Forest fire mobilization and uptake of metals by biota temporarily exacerbates impacts of legacy mining.

While wildfires are a natural occurrence and ecosystems have evolved with fire, a changing climate is extending the wildfire season increasing the number, size and severity of fires in the western United States. In 2018, wildfire consumed 30% more area in the Western United States than the average ten years prior, and 76% more area than the average twenty years ago. These recent wildfires have impacted communities in the southwestern Rocky Mountains. In 2018, the 416 Fire, burned over 21,000 ha of public and private lands in southwestern Colorado. The 416 Fire is uniquely located in a legacy mine region in Colorado. The fire occurred in the Animas River watershed, which was already recovering from impacts of the Gold King Mine release of 2015. Three years of water quality monitoring after the 416 Fire have demonstrated elevated total and dissolved metal concentrations downstream of the burn area in Hermosa Creek and the Animas River. Following high-intensity rainstorm events, concentrations of metals such as aluminum, cadmium, iron, lead, manganese and zinc were significantly higher compared to pre-fire conditions in the burned watershed, and several metals often exceeded water quality standards for aquatic life. Macroinvertebrate monitoring in the Animas River and the main fire-impacted tributary indicate substantially altered insect communities. Macroinvertebrate tissue samples, with high concentrations of aluminum, iron, lead and nickel provide evidence that metals observed in the water column of fire-impacted streams were transferred to the benthic communities. In contrast, algae tissue from below the fire did not have elevated metals. High sediment volumes with absorbed metals from mineral rich and mined hillsides were transported to the streams and their aquatic ecosystems after the fire. Results from this study highlight the post-fire mobilization of naturally occurring metals to streams that already experience elevated metals from legacy mines, and will help in development of mitigation efforts in downstream communities.

Resilience of terrestrial and aquatic fauna to historical and future wildfire regimes in western North America.

Wildfires in many western North American forests are becoming more frequent, larger, and severe, with changed seasonal patterns. In response, coniferous forest ecosystems will transition toward dominance by fire-adapted hardwoods, shrubs, meadows, and grasslands, which may benefit some faunal communities, but not others. We describe factors that limit and promote faunal resilience to shifting wildfire regimes for terrestrial and aquatic ecosystems. We highlight the potential value of interspersed nonforest patches to terrestrial wildlife. Similarly, we review watershed thresholds and factors that control the resilience of aquatic ecosystems to wildfire, mediated by thermal changes and chemical, debris, and sediment loadings. We present a 2-dimensional life history framework to describe temporal and spatial life history traits that species use to resist wildfire effects or to recover after wildfire disturbance at a metapopulation scale. The role of fire refuge is explored for metapopulations of species. In aquatic systems, recovery of assemblages postfire may be faster for smaller fires where unburned tributary basins or instream structures provide refuge from debris and sediment flows. We envision that more-frequent, lower-severity fires will favor opportunistic species and that less-frequent high-severity fires will favor better competitors. Along the spatial dimension, we hypothesize that fire regimes that are predictable and generate burned patches in close proximity to refuge will favor species that move to refuges and later recolonize, whereas fire regimes that tend to generate less-severely burned patches may favor species that shelter in place. Looking beyond the trees to forest fauna, we consider mitigation options to enhance resilience and buy time for species facing a no-analog future.

Simulating the thermal impact of substrate temperature on ecological restoration in shallow urban rivers.

Managing river temperature in highly urbanized stream systems is critical for maintaining aquatic ecosystems and associated beneficial uses. In this work, we updated and utilized a mechanistic river temperature model, i-Tree Cool River, to evaluate the cooling impacts of two ecological restoration scenarios: (1) an alternative streambed material limecrete and (2) shading effects of tree planting in riparian areas. The i-Tree Cool River model was modified to account for diurnal fluctuations of streambed temperature, which is relevant in shallow urban streams where lack of natural shading combined with low heat capacity of the water column can make diurnal fluctuations relatively extreme. The model was calibrated and validated on a 4.2 km reach of Compton Creek in the Los Angeles River watershed, California. Two native fish, arroyo chub (Gila orcuttii) and unarmored threespine stickleback (Gasterosteus aculeatus williamsoni), were considered the target species for assessing thermal habitat suitability. Key findings include: (1) model performance was improved when accounting for diurnal fluctuations in bed temperature (R2 increased from 0.43 to 0.68); and (2) substrate rehabilitation and tree planting can potentially reduce summertime temperatures to within the documented spawning temperature thresholds for the focal fish species. Using limecrete as an alternative material for the concrete bottom decreased the median river temperature metrics: maximum weekly maximum, maximum weekly average, and minimum weekly minimum temperatures by an average of 3 °C (13%) to 20.4 °C, 19.7 °C, and 17.8 °C, respectively. Tree planting in the riparian corridor decreased the average river temperature metrics by an average of 0.9 °C (4%) to 22.7 °C, 22 °C, and 19 °C, respectively. Combining the two scenarios decreased the river temperature metrics by an average of 4 °C (18%) to 18.2 °C. Therefore, water temperature would not be a limiting factor in potential reintroduction of the focal fish species to Compton Creek if restoration were implemented. Implications of this work could be used by urban forest and water managers for restoring thermally polluted rivers in other urban areas.