Southern Rocky Research Articles

Short‐term effects of wildfire on high elevation stream‐riparian food webs

Understanding how wildfires affect food web structure and function remains an important challenge, especially at high elevations that historically have burned infrequently. In particular, fires may alter the magnitude of reciprocal cross‐ecosystem subsidies, leading to indirect effects on aquatic and terrestrial consumers. We quantified characteristics of high‐elevation (2500–3000 m) stream‐riparian food webs at 10 locations in the southern Rocky Mountains less than one year following high‐intensity, stand‐replacing wildfires. Using a paired ‘burned−unburned' stream survey design, we assessed benthic periphyton, aquatic macroinvertebrate community structure, trout population characteristics, trout stomach contents, inputs and emergence of insects to and from streams, and abundance of predatory riparian spiders that consume aquatic insects. Benthic macroinvertebrate density, flux of emerging aquatic insects, and riparian spider abundances were lower at burned sites. Fluxes of insect inputs entering the stream did not differ with burn status, despite the loss of riparian vegetation due to fire. Trout were somewhat less abundant, but larger on average at burned sites and did not differ in body condition. These results suggest mortality of smaller trout from fire disturbance and/or recolonization of burned sites by larger individuals. Trout showed subtle changes in diet composition with burn status, but no change in biomass or number of prey consumed. In general, burned sites showed greater variation in community characteristics than unburned sites, which may reflect differences in the timing and magnitude of post‐fire flooding, erosion, and scouring of the stream bed. Taken together, our results suggest that short‐term effects of fire disturbance strongly altered some food web responses, but others appeared relatively resilient, which is notable given the high severity of the wildfires in the study area.

Optimal management decisions are robust to unknown dynamics in an amphibian metapopulation plagued by disease

AbstractIdentifying conservation actions to recover threatened species can be challenging due to many ecological uncertainties. For example, major threats to a species' conservation are commonly known or suspected, but the specific impacts on population or metapopulation dynamics can be uncertain. This is frequently the case with emerging infectious diseases, including chytridiomycosis, a global driver of amphibian population declines caused by the fungal pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans. While these diseases are known to cause amphibian declines and extirpations, the mechanisms of their landscape‐scale spread are still largely unknown. Such uncertainty can lead to inaction which may jeopardize timely recovery of a species. Decision analysis is a pragmatic approach to making transparent and defensible decisions while dealing with uncertainties. We investigated whether optimal actions aimed at recovering boreal toad (Anaxyrus boreas boreas) metapopulations in the southern Rocky Mountains are robust to the unknown dynamics of Bd spread using value of information and regret analyses. Value of information is a decision‐analytic tool for calculating the value of new information in terms of performance on management objectives, while regret measures the cost of acting under incorrect information. We further conducted a stochastic sensitivity analysis to identify the relative effects of metapopulation parameters on system dynamics. We found optimal actions were robust to the unknown dynamics of Bd spread. While boreal toad breeding occurrence is highly sensitive to Bd distribution, the optimal decision is not. Resolving the unknown dynamics of Bd spread would lead to a minimal gain of less than one breeding toad subpopulation at the end of 50 years, given the currently available management actions. Applying a decision‐analytic framework coupled with value of information and regret analyses can help frame how uncertainties affect decisions in a way that empowers decision makers.

Regional-scale resistivity structure of the middle and lower crust and uppermost mantle beneath the southeastern Canadian Cordillera and insights into its causes

SUMMARYSubduction zones are recognized as an important class of plate boundaries and are the location of a number of important geological processes. They are also important because of the mineral and geothermal energy resources formed by plate convergence. While subduction zones around the world have a number of common features, there are also significant differences among them. The Cascadia subduction zone in southern British Columbia is characterized by a relatively hot subducting plate, and a broad backarc region that is believed to exhibit a shallow, convecting asthenosphere. The magnetotelluric (MT) method is a useful tool to study subduction zones and backarc regions because measurements of subsurface resistivity are sensitive to the presence of fluids. A number of previous MT studies have taken place in this region, but they were limited to a 2-D approach to data analysis. As the MT method has developed, it has become clear that there is a significant advantage to using a 3-D approach to data analysis. This paper presents the first regional-scale 3-D resistivity model of the southern Canadian Cordillera and provides new insights into the lithospheric structure and the distribution of fluids. The southeastern Canadian Cordillera has high heat flow and numerous thermal springs, the locations of which are often controlled by faults. However, the deeper thermal structure and origin of the fluids are poorly understood. To develop an improved understanding of the structure of this area, MT data measured at 331 locations were used to create a 3-D model of subsurface electrical resistivity. This study is primarily focused on the Omineca and Foreland morphogeological belts in southeastern British Columbia, which are separated by the southern Rocky Mountain Trench. The resistivity model is presented to a depth of 100 km and a number of conductive features are observed in the crust and uppermost mantle of the southeastern Cordillera. The locations of these conductors broadly matched previously reported conductors, but the 3-D inversion revealed new details of their geometry. The previously reported Canadian Cordilleran Regional conductor was modelled as a number of discrete conductors in the depth range 15–55 km beneath the Omineca belt. Temperatures approximately in the range 400–700 °C are expected at depths of 15–26 km and saline aqueous fluids are likely the cause of the low resistivity. Temperatures approximately in the range 700–1300 °C are expected at depths of 26–55 km and small volumes of partial melt may explain the low resistivity. The Southern Alberta–British Columbia conductor, Red Deer conductor and Loverna conductor were imaged as a single connected conductor, whose low resistivity is likely caused by sulphide mineralization. A group of conductors was imaged near the southern Rocky Mountain Trench in the depth range 10–70 km and their low resistivity is likely caused by interconnected saline fluids and possibly interconnected graphite films. To understand if the distribution of thermal springs was correlated with the 3-D resistivity model, a statistical study was undertaken. This showed no clear correlation between crustal conductance and the distribution of thermal springs.

Model & Data Files for Analyses in "Differential impacts of spruce beetle outbreaks on snowshoe hares and red squirrels in the southern Rocky Mountains"

Model & Data Files for Analyses in "Differential impacts of spruce beetle outbreaks on snowshoe hares and red squirrels in the southern Rocky Mountains"

Bark beetle impacts on forest evapotranspiration and its partitioning

Insect outbreaks affect forest structure and function and represent a major category of forest disturbance globally. However, the resulting impacts on evapotranspiration (ET), and especially hydrological partitioning between the abiotic (evaporation) and biotic (transpiration) components of total ET, are not well constrained. As a result, we combined remote sensing, eddy covariance, and hydrological modeling approaches to determine the effects of bark beetle outbreak on ET and its partitioning at multiple scales throughout the Southern Rocky Mountain Ecoregion (SRME), USA. At the eddy covariance measurement scale, 85 % of the forest was affected by beetles, and water year ET as a fraction of precipitation (P) decreased by 30 % relative to a control site, with 31 % greater reductions in growing season transpiration relative to total ET. At the ecoregion scale, satellite remote sensing masked to areas of >80 % tree mortality showed corresponding ET/P reductions of 9–15 % that occurred 6–8 years post-disturbance, and indicated that the majority of the total reduction occurred during the growing season; the Variable Infiltration Capacity hydrological model showed an associated 9–18 % increase in the ecoregion runoff ratio. Long-term (16–18 year) ET and vegetation mortality datasets extend the length of previously published analyses and allowed for clear characterization of the forest recovery period. During that time, transpiration recovery outpaced total ET recovery, which was lagged in part due to persistently reduced winter sublimation, and there was associated evidence of increasing late summer vegetation moisture stress. Overall, comparison of three independent methods and two partitioning approaches demonstrated a net negative impact of bark beetles on ET, and a relatively greater negative impact on transpiration, following bark beetle outbreak in the SRME.

Valve Morphology and Ultrastructure of the Enigmatic Diatom Cymbopleura semirecta (Manguin ex Kociolek & Reviers) Bahls

We report the occurrence of the rare and enigmatic diatom species Cymbopleura semirecta (Manguin ex Kociolek & Reviers) Bahls here for the first time from the southern Rocky Mountains. We review the current systematics and historic taxonomic placement of this taxon and its placement in both Cymbella and Cymbopleura. Valve morphology and ultrastructure are presented based on light (LM) and scanning electron microscope (SEM)_investigations. LM and SEM observations show this species lacks apical pore fields, a key feature helping to definitively place this taxon in Cymbopleura. We discuss the distribution of this species in higher latitude freshwater environments.

Prescribed fire is associated with increased floral richness and promotes short‐term increases in bee biodiversity in the ponderosa pine forest of the Southern Rocky Mountains

Abstract Managed low‐severity surface fires are frequently implemented in efforts to restore disturbance processes to forests of North America; although the effects of managed fire on forest structure are well‐studied, few studies investigate whether these disturbances cascade to impact pollinator communities. We analysed bee‐habitat relationships in fire‐treated (1‐ and 3‐years post‐treatment) and non‐treated ponderosa pine stands in Colorado to test wild bee population responses. Observed bee richness and α‐diversity were highest in stands 1‐year post‐fire and had more Anthophora, Bombus, Osmia and Lasioglossum spp. in comparison to 3‐year post‐fire and non‐treated stands. Bee functional groups were responsive to treatments, with more below‐ground nesting taxa present in stands 3 years post‐fire. Floral richness was the highest mid‐growing season (June, July) and within 1‐year post‐fire stands. A model analysing the effects of foraging and nesting habitat variation on bee assemblages indicated positive association between floral richness and bee α‐diversity, but negative relationships with stand basal area. Nesting habitat was not associated with variation in bee assemblages. We conclude that managed fire has positive short‐term effects on bee biodiversity that are likely mediated by floral richness. However, these effects were not detectable by 3 years post‐treatment in the southern Rocky Mountain region.

Xylem Traumatic Resin Duct Formation in Response to Stem Fungal Inoculation in Douglas-Fir and Lodgepole Pine

Xylem traumatic resin ducts (TRDs) in Douglas-fir form in response to mechanical injury, fire, and root pathogens, but it is unknown if these form at the stem in response to bark-beetle-associated fungi. Meanwhile, TRDs are rarely documented in lodgepole pine. In the southern Rocky Mountains, TRD formation in the two species from sterile (Control) and fungal inoculation treatments (Aggressive, Weak (Douglas-fir only)) were compared; predicting the following: (1) both trees would produce TRDs in response to fungal treatments; (2) in Douglas-fir, Aggressive treatment would promote denser and larger TRDs than Weak or Control treatments; and (3) interspecifically, Douglas-fir would produce a higher density of TRDs than lodgepole pine in Aggressive treatments. Two months post-treatment, the position of TRDs indicated these were only induced on all Douglas-fir treatments. Aggressive and Weak treatments had similar responses, except a second TRD line formed in two Douglas-fir Aggressive treatments. Douglas-fir produced >7× more resin ducts that were twice the size of those in lodgepole pine. Douglas-fir’s stronger induced response indicates better resistance traits against bark beetle fungal associate colonization. Understanding the characteristics of TRD produced in reaction to specific damage in Douglas-fir can improve past disturbance reconstructions and explain interspecific tree response differences conducive to bark beetle resistance.

Overlapping outbreaks of multiple bark beetle species are rarely more severe than single‐species outbreaks

AbstractBiotic disturbances that overlap in space and time may result in important shifts in forest structure and composition, with potential effects on many ecosystem services. Starting in the late 1990s, outbreaks of multiple bark beetle species caused widespread mortality of three co‐occurring conifer species in the ca. 40,000‐km2 subalpine zone of the southern Rocky Mountains (SRM), USA. To better understand the implications of such outbreaks, our goal was to determine if overlapping outbreaks of multiple bark beetle species caused greater tree mortality than single‐species outbreaks in stands with multiple susceptible host tree species. We mapped stand susceptibility to outbreaks of spruce beetle (SB, Dendroctonus rufipennis), mountain pine beetle (MPB, Dendroctonus ponderosae), and western balsam bark beetle (WBBB, Dryocoetes confusus) by combining aerial survey data and forest composition variables in a random forest modeling framework. Then, we used existing maps of cumulative forest mortality from bark beetles to investigate the extent and severity of overlapping outbreaks from 1999 to 2019. We found that 46% of stands with two or more of the three studied hosts species—Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta var. latifolia), or subalpine fir (Abies lasiocarpa)—were susceptible to overlapping outbreaks (25% of all sampled stands). Of those stands, 31% experienced outbreaks of two or more beetle species. Stands affected by outbreaks of both MPB and SB had higher tree mortality than stands affected by one species alone, though stands susceptible to both MPB and SB were uncommon (<4% of all sampled stands). No other combinations of beetle outbreaks increased tree mortality above levels caused by single‐species outbreaks. Thus, contrary to expectations, overlapping outbreaks were rarely more severe than single‐species outbreaks in the SRM. This suggests that diverse forest communities may buffer against the most severe effects of bark beetle outbreaks, even during warm, dry conditions.

Changes in wildfire occurrence and risk to homes from 1990 through 2019 in the Southern Rocky Mountains,USA

AbstractWildfires and housing development have increased since the 1990s, presenting unique challenges for wildfire management. However, it is unclear how the relative influences of housing growth and changing wildfire occurrence have altered risk to homes, or the potential for wildfire to threaten homes. We used a random forests model to predict burn probability in relation to weather variables at 1‐km resolution and monthly intervals from 1990 through 2019 in the Southern Rocky Mountains ecoregion. We quantified risk by combining the predicted burn probabilities with decadal housing density. We then compared the predicted burn probabilities and risk across the study area with observed values and quantified trends. Finally, we evaluated how housing growth and changes in burn probability influenced risk individually and combined. Fires burned 9055 km2and exposed more than 8500 homes from 1990 to 2019. Observed burned area increased 632% from the 1990s to the 2000s, which combined with housing growth, resulted in a 1342% increase in homes exposed. Increases continued in the 2010s but at lower rates; burned area by 65% and exposure by 32%. The random forests model had excellent fit and high correlation with observations (AUC = 0.88 andr = 0.9). Observed values were within the 95% uncertainty interval for all years except 2016 (burned area) and 2000 (exposure). However, our model overpredicted in years with low observed burned area and underpredicted in years with high observed burned area. Overpredictions in risk resulted in lower rates of change in predicted risk compared with change in observed exposure. Increases in risk between the 1990s and 2000s were primarily due to warmer and drier weather conditions and secondarily because of housing growth. However, increases between the 2000s and 2010s were primarily due to housing growth. Our modeling approach identifies spatial and temporal patterns of wildfire potential and risk, which is critical information to guide decision‐making. Because the drivers behind risk shift over time, strategies to mitigate risk may need to account for multiple drivers simultaneously.

Efficacy of prescribed fire as a fuel reduction treatment in the Colorado Front Range

Prescribed fires are an important management tool for reducing fuels and returning fire to the landscape. However, rarely are changes in fuels fully quantified using pre- to post-prescribed fire measurements and those studies that do exist show variable results. In the southern Rockies, little literature exists on the impacts of prescribed fires; thus we examined multiple prescribed fires in northern Colorado to understand fire effects and changes in fuel complexes. Most prominently, prescribed fires influenced litter, duff, and rotten coarse woody debris but did not influence other surface fuels. Canopy base height increased and tree density decreased, while basal area was relatively unimpacted. Season of burning impacted fire effects as substrate burn severity, bole char, and crown volume scorch were highest in summer and fall. Continued monitoring of prescribed fires is critical to understand the influence of prescribed fire on wildfires and ultimately improve prescribed fire outcomes.

An aridity threshold model of fire sizes and annual area burned in extensively forested ecoregions of the western USA

Wildfire occurrence varies among regions and through time due to the long-term impacts of climate on fuel structure and short-term impacts on fuel flammability. Identifying the climatic conditions that trigger extensive fire years at regional scales can enable development of area burned models that are both spatially and temporally robust, which is crucial for understanding the impacts of past and future climate change. We identified region-specific thresholds in fire-season aridity that distinguish years with limited, moderate, and extensive area burned for 11 extensively forested ecoregions in the western United States. We developed a new area burned model using these relationships and demonstrate its application in the Southern Rocky Mountains using climate projections from five global climate models (GCMs) that bracket the range of projected changes in aridity. We used the aridity thresholds to classify each simulation year as having limited, moderate, or extensive area burned and defined fire-size distributions from historical fire records for these categories. We simulated individual fires from a regression relating fire season aridity to the annual number of fires and drew fire sizes from the corresponding fire-size distributions. We project dramatic increases in area burned after 2020 under most GCMs and after 2060 with all GCMs as the frequency of extensive fire years increases. Our adaptable model can readily incorporate new observations (e.g., extreme fire years) to directly address the non-stationarity of fire-climate relationships as climatic conditions diverge from past observations. Our aridity threshold fire model provides a simple yet spatially robust approach to project regional changes in area burned with broad applicability to ecosystem and vegetation simulation models.

A FLORISTIC INVENTORY OF TWO BOULDER COUNTY OPEN SPACE PARCELS: HEIL VALLEY RANCH AND HALL RANCH, COLORADO, U.S.A

An inventory of the vascular plants of Heil Valley Ranch and Hall Ranch in the north foothills of Boulder County was completed during 2017 and 2018, prior to a catastrophic forest fire that severely impacted this area in the fall of 2020. Heil Valley Ranch and Hall Ranch are keystone units of the Boulder County Parks and Open Space system. These large parcels (∼10,000 acres/4047 ha) represent a significant portion of publicly accessible open space in the county and represent an exemplary legacy of land preservation in the region. The biologically diverse foothill landscapes represent the ecotone between the High Plains and the Southern Rocky Mountains. Vital transition zones in the area harbor globally vulnerable and critically imperiled plant associations, the most detailed categorization described by the US National Vegetation Classification (2021). A total of 607 botanical specimens were collected over the course of 39 separate visits. There are 381 species from 82 families of flowering plants represented at the study area, with the greatest diversity occurring in Poaceae and the Asteraceae. Of these, 319 are native, 62 are non-native, and 7 species are of conservation concern. The present study is the first floristic study of these parcels and provides baseline data for future work in the area, as well as information for comparison to floristic works in the broader Front Range foothills region. Together, these studies inform conservation policy at a time when biodiversity is threatened both globally and locally. As the Front Range megaregion rapidly expands in human population, biological inventories will provide critical data to promote the expansion of conservation areas in parallel with urban growth.

Modern dextral strain controls active hydrothermal systems in the southeastern Canadian Cordillera

Thermal springs in the southeastern Canadian Cordillera occur in association with several major fault zones, which may permit deep circulation of fluid through fractured reservoirs to depths greater than 2 km. Both the current stress field and the most recent kinematics of these faults likely play a significant role in localizing hydrothermal upwellings but are poorly resolved. In this paper, we present new data from structural mapping along the Columbia River, Slocan Lake, Purcell Trench, Southern Rocky Mountain Trench, and Redwall faults. Fault plane and slickenfiber orientations measured in the field indicate a previously unidentified, post-Eocene phase of dextral strike-slip kinematics on these faults, which have historically been mapped primarily as Eocene extensional structures. The NE−SW maximum principal stress required for these kinematics shares a similar orientation to the present-day stress field derived from crustal earthquake focal mechanisms, which suggests dextral slip may be a recent and ongoing development that continues at the present time. On a regional scale, there is a positive correlation between the locations of springs and regions with elevated seismicity. At the local scale, geothermal upwellings may be controlled by local zones of enhanced permeability including fault intersections and strain-transfer zones. The identification of favorable structural settings may enable discovery of hidden (a.k.a. blind) geothermal energy resources.

Revision of Quedius sensu stricto (Coleoptera: Staphylinidae)

We here present the first integrative revision of the subgenus Quedius Stephens, 1829 sensu stricto where taxonomic decisions are based on morphology, genomic phylogeny (published elsewhere) and single locus DNA evidence. The subgenus is restricted to the Holarctic region and includes some of the largest, most robust members of the genus Quedius. For species delimitation, a total of 200 COI barcodes covering nearly all species within the subgenus were evaluated through phylogenetic, cluster and network analysis. Taxonomic, distributional and bionomic data, all hitherto published and new are synthesized and an identification key is constructed for all species. All 20 species of Quedius s. str. are divided into five species groups based on genomic phylogeny, morphology, distributional patterns and practical considerations. Overall, there was good congruence among various lines of evidence. As exceptions, high COI barcode variability was found within the wingless and patchily distributed Q. unicolor Kiesenwetter, 1847 and Q. sundukovi Smetana, 2003 (3 and 5 BOLD BINs, respectively) from the Q. molochinus-group without corresponding morphological or geographic patterns. On the contrary, in the Q. pallipes-group very little divergence in the COI barcode was found between clearly morphologically separated species. From the taxonomically challenging Nearctic Quedius molochinoides-group, Quedius altanai Hansen & Brunke sp. nov. is described as new species from the central and southern Rockies, rendering Q. lanei Hatch, 1957 to be restricted to the eastern foothills of the Cascades, Sierra Nevada and Blue Mountains. In the case of Q. subunicolor and Q. altaicus, continuous morphological and COI variation firmly established the earlier suspected synonymy Quedius subunicolor Korge, 1961 = Quedius altaicus Korge, 1962 syn. nov. Other new synonyms established here are: Quedius pallipes Lucas, 1849 = Quedius simplicifrons Fairmaire, 1861 syn. nov. = Quedius levasseuri Coiffait, 1964 syn. nov.; Quedius hispanicus Bernhauer, 1898 stat. reinstituted = Quedius cobosi Coiffait, 1964 syn. nov.; Quedius fuliginosus (Gravenhorst, 1802) = Quedius latus Hochhuth, 1851 syn. nov. = Quedius viduus Sawada, 1965, syn. nov. Quedius gracilis Stephens, 1832 syn. rev. is moved from synonymy with Q. fuliginosus and placed into synonymy with Q. curtipennis Bernhauer, 1908, the younger being the valid name due to prevailing usage in accordance with the ICZN Article 23.9; Quedius hammianus Sharp, 1911 syn. revid., Q. secundus Last, 1952 syn. revid., and Q. rufulus Blümml, 1898 syn. revid., are moved from synonymy with Q. simplicifrons and placed into synonymy with Q. hispanicus Bernhauer, 1898; Quedius sardous Gridelli, 1924 syn. revid., and Q. leonhardi Bernhauer, 1914 syn. revid., are moved from synonymy with Q. molochinus (Gravenhorst, 1806) to synonymy with Q. pallipes. Human mediated dispersals, most likely through the historical transport of ship ballast, are noted as regular phenomenon for some members of the subgenus. Quedius fuliginosus, Q. curtipennis and Q. molochinus, introduced from the Palearctic to the Nearctic, are the most noticeable examples.

Formation Mechanisms of the Mesoscale Environment Conducive to a Downslope Windstorm over the Cuyamaca Mountains Associated with Santa Ana Wind during the Cedar Fire (2003)

Abstract Numerical simulations were conducted to investigate the upstream environment’s impacts on the airflow over the lee slope of the Cuyamaca Mountains (CM) near San Diego, California, during the Cedar Fire that occurred from 25 to 29 October 2003. The upstream environment was largely controlled by a southwest–northeast-oriented upper-tropospheric jet streak that rotated around a positively tilted ridge within the polar jet stream. Three sequential dynamical processes were found to be responsible for modifying the mesoscale environment conducive to low-level momentum and dry air that sustained the Cedar Fire. First, the sinking motion associated with the indirect circulation of the jet streak’s exit region strengthened the midtropospheric flow over the southern Rockies and the lee slope of the Sawatch and San Juan Ranges, thus modestly affecting the airflow by enhancing the downslope wind over the CM. Second, consistent with the coupling process between the upper-level sinking motion, downward momentum transfer, and developing lower-layer mountain waves, a wave-induced critical level over the mountain produced wave breaking, which was characterized by a strong turbulent mixed region with a wind reversal on top of it. This critical level helped to produce severe downslope winds leading to the third stage: a hydraulic jump that subsequently enhanced the downstream extent of the strong winds conducive to the favorable lower-tropospheric environment for rapid fire spread. Consistent with these findings was the deep-layer resonance between the mountain surface and tropopause, which had a strong impact on strengthening the severe downslope winds over the lee slope of the CM accompanying the elevated strong easterly jet at low levels.

Fluorine-rich mafic lower crust in the southern Rocky Mountains: The role of pre-enrichment in generating fluorine-rich silicic magmas and porphyry Mo deposits

AbstractFluorine-rich granites and rhyolites occur throughout the southern Rocky Mountains, but the origin of F-enrichment has remained unclear. We test if F-enrichment could be inherited from ancient mafic lower crust by: (1) measuring amphibole compositions, including F and Cl contents, of lower crustal mafic granulite xenoliths from northern Colorado to determine if they are unusually enriched in halogens; (2) analyzing whole-rock elemental and Sr, Nd, and Pb isotopic compositions for upper crustal Cretaceous to Oligocene igneous rocks in Colorado to evaluate their sources; and (3) comparing batch melting models of mafic lower crustal source rocks to melt F and Cl abundances derived from biotite data from the F-rich silicic Never Summer batholith. This approach allows us to better determine if the mafic lower crust was pre-enriched in F, if it is concentrated enough to generate F-rich anatectic melts, and if geochemical data support an ancient lower crustal origin for the F-rich rocks in the southern Rocky Mountains.Electron microprobe analyses of amphibole in lower crustal mafic granulite xenoliths show they contain 0.56–1.38 wt% F and 0.45–0.73 wt% Cl. Titanium in calcium amphibole thermometry indicates that the amphiboles equilibrated at high to ultrahigh temperature conditions (805 to 940 °C), and semiquantitative amphibole thermobarometry indicates the amphiboles equilibrated at 0.5 to 1.0 GPa prior to entrainment in magmas during the Devonian. Mass balance calculations, based on these new measurements, indicate parts of the mafic lower crust in Colorado are at least 3.5 times more enriched in F than average mafic lower crust. Intrusions coeval with the Laramide Orogeny (75 to 38 Ma) pre-date F-rich magmatism in Colorado and have Sr and Nd isotopic compositions consistent with mafic lower crust ± mantle sources, but many of these intrusions contain elevated Sr/Y ratios (>40) that suggest amphibole was a stable phase during magma generation. The F-rich igneous rocks from the Never Summer igneous complex and Colorado Mineral Belt also have Sr and Nd isotopic compositions that overlap with the lower crustal mafic granulite xenoliths, but they have lower Sr/Y, higher Nb and Y abundances, and distinctly less radiogenic 206Pb/204Pbi compositions than preceding Laramide magmatism. Batch melt modeling indicates low-degree partial melts derived from rocks similar to the mafic lower crustal xenoliths we analyzed can yield silicic melts with >2000 ppm F, similar to estimated F melt concentrations for silicic melts that are interpreted to be parental to evolved leucogranites.We suggest that F-rich silicic melts in the southern Rocky Mountains were sourced from garnet-free mafic lower crust, and that fluid-absent breakdown of amphibole in ultrahigh temperature metamorphic rocks was a key process in their generation. Based on the composition of high-F amphibole measured from lower crustal xenoliths, the temperature of amphibole breakdown and melt generation for these F-enriched source rocks is likely >100 °C higher than similar lower crust with low or average F abundances. As such, these source rocks only melted during periods of unusually high heat flow into the lower crust, such as during an influx of mantle-derived magmas related to rifting or the post-Laramide ignimbrite flare-up in the region. These data have direct implications for the genesis of porphyry Mo mineralization, because they indicate that pre-enrichment of F in the deep crust could be a necessary condition for later anatexis and generation of F-rich magmas.

Thermal tolerance in cutthroat trout of the southern Rocky Mountains

With temperatures expected to rise across the southern Rocky Mountains, the ability of native fishes to tolerate stream warming has become a critical concern for those tasked with preserving coldwater species. We used common garden experiments to evaluate the thermal tolerance of cutthroat trout ( Oncorhynchus clarkii) fry from five populations important to managers representing three sub-species. Critical thermal maxima (CTMs) were evaluated through traditional exposure trials, while optimal growth and ultimate upper incipient lethal temperatures (UUILTs) were examined over the course of 21-day trials at six static temperature treatments. Whereas CTMs differed among populations (mean = 27.91 °C, SD = 0.35 °C), UUILTs did not (mean = 24.40 °C, SD = 0.04 °C). Comparison of cubic temperature-growth functions to the traditional quadratic functions showed that adding a third-order term for temperature can improve model fit, and revealed substantial differences in optimal growth temperatures (15.4–18.3 °C). Knowledge of these thermal tolerance thresholds will help to predict the consequences of a warming climate, identify suitable habitats for repatriation, and inform water quality temperature standards established to protect these fish into the future.

Rocky Mountain forests are poised to recover following bark beetle outbreaks but with altered composition

AbstractAmplified by warming temperatures and drought, recent outbreaks of native bark beetles (Curculionidae: Scolytinae) have caused extensive tree mortality throughout Europe and North America. Despite their ubiquitous nature and important effects on ecosystems, forest recovery following such disturbances is poorly understood, particularly across regions with varying abiotic conditions and outbreak effects.To better understand post‐outbreak recovery across a topographically complex region, we synthesized data from 16 field studies spanning subalpine forests in the Southern Rocky Mountains, USA. From 1997 to 2019, these forests were heavily affected by outbreaks of three native bark beetle species (Dendroctonus ponderosae,Dendroctonus rufipennisandDryocoetes confusus). We compared pre‐ and post‐outbreak forest conditions and developed region‐wide predictive maps of post‐outbreak (1) live basal areas, (2) juvenile densities and (3) height growth rates for the most abundant tree species – aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta) and subalpine fir (Abies lasiocarpa).Beetle‐caused tree mortality reduced the average diameter of live trees by 28.4% (5.6 cm), and species dominance was altered on 27.8% of field plots with shifts away from pine and spruce. However, most plots (82.1%) were likely to recover towards pre‐outbreak tree densities without additional regeneration. Region‐wide maps indicated that fir and aspen, non‐host species for bark beetle species with the most severe effects (i.e.Dendroctonusspp.), will benefit from outbreaks through increased compositional dominance. After accounting for individual size, height growth for all conifer species was more rapid in sites with low winter precipitation, high winter temperatures and severe outbreaks.Synthesis. In subalpine forests of the US Rocky Mountains, recent bark beetle outbreaks have reduced tree size and altered species composition. While eventual recovery of the pre‐outbreakforest structureis likely in most places, changes inspecies compositionmay persist for decades. Still, forest communities following bark beetle outbreaks are widely variable due to differences in pre‐outbreak conditions, outbreak severity and abiotic gradients. This regional variability has critical implications for ecosystem services and susceptibility to future disturbances.

Limited seed viability in long-dead serotinous lodgepole pine trees in the Southern Rockies, USA

Serotinous cones, those that remain closed until heated, confer post-disturbance resilience on many lodgepole pine forests throughout the Southern Rockies. The record-breaking extent of wildfires in northern Colorado and southern Wyoming in 2020 raised concerns about tree regeneration in areas where crown fires burned lodgepole pine stands that had experienced high levels of mortality during bark beetle outbreaks occurring more than a decade previously. We measured seed germination and assessed the relationship between cone age and seed viability for serotinous cones on beetle-killed lodgepole pine in eight stands adjacent to four of those wildfires (n = 1160 cones). On average, germination was 34 % with three viable germinants produced per cone. Seed germination declined and the proportion of cones that contained no seed increased across the age range of cones sampled (17–51 yrs at the time of the 2020 wildfires). Germination of seed retained within the canopy seedbank of these long-dead stands was roughly half of that measured on live or recently killed lodgepole pine. Though pre-fire tree mortality from bark beetles can exceed 90 % in this area, serotinous cones on the remaining live trees and cones buried in the soil seedbank will contribute viable lodgepole seeds for tree regeneration on some burned landscapes. The decreased germinant levels from these cones may regenerate into pine stands that are less dense than after harvesting, bark beetles or wildfire alone, but they will likely create forest structure within a range of conditions expected for this region.