California Basin Research Articles

Future-Year Ozone Isopleths for South Coast, San Joaquin Valley, and Maryland

Many areas of the United States are working toward achieving the 2015 ozone National Ambient Air Quality Standard (NAAQS) attainment level. The objective of this study was to develop future-year (2030) volatile organic compounds and nitrogen oxides (VOC-NOx) isopleth diagrams of the 4th highest maximum daily 8-h average ozone design value concentrations at monitors of interest in the South Coast Air Basin (SoCAB) and San Joaquin Valley (SJV) in California, and in Maryland. The simulation results showed there would be attainment of the 2015 ozone NAAQS in 2030 without further controls at the selected monitors: 27% in SoCAB, 57% in SJV, and 100% in Maryland. The SoCAB ozone isopleths developed in this study were compared with those reported in the South Coast Air Quality Management District 2016 Air Quality Management Plan. There are several differences between the two modeling studies, the results are qualitatively similar for most of the monitors in the relative amounts of additional emission reductions needed to achieve the ozone NAAQS. The results of this study provide insight into designing potential control strategies for ozone attainment in future years for areas currently in non-attainment. Additional photochemical modeling using these strategies can then provide confirmation of the effectiveness of the controls.

Widespread Distribution of Ceratonova shasta (Cnidaria: Myxosporea) Genotypes Indicates Evolutionary Adaptation to its Salmonid Fish Hosts.

The distribution of the freshwater myxozoan parasite Ceratonova shasta in the Pacific Northwest of North America is limited to overlap in the ranges of its 2 hosts: the polychaete Manyunkia sp., and Pacific salmonids. Studies in the Klamath River (Oregon/California) and Deschutes River (Oregon), showed that the parasite population is comprised of multiple sympatric genotypes, some of which correlate with particular salmonid host species and with differences in clinical disease in those hosts. The 3 primary genotypes O, I, and II are defined by the number of a specific tri-nucleotide repeat in the internal transcribed spacer-1 region. To understand the spatial extent of host-parasite genotype patterns, we sequenced the parasite from 448 salmonid fishes from river basins in California, Oregon, Washington, Idaho, and British Columbia, Canada. We sampled intestinal tissues from 6 species of salmon and trout, both those that exist naturally with the parasite (sympatric) and those that do not naturally co-occur with the parasite and were exposed artificially in cages (allopatric). In most river basins we detected the same primary C. shasta genotypes that were described from the Klamath and Deschutes rivers, and we did not detect any novel primary genotypes. Host-parasite genotype patterns were consistent with previous data: genotype O was found in sympatric trout only; genotype I predominantly in Chinook salmon, and genotype II in all 6 fish species but dominant in coho salmon. Our findings of widespread, consistent host-parasite genotype patterns support the hypothesis that C. shasta has a long evolutionary history with salmonid fishes in the Pacific Northwest, and impels additional studies to determine if these parasite genotypes should be considered different species.

Controls on nitrogen transformation rates on restored floodplains along the Cosumnes River, California

Levee construction results in the systematic replumbing of river systems and reduces the frequency of floodplain inundation, which impacts nutrient delivery and transformations in floodplains. Floodplain restoration via levee removal affects downstream water quality by restoring soil microbial metabolic pathways such as denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA). Although these metabolisms are important for the nitrogen cycle, few studies have quantified the contribution of all three pathways to nitrate retention or loss in restored floodplains. The objectives of this study were to quantify the relevance of denitrification, anammox and DNRA to nitrogen retention, characterize the hydrologic conditions most favorable to each pathway, and estimate the potential for floodplain restoration to improve nitrogen cycling in the Cosumnes River watershed. To address these goals, we simulated flood conditions in soil mesocosms collected from two floodplains where levees were breached in 1997 and 2014 along the Lower Cosumnes River in the San Joaquin Basin of California. River water enriched with K15NO3 tracer was pumped into each mesocosm at a constant rate for a period of 3 months. Samples were collected from the surface water and soil pore water for measurements of NO3−, NO2−, and NH4+ concentrations, and δ15N of dissolved gases (N2 and N2O). To the best of our knowledge, this study reports the highest relative contribution to N2 production due to anammox for freshwater systems (41 to 84%) to date. High anammox rates were associated with heterogeneous grain size distribution across depth and high nitrification rates. We quantify the capacity of restored floodplain soils with distinct textural and chemical characteristics to retain or release nitrogen during large and small floods in a particular water year.

On distinguishing snowfall from rainfall using near‐surface atmospheric information: Comparative analysis, uncertainties and hydrologic importance

The accurate estimation of precipitation phase has broad applications. In this study, we compared the skill of using various atmospheric variables and their combinations as predictors in accurately identifying surface precipitation phase, determined uncertainties associated with commonly used fixed temperature thresholds, and explored the sensitivity of hydrologic model output to uncertainty in precipitation phase using two case‐studies. The results suggest that among all single predictors, wet‐bulb temperature yields the highest skill score for determining precipitation phase and can reduce uncertainties due to regional differences, especially compared to the commonly used near‐surface air temperature. However, addition of good‐quality near‐surface wind speed measurement to dew‐point temperature and pressure showed slightly higher skill than wet‐bulb temperature. We showed that the scale mismatch between temperature from stations and gridded products can cause large uncertainties in determining precipitation phase, especially in regions with rugged topography. Such uncertainties need to be considered when the relationships developed based on station data are applied to remote‐sensing observations and model‐generated data to separate rain from snowfall. The sensitivity of hydrologic model outputs to uncertainty in precipitation phase delineation was also assessed over two major basins in California by modifying default near‐surface temperatures used in the Variable Infiltration Capacity (VIC) model. It was found that regional and scaling uncertainties in determining temperature thresholds can largely influence the accuracy of simulated downstream runoff and snow water equivalent (SWE) (e.g. up to 40% change in SWE for 2 °C shift in temperature threshold). Therefore, to reduce simulation uncertainties, it is important to improve rain–snow partitioning methods, consider regional variabilities in determining temperature thresholds, and perform the analysis at the highest possible resolutions to mitigate scale‐related uncertainties.

Histopathology and external examination of heavily parasitized Lost River Sucker Deltistes luxatus (Cope 1879) and Shortnose Sucker Chasmistes brevirostris (Cope 1879) from Upper Klamath Lake, Oregon.

Shortnose Sucker (Chasimistes brevirostris) and Lost River Sucker (Deltistes luxatus) are endemic to the Upper Klamath Basin of Southern Oregon and Northern California, and their populations are in decline. We used histopathology and external examination of 140 and external examination only of 310 underyearling suckers collected in 2013, 2015 and 2016 to document pathological changes, particularly those relating to parasites. The most severe infection was caused by a Contracaecum sp., infecting the atrium of 8%-33% of Shortnose Suckers. The most prevalent infections were caused by Bolbophorus sp. metacercariae in the muscle of Shortnose Suckers (21%-63%) and Lernaea cyprinacea in the skin and muscle of Lost River Suckers (30%-81%). Histology detected Bolbophorus in only 5% of cases where it was not seen externally. Three myxozoans were observed; a Parvicapsula sp. in the renal tubules (10%), a Myxobolus sp. in the intestinal mucosa (2%) and an unusual multicellular, presporogonic myxozoan in the intestinal lumen of one sucker. Severe gill epithelial hyperplasia was observed in several fish collected in 2016. Trichodinids and Ichthyobodo sp. were observed on some of the gills, but absent in many of the fish with severe lesions. A histiocytic sarcoma was observed in sucker.

Controlling the Narrative: A Comparative Examination of Gendered Publishing Trends in the SCA and Beyond

This article explores the relationship between gender and authorship in conference presentations and publications as a lens to examine current disciplinary sociopolitics and the relative contributions of men and women in California archaeology. I contextualize this analysis within a broader comparison to other regions, including Southeastern archaeology as well as the North American archaeology community at large. I also examine how occupational affiliation in different sectors of archaeology (including academic, agency, museums, private sector/CRM, and others) bears on publishing trends. An evaluation of publishing trends serves as a means to investigate academic merit and visibility, along with the production and validation of knowledge in California archaeology and beyond. Despite growing parity in the numbers of women represented in professional organizations and presenting research at regional and national conferences, disparities remain with respect to publication in peer-reviewed journals, including in Journal of California and Great Basin Anthropology, California Archaeology, Southeastern Archaeology, and American Antiquity. I explore possible reasons for these disparities, including links to occupational affiliation, and conclude with some recommendations for change.

Reactive uptake of ammonia by secondary organic aerosols: Implications for air quality

Reactions between ammonia (NH3) and organic compounds containing carbonyl functional groups in aerosol particles can form organic products that are less basic than NH3 and are thus unable to neutralize efficiently nitric and sulfuric acids. In this exploratory study, the University of California, Irvine - California Institute of Technology (UCI-CIT) model is used to investigate the potential air quality impacts of including of the chemical uptake of NH3 by secondary organic aerosols (SOA) in a regional airshed model. A surface reaction of NH3 with SOA is implemented into the model to determine the impact of this process on NH3 and PM2.5 concentrations in the South Coast Air Basin of California (SoCAB). Air quality simulations are conducted using uptake coefficients ranging from 10−5 to 10−2 to explore the sensitivity of changes in NH3 and PM2.5 concentrations to the magnitude of the uptake coefficient. Results indicate that the chemical uptake of NH3 by SOA can potentially deplete gaseous NH3 concentrations, causing indirect reductions in the amount of ammonium nitrate and ammonium sulfate in particulate matter. The magnitude of the impact on NH3 and PM2.5 concentrations exhibits a strong but non-linear dependence on the value of the uptake coefficient, with evidence for small but notable impacts on air quality even with the lowest assumed uptake coefficient of 10−5.

PM2.5 and ozone, indicators of air quality, and acute deaths in California, 2004–2007

Since the London Great Smog of 1952 was estimated to have killed over 4000 people, scientists have studied the relationship between air quality and acute mortality. Currently, the association between air quality and acute deaths is usually taken as evidence for causality. As air quality has markedly improved since 1952, do contemporary datasets support this view? We use a large dataset, eight air basins in California for the years 2004–2007, to examine the possible association of ozone and PM2.5 with acute deaths after statistically removing seasonal and weather effects. Our analysis dataset is available on request. We conducted a regression-corrected, case-crossover analysis for all non-accidental deaths age 75 and older. We used stepwise regression to examine three causes of death. After seasonal and weather adjustments, there was essentially no predictive power of ozone or PM2.5 for acute deaths. The case-crossover analysis produced odds ratio very close to 1.000 (no effect). The very narrow confidence limits indicated good statistical power. We study recent air quality in both time-stratified, symmetric, bidirectional case-crossover and time series regression and both give consistent results. There is no statistically significant association between either ozone or PM2.5 and acute human mortality. In the absence of an association, causality is in question.

Impacts of groundwater management on energy resources and greenhouse gas emissions in California

California faces significant energy and water infrastructure planning challenges in response to a changing climate. Immediately following the most severe recorded drought, the state experienced one of its wettest water years in recorded history. Despite the recent severe wet weather, much of the state's critical groundwater systems have not recovered from the drought. The recent Sustainable Groundwater Management Act (SGMA) aims to eliminate future depletion risks, but may force California basins to seek alternative water sources by limiting groundwater withdrawals during droughts. These alternative water resources, such as recycled water or desalination, can have significantly higher energy demands in treatment and supply than local groundwater or surface water resources.This research developed potential scenarios of water supply sources for five overdrafted groundwater basins, and modeled the impacts of these scenarios on energy demands and greenhouse gas (GHG) emissions for water supply systems. Our results reveal that energy demands and GHG emissions in different water supply scenarios can vary substantially between basins, but could increase statewide energy consumption as much as 2% and GHG emissions by 0.5. These results highlight the need to integrate these energy and GHG impacts into water resource management. Better understanding these considerations enables water supply planners to avoid potential unintended consequences (i.e., increased energy demands and GHG emissions) of enhancing drought resilience.

Globally scalable alpine snow metrics

Horizontal and altitudinal redistribution of snow by wind transport and avalanches can be important controls on small- and large-scale snow accumulation patterns that control meltwater supply in alpine environments. Redistribution processes control the spatial variability of snow accumulation, which not only controls meltwater supply, but also regulates snowmelt timing, duration, and rates, as well as snow-covered area depletion and the variable contributing area for meltwater runoff generation. However, most hydrological models and land surface schemes do not consider snow redistribution processes, and those that do are difficult to verify without spatially distributed snow depth measurements. These are rarely available in both high resolution and covering large scales. As an increased number of hydrological models include snow redistribution processes there is a need for additional snowcover metrics to verify snow redistribution schemes over large areas using readily available data.This study develops novel high-resolution (20 m), snowcover indices from remotely sensed imagery (Landsat-8 and Sentinel-2) to evaluate snow redistribution models over alpine areas without in-situ or airborne snow observations. A snowcover absence (SA) index, calculated from snow-free areas during the winter, identifies areas of wind erosion or avalanche source areas. A snowcover persistence (SP) index, calculated from snow-covered areas during the summer, identifies snow deposition in drifts and avalanche deposits.The snowcover indices captured the relative differences in surface observations of snow presence and absence between exposed and sheltered sites on an intensely instrumented ridge in the Canadian Rockies Hydrological Observatory. Within the Tuolumne River Basin in central California (1100 km2), the SP index captured roughly half of the spatial variability (R2 = 0.49 to 0.56) in peak SWE as estimated from airborne LiDAR-derived snow depths. At the individual mountain ridge scale (~800 m), variability in both ablation and snow redistribution controlled the SP patterns over 7979 ridges. Differences in shortwave irradiance explained 76% of the SP variance across ridges, but could not explain smaller-scale (~100 m) SP peaks that are associated with snowdrifts and avalanche deposits. The snowcover indices can be used to evaluate snow redistribution models of the finer scale impacts of snow redistribution by wind and gravity as long as the larger scale influences of spatially variable solar irradiance effects are also simulated.

Air quality impacts of implementing emission reduction strategies at southern California airports

Reducing aviation emissions will be a major concern in the coming years, as the relative contribution of aviation to overall emissions is projected to increase in the future. The South Coast Air Basin of California (SoCAB) is an extreme nonattainment area with many airports located upwind of the most polluted regions in the basin. Techniques to reduce aviation emissions have been studied in the past, and strategies that can be implemented at airports include taxi-out times reduction, ground support equipment electrification and aviation biofuel implementation. These strategies have been analyzed only at the national scale, their effectiveness to improve air quality within the SoCAB given the local meteorology and chemical regimes is unclear. This work studies how the adoption of the techniques at commercial SoCAB airports affect ozone (O3) and fine particulate matter (PM2.5) concentrations. In addition, potential impacts on public exposure to PM2.5 and O3 resulting from changes in the concentration of these pollutants are estimated. In addition, the work calculates aviation emissions for each scenario and simulate the transport and atmospheric chemistry of the pollutants using the Community Multiscale Air Quality (CMAQ) model. The simultaneous application of all reduction strategies is projected to reduce the aviation-attributable population weighted ground-level PM2.5 by 36% in summer and 32% in winter. On the other hand, O3 increases by 16% in winter. Occurring mostly in densely populated areas, the decrease in ground-level PM2.5 would have a positive health impact and help the region achieve attainment of national ambient air quality standards.

Simulated sensitivity of secondary organic aerosol in the South Coast Air Basin of California to nitrogen oxides and other chemical parameters

ABSTRACTSensitivity of secondary organic aerosol (SOA) concentrations in the South Coast Air Basin (SoCAB) of California to nitrogen oxide (NOx) emission is simulated using gas-phase chemistry and gas-particle partitioning modules. These modules are implemented into a three-dimensional air quality model applied for high-pollution summer meteorology and 2008 emissions. To test sensitivity, NOx emissions in all locations and at all times are scaled by factors ranging from 0.1 to 10.0 in separate model runs. The basin-wide average SOA concentration exhibits a ‘turnover’ NOx emission multiplicative factor, above and below which the average SOA concentration decreases. For the entire SoCAB, this critical NOx emission factor is ∼0.3; while the magnitude of SOA concentrations changes with time, this peak value (∼0.2–0.3) appears to be relatively independent of the hour of the simulated day. When considering individual locations within the SoCAB, this peak factor shows a slightly broader range. Projected emission...

Genetically distinct populations of the pygmy rabbit (Brachylagus idahoensis) in the Mono Basin of California

We examined patterns of genetic variation and diversity of extant pygmy rabbit (Brachylagus idahoensis) populations across the species' current range in Nevada and California. Our aims were to determine population genetic structure and levels of diversity across the southern portion of the species' range. We genotyped 13 microsatellite loci from 194 fecal samples collected across 14 localities. Our Bayesian cluster analyses found 2 genetically distinct groups: 1 in the Mono Basin of California and the other encompassing all remaining Nevada Great Basin populations. Considering only the Nevada Great Basin group, we found 4 minimally divergent groups that overlap spatially with many individuals maintaining composite genomes with representation from multiple genetic groups. On average, the Mono Basin populations maintain lower levels of genetic diversity than the Nevada Great Basin populations as measured by both heterozygosity and numbers of alleles. The Mono Basin contains the only populations of pygmy rabbits remaining in California. Given their geographic isolation from other known populations and their genetic distinction, these remnant, peripheral populations warrant special attention to ensure maintenance of genetic variation and continued viability.

Full Coupling Between the Atmosphere, Surface, and Subsurface for Integrated Hydrologic Simulation

AbstractAn ever increasing community of earth system modelers is incorporating new physical processes into numerical models. This trend is facilitated by advancements in computational resources, improvements in simulation skill, and the desire to build numerical simulators that represent the water cycle with greater fidelity. In this quest to develop a state‐of‐the‐art water cycle model, we coupled HydroGeoSphere (HGS), a 3‐D control‐volume finite element surface and variably saturated subsurface flow model that includes evapotranspiration processes, to the Weather Research and Forecasting (WRF) Model, a 3‐D finite difference nonhydrostatic mesoscale atmospheric model. The two‐way coupled model, referred to as HGS‐WRF, exchanges the actual evapotranspiration fluxes and soil saturations calculated by HGS to WRF; conversely, the potential evapotranspiration and precipitation fluxes from WRF are passed to HGS. The flexible HGS‐WRF coupling method allows for unique meshes used by each model, while maintaining mass and energy conservation between the domains. Furthermore, the HGS‐WRF coupling implements a subtime stepping algorithm to minimize computational expense. As a demonstration of HGS‐WRF's capabilities, we applied it to the California Basin and found a strong connection between the depth to the groundwater table and the latent heat fluxes across the land surface.

Effects of the distribution and evolution of the coefficient of friction along a fault on the assessment of the seismic activity associated with a hypothetical industrial-scale geologic CO2 sequestration operation

Carbon capture and storage (CCS) in geological formations is considered as a promising option that could limit CO2 emissions from human activities into the atmosphere. However, there is a risk that pressure buildup inside the storage formation can induce slip along preexisting faults and create seismic event felt by the population. To prevent this to happen a geomechanical fault stability analysis should be performed, considering uncertainties of input parameters. In this paper, we investigate how the distribution of the coefficient of friction and the applied frictional law could influence the assessment of fault stability and the characteristics of potential injection-induced seismic events. Our modelling study is based on a hypothetical industrial-scale carbon sequestration project located in the Southern San Joaquin Basin in California, USA, where the stability on a major (25km long) fault that bounds the sequestration site is assessed during 50 years of CO2 injection. We conduct nine simulations in which the distributions of the coefficients of static and dynamic friction are changed to simulate a hardening and softening phase before and during rupture. Our main findings are: (i) variations in friction along the fault have an important effect on the predicted seismic activity, with maximum magnitude ranging from 1.88 to 5.88 and number of seismic events ranging from 338 to 3272; (ii) the extreme values of the coefficient of friction (lowest and highest) present along the rupture area control how much stress is accumulated before rupture; and (iii) an argillaceous caprock can prevent the development of large magnitude seismic events but favor the occurrence of a large number of smaller events.

Spatial Modeling for Resources Framework (SMRF): A modular framework for developing spatial forcing data for snow modeling in mountain basins

In the Western US and many mountainous regions of the world, critical water resources and climate conditions are difficult to monitor because the observation network is generally very sparse. The critical resource from the mountain snowpack is water flowing into streams and reservoirs that will provide for irrigation, flood control, power generation, and ecosystem services. Water supply forecasting in a rapidly changing climate has become increasingly difficult because of non-stationary conditions. In response, operational water supply managers have begun to move from statistical techniques towards the use of physically based models. As we begin to transition physically based models from research to operational use, we must address the most difficult and time-consuming aspect of model initiation: the need for robust methods to develop and distribute the input forcing data. In this paper, we present a new open source framework, the Spatial Modeling for Resources Framework (SMRF), which automates and simplifies the common forcing data distribution methods. It is computationally efficient and can be implemented for both research and operational applications. We present an example of how SMRF is able to generate all of the forcing data required to a run physically based snow model at 50–100 m resolution over regions of 1000–7000 km2. The approach has been successfully applied in real time and historical applications for both the Boise River Basin in Idaho, USA and the Tuolumne River Basin in California, USA. These applications use meteorological station measurements and numerical weather prediction model outputs as input. SMRF has significantly streamlined the modeling workflow, decreased model set up time from weeks to days, and made near real-time application of a physically based snow model possible.

Dynamic Evaluation of Photochemical Grid Model Response to Emission Changes in the South Coast Air Basin in California

This paper describes a study to evaluate the capability of a photochemical grid modeling system to predict changes in ozone concentrations in response to emission changes over a period of several years. The development of regulatory emission control plans to meet air quality standards primarily relies on modeled projections of future-year air quality, although a weight of evidence approach (which takes into account a number of factors including modeling results, model evaluation and other pertinent information such as ambient trends) is recommended and is also typically used as part of the attainment demonstration. Thus, it is important to determine if the modeling system used to project future-year quality can correctly simulate ozone responses to the projected emissions reductions. Uncertainties and errors in modeled projections can lead to erroneous estimates of emissions controls required to attain the standards. We use two existing regulatory modeling databases, employed for forecasting future-year air quality in the South Coast Air Basin (SoCAB) of California, for a number of historical years to evaluate the ability of the system to accurately simulate the observed changes in air quality over a multi-year period. The evaluation results with the older (2012) database show that the modeling system consistently under-predicts the reductions in ozone in response to emission reductions over the years. Model response improves with the newer (2016) database with good agreement at some locations, but the system still tends to under-predict ozone responses by as much as a factor of 2 in recent years for the Basin maximum ozone design value. This suggests that future-year estimates of ozone design values may be overly conservative, resulting in emission controls that are technologically challenging or very expensive to implement. The development of better emission inventories and model inputs is recommended to develop a modeling system that more accurately responds to emission changes. Future regulatory planning should include dynamic evaluation in addition to the traditional operational evaluation of the model to provide more confidence to all stakeholders that the resulting policy decisions are necessary to attain the air quality standards and to protect public health.

Availability of high-magnitude streamflow for groundwater banking in the Central Valley, California

California’s climate is characterized by the largest precipitation and streamflow variability observed within the conterminous US This, combined with chronic groundwater overdraft of 0.6–3.5 km3 yr−1, creates the need to identify additional surface water sources available for groundwater recharge using methods such as agricultural groundwater banking, aquifer storage and recovery, and spreading basins. High-magnitude streamflow, i.e. flow above the 90th percentile, that exceeds environmental flow requirements and current surface water allocations under California water rights, could be a viable source of surface water for groundwater banking. Here, we present a comprehensive analysis of the magnitude, frequency, duration and timing of high-magnitude streamflow (HMF) for 93 stream gauges covering the Sacramento, San Joaquin and Tulare basins in California. The results show that in an average year with HMF approximately 3.2 km3 of high-magnitude flow is exported from the entire Central Valley to the Sacramento-San Joaquin Delta often at times when environmental flow requirements of the Delta and major rivers are exceeded. High-magnitude flow occurs, on average, during 7 and 4.7 out of 10 years in the Sacramento River and the San Joaquin-Tulare Basins, respectively, from just a few storm events (5–7 1-day peak events) lasting for 25–30 days between November and April. The results suggest that there is sufficient unmanaged surface water physically available to mitigate long-term groundwater overdraft in the Central Valley.

Is diversionary feeding an effective tool for reducing human–bear conflicts? Case studies from North America and Europe

Diversionary feeding uses food to lure animals away from areas where they are unwanted or could cause conflicts with people. With bears (Ursidae) increasingly attracted to human food sources worldwide, diversionary feeding represents a seemingly logical and publicly acceptable means of alleviating conflicts. Feeding wildlife is widely practiced in Europe to enhance hunting and reduce conflicts, but feeding of bears is discouraged across North America. The efficacy and potential side-effects of bear feeding remain an open question because of a lack of rigorous studies. Here we examine 5 case studies from which we attempt to draw inferences about feeding as a conflict-mitigation strategy. Studies included U.S. national parks, where after bear feeding was banned conflicts were reduced; Aspen, Colorado, where lucrative dumpsters in town did not divert bears from using human-related foods at other sources; rural Minnesota, where results of intentional feeding of a small sample of bears were confounded with other variables; the Tahoe Basin of California–Nevada, where an emergency feeding effort during a drought-caused food failure seemed to reduce conflicts within approximately 1 km of the feeding site; and Slovenia, where a high density of feeders at established locations seemed to divert bears from using settlements during autumn hyperphagia. Although none of these studies were true experiments with treatments and controls, the range of circumstances yielded insights into when feeding could be effective: when food demands are not readily met by natural foods; when the provisioned food is easily found outside the potential conflict area; when the food is attractive; and when bears do not associate the feeding with people. However, long-term feeding may increase bear population size, which may increase conflicts overall, or trigger a demand for population control. Diversionary feeding, if used, should be conducted as an adaptive management strategy by professionals so as to learn more about factors influencing its effectiveness.

Curtailing succession: Removing conifers enhances understory light and growth of young aspen in mixed stands around Lake Tahoe, California and Nevada, USA

Aspen is a keystone species that is relatively rare in the Sierra Nevada Mountains and is undergoing succession to conifers in the absence of disturbance. In the presence of more conifers, recruitment of young aspen to replace aging overstory aspen is failing and diverse understory plant communities are being lost. We studied vegetation attributes and understory light in nine aspen-conifer stands in the Lake Tahoe Basin in California and Nevada, USA. Within each stand we measured and mapped trees in a one-hectare study area and collected 27–30 hemispherical canopy images on a grid that were used to estimate forest canopy variables and the amount of light reaching the understory. We also measured percent cover of herbaceous vegetation on the same sample grid. After restoration treatments cut smaller conifer trees at six of the nine sites, we repeated canopy images and began monitoring the growth of young aspen adjacent to sample grid points. Less light was reaching the understory beneath areas of forest canopy dominated by true firs. Understory light was enhanced by removal of conifers (pine and fir species). Stand density and species composition of trees surrounding a grid point where each hemispherical image was taken were useful predictors of understory light and forest canopy variables, herbaceous vegetation cover, and the morphology and growth of young aspen. Less herbaceous vegetation was found in the vicinity of true firs and pines, particularly in areas with less light being transmitted to the forest floor. In areas where the overstory was pure aspen, herbaceous vegetation cover remained high across the range of understory light levels measured. Greater conifer removal treatment intensity, in terms of proportion of stand density cut, enhanced crown ratio and growth of young aspen. Height and diameter increment of young aspen was negatively impacted by the presence of true fir trees in the vicinity. If the objectives of forest management include promoting understory vegetation cover or vigorous aspen regeneration, then removing conifers (especially true firs) from aspen-conifer stands appears to be a viable management tool; using such an approach should focus on removing as many conifers as possible within approximately 8–11m of areas where an enhancement of understory light, herbaceous vegetation, and the growth and vigor of young aspen is desired.