Historical Range Of Variability Research Articles

Impacts and underlying factors of landscape-scale, historical disturbance of mountain forest identified using archival documents

Large areas of temperate mountain Norway spruce (Picea abies) forests in Central Europe have been disturbed by windstorms and subsequent bark beetle (Ips typographus) outbreaks in recent years. The impact of these events has been severe, with millions of canopy trees dying in nature reserves, where salvage logging was not conducted. The occurrence of these windstorms has raised the question of whether such events were within the historical range of variability (HRV) of forest dynamics in Central Europe, where disturbances were traditionally perceived as non-natural and without strong relevance for forest development. To answer this question, we analyzed the available historical forest management maps documenting large-scale disturbance resulting from windthrow events in the years 1868–1870 in spruce- and beech- (Fagus sylvatica) dominated forests in the Bohemian Forest region (Šumava Mts., Czech Republic and Bayerischer Wald, Germany). We created a cross-boundary forest database covering 54 974ha and containing information about tree species composition and age, and the severity of the 1868–1870 disturbance, considering the cumulative effect of the windthrow, subsequent bark beetle outbreak, and salvage logging. The age structure of the forests before the disturbances was unbalanced, with stands of 80–120years underrepresented, and covering only 9% of the area, and stands older than 120years, historically classified as old growth, covering 26% of the area. Within the decade that included the windstorms and their aftermath, 40% of the stands in the mountain range were at least partly disturbed, with significant effect on the oldest stands. To identify important factors responsible for the severity of disturbance, we constructed regression models relating severity to two groups of explanatory variables: forest stand characteristics and environmental attributes (mainly topographic factors). Overall, stand age was identified as the most important driver of disturbance severity across the landscape, with the oldest trees most susceptible. The high importance of age for disturbance severity showed the role of forest age structure in determining the scale of disturbances resulting from windstorms and associated bark beetle outbreaks. Nevertheless, despite the documented occurrence of frequent large disturbances during the two centuries that preceded the 1868–1870 events, old growth accounted for 26% of the area, making it clear that both large scale disturbance and old-growth forest are within the HRV of mountain spruce forest dynamics in Central Europe.

Characterizations of anthropogenic disturbance patterns in the mixedwood boreal forest of Alberta, Canada

Ecosystem-based management (EBM) has emerged as a dominant paradigm for the Canadian boreal forest. One of the principles of EBM is to maintain ecosystem function by means of management activities that approximate the historic patterns or processes responsible for maintaining a range of landscape conditions. This ideal has been manifested as planning schemes are shifting away from traditional sustained yield harvests toward designs based on historic wildfire disturbance patterns. Wildfire disturbance patterns represent a coarse-filter management strategy, and are well-suited to the boreal forests of Canada. Forest management professionals in the boreal have been leaders in adopting these strategies over the past decade. However, two key questions remain unanswered: (1) to what degree have these forest management efforts resulted in disturbance patterns that resemble wildfire burning patterns?; and (2) to what degree do the other sources of anthropogenic disturbance activities align with historic wildfire patterns? In this paper, an existing knowledge of historic range of variability (HRV) of wildfire patterns and the NEPTUNE (Novel Emulation Pattern Tool for Understanding Natural Events) decision support tool were used to test both questions.The results suggest that forest harvest disturbances better approximated historic disturbance patterns than did energy extraction disturbances, though in both cases some of the metrics were beyond the HRV. Significant differences were found between traditional dispersed patterns (e.g., multi-pass harvesting) and the more recent aggregate harvest (e.g., single-pass) designs. Aggregate harvests were characterized by low proportional area in matrix remnants, moderate levels of combined island and matrix remnants, and a high proportional area in the single largest disturbed patch (LDP). Dispersed harvests tended to have a higher proportional area in matrix remnants and better approximated the HRV in terms of proportional area in proportional island remnants area and largest island remnant. Aggregate harvest patterns did not perform as well for a few metrics such as proportional area in island remnants, mean island remnant size and largest island remnant. Overall, the results suggest that aggregate harvest designs better approximated key HRV patterns such as proportional matrix remnants and LDP, than did dispersed harvest designs on the landscape. The results also suggest that forest harvesting was significantly more effective at approximating historic disturbance patterns than the activities of the energy sector. Energy sector disturbances were smaller and had fewer island remnants than the HRV. The composition of surviving remnant trees within anthropogenic disturbance events (i.e., matrix and island remnants) remains a critical area of research for approximating HRV patterns.

Wilderness is dead: Whither critical zone studies and geomorphology in the Anthropocene?

Abstract Numerous studies document the extent and intensity of human appropriation of ecosystem services and the manipulation of Earth's surface and fluxes of water, sediment and nutrients within the critical zone of surface and near-surface environments. These studies make it increasingly clear that wilderness is effectively gone. This paper explores the implications for critical zone studies and management from a geomorphic perspective. Geomorphologists possess knowledge of the long history of human alteration of the critical zone. This knowledge can be applied to characterizing: historical range of variability and reference conditions; fluxes of matter and energy; and integrity and sustainability of critical zone environments. Conceptual frameworks centered on connectivity, inequality, and thresholds or tipping points are particularly useful for such characterizations, as illustrated by a case study of beaver meadows in the Front Range of Colorado, USA. Specifically, for connectivity, inequality, and thresholds, geomorphologists can identify the existence and characteristics of these phenomena, quantify and predict changes resulting from past or future human manipulations, and recommend actions to restore desirable conditions or prevent development of undesirable conditions. I argue that we should by default assume that any particular landscape has had greater rather than lesser human manipulation through time. This history of manipulation continues to influence critical zone process and form, and geomorphologists can use knowledge of historical context in a forward-looking approach that emphasizes prediction and management.

Bounded ranges of variation as a framework for future conservation and fire management

Alterations in natural fire patterns have negatively affected fire-prone ecosystems in many ways. The historical range of variability (HRV) concept evolved as a management target for natural vegetation composition and fire regimes in fire-prone ecosystems. HRV-based management inherently assumes that ecosystem resilience is reflected in observed ranges of past vegetation and fire dynamics, typically without knowledge of where thresholds exist beyond these dynamics. Given uncertainty in future conditions, some have argued that HRV may not adequately reflect ecosystem resilience to future fire activity. We suggest a refinement that includes concepts from the thresholds of potential concern (TPC) framework, which emphasizes tipping points at the extremes of ecosystem dynamics and other socially unacceptable outcomes. We propose bounded ranges of variation (BRV), an approach focused on building resilience by using historical information, but also by identifying socio-ecological thresholds to avoid and associated management action triggers. Here, we examine nonnative species and carbon sequestration as examples of how the BRV framework could be used in the context of conservation and fire management.

Forest fire management, climate change, and the risk of catastrophic carbon losses

Approaches to management of fireprone forests are undergoing rapid change, driven by recognition that technological attempts to subdue fire at large scales (fire suppression) are ecologically and economically unsustainable. However, our current framework for intervention excludes the full scope of the fire management problem within the broader context of fire−vegetation−climate interactions. Climate change may already be causing unprecedented fire activity, and even if current fires are within the historical range of variability, models predict that current fire management problems will be compounded by more frequent extreme fire-conducive weather conditions (eg Fried et al. 2004). Concern about climate change has also made the mitigation of greenhouse-gas (GHG) emissions and increased carbon (C) storage a priority for forest managers.

呼中林区森林景观的历史变域模拟及评价

在大兴安岭地区呼中区,将物种的年龄和分布信息进行随机化处理后,应用空间直观景观模型LANDIS对森林景观进行长时间(2500a)模拟,取物种演替稳定时间段作为模拟历史变域的数据来源。分别在景观水平和年龄类型水平上利用景观指数空间分析、主成分分析和核密度估计方法分析景观格局历史变域的模拟结果,并在二维空间坐标内,将研究区1990年、2000年森林景观特征与之比较。结果表明,各树种面积比例在模拟900a后都达到稳定状态,可作为无干扰条件下森林景观特征的历史变域;由于长期采伐,研究区1990年景观的斑块面积和破碎化程度都偏离了该历史变域,其中,过熟林偏离最明显,其斑块面积远小于历史变域,破碎化程度非常严重;虽然1990年后的采伐管理方案比之前的更为合理,但2000年的森林景观仍旧继续偏离历史变域。;After randomizing the input information of age and distribution of forest species we simulated the long-term (2500 years) dynamic of forest landscape pattern of huzhong area in Great Xing'an Mountains using spatially explicit landscape model (LANDIS), Then we took the period when simulation results reached stable as the time range to estimate the historical range of variability (HRV). Furthermore, we analyzed the relationship between HRV and the forest landscape pattern of 1990 and 2000 using principal component analysis and kernel density estimation methods. The simulation results showed that the succession of almost all forest species arrived to their stable status at around 900th year of the simulation, and the period afterwards was utilized to estimate the HRV; as to the characteristics of forest, landscape pattern in 1990 fell out of the HRV in area of patches and the degree of landscape fragmentation, especially mature and over-mature forest deviated significantly from HRV in patch area and fragmentation; though forest harvest management was better planned after 1990, the forest landscape pattern in 2000 still deviated from the HRV.

Guest Editorial: Introduction to special issue on historical range of variability

Earth Surface Processes and LandformsVolume 38, Issue 2 p. 213-216 ESEX Commentary Guest Editorial: Introduction to special issue on historical range of variability First published: 26 October 2012 https://doi.org/10.1002/esp.3357Citations: 5Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume38, Issue2February 2013Pages 213-216 RelatedInformation

Evaluating channel response to an extreme sedimentation event in the context of historical range of variability: Upper Colorado River, USA

ABSTRACTIn May 2003, a breach in a large irrigation ditch within Rocky Mountain National Park (RMNP) initiated a debris flow that entered Lulu Creek and the Colorado River, where 36 000 m3 of sediment substantially altered channel forms and processes. We present a proof of concept to understand whether the 2003 disturbance is within the historical range of variability (HRV), and whether the recovery potential of the system is sufficient to adapt to the disturbance. Flow and sediment regimes, and channel morphology and stability were monitored on Lulu Creek and the Colorado River from 2004 to 2011. Dominant channel response following the debris flow within Lulu Creek included step development, bed armoring, and channel widening. Step height‐to‐length ratios (H/L) for three reaches on Lulu Creek are outside the HRV of reference channels, with one reach approaching reference conditions. Erosion of approximately 23% of the debris fan volume occurred as a result of the long duration 2011 peak flow. Sediment within the Lulu Creek fan will persist for ~30–190 years, assuming current maximum and mean removal rates. Planform changes on the Colorado River since the debris flow include an increase in single‐thread geometries, with braided reaches where bar deposition occurred. Bedload transport and grain‐size analysis of bedload indicate translational spreading of a sand wave front with a dispersive component in steeper reaches. Lulu Creek is returning to a condition of natural variability, but the Colorado River is outside the HRV expected for steep‐gradient, pool‐riffle channels. Applying HRV to a situation where management questions require a longer term perspective, and pre‐disturbance baseline data are limited, is a useful approach. The HRV analysis facilitates a better understanding of site variability and delineates the range of possibilities of channel form and process to achieve management goals. Copyright © 2012 John Wiley & Sons, Ltd.

A Bayesian Approach to Landscape Ecological Risk Assessment Applied to the Upper Grande Ronde Watershed, Oregon

ABSTRACT We present a Bayesian network model based on the ecological risk assessment framework to evaluate potential impacts to habitats and resources resulting from wildfire, grazing, forest management activities, and insect outbreaks in a forested landscape in northeastern Oregon. The Bayesian network structure consisted of three tiers of nodes: landscape disturbances, habitats, and the ecological resources or endpoints of interest to land managers. Nodes at each tier were linked to lower nodes if ecological and spatial relationships existed between them. All parameters had four potential discrete states: zero, low, medium, and high. Our model reliably predicted probable risk to habitats and endpoints from natural and anthropogenic disturbances. The disturbances most likely to transform habitats and effect ecological resources were forest management and wildfire. Of the six habitats, moist forest (characterized by Douglas fir and grand fir) was found to be at greatest risk of ecological impacts. The management endpoint with the highest likelihood of impact was historical range of variability (HRV) for salmon habitat, followed by recreation (hunting native ungulates) and HRV wildfire. We found that the Bayesian approach to ecological risk assessment was a useful method to assess potential impacts to ecological resources resulting from forest management and natural disturbances.

Fire in managed forests of eastern Canada: Risks and options

In this era of climate change, understanding past and predicting future fire activity are scientific challenges that are central to the development of sustainable forest management practices and policies. Such objectives, however, are difficult to achieve for several reasons. Uncertainties about future fire activity can be superimposed on the short time period covered by existing meteorological data and fire statistics, from which a historical range of variability can be determined. Regional fire activity is also tremendously variable over time, such that contemporary fire records cannot provide information on the full range of fire activity variability a given forest experienced and adapted to. This factor is increasingly important when it comes to determining the resilience of boreal forests to changes in climate and disturbance regimes. In this paper, we present a synthesis of past, present and future trends in seasonal fire danger and fire activity based on data gathered in eastern Canadian boreal forests over the last 20years, and we provide a critical assessment of the ability to conduct sustainable forest management over the 21st century. The data synthesis provides compelling evidence of a synchronous pattern of decreasing fire-conducive climatic conditions and activity of large fire seasons over the last 2000years in the eastern coniferous boreal forest. Model simulations suggest that the climate will become drier in upcoming decades, driving future fire activity close to the upper bound of the pre-industrial range of variability. The effects of increasing fire incidence cumulated with forest harvesting may thus pose a risk to forest resilience in the future. This ecological knowledge should help us to define forest management strategies and practices considering future fire activity changes forecasted under climate change. Development of alternative silvicultural interventions that would emulate secondary disturbances (e.g. wind, insects) rather than fire would be necessary to maintain pre-industrial forest characteristics (e.g. composition and age class distribution), and associated forest resilience.

A Pilot Application of Regional Scale Risk Assessment to the Forestry Management of the Upper Grande Ronde Watershed, Oregon

ABSTRACT An issue in forestry management has been the integration of a variety of different information into a threat analysis or risk assessment. In this instance, regional scale risk assessment was applied to the Upper Grande Ronde watershed in eastern Oregon to examine the potential of risk assessment for use in the management of broad landscapes. The site was a focus of study for the U.S. Forest Service through the Interior Northwest Landscape Analysis System (INLAS) project. In the study, a range of stressors, habitats, and endpoints were identified from previous studies in the watershed, and endpoints were determined from meeting with the primary stakeholder, the U.S. Forest Service. These endpoints were focused around the historic range of variability (HRV) defined for the area. The relative risk model (RRM) incorporating a Monte Carlo analysis was used as the analysis tool. The risk model output showed the HRV fire regime was the endpoint most at risk. The results of this analysis were compared to the Wallowa-Whitman National Forest prioritization of watershed restoration analysis. The RRM demonstrated similar results but with a better accounting for uncertainty. From this trial the RRM has proven to be a potential management tool for forestry management.

A legacy of climate and catchment change: the real challenge for wetland management

Wetland managers are faced with an array of challenges when restoring ecosystems at risk from changing climate and human impacts, especially as many of these processes have been operating over decadal–millennial timescales. Variations in the level and salinity of the large crater lakes of western Victoria, as revealed over millennia by the physical, chemical and biological evidence archived in sediments, attest to extended periods of positive rainfall balance and others of rainfall deficit. The recent declines in the depth of these lakes have been attributed to a 15% decline in effective rainfall since AD 1859. Whilst some sites reveal state shifts following past droughts, the response of most wetlands to millennial-scale climatic variations is muted. Regional wetland condition has changed comprehensively, however, since European settlement, on account of extensive catchment modifications. These modifications appear to have reduced the resilience of wetlands limiting their capacity to recover from the recent ‘big dry’. These sedimentary archives reveal most modern wetlands to be outside their historical range of variability. This approach provides a longer-term context when assessing wetland condition and better establishes the restoration challenge posed by the impact of climate change and variability and human impacts.

Comparison of the Higher-Severity Fire Regime in Historical (A.D. 1800s) and Modern (A.D. 1984–2009) Montane Forests Across 624,156 ha of the Colorado Front Range

There are concerns that recent fires, following a century of land uses, are burning in dry western forests in an uncharacteristic manner with large patches of higher-severity fire affecting long-term ecosystem dynamics. For example, it is well documented that a mixed-severity fire regime predominated over montane forests of the Colorado Front Range. However, much about the historical fire regime is unknown including the size, frequency, and distribution of higher-severity fires. We addressed these questions utilizing data from the original land surveyors who recorded locations of burned timber along survey lines resulting in a coarse-scale transect of fire occurrence across 624,156 ha. We reconstructed higher-severity burn patches, size distribution, and fire rotation for the 1800s (A.D. 1809–1883) and compared to the characteristics of modern fires over a recent 26-year period (A.D. 1984–2009) taken from remotely sensed data. We found the historical geometric mean higher-severity patch was 170.9 ha and the maximum patch size was 8,331 ha; the higher-severity fire rotation was 248.7 years. In addition, we confirmed that higher-severity fires were historically less common at elevations below 2,200 m. Modern fires had a geometric mean patch size of 90.0 ha (patches >20 ha) and a maximum size of 5,183 ha; the higher-severity fire rotation was 431 years. The distributions of higher-severity patches were only 63.5% similar, as the historical distribution had fewer small patches and more large patches. The mixed-severity fire regime, historically, included a significant portion of higher-severity fire and large burn patches; modern fires appear to be within the range of historical variability.

Spatially extensive reconstructions show variable‐severity fire and heterogeneous structure in historical western United States dry forests

ABSTRACTAim Wildfire is often considered more severe now than historically in dry forests of the western United States. Tree‐ring reconstructions, which suggest that historical dry forests were park‐like with large, old trees maintained by low‐severity fires, are from small, scattered studies. To overcome this limitation, we developed spatially comprehensive reconstructions across 927,000 ha in four landscapes, using a new method based on land surveys fromc. 1880.Location Dry forests of the western United States.Methods We reconstructed forest structure for four large dry‐forest landscapes using forest descriptions and tree data from historical land surveys. Using multiple elements of historical forest structure from this study along with corroborating information from tree‐ring studies, we were able to interpret past forest dynamics. Hypotheses concerning historical structure and dynamics were then tested.Results These reconstructions show that dry forests were structurally variable, containing from 20 to over 1000 trees ha−1and some dense understoreys of shrubs and small trees. Park‐like stands of large trees maintained by low‐severity fire predominated only in parts of the study landscapes. Only 3, 12, 40 and 62% of the four landscapes fit a low‐severity fire model; 38–97% had evidence ofhigher‐severity (mixed‐ and high‐severity) fire. Some large modern wildfires (e.g. Rodeo‐Chediski), perceived as catastrophic, had fire severity congruent with historical variability.Main conclusions Spatially extensive reconstructions from the late 1800s show that these forests were structurally variable, including areas of dense forests and understorey trees and shrubs, and fires varied in severity, including 15–65% high‐severity fire. A set of laws, policies and initiatives that aim to uniformly reduce fuels and fire severity is likely to move many of these forests outside their historical range of variability with adverse effects on biological diversity. Macroscale survey‐based reconstructions and palaeoecological studies reveal thathigher‐severity fires were and are a part of the normal dynamics of dry forests.

Historical Stand-Replacing Fire in Upper Montane Forests of the Madrean Sky Islands and Mogollon Plateau, Southwestern USA

The recent occurrence of large fires with a substantial stand-replacing component in the southwestern United States (e.g., Cerro Grande, 2000; Rodeo-Chedeski, 2002; Aspen, 2003; Horseshoe 2, Las Conchas, and Wallow, 2011) has raised questions about the historical role of stand-replacing fire in the region. We reconstructed fire dates and stand-replacing fire patch sizes using four lines of tree-ring evidence at four upper montane forest sites (>2600 m) in the Madrean Sky Islands and Mogollon Plateau of Arizona and New Mexico, USA. The four lines of tree-ring evidence include: (1) quaking aspen (Populus tremuloides) and spruce-fir age structure, (2) conifer death dates, (3) traumatic resin ducts and ring-width changes, and (4) conifer fire scars. Pre-1905 fire regimes in the upper montane forest sites were variable, with drier, south-facing portions of some sites recording frequent, low-severity fire (mean fire interval of all fires ranging from 5 yr to 11 yr among sites), others burning with stand-replacing severity, and others with no evidence of fire for >300 yr. Reconstructed fires at three of the four sites (Pinaleno Mountains, San Francisco Peaks, and Gila Wilderness) had stand-replacing fire patches >200 ha, with maximum patch sizes ranging from 286 ha in mixed conifer-aspen forests to 521 ha in spruce-fir forests. These data suggest that recent stand-replacing fire patches as large as 200 ha to 500 ha burning in upper elevation (>2600 m) mixed conifer-aspen and spruce-fir forests may be within the historical range of variability.

Both topography and climate affected forest and woodland burn severity in two regions of the western US, 1984 to 2006

Fire is a keystone process in many ecosystems of western North America. Severe fires kill and consume large amounts of above- and belowground biomass and affect soils, resulting in long-lasting consequences for vegetation, aquatic ecosystem productivity and diversity, and other ecosystem properties. We analyzed the occurrence of, and trends in, satellite-derived burn severity across six ecoregions in the Southwest and Northwest regions of the United States from 1984 to 2006 using data from the Monitoring Trends in Burn Severity project. Using 1,024 fires from the Northwest (4,311,871 ha) and 497 fires from the Southwest (1,434,670 ha), we examined the relative influence of fine-scale topography and coarse-scale weather and climate on burn severity (the degree of change from before the fire to one year after) using the Random Forest machine learning algorithm. Together, topography, climate, and weather explained severe fire occurrence with classification accuracies ranging from 68% to 84%. Topographic variables were relatively more important predictors of severe fire occurrence than either climate or weather variables. Predictability of severe fire was consistently lower during years with widespread fires, suggesting that local control exerted by topography may be overwhelmed by regional climatic controls when fires burn in dry conditions. Annually, area burned severely was strongly correlated with area burned in all ecoregions (Pearson's correlation 0.86–0.97; p < 0.001), while the proportion of area burned severely was significantly correlated with area burned only in two ecoregions (p ≤ 0.037). During our short time series, only ecoregions in the Southwest showed evidence of a significant increase (p ≤ 0.036) in annual area burned and area burned severely, and annual proportion burned severely increased in just one of the three Southwest ecoregions. We suggest that predictive mapping of the potential for severe fire is possible, and will be improved with climate data at the scale of the topographic and Landsat-derived burn severity data. Although severity is a value-laden term implying negative ecosystem effects, we stress that severity can be objectively measured and recognize that high severity fire is an important ecological process within the historical range of variability in some ecosystems.

Historic range of variability in geomorphic processes as a context for restoration: Rocky Mountain National Park, Colorado, USA

ABSTRACTEvaluation of historic range of variability (HRV) is an effective tool for determining baseline conditions and providing context to researchers and land managers seeking to understand and enhance ecological function. Incorporating HRV into restoration planning acknowledges the dynamic quality of landscapes by allowing variability and disturbance at reasonable levels and permitting riverine landscapes to adapt to the physical processes of their watersheds. HRV analysis therefore represents a practical (though under‐utilized) method for quantifying process‐based restoration goals. We investigated HRV of aggradational processes in the subalpine Lulu City wetland in Rocky Mountain National Park to understand the impacts of two centuries of altered land use and to guide restoration planning following a human‐caused debris flow in 2003 that deposited up to 1 m of sand and gravel in the wetland. Historic aerial photograph interpretation, ground penetrating radar surveys, and trenching, coring, and radiocarbon dating of valley‐bottom sediments were used to map sediment deposits, quantify aggradation rates, and identify processes (in‐channel and overbank fluvial deposition, direct hillslope input, beaver pond filling, peat accumulation) creating alluvial fill within the wetland. Results indicate (i) the Lulu City wetland has been aggrading for several millennia, (ii) the aggradation rate of the past one to two centuries is approximately six times higher than long‐term pre‐settlement averages, (iii) during geomorphically active periods, short‐term aggradation rates during the pre‐settlement period were probably much higher than the long‐term average rate, and (iv) the processes of aggradation during the last two centuries are the same as historic processes of aggradation. Understanding the HRV of aggradation rates and processes can constrain management and restoration scenarios by quantifying the range of disturbance from which a landscape can recover without active restoration. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Using a historical aerial photograph analysis to inform trout habitat restoration efforts

ABSTRACTRestoration of the upper Strawberry River included bank stabilization techniques because it was assumed that excessive bank erosion was degrading spawning habitat for Bonneville cutthroat trout (BCT). Using a long‐term aerial photograph record, the historical range of variability in bank erosion rates and channel geometry was determined, and this information was used to assess present‐day conditions and the rationale for restoration. Relative to historical variability, the channel planform was relatively stable and bank erosion rates were the lowest recorded in the post‐disturbance era. Although a historical loss of riparian vegetation coincided with a shift to a wider and more sinuous channel, lateral migration rates declined and the channel narrowed as riparian cover increased in the decades before restoration, indicating a process of natural recovery. Furthermore, it was found that the percentage of fine sediment in the streambed before restoration was insufficient to affect BCT spawning success. Together these results suggest that bank erosion and fine sediment did not affect the quality of spawning habitat or the abundance of BCT on the upper Strawberry River. The results highlight how a historical analysis can be used to identify the sources of habitat degradation and inform the selection of restoration goals and strategies. Copyright © 2011 John Wiley &amp; Sons, Ltd.

What should these rivers look like? Historical range of variability and human impacts in the Colorado Front Range, USA

ABSTRACTHistorical range of variability (HRV) describes the range of temporal and spatial variations in river variables such as flow regime or channel planform prior to intensive human alteration of the ecosystem. In mountainous river networks, HRV is most usefully applied to spatially differentiated geomorphic process domains with distinctive form and process. Using the Colorado Front Range as an example, three examples of how knowledge of HRV can assist river management and restoration are discussed. The examples involve instream wood load and channel morphology, beaver colonies and valley‐bottom form and process, and flow thresholds in regulated rivers. The question of what a river should look like – that is, what range of process and form the river included prior to intensive human alteration – can be addressed by (i) placing the river within a process domain, (ii) establishing correlations between form parameters that can be remotely sensed and reach‐scale process and form, so that the spatial extent, connectivity, and rarity of process domains within a river network or a region can be quickly assessed, (iii) inferring characteristics of the river prior to intensive alteration by documenting characteristics of the least altered reference rivers and by using proxy indicators of pre‐alteration conditions, and (iv) establishing process thresholds that must be exceeded to maintain form (e.g. flow thresholds to mobilize bed sediment). Once this context has been established, resource managers can better evaluate the options for restoring altered riverine form and function. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Climate variability and projected change in the western United States: regional downscaling and drought statistics

Climate change in the twenty-first century, projected by a large ensemble average of global coupled models forced by a mid-range (A1B) radiative forcing scenario, is downscaled to Climate Divisions across the western United States. A simple empirical downscaling technique is employed, involving model-projected linear trends in temperature or precipitation superimposed onto a repetition of observed twentieth century interannual variability. This procedure allows the projected trends to be assessed in terms of historical climate variability. The linear trend assumption provides a very close approximation to the time evolution of the ensemble-average climate change, while the imposition of repeated interannual variability is probably conservative. These assumptions are very transparent, so the scenario is simple to understand and can provide a useful baseline assumption for other scenarios that may incorporate more sophisticated empirical or dynamical downscaling techniques. Projected temperature trends in some areas of the western US extend beyond the twentieth century historical range of variability (HRV) of seasonal averages, especially in summer, whereas precipitation trends are relatively much smaller, remaining within the HRV. Temperature and precipitation scenarios are used to generate Division-scale projections of the monthly palmer drought severity index (PDSI) across the western US through the twenty-first century, using the twentieth century as a baseline. The PDSI is a commonly used metric designed to describe drought in terms of the local surface water balance. Consistent with previous studies, the PDSI trends imply that the higher evaporation rates associated with positive temperature trends exacerbate the severity and extent of drought in the semi-arid West. Comparison of twentieth century historical droughts with projected twenty-first century droughts (based on the prescribed repetition of twentieth century interannual variability) shows that the projected trend toward warmer temperatures inhibits recovery from droughts caused by decade-scale precipitation deficits.