[1] We document effects of postfire forest conditions on snow accumulation, albedo, and ablation in the Oregon Cascades. We measured snow water equivalent, solar radiation, snow albedo, and snowpack surface debris at a pair of burned and unburned forest plots. Snow accumulation was greater in the burned forest; however, the snowpack disappeared 23 days earlier and had twice the ablation rate than in the unburned forest. Snow albedo was 40% lower in the burned forest during ablation, while approximately 60% more solar radiation reached the snow surface, driving a 200% increase in net shortwave radiation. Significant amounts of pyrogenic carbon particles and larger burned woody debris shed from standing charred trees accumulated on the snowpack and darkened its surface. Spatial analysis showed that across the Western U.S., 80% of all forest fires occurred in the seasonal snow zone, and were 4.4 times larger than fires outside the seasonal snow zone. Citation: Gleason, K. E., A. W. Nolin, and T. R. Roth (2013), Charred forests increase snowmelt: Effects of burned woody debris and incoming solar radiation on snow ablation, Geophys. Res. Lett., 40,[4654][4655][4656][4657][4658][4659][4660][4661]

Forests around the world are experiencing increasingly severe droughts and elevated competitive intensity due to increased tree density. However, the influence of interactions between drought and competition on forest growth remains poorly understood. Using a unique dataset of stand‐scale dendrochronology sampled from 6405 trees, we quantified how annual growth of entire tree populations responds to drought and competition in eight, long‐term (multi‐decadal), experiments with replicated levels of density (e.g., competitive intensity) arrayed across a broad climatic and compositional gradient. Forest growth (cumulative individual tree growth within a stand) declined during drought, especially during more severe drought in drier climates. Forest growth declines were exacerbated by high density at all sites but one, particularly during periods of more severe drought. Surprisingly, the influence of forest density was persistent overall, but these density impacts were greater in the humid sites than in more arid sites. Significant density impacts occurred during periods of more extreme drought, and during warmer temperatures in the semi‐arid sites but during periods of cooler temperatures in the humid sites. Because competition has a consistent influence over growth response to drought, maintaining forests at lower density may enhance resilience to drought in all climates.

SignificanceOur measurements show a tripling of iodine in Alpine ice between 1950 and 1990. A 20th century increase in global iodine emissions has been previously found from model simulations, based on laboratory studies, but, up to now, long-term iodine records exist only in polar regions. These polar records are influenced by sea ice processes, which may obscure global iodine trends. Our results suggest that the increased iodine deposition over the Alps is consistent with increased oceanic iodine emissions coupled with a change in the iodine speciation, both driven by increasing anthropogenic NOx emissions. In turn, the recent increase of iodine emissions implies that iodine-related ozone loss in the troposphere is more active now than in the preindustrial period.

Forest fires are increasing across the American West due to climate warming and fire suppression. Accelerated snow melt occurs in burned forests due to increased light transmission through the canopy and decreased snow albedo from deposition of light-absorbing impurities. Using satellite observations, we document up to an annual 9% growth in western forests burned since 1984, and 5 day earlier snow disappearance persisting for >10 years following fire. Here, we show that black carbon and burned woody debris darkens the snowpack and lowers snow albedo for 15 winters following fire, using measurements of snow collected from seven forested sites that burned between 2002 and 2016. We estimate a 372 to 443% increase in solar energy absorbed by snowpacks occurred beneath charred forests over the past two decades, with enhanced post-fire radiative forcing in 2018 causing earlier melt and snow disappearance in > 11% of forests in the western seasonal snow zone.

Abstract:As large, high-severity forest fires increase and snowpacks become more vulnerable to climate change across the western USA, it is important to understand post-fire disturbance impacts on snow hydrology. Here, we examine, quantify, parameterize, model, and assess the post-fire radiative forcing effects on snow to improve hydrologic modelling of snow-dominated watersheds having experienced severe forest fires. Following a 2011 high-severity forest fire in the Oregon Cascades, we measured snow albedo, monitored snow, and micrometeorological conditions, sampled snow surface debris, and modelled snowpack energy and mass balance in adjacent burned forest (BF) and unburned forest sites. For three winters following the fire, charred debris in the BF reduced snow albedo, accelerated snow albedo decay, and increased snowmelt rates thereby advancing the date of snow disappearance compared with the unburned forest. We demonstrate a new parameterization of post-fire snow albedo as a function of days-since-snowfall and net snowpack energy balance using an empirically based exponential decay function. Incorporating our new post-fire snow albedo decay parameterization in a spatially distributed energy and mass balance snow model, we show significantly improved predictions of snow cover duration and spatial variability of snow water equivalent across the BF, particularly during the late snowmelt period. Field measurements, snow model results, and remote sensing data demonstrate that charred forests increase the radiative forcing to snow and advance the timing of snow disappearance for several years following fire.

This study evaluated the spatial variability of trends in simulated snowpack properties across the Rio Grande headwaters of Colorado using the SnowModel snow evolution modeling system. SnowModel simulations were performed using a grid resolution of 100 m and 3-hourly time step over a 34-year period (1984 – 2017). Atmospheric forcing was provided by the Phase 2 North American Land Data Assimilation System, and the simulations accounted for temporal changes in forest canopy from bark-beetle and wildfire disturbances. Annual summary values of simulated snowpack properties (snow metrics; e.g., peak snow water equivalent (SWE), snowmelt rate and timing, and snow sublimation) were used to compute trends across the domain. Trends in simulated snow metrics varied depending on elevation, aspect, and land cover. Statistically significant trends did not occur evenly within the basin, and some areas were more sensitive than others. In addition, there were distinct trend differences between the different snow metrics. Upward trends in mean winter air temperature were 0.3°C decade-1, and downward trends in winter precipitation were -52 mm decade-1. Middle elevation zones, coincident with the greatest volumetric snow water storage, exhibited the greatest sensitivity to changes in peak SWE and snowmelt rate. Across the Rio Grande headwaters, snowmelt rates decreased by 20 percent decade-1, peak SWE decreased by 14 percent decade-1, and total snowmelt quantity decreased by 13 percent decade-1. These snow trends are in general agreement with widespread snow declines that have been reported for this region. This study further quantifies these snow declines and provides trend information for additional snow variables across a greater spatial coverage at finer spatial resolution.

As climate warms, snow-water storage is decreasing while forest fires are increasing in extent, frequency, and duration. The majority of forest fires occur in the seasonal snow zone across the western US. Yet, we do not understand the broad-scale variability of forest fire effects on snow-water storage and water resource availability. Using pre- and post-fire data from 78 burned SNOTEL stations, we evaluated post-fire shifts in snow accumulation (snow-water storage) and snowmelt across the West and Alaska. For a decade following fire, maximum snow-water storage decreased by over 30 mm, and the snow disappearance date advanced by 9 days, and in high severity burned forests snowmelt rate increased by 3 mm/day. Regionally, forest fires reduced snow-water storage in Alaska, Arizona, and the Pacific Northwest and advanced the snow disappearance date across the Rockies, Western Interior, Wasatch, and Uinta mountains. Broad-scale empirical results of forest fire effects on snow-water storage and snowmelt inform natural resource management and modeling of future snow-water resource availability in burned watersheds.

Pollutants emitted by industrial processes are deposited across the landscape. Ice core records from mid-latitude glaciers located close to emission sources document the history of local-toregional pollution since preindustrial times. Such records underpin attribution of pollutants to specific emission sources critical to developing abatement policies. Previous ice core studies from the Alps document the overall magnitude and timing of pollution related to nitrogen and sulfur-derived species, as well as a few metals including lead. Here, we used subannually resolved measurements of vanadium (V) and molybdenum (Mo) in two ice cores from Col du Dome (French Alps), as well as atmospheric transport and deposition modeling, to investigate sources of pollution in the free European troposphere. The noncrustal V and Mo (ncV, ncMo) components were calculated by subtracting the crustal component from the total concentration. These ice core results showed a 32-fold increase in ncV and a 69-fold increase in ncMo from the preindustrial era (pre-1860) to the industrial concentration peaks. Anthropogenic V and Mo emissions in Europe were estimated using emission factors from oil and coal consumption and atmospheric transport and deposition modeling. When comparing ice core data to estimated anthropogenic V and Mo emissions in Europe, V was found to be sourced primarily from oil combustion emissions. Conversely, coal and oil combustion estimated emissions did not agree with the measured ice core Mo concentrations, suggesting that other anthropogenic Mo sources dominated coal-burning emissions, particularly after the 1950s. Noncoal-burning sources of Mo may include metallurgy although emission factors are poorly known. Plain Language Summary Industrial activities release pollutants that are transported and deposited across the landscape. Such pollutants include metals that can impact wildlife, the environment, and human health. Vanadium and molybdenum metals are essential elements for life but have possible harmful effects at high concentrations. Historical anthropogenic emissions of these two metals remain uncertain. Here, we analyzed vanadium and molybdenum in two ice cores extracted from Col du Dome, located in the French Alps. Results showed an increase in pollution-sourced vanadium and molybdenum from the preindustrial era to the 1970s and 1980s. Anthropogenic vanadium and molybdenum deposition at the core site were estimated using previously published emission factors and consumption of fossil fuels combined with atmospheric transport and deposition modeling. When comparing ice core vanadium measurements to estimated deposition from anthropogenic emissions from oil combustion, good agreement was observed-supporting that vanadium pollution in the ice was sourced primarily from oil combustion in Europe. Conversely, prior thinking was that molybdenum emissions were dominated by coal combustion, but estimated deposition from fossil fuel emissions did not agree with the ice core data. We propose instead that the processing of mo...