland. Our plot data showed that geomorphic disturbance intensities permanently changed from high/moderate to low/stable when vegetation reached cover values of around 40%. Around this cover value, species with response and effect traits adapted to geomorphic disturbances dominated. This suggests that such species can act as “biogeomorphic” ecosystem engineers that stabilize ground through positive feedback loops. Across floristic regions, biogeomorphic ecosystem engineer traits creating ground stabilization, such as mat growth and association with mycorrhiza, are remarkably similar. Nonmetric multidimensional scaling revealed a linked sequence of decreasing geomorphic disturbance intensities and changing species composition from pioneer to late successional species. We interpret this linked geomorphic disturbance‐vegetation succession sequence as “biogeomorphic succession,” a common successional pathway in unstable river and coastal ecosystems across the world. Soil and vegetation development were related to this sequence and only advanced once biogeomorphic ecosystem engineer species covered 40%–45% of a plot, indicating a crucial role of biogeomorphic ecosystem engineer stabilization. Different topoclimatic conditions could explain variance in biogeomorphic succession timescales and ecosystem engineer root traits between the glacier forelands. As glacier foreland ground is widely unstable, we propose to consider glacier forelands as “biogeomorphic ecosystems” in which ecosystem structure and function are shaped by geomorphic disturbances and their feedbacks with adapted plant species, similar to rivers and coasts.

Identifying failure mechanisms of native riparian forest regeneration in a variable-width floodplain using a spatially-distributed riparian forest recruitment model

Native riparian forests provide the physical structure, nutrient inputs, and habitat elements necessary to support ecosystem function for many aquatic and terrestrial species. However, regeneration of these important ecotones is being negatively impacted by anthropogenic and climatic pressures, driving managers to consider creative ways to increase native forest recruitment. Decision-making tools currently have limited capacity to assist native forest management and restoration since most cannot identify which recruitment mechanisms fail or where those failures occur on the floodplain. In response, we reconfigured a validated spatially-distributed riparian forest recruitment model to identify the limiting recruitment mechanisms and delineate their spatial extent on the floodplain to guide and prioritize management and restoration efforts. Here we focus on native Salicaceae species, whose seed dispersal mechanism is primarily driven by hydrochory.Results indicate that the success of individual recruitment parameters in semi-confined river systems changes with both hydrologic condition and relative floodplain width (Wfp∗ = Wfloodplain/Wchannel). Overall, the disturbance mechanism (shear stress), required to create bare-soil conditions, was the most significant limitation to native forest recruitment in the system. Greater potential riparian recruitment occurs in wet hydrologic years than in average hydrologic years owing to greater floodplain inundation, but inundation alone is a poor success metric since recruitment requires an appropriate sequence of geomorphic disturbance, seed delivery, and water availability over the growing season. Consequently, narrow floodplains (Wfp∗< ∼3) have greater recruitment efficiency, potential recruitment per available floodplain area, than wide floodplains (Wfp∗> ∼3) but wide floodplains provide greater total potential recruitment area during wet hydrologic conditions. Model output maps that delineated individual and combined failure mechanisms during average and wet hydrologic years in the managed South Fork Boise River, USA were used to identify areas on the floodplain for potential restoration actions.

Riparian recruitment persists after damming: Environmental flows and coupled colonization of cottonwoods and willows following floods along a dryland river

AbstractFor riparian woodlands, occasional floods provide geomorphic disturbance that creates barren colonization sites, and river stage patterns that enable seedling establishment. To investigate impacts from river damming and instream flow regulation on these processes, we studied the lower Red Deer River in the semi‐arid region of Alberta, Canada. Dickson Dam was implemented in 1983 and although major flood peaks in June 2005 and June 2013 were attenuated by about one‐quarter, substantial seedling establishment of plains cottonwoods (Populus deltoides) was observed. Cross‐sectional transects in 2014 and 2015 revealed the reproductively mature cottonwood band from 2005, while the 2013 colonization was limited to low elevations of 0.3–1.3 m above the base river stage. Seedling numbers and the elevational ranges were probably reduced by abrupt (&gt;4 cm/day) river stage reductions in July 2005 and in July 2013. In addition to direct seedling establishment on barren sandbars, we observed “coupled colonization” with cottonwood recruitment within sparse patches of sandbar willows (Salix exigua). The flood‐tolerant willows stabilize the bars and increase aggradation through sediment trapping. Thus, cottonwood colonization persisted after damming, indicating that the pattern of downstream flow regulation was important, rather than damming per se. To sustain riparian recruitment along regulated rivers in dry regions, we recommend: (1) that floods be allowed as feasible, (2) higher river stages during seed dispersal, (3) flow ramping (gradual summer recession) for seedling survival, (4) sufficient growing season flows to avoid drought‐induced mortality, and (5) that willows be encouraged as well as cottonwoods.

Changes in Alpine Butterfly Communities during the Last 40 Years.

Simple SummaryThe Alps are among the most vulnerable ecosystems to climate change, as these modifications take place at a faster pace at higher elevations. Since butterflies are ideal model systems to investigate species responses to climate and habitat changes, we monitored a butterfly community of the Valasco valley in the Maritime Alps (NW Italy) by three sampling events covering a period longer than 40 years. We observed an overall increase in mobile, tolerant and thermophilous species, which eventually might cause an overall loss of community distinctiveness. The variations observed in the butterfly community can be explained by the notable increase in maximum temperatures and the reduction of grasslands, along with the increase of woodlands, supporting the hypothesis that local warming and land-use changes have ultimately affected the butterfly community composition.Our work aims to assess how butterfly communities in the Italian Maritime Alps changed over the past 40 years, in parallel with altitudinal shifts occurring in plant communities. In 2019, we sampled butterflies at 7 grassland sites, between 1300–1900 m, previously investigated in 2009 and 1978, by semi-quantitative linear transects. Fine-scale temperature and precipitation data elaborated by optimal interpolation techniques were used to quantify climate changes. The changes in the vegetation cover and main habitat alterations were assessed by inspection of aerial photographs (1978–2018/1978–2006–2015). The vegetation structure showed a marked decrease of grassland habitats and an increase of woods (1978–2009). Plant physiognomy has remained stable in recent years (2009–2019) with some local exceptions due to geomorphic disturbance. We observed butterfly ‘species substitution’ indicating a general loss in the more specialised and a general gain in more tolerant elements. We did not observe any decrease in species richness, but rather a change in guild compositions, with (i) an overall increased abundance in some widespread and common lowland species and (ii) the disappearance (or strong decrease) of some alpine (high elevation) species, so that ‘resilience’ could be just delusive. Changes in butterfly community composition were consistent with predicted impacts of local warming.

Permafrost Causes Unique Fine‐Scale Spatial Variability Across Tundra Soils

AbstractSpatial analysis in earth sciences is often based on the concept of spatial autocorrelation, expressed by W. Tobler as the first law of geography: “everything is related to everything else, but near things are more related than distant things." Here, we show that subsurface soil properties in permafrost tundra terrain exhibit tremendous spatial variability. We describe the subsurface variability of soil organic carbon (SOC) and ground ice content from the centimeter to the landscape scale in three typical tundra terrain types common across the Arctic region. At the soil pedon scale, that is, from centimeters to 1–2 m, variability is caused by cryoturbation and affected by tussocks, hummocks and nonsorted circles. At the terrain scale, from meters to tens of meters, variability is caused by different generations of ice‐wedges. Variability at the landscape scale, that is, ranging hundreds of meters, is associated with geomorphic disturbances and catenary shifts. The co‐occurrence and overlap of different processes and landforms creates a spatial structure unique to permafrost environments. The coefficient of variation of SOC at the pedon scale (21%–73%) exceeds that found at terrain (17%–66%) and even landscape scale (24%–67%). Such high values for spatial variation are otherwise found at regional to continental scale. Clearly, permafrost soils do not conform to Tobler's law, but are among the most variable soils on Earth. This needs to be accounted for in mapping and predictions of the permafrost carbon feedbacks through various ecosystem processes. We conclude that scale deserves special attention in permafrost regions.

Sediment yield from a forested mountain basin in inland Pacific Northwest: Rates, partitioning, and sources

Sediment yield estimates, combined with information regarding sediment sources and partitioning (bedload vs. suspended load), can provide important insights into geomorphic character of the landscape of interest. This study reports such an analysis, conducted in a forested mountain basin in inland Pacific Northwest using reservoir deposits, ground surveys, and repeat LiDAR mapping. Our research generated three key conclusions. First, we estimated mean specific yield of clastic sediment from the study basin during the last century as ~51 Mg km−2 a−1. In the context of a data compilation from mountain basins in northwestern North America, this value was among the highest for inland basins, and among the lowest when compared with coastal basins. Second, a sediment source analysis provided valuable clues regarding the relative importance of various geomorphic processes operating in the system under study. These findings were consistent with our hypothesis that anthropogenic disturbances may be an important factor that conditions sediment yield from the basin. A mix of quantitative and qualitative evidence suggested that the estimated sediment yield value reflects a transient increase associated with past timber harvest, road construction, and large wood removal. Legacies of these disturbances appear to have the opposite effect on contemporary processes, limiting lateral activity of the channel as well as hillslope-channel connectivity. Third, our partitioning procedure revealed that bedload constituted approximately a third of the total clastic load exported from the basin. This finding suggests that, in this and similar fluvial systems, sediment yield recovery following major geomorphic disturbances could be protracted, as a considerable portion of mobilized sediment that moves as bedload is routed and evacuated from the basin for years or decades after rapid flushing of suspended material. Moreover, this finding indicates that the common assumption that bedload constitutes 10–20% of the total load can lead to underestimation in mountain basins.

Floodplain forest dynamics: Half‐century floods enable pulses of geomorphic disturbance and cottonwood colonization along a prairie river

AbstractIn dry ecoregions, trees are restricted to river valley floodplains where river water supplements the limited local precipitation. Around the Northern Hemisphere, cottonwoods, riparian poplars, are often predominant trees in floodplain forests and these ecological specialists require floods that create and saturate sand and gravel bars, enabling seedling recruitment. By pairing the interpretation of aerial photographs at approximately decade intervals with dendrochronology, we explored the coordination between river floods, geomorphic disturbance and colonization of plains cottonwoods (Populus deltoides) over eight meanders along the Red Deer River in the semi‐arid prairie of western Canada. This river has a relatively natural flow regime and minimal human alteration through the World Heritage Site of Dinosaur Provincial Park. We found that the 50‐year flood of 1954 increased channel migration and produced extensive accretion with downstream expansion of meander lobes and some channel infilling, which was followed by prolific cottonwood colonization. Those processes accompanied the major flood, while bank erosion and cottonwood losses were more gradual and continuous over the past half‐century. Results indicated even greater floodplain and woodland development after an earlier 100‐year flood in 1915. Each flood produced an arcuate band of mature cottonwoods and there were five to seven progressively older woodland bands across the floodplain, with each cottonwood age grouping increasing by about a half‐century. The 700 m wide floodplain was progressively reworked by the river through pulses of channel movement and floodplain and woodland development over approximately 250 years and correspondingly, the oldest cottonwoods were about 250 years old.

Impact of geomorphic disturbance on spatial variability of soil CO2 flux within a depositional landform

AbstractLandform structure‐dependent erosive soil processes impact on the physical, chemical, and biological properties of arid and semiarid areas soils through the redeposition of erosional soils and, consequently, play a role in changes to the global carbon cycle. Understanding the biophysical mechanisms that influence the balance of soil C flux is important for predicting the responses of dryland soilscapes to many forms of environmental change. This study was done along an upper‐to‐lower alluvial fan gradient, where debris flows, representing one of the most common erosive processes of dryland soils, cause local‐scale changes in soil biotic–abiotic patterns. In this empirically founded study, we assessed (a) the variations in soil CO2 flux and some related physiochemical variables such as soil organic–inorganic C, soil C storage, exchangeable cations, pH, electrical conductivity, and volumetric water content and (b) whether these variations in soil CO2 flux and its association with the above‐mentioned soil factors remained constant at three different positions in altitude along the slope of the alluvial fan. Our findings indicated significant differences according to slope position in terms of the flux rate of soil CO2 and associated physiochemical properties. The application of multiple regression demonstrated that the higher soil CO2 flux rates in the upper and middle fan positions are significantly positively correlated with organic C content. Despite the development of biological crusts on the lower fan sediments, they exhibited the lowest rate of soil CO2 flux. Further, low CO2 flux in the lower fan soils was found to be significantly negatively related to pH and inorganic C. This anomaly may be attributed to the alkalinity of the environment formed by the deposition of fine particle sediments on the lower fan position and the fact that CO2 generated by the respiration of mosses may favor the exchange of organic C to carbonate production. Our findings underline the paradoxical impacts of debris flow disturbances on soil C dynamics. This erosive soil disturbance, together with the asymmetric resource redistribution and subsequent variations in the functioning of adjacent alluvial fan positions, can simultaneously provide ‘hot spots’ of reservoir and flux of soil C within different positions of a depositional landform.

Modeling the impact of dam removal on channel evolution and sediment delivery in a multiple dam setting

Dam removal can generate geomorphic disturbances, including channel bed and bank erosion and associated abrupt/pulsed release and downstream transfer of reservoir sediment, but the type and rate of geomorphic response often are hard to predict. The situation gets even more complex in systems which have been impacted by multiple dams and a long and complex engineering history. In previous studies one-dimensional (1-D) models were used to predict aspects of post-removal channel change. However, these models do not consider two-dimensional (2-D) effects of dam removal such as bank erosion processes and lateral migration. In the current study the impacts of multiple dams and their removal on channel evolution and sediment delivery were modeled by using a 2-D landscape evolution model (CAESAR-Lisflood) focusing on the following aspects: patterns, rates, and processes of geomorphic change and associated sediment delivery on annual to decadal timescales. The current modeling study revealed that geomorphic response to dam removal (i.e., channel evolution and associated rates of sediment delivery) in multiple dam settings is variable and complex in space and time. Complexity in geomorphic system response is related to differences in dam size, the proximity of upstream dams, related buffering effects and associated rates of upstream sediment supply, and emerging feedback processes as well as to the presence of channel stabilization measures. Modeled types and rates of geomorphic adjustment, using the 2-D landscape evolution model CAESAR-Lisflood, are similar to those reported in previous studies. Moreover, the use of a 2-D method showed some advantages compared to 1-D models, generating spatially varying patterns of erosion and deposition before and after dam removal that provide morphologies that are more readily comparable to field data as well as features like the lateral re-working of past reservoir deposits which further enables the maintenance of sediment delivery downstream.

A dam in the drylands: Soil geomorphic treatments facilitate recruitment of the endangered Santa Ana River woolly star

AbstractThe long‐term management of aridland riparian ecosystems impacted by dams is crucial to reduce losses of biodiversity, reduce extinction risks for species, and restore ecosystem services. When dams preclude natural flow, safeguarding aridland riparian ecosystems adapted to infrequent, catastrophic floods poses additional challenges owing to the need to consider biological and pattern legacies. Seven Oaks Dam (California, USA) eliminated the occurrence of flooding, scouring, and deposition across the rare plant community downstream. This Riversidian alluvial fan sage scrub or alluvial scrub includes one of the most endangered plants in California, Eriastrum densifolium spp. sanctorum (Santa Ana River woolly star, hereafter called woolly star). In this study, we evaluated the impact of six soil geomorphic treatments on alluvial scrub and woolly star re‐establishment after 5, 7.5, and 13 yr. We implemented a complete randomized block design, with each block incorporating six treatments: cleared, diked, cut, filled‐10 (10 cm soil), filled‐20 (20 cm), and filled‐30 (30 cm), mimicking one or more physical disturbances (pattern legacy) occurring after a natural flood event. We performed plant community surveys (cover, abundance, maturity, invasibility, diversity) on full plots in 2006, representing 7.5 yr of response from the original 1999 treatment, and on half‐plots in 2012, representing 5 yr of response following re‐disturbance in 2007, and 13 yr of response on half‐plots left intact since 1999. We found very limited recruitment of woolly star into control plots (1.2% cover). By contrast, the cut treatment showed consistently higher cover of woolly star (25.3%, 53.4%, 14.3%), after 5, 7.5, and 13 yr, respectively. Other treatments showed responses ranging between these extremes. Similar results were found for total native cover and diversity. Woolly star cover was inversely related to alien grass cover, suggesting that exotic grass invasions inhibit early recovery of the perennial. The re‐establishment of base flows and flood pulses or soil geomorphic disturbances that mimic pattern legacies associated with infrequent, catastrophic flooding could be implemented to sustain downstream ecosystems that include woolly star populations. In the absence of such actions in the Santa Ana River floodplain, the persistence of woolly star as a species appears unlikely.

Using Machine Learning to Predict Geomorphic Disturbance: The Effects of Sample Size, Sample Prevalence, and Sampling Strategy

AbstractAdvances in data acquisition and statistical methodology have led to growing use of machine‐learning methods to predict geomorphic disturbance events. However, capturing the data required to parameterize these models is challenging because of expense or, more fundamentally, because the phenomenon of interest occurs infrequently. Thus, it is important to understand how the nature of the data used to train predictive models influences their performance. Using a database of cliff failure prediction and associated covariates from Auckland, New Zealand, we assess the performance of seven machine‐learning algorithms under different sampling strategies. Three sampling components are investigated: (i) the number of data points used in model training (sample size), (ii) the prevalence of occurrences (presences) in the data, and (iii) random versus spatial sampling strategy. Across the seven algorithms, small sample sizes can produce models that perform relatively well, especially if the prime concern is identifying key predictors rather than quantifying risk or predicting categorical outcome. Our analyses show that for the same effort (i.e., number of samples), sampling around multiple locations provides better predictions than sampling at just one or a few locations. Predictive performance may be further improved by considering issues such as the nature of what absences actually represent and paying careful attention to decisions about hyperparameter tuning, training‐testing data splits, and threshold optimization. It is well known that big data can inform complex data‐driven modeling, but here we show that careful sampling can facilitate informative event prediction even from small data.

Unravelling the recent dynamics of headwaters based on a combined dendrogeomorphic approach (a case study from the Sudetes Mts., Czech Republic)

ABSTRACTHeadwater catchments are frequently prone to debris flows/floods. Dendrogeomorphic methods allow for the accurate dating of the frequencies and spatial patterns of these events. Nevertheless, a combined approach based on the sampling of increment cores from tree stems and the extraction of cross-sections of scarred roots has been rarely used together at one site to determine the headwater dynamics. Such sampling strategy was performed in the Rudohorský potok catchment (the Hrubý Jeseník Mountains, Eastern Sudetes, Czech Republic) to compile the debris flow/flood chronology and to describe the detailed spatial patterns in the studied sub-catchments. In total, 44 events of increased hydrogeomorphic activity during the last 110 years were identified based on the analysis of 860 growth disturbances from 322 trees and 85 roots. The largest events (debris flows) occurred in 1921, 1951, 1965, 1975, 1991, 1997, 2001 and 2010. Higher dynamics of hydrogeomorphic processes were investigated in the sub-catchment affected by deep-seated rockslides. The inclusion of root analyses facilitated the completion of the event chronology mainly during the last 20 years. With respect to the decreasing sensitivity of tree stems with increasing age to the recording of geomorphic disturbances, the root analysis helped to better illustrate the spatial imprint of recent debris flow events (especially the most recent one in 2010).

Climatic, geomorphologic and hydrologic perturbations as drivers for mid- to late Holocene development of ice-wedge polygons in the western Canadian Arctic.

Ice‐wedge polygons are widespread periglacial features and influence landscape hydrology and carbon storage. The influence of climate and topography on polygon development is not entirely clear, however, giving high uncertainties to projections of permafrost development. We studied the mid‐ to late Holocene development of modern ice‐wedge polygon sites to explore drivers of change and reasons for long‐term stability. We analyzed organic carbon, total nitrogen, stable carbon isotopes, grain size composition and plant macrofossils in six cores from three polygons. We found that all sites developed from aquatic to wetland conditions. In the mid‐Holocene, shallow lakes and partly submerged ice‐wedge polygons existed at the studied sites. An erosional hiatus of ca 5000 years followed, and ice‐wedge polygons re‐initiated within the last millennium. Ice‐wedge melt and surface drying during the last century were linked to climatic warming. The influence of climate on ice‐wedge polygon development was outweighed by geomorphology during most of the late Holocene. Recent warming, however, caused ice‐wedge degradation at all sites. Our study showed that where waterlogged ground was maintained, low‐centered polygons persisted for millennia. Ice‐wedge melt and increased drainage through geomorphic disturbance, however, triggered conversion into high‐centered polygons and may lead to self‐enhancing degradation under continued warming.

Interactive effect of soil moisture and temperature regimes on the dynamics of soil organic carbon decomposition in a subarctic tundra soil

Geomorphic disturbances to surrounding terrain induced by thermal degradation of permafrost often lead to surface ponding or soil saturation. However, interactions between soil moisture and temperature on belowground carbon processes are not fully understood. We conducted batch incubation for three temperature treatments [constant freezing (CF), constant thawing (CT), and fluctuating temperatures (FTC)] and two soil moisture conditions (ponded and unsaturated). Extracellular enzyme activity was higher under ponded conditions than under unsaturated conditions, resulting in higher dissolved organic carbon (DOC) levels for ponded conditions. More CO2 and less CH4 were emitted under unsaturated conditions than under ponded conditions. Carbon dioxide emission was similar for CT and FTC treatments regardless of moisture conditions. However, CH4 emission was higher under ponded conditions than under unsaturated conditions for CT treatments, but was very low for FTC treatments regardless of moisture conditions. Little CO2 and CH4 were produced in CF treatments. Despite similar CO2 and CH4 emission levels for CT and FTC treatments, lower DOC levels were observed in the latter, indicating slower soil organic carbon (SOC) decomposition. Similar DOC variation patterns between CT and CF treatments indicated that SOC decomposition was considerable and further degradation to CO2 or CH4 was negligible even for CF treatments. The SOC decomposition and CO2 and CH4 emissions were considerable for FTC treatments. Our results suggest that labile-C produced during SOC decomposition in seasonally frozen soils and permafrost may provide supplemental substrates that would enhance the positive feedback to climate change with rising temperatures and wetter conditions.

Assessing the geomorphic disturbance from fires on coastal dunes near Esperance, Western Australia: Implications for dune de-stabilisation

Abstract Fire is commonly listed as a contributing disturbance to dune re-activation. This paper aims to characterise post-fire disturbance to vegetation and soil surface, and aeolian activity on coastal dunes. Field data were collected in February 2016 at two sites on coastal dunes near Esperance, Western Australia (WA) after recent wildfires in November 2015 and January 2016. We measured wind profiles at burnt and unburnt sites, and assessed recent sand movement, protective covering and burn severity. We also used remote sensing and on-site photos to monitor local patterns of short term biomass recovery. Results suggest that burnt vegetation enables near surface winds to flow with a similar profile shape to bare surfaces. Speed-up ratios (SR) were higher by 5–120% on burnt surfaces when compared with vegetated. However, burnt vegetation did not show the same topographic acceleration as bare dunes. This decelerating effect correlated with surface-level ground cover after removing topographically sheltered data points (r2 = 0.8, p 40%. At one burnt transect a high burn intensity may have inhibited short term germination and re-sprouting. Fire as the sole disturbance is not a major threat to the stability of these dunes, however, extreme burn intensities may leave dunes susceptible to further non-fire disturbance events.

Variations in spatial patterns of soil-vegetation properties and the emergence of multiple resilience thresholds within different debris flow fan positions

Debris flow fans are non-equilibrium landforms resulting from the spatial variations of debris flows deposited on them. This geomorphic disturbance involving the asymmetric redistribution of water and sediment may create spatially heterogeneous patterns of soil-vegetation along landforms. In this research, founded on field-based observations, we characterized the spatial patterns of some soil (e.g., particle size distribution including fine and coarse covers, and infiltration capacity) and vegetation (e.g., plant distance, vegetation density, patch size, and average number of patches) properties within different debris flow fan positions (Upper, Middle, and Lower fan) located at the base of the Binaloud Mountain hillslope in northeastern Iran. Thereafter, using a mathematical model of dry land vegetation dynamics, we calculated response trends of the different positions to the same environmental harshness gradient. Field measurements of soil-vegetation properties and infiltration rates showed that the asymmetric redistribution of debris flow depositions can cause statistically significant differences (P<0.05) in the spatial patterns of soil and eco-hydrological characteristics along different landform positions. The results showed that mean plant distance, mean vegetation density, and the average number of patches decreased as the coarse covers increased toward the Lower fan plots. Conversely, an increase in infiltration rate was observed. The simulation results on the aerial images taken from different positions, illustrated that positions with a heterogeneous distribution of vegetation patterns were not desertified to the same degree of aridity. Thus, the Middle and Lower positions could survive under harsher aridity conditions, due to the emergence of more varied spatial vegetation patterns than at the Upper fan position. The findings, based on a combined field and modeling approach, highlighted that debris flow as a geomorphic process with the asymmetric distribution of depositions on the gentle slope of an alluvial fan, can incur multiple resilience thresholds with different degrees of self-organization under stressful conditions over the spatial heterogeneities of soil-dependent vegetation structures.

Impacts of Geomorphic Disturbances on Plant Colonization in Ebba Valley, Central Spitsbergen, Svalbard

AbstractGlobal warming observed nowadays causes an increase in geomorphic activity in polar regions. Within the areas influenced by cold climatic conditions, relief dynamics and vegetation development are the main landscape shaping processes. The study is limited to the Ebba Valley (78°43’N; 16°37’E) in central Spitsbergen (Svalbard), where geomorphologic observations and vegetation sampling were conducted in 2007. The valley was divided into three zones differentiated by dominating geomorphic activity and stability of deposits. The settlement and the evolution of plant cover have been documented there. The main factors that control well developed vegetation cover within raised marine terraces are frost heave and solifluction. In deeper parts of the valley, aeolian processes dominate and high differentiation of microsite conditions causes high variability in plant coverage. The area close to the Ebba glacier marginal zone is characterized by initial stages of plant colonisation where disturbance to vegetation is mainly caused by hydrological processes.