Colorado Front Range Research Articles

Geomorphic characteristics influencing post-fire river response in mountain streams

As wildfires increase in frequency and severity, there is a growing interest in understanding river response to the wildfire disturbance cascade. Numerous headwater mountain catchments within the Cache la Poudre (Poudre) River basin in the Colorado Front Range, USA burned severely and extensively during the 2020 Cameron Peak fire. Debris flows and flash floods occurred in many of these catchments triggered by convective storms after the fire. The downstream effects of the floods and sediment varied along a continuum from attenuated and largely contained within the catchment, to releasing substantial volumes of water and sediment to the Poudre River. We conceptualize these catchments as exhibiting decreasing absorbance of post-fire disturbance along the continuum described above based on the geomorphic evidence of relative sediment export. We conceptualize characteristics on different spatial scales as driving or resisting response to disturbance and therefore impacting the absorbing capacity (ability to attenuate post-fire fluxes) of the catchment. As the magnitude of resisting characteristics increases at the catchment, inter- and intra- reach scales, we hypothesize that a catchment will increasingly absorb the impacts of the wildfire disturbance cascade. We conducted longitudinally continuous surveys to measure reach-scale characteristics within each study catchment. We focus on the catchment- and reach-scale geomorphic, vegetation, and burn characteristics. The floods observed at the study catchments illustrate fire influencing the elevation above which rainfall-induced flooding occurs due to the efficient conveyance of water from hillslopes to channels after wildfire. Results suggest that inter- and intra-reach spatial heterogeneity are better aligned with absorbance capacity than catchment-scale characteristics: greater absorbing capacity is associated with greater longitudinal variations in floodplain/channel width and more reaches with wide floodplains, low channel gradients, beaver-modified topography, and multi-stem deciduous vegetation. We connect the capacity to absorb the impacts of disturbance as informing the catchment response to wildfire disturbance.

Downslope Wind Verification of the National Blend of Models v4.0 Across the Northern Front Range of Colorado During the 2020/2021 Cool Season

Downslope Wind Verification of the National Blend of Models v4.0 Across the Northern Front Range of Colorado During the 2020/2021 Cool Season

Quantifying Aspect‐Dependent Snowpack Response to High‐Elevation Wildfire in the Southern Rocky Mountains

AbstractIncreasing wildfire frequency and severity in high‐elevation seasonal snow zones presents a considerable water resource management challenge across the western United States (U.S.). Wildfires can affect snowpack accumulation and melt patterns, altering the quantity and timing of runoff. While prior research has shown that wildfire generally increases snow melt rates and advances snow disappearance dates, uncertainties remain regarding variations across complex terrain and the energy balance between burned and unburned areas. Utilizing paired in situ data sources within the 2020 Cameron Peak burn area on the Front Range of Colorado, U.S., during the 2021–2022 winter, we found no significant difference in peak snow water equivalent (SWE) magnitude between burned and unburned areas. However, the burned south aspect reached peak SWE 22 days earlier than burned north. During the ablation period, burned south melt rates were 71% faster than unburned south melt rates, whereas burned north melt rates were 94% faster than unburned north aspects. Snow disappeared 7–11 days earlier in burned areas than unburned areas. Net energy differences at the burned and unburned weather station sites were seasonally variable, the burned area snowpack lost more net energy during the winter, but gained more net energy during the spring. Increased incoming shortwave radiation at the burned site was 6x more impactful in altering the net shortwave radiation balance than the decline in surface albedo. These findings emphasize the need for post‐wildfire water resource planning that accounts for aspect‐dependent differences in energy and mass balance to accurately predict snowpack storage and runoff timing.

Landslide susceptibility mapping using physics-guided machine learning: a case study of a debris flow event in Colorado Front Range

Landslide susceptibility mapping using physics-guided machine learning: a case study of a debris flow event in Colorado Front Range

Defining the Pathobiomes Associated with Drippy Blight in Colorado and Drippy Nut in California

Drippy blight, an emergent bacterial disease of oaks, was described recently from urban oaks in the Front Range of Colorado, USA. This disease, which causes branch dieback and oozing of bacterial exudates from cankers, is caused by Lonsdalea quercina and primarily affects red oaks, with Northern red oak (Quercus rubra) the most susceptible. Drippy nut is a similar, less acute condition that exists in California, USA and affects acorns of interior live oak (Quercus wislizenii) and coastal live oak (Quercus agrifolia). The objective of this study was to compare the microbial communities of drippy blight in Colorado to those of drippy nut in California, and determine if other pathogenic fungal or bacterial taxa are associated with the diseases. Symptomatic and asymptomatic tissues were sampled in California and Colorado, and metagenomic analyses were performed to describe the fungal and bacterial communities. We found a suite of bacterial species, dominated by taxa within the genus Zymobacter, that were associated with symptomatic tissues for both drippy nut and drippy blight. L. quercina is the main pathogenic associate of both diseases. We found fungal yeasts associated with symptomatic tissues, and although not pathogens, these taxa were present in the pathobiome of drippy nut and drippy blight in California and Colorado. We did, however, identify taxa belonging to Gnomoniopsis, a known pathogen, associated with California symptomatic samples. Our study found an overlapping suite of bacterial associates of drippy blight and drippy nut, though more variation occurred within the fungal genera associated with these oak diseases.

An ichnotaxonomic assessment of the Cretaceous Dakota Group, Front Range, Colorado, USA, and its comparison to other Western interior seaway deposits

The Aptian-Cenomanian Dakota Group along the Colorado Front Range is known for its dinosaur tracks; however, it also contains extensive invertebrate marine and continental trace fossils. The Dakota Group in Colorado is subdivided into the Lytle, Plainview, Glencairn (Cañon City), Skull Creek Shale (Denver, Fort Collins), and Muddy formations. Thirty-two invertebrate ichnogenera and 34 ichnospecies were identified: Archaeonassa, Arenicolites, Asterosoma, Asthenopodichnium, Aulichnites, Chondrites, Cochlichnus, Conichnus, Cruziana, Cylindrichnus, Diplocraterion, Gyrolithes, Lockeia, Macaronichnus, Margaritichnus, Naktodemasis, Ophiomorpha, Palaeophycus, Planolites, Protovirgularia, Rhizocorallium, Rosselia, Rusophycus, Schaubcylindrichnus, Scolicia, Skolithos, Taenidium, Teichichnus, Teredolites, Thalassinoides, Treptichnus, and Zoophycos. Six tetrapod ichnogenera and three ichnospecies were identified: Caririchnium, Dromaeosauripus, Hatcherichnus, Ostendichnus, Magnoavipes, and Tetrapodosaurus. Three tetrapod ichnogenera Chelonipus, Ignotornis, and Mehliella have been reported at other sites in the Dakota Group, but none were found at our study sites. Rhizohaloes are reported in the Muddy Formation. Three vertebrate ichnogenera and two ichnospecies were previously described elsewhere by other workers. The Dakota Group trace fossils comprise eight ichnocoenoses–Caririchnium, Diplocraterion, Lockeia, Naktodemasis, Rhizohalo, Scolicia, Skolithos-Teichichnus, and Zoophycos–representing dwelling, deposit- and filter-feeding, and locomotion behaviors of plants, invertebrates, and tetrapods. Twelve previously unreported ichnogenera were identified: Archaeonassa, Asthenopodichnium, Conichnus, Cylindrichnus, Gyrolithes, Macaronichnus, Naktodemasis, Protovirgularia, Rusophycus, Taenidium, Treptichnus, and Zoophycos. This study is the first systematic ichnotaxonomic assessment of the invertebrate trace fossils of the Dakota Group along the Colorado Front Range, which has the highest reported ichnodiversity when compared to other Western Interior Seaway deposits.

Inorganic Nitrogen Gas‐Aerosol Partitioning in and Around Animal Feeding Operations in Northeastern Colorado in Late Summer 2021

AbstractAmmonia (NH3) from animal feeding operations (AFOs) is an important source of reactive nitrogen in the US, but despite its ramifications for air quality and ecosystem health, its near‐source evolution remains understudied. To this end, Phase I of the Transport and Transformation of Ammonia (TRANS2Am) field campaign was conducted in the northeastern Colorado Front Range in summer 2021 and characterized atmospheric composition downwind of AFOs during 10 research flights. Airborne measurements of NH3, nitric acid (HNO3), and a suite of water‐soluble aerosol species collected onboard the University of Wyoming King Air research aircraft present an opportunity to investigate the sensitivity of particulate matter (PM) formation to AFO emissions. We couple the observations with thermodynamic modeling to predict the seasonality of ammonium nitrate (NH4NO3) formation. We find that during TRANS2Am northeastern Colorado is consistently in the NH3‐rich and HNO3‐limited NH4NO3 formation regime. Further investigation using the Extended Aerosol Inorganics Model reveals that summertime temperatures (mean: 23°C) of northeastern Colorado, especially near the surface, inhibit NH4NO3 formation despite high NH3 concentrations (max: ≤114 ppbv). Finally, we model spring/autumn and winter conditions to explore the seasonality of NH4NO3 formation and find that cooler temperatures could support substantially more NH4NO3 formation. Whereas NH4NO3 only exceeds 1 μg m−3 ∼10% of the time in summer, modeled NH4NO3 would exceed 1 μg m−3 61% (88%) of the time in spring/autumn (winter), with a 10°C (20°C) temperature decrease relative to the campaign.

Using Financial Contracts to Facilitate Informal Leases Within a Western United States Water Market Based on Prior Appropriation

AbstractThe ability to reallocate water to higher‐value uses during drought is an increasingly important “soft‐path” tool for managing water resources in an uncertain future. In most of the Western United States, state‐level water market institutions that enable reallocation also impose substantial transaction costs on market participants related to regulatory approval and litigation. These transaction costs can be prohibitive for many participants in terms of both costs and lengthy approval periods, limiting transfers and reducing allocation efficiency, particularly during drought crises periods. This manuscript describes a mechanism to reduce transaction costs by adapting an existing form of informal leases to facilitate quicker and less expensive transfers among market participants. Instead of navigating the formal approval process to lease a water right, informal leases are financial contracts for conservation that enable more junior holders of existing rights to divert water during drought, thereby allowing the formal transfer approval process to be bypassed. The informal leasing approach is tested in the Upper Colorado River Basin, where drought and institutional barriers to transfers lead to frequent shortages for urban rights holders along Colorado's Front Range. Informal leases are facilitated via option contracts that include adaptive triggers and that define volumes of additional, compensatory, releases designed to mitigate impacts to instream flows and third parties. Results suggest that more rapid reallocation of water via informal leases could have resulted in up to $222 million in additional benefits for urban rights holders during the historical period 1950–2013.

Creating Boundary Objects Supports Knowledge Co-development Processes: A Case Study Evaluation from the Colorado Front Range

Abstract This qualitative case study evaluates manager and researcher perceptions of the impact of a place-based, collaborative knowledge co-development process and examines the outcomes of that co-development for changes to management approaches. The USDA Forest Service (Forest Service) Rocky Mountain Research Station General Technical Report 373 (GTR-373) is a codeveloped science synthesis that functions as a boundary object providing a framework for planning, designing, and implementing management action for restoration of ponderosa and dry mixed-conifer forests. The process of creating and socializing the GTR-373 framework fostered continual knowledge exchange and engagement between researchers and managers across different organizations and levels of decision-making. This built trust in the information, improved justification for management action, developed a common foundation for cross-boundary implementation, and increased communication. The framework has been applied across jurisdictions and has been used as a foundational tool for training staff and designing projects. However, adapting the GTR-373 framework across scales remains challenging. Study Implications: This qualitative case study evaluates a co-development process in which researchers and managers from multiple organizations and agencies collaborated to produce a science-informed restoration framework to support forest management on the Colorado Front Range (GTR-373). The process built trust, improved justification for management action, developed a common foundation for implementation, and increased communication. However, cross-boundary management across spatial scales remains challenging, and managers interpret information through organizational mandates and site-specific context. Sustained collaboration between researchers and land managers can help make science actionable and relevant at the appropriate scale for planning and management across expertise and jurisdictional boundaries.

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.

Stable Isotopes Constrain Water Seepage From Gnammas Into Bare Granitic Bedrock

AbstractSubtle bedrock depressions called gnammas allow the water in ephemeral pools to maintain contact with bare rock, thus serving as natural rock‐weathering experiments. Following filling by precipitation, evaporation is often assumed to be the sole process of water loss from gnammas. We evaluated this assumption by monitoring evolving stable isotope compositions of gnamma waters hosted in granite of Colorado's Front Range. Surprisingly, we found that a significant fraction of the water was lost by seepage through the underlying bedrock. Seepage dominated, with only 10%–20% loss by evaporation. We propose a conceptual model of gnamma formation in which enhanced weathering of the bedrock beneath the gnamma increases the water holding capacity and permeability of the underlying rock that in turn promotes efficient water loss through seepage and further weathering of the surrounding rock. This model has implications for bare‐rock weathering and hence the evolution of landscapes over geologic timescales.

Water‐rock interactions drive chemostasis

AbstractThe western U.S. is experiencing shifts in recharge due to climate change, and it is currently unclear how hydrologic shifts will impact geochemical weathering and stream concentration–discharge (C–Q) patterns. Hydrologists often use C–Q analyses to assess feedbacks between stream discharge and geochemistry, given abundant stream discharge and chemistry data. Chemostasis is commonly observed, indicating that geochemical controls, rather than changes in discharge, are shaping stream C–Q patterns. However, few C–Q studies investigate how geochemical reactions evolve along groundwater flowpaths before groundwater contributes to streamflow, resulting in potential omission of important C–Q controls such as coupled mineral dissolution and clay precipitation and subsequent cation exchange. Here, we use field observations—including groundwater age, stream discharge, and stream and groundwater chemistry—to analyse C–Q relations in the Manitou Experimental Forest in the Colorado Front Range, USA, a site where chemostasis is observed. We combine field data with laboratory analyses of whole rock and clay x‐ray diffraction and soil cation‐extraction experiments to investigate the role that clays play in influencing stream chemistry. We use Geochemist's Workbench to identify geochemical reactions driving stream chemistry and subsequently suggest how climate change will impact stream C–Q trends. We show that as groundwater age increases, C–Q slope and stream solute response are not impacted. Instead, primary mineral dissolution and subsequent clay precipitation drive strong chemostasis for silica and aluminium and enable cation exchange that buffers calcium and magnesium concentrations, leading to weak chemostatic behaviour for divalent cations. The influence of clays on stream C–Q highlights the importance of delineating geochemical controls along flowpaths, as upgradient mineral dissolution and clay precipitation enable downgradient cation exchange. Our results suggest that geochemical reactions will not be impacted by future decreasing flows, and thus where chemostasis currently exists, it will continue to persist despite changes in recharge.

Validation of MUSES NH3 observations from AIRS and CrIS against aircraft measurements from DISCOVER-AQ and a surface network in the Magic Valley

Abstract. Ammonia is a significant precursor of PM2.5 particles and thus contributes to poor air quality in many regions. Furthermore, ammonia concentrations are rising due to the increase of large-scale, intensive agricultural activities, which are often accompanied by greater use of fertilizers and concentrated animal feedlots. Ammonia is highly reactive and thus highly variable and difficult to measure. Satellite-based instruments, such as the Atmospheric Infrared Sounder (AIRS) and the Cross-Track Infrared Sounder (CrIS), have been shown to provide much greater temporal and spatial coverage of ammonia distribution and variability than is possible with in situ networks or aircraft campaigns, but the validation of these data is limited. Here we evaluate MUSES (multi-spectra, multi-species, multi-sensors) ammonia retrievals from AIRS and CrIS against ammonia measurements from aircraft in the California Central Valley and in the Colorado Front Range. These are small datasets taken over high-source regions under very different conditions: winter in California and summer in Colorado. Direct comparisons of the surface values of the retrieved profiles are biased very low in California (∼ 40 ppbv) and slightly high in Colorado (∼ 4 ppbv). This bias appears to be primarily due to smoothing error, since applying the instrument operator effectively reduces the bias to zero; even after the smoothing error is accounted for, the average uncertainty at the surface is in the 20 %–30 % range. We also compare 3 years of CrIS ammonia against an in situ network in the Magic Valley in Idaho We show that CrIS ammonia captures both the seasonal signal and the spatial variability in the Magic Valley, although it is biased low here also. In summary, this analysis substantially adds to the validation record but also points to the need for more validation under many different conditions and at higher altitudes.

Glacial Cover Assessment from 1993 to 2021 Using Normalized Difference Snow Index with Landsat Imagery in the Colorado Front Range, USA

Glacial Cover Assessment from 1993 to 2021 Using Normalized Difference Snow Index with Landsat Imagery in the Colorado Front Range, USA

The edge of a Permian erg: Eolian facies and provenance of the Lyons Sandstone in northern Colorado

ABSTRACT Facies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2, in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km2, and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.

The 30 December 2021 Colorado Front Range windstorm and Marshall Fire: Evolution of surface and 3-d structure, NWP guidance, NWS forecasts and decision support

Abstract The Marshall Fire on 30 December 2021 became the most destructive wildfire cost-wise in Colorado history as it evolved into a suburban firestorm in southeastern Boulder County, driven by strong winds and a snow-free and drought-influenced fuel state. The fire was driven by a strong downslope windstorm that maintained its intensity for nearly eleven hours. The southward movement of a large-scale jet axis across Boulder County brought a quick transition that day into a zone of upper-level descent, enhancing the mid-level inversion providing a favorable environment for an amplifying downstream mountain wave. In several aspects, this windstorm did not follow typical downslope windstorm behavior. NOAA rapidly updating numerical weather prediction guidance (including the High-Resolution Rapid Refresh) provided operationally useful forecasts of the windstorm, leading to the issuance of a high-wind warning (HWW) for eastern Boulder County. No Red Flag Warning was issued due to a too restrictive relative humidity criterion (already published alternatives are recommended); however, owing to the HWW, a county-wide burn ban was issued for that day. Consideration of spatial (vertical and horizontal) and temporal (both valid time and initialization time) neighborhoods allows some quantification of forecast uncertainty from deterministic forecasts – important in real-time use for forecasting and public warnings of extreme events. Essentially, dimensions of the deterministic model were used to roughly estimate an ensemble forecast. These dimensions including run-to-run consistency are also important for subsequent evaluation of forecasts for small-scale features such as downslope windstorms and the tropospheric features responsible for them, similar to forecasts of deep, moist convection and related severe weather.

Geomorphic complexity influences coarse particulate organic matter transport and storage in headwater streams

Coarse particulate organic matter (CPOM; organic matter 1–100 mm in diameter, excluding small wood) stored in streams provides an important energy source for aquatic ecosystems, and CPOM transport provides downstream energy subsidies and is a pathway for watershed carbon export. However, we lack understanding of the magnitude of and processes influencing CPOM storage and transport in headwater streams. We assessed how geomorphic complexity and hydrologic regime influence CPOM transport and storage in the Colorado Front Range, USA. We compared CPOM transport during snowmelt in a stream reach with high retentive feature (e.g., wood, cobbles, and other features) frequency to a reach with low retentive feature frequency, assessing how within-a-reach geomorphic context influences CPOM transport. We also compared CPOM transport in reaches with differing valley geometry (two confined reaches versus a wide, multi-thread river bead) to assess the influence of geomorphic variations occurring over larger spatial extents. Additionally, we compared CPOM storage in accumulations in reaches (n = 14) with flowing water or dry conditions in late summer and investigated how small pieces of organic matter [e.g., woody CPOM and small wood (>1 min length and 0.05–1 min diameter or 0.5–1 min length and >0.1 min diameter)] influence CPOM storage. We found that within-a-reach retentive feature frequency did not influence CPOM transport. However, valley geometry influenced CPOM transport, with a higher CPOM transport rate (median: 1.53 g min−1) downstream of a confined stream reach and a lower CPOM transport rate (median: 0.13 g min−1) downstream of a low gradient, multi-thread river bead. Additionally, we found that particulate organic carbon (POC) export (0.063 Mg C) in the form of CPOM was substantially lower than dissolved organic carbon (DOC) export (12.3 Mg C) in one of these headwater streams during the 2022 water year. Dry reaches stored a higher volume of CPOM (mean = 29.18 m3 ha−1) compared to reaches with flowing water (15.75 m3 ha−1), and woody CPOM pieces trapped 37% of CPOM accumulations. Our results demonstrate that the influence of geomorphic context on CPOM transport depends on the scale and type of geomorphic complexity, POC may be lower than DOC export in some headwater streams, and small woody organic material is important for trapping CPOM small streams.

Mechanical forest restoration treatments stimulate understory plants in the Colorado Front Range

Restoration efforts are underway in dry conifer forests across the western United States to increase their resilience to wildfire and other disturbances. Because such treatments typically decrease overstory density and homogeneity, they can also drive changes in the understory plant community. Past studies of post-treatment changes in understories have found variable results over short time frames and across regions, highlighting the need to study longer-term, region-specific responses. We investigated whether mechanical restoration treatments benefited understory plants in dry conifer forests of the Colorado Front Range, and what biotic and abiotic variables modified understory plant responses in treated areas. We analyzed data collected 1–2 years pre-treatment, 1–2 years post-treatment, and 4–6 years post-treatment in 168 plots, which were distributed across 8 sites and 16 pairs of treated and nearby untreated areas. Treatments were implemented by removing trees with heavy machinery or hand tools (i.e., thinning). By the time treatments were 4–6 years old, native understory plant cover was 1.7 times higher in treated compared to untreated plots, and native richness was 1.1 times higher. Heightened cover and richness values in treated plots were not driven by a single native plant functional group, but by a large portion of the native community; long-lived, graminoid, vegetatively spreading, and non-vegetatively spreading plants all had higher cover in treated plots, while short-lived, long-lived, forb, graminoid, and non-vegetatively spreading plants had higher richness in treated plots. Non-native plants showed 3.1 times higher cover and 4.4 times higher richness in treated compared to untreated plots at 4–6 years post-treatment, but were present at very low levels (e.g., ≤ 0.5% mean cover in either treatment). Greater native plant cover and richness at 4–6 years post-treatment were associated with lower overstory basal areas that resembled 19th-century forest structural conditions for the landscape. Contrary to expectations, a long-term measure of moisture availability (i.e., 30-year average climatic water deficit) was not a strong predictor of native cover or richness in treated plots 4–6 years post-treatment; rather, they were better predicted by moisture availability during the spring months prior to sampling. Overall, the consistent and enduring stimulation of cover and richness of native understory plants after mechanical treatments, with only limited invasion from non-native species, illustrates the important benefits of ongoing restoration activities in dry conifer forests of the Colorado Front Range.

Chronology and erosion rate of the Pinedale glaciation, Colorado Front Range (USA), inferred from the sedimentary record of glacial Lake Devlin

Glacial and periglacial sediments and landforms record the chronology of glaciation and amount of Pleistocene erosion during colder periods that added substantially to global sediment budgets and contributed to the global CO2 cycle. The now-drained glacial Lake Devlin, dammed in a Front Range tributary valley by a glacier in the North Branch of Boulder Creek (Colorado, USA) preserves an important sedimentary archive of the ca. 32−14 ka Pinedale glaciation, recording both paleoclimate information and an integrated measure of glacial and periglacial erosion rates over a full glacial cycle. Despite rapid erosion of fine-grained deposits after the lake drained, most sediment generated during Pinedale time remains as legacy deposits in the catchment. Geomorphic evidence and dating of glaciolacustrine sediment from surface exposures demonstrate that the ca. 30 ka Pinedale glacial advance was nearly as extensive as the local Late Glacial Maximum at ca. 20 ka. Sedimentary archives dated by 14C, optically stimulated luminescence, and cosmogenic nuclides extend earlier studies (Madole et al., 1973) of pollen and magnetic susceptibility (MS) in cores from the glaciolacustrine deposits of Lake Devlin and of Pinedale climate. Records suggest short-term warming and biotic change at ca. 15 ka after ∼14 kyr of cold, dry conditions punctuated by MS peaks at ca. 26.5 ka, 20 ka, and 16.5 ka. Lake Devlin drained catastrophically after ca. 14 ka, millennia after ice had retreated upvalley from the lateral moraine that dammed the lake. Sediment production during the Pinedale was equivalent to a periglacial and glacial erosion rate of ∼70 mm kyr−1, several times higher than long-term rates in the adjacent Front Range, but much lower than rates measured where modern glaciers are eroding weak bedrock in zones of rapid rock uplift, such as SSE Alaska, USA. Data from the Lake Devlin basin contribute to contemporary discussions of how glacial erosion influences the global CO2 cycle.

Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

AbstractAncient magnetization(s), often recorded by hematite (Fe2O3), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations.