Frequency, magnitude and connectivity of post-wildfire rainfall-runoff and sediment transport, The

2019 Fall.Includes bibliographical references.Wildfire increases the likelihood of runoff, erosion, and downstream sedimentation in many of the watersheds that supply water for communities across the western U.S. The goal of this research was to examine the complex interactions between fire, rainfall and landscape properties (e.g., burn severity, topography) across scales from hillslopes to watersheds. The research combines both regional data analysis and field monitoring to examine the frequency, magnitude and connectivity of post-fire rainfall-runoff events and associated sediment delivery. In the first part of this study (chapter 2), the goal was to quantify rainfall thresholds that cause runoff and sediment delivery across multiple fires, years post-fire, spatial scales, and mulch treatments in the Colorado Front Range. Rain intensity thresholds were identified for plots, hillslopes, and watersheds across three Colorado Front Range fires. Thresholds did not significantly differ among fires for any year post-fire, but were significantly different between spatial scales and years post-fire. Thresholds increased with time since burn likely due to vegetation regrowth, litter accumulation and recovery of soil infiltration capacity. The frequency of storms exceeding thresholds for runoff and erosion was mapped across Colorado to provide a tool for identifying areas most vulnerable to post-fire runoff and sediment delivery and prioritizing post-fire treatments. In chapter three, the goal was to improve understanding of the catch efficiency of sediment fences commonly used to measure post-fire hillslope erosion. During post-fire year two (2014) of the 2012 High Park Fire four sediment fences were modified to collect and measure both the sediment deposited behind the fence and the amount of runoff and sediment that overtopped the fence. Sediment fence catch efficiency ranged from 28-100% for individual events and from 38-94% across the sampling season. Increasing rainfall intensities were correlated with greater runoff and total sediment loads and lower sediment fence catch efficiencies. Enrichment ratios indicate that the sediment behind the fence was significantly enriched in sand relative to the hillslope soil samples. These results indicate that sediment fences underestimate sediment yields and demonstrate how sediment particle sizes may be sorted en route to the stream network. In chapter four, the goal was to examine connectivity between hillslopes and channel networks. Runoff and sediment from nested hillslopes (n = 31) and catchments (n = 12) were assessed for two rainfall events with different duration and intensity during post-fire year three (2015) of the High Park Fire to determine the factors affecting connectivity. The first event had a return interval of <1 year with low intensity rainfall over an average of 11 hours, whereas the second event had high intensity rainfall that lasted for an average of 1 hour with a maximum return interval of 10 years. The lower intensity event led to low hillslope sediment yields and widespread channel incision. The higher intensity event led to infiltration excess overland flow, high sediment yields and in-stream sediment deposition and fining. During both events, the percent of a catchment that burned at high severity was positively correlated with sediment delivery ratios and area-normalized absolute channel change. Overall, this research demonstrated that the rainfall events and thresholds associated with the generation of post-fire runoff and sediment transport vary with spatial scale and time since burn. In addition, not every threshold-exceeding event will produce the same type of response due to the complex and transient nature of post-fire responses from hillslope to watershed scale. Increasing our understanding of post-fire responses and connectivity will therefore require nested multi-scale monitoring over time to determine how sediment moves to and through channel networks

Controls on Streamflow Densities in Semiarid Rocky Mountain Catchments

Developing accurate stream maps requires both an improved understanding of the drivers of streamflow spatial patterns and field verification. This study examined streamflow locations in three semiarid catchments across an elevation gradient in the Colorado Front Range, USA. The locations of surface flow throughout each channel network were mapped in the field and used to compute active drainage densities. Field surveys of active flow were compared to National Hydrography Dataset High Resolution (NHD HR) flowlines, digital topographic data, and geologic maps. The length of active flow declined with stream discharge in each of the catchments, with the greatest decline in the driest catchment. Of the tributaries that did not dry completely, 60% had stable flow heads and the remaining tributaries had flow heads that moved downstream with drying. The flow heads were initiated at mean contributing areas of 0.1 km2 at the lowest elevation catchment and 0.5 km2 at the highest elevation catchment, leading to active drainage densities that declined with elevation and snow persistence. The field mapped drainage densities were less than half the drainage densities that were represented using NHD HR. Geologic structures influenced the flow locations, with multiple flow heads initiated along faults and some tributaries following either fault lines or lithologic contacts.</p

The Ecosystem Services and Biodiversity of Novel Ecosystems: A literature review

Scientists, policy makers, and managers use ecosystem services and biodiversity metrics to inform management goals of novel ecosystems. Fragmented knowledge of the ecosystem services provided by novel ecosystems contributes to disagreement over these systems and how they should be managed. To address this gap, we conducted a systematic review of refereed articles to understand how novel ecosystems have changed ecosystem services and biodiversity. Despite anthropogenic drivers of change, we found that the literature on novel ecosystems is focused on ecological rather than social aspects of novel systems. Our review highlights the frequency that novel ecosystems enhance both ecosystem services and biodiversity. More than two-thirds of studies reported biodiversity equal to or above the reference state, while the portion of studies reporting increased cultural, provisioning, and regulating services was even greater. Still, we urge caution in interpreting these trends, as they exist in part due to degraded ecosystem baselines and inconsistent framing. Finally, the wide range of management recommendations we reviewed reflects both the diversity of novel ecosystems and substantial disagreement among researchers and managers about what novel ecosystems actually mean for society

The ecosystem services and biodiversity of novel ecosystems: A literature review

Scientists, policy makers, and managers use ecosystem services and biodiversity metrics to inform management goals of novel ecosystems. Fragmented knowledge of the ecosystem services provided by novel ecosystems contributes to disagreement over these systems and how they should be managed. To address this gap, we conducted a systematic review of refereed articles to understand how novel ecosystems have changed ecosystem services and biodiversity. Despite anthropogenic drivers of change, we found that the literature on novel ecosystems is focused on ecological rather than social aspects of novel systems. Our review highlights the frequency that novel ecosystems enhance both ecosystem services and biodiversity. More than two-thirds of studies reported biodiversity equal to or above the reference state, while the portion of studies reporting increased cultural, provisioning, and regulating services was even greater. Still, we urge caution in interpreting these trends, as they exist in part due to degraded ecosystem baselines and inconsistent framing. Finally, the wide range of management recommendations we reviewed reflects both the diversity of novel ecosystems and substantial disagreement among researchers and managers about what novel ecosystems actually mean for society. Keywords: Novel-ecosystems, Biodiversity, Ecosystem-services, Trade-offs, Revie

The role of warm, dry summers and variation in snowpack on phytoplankton dynamics in mountain lakes

Climate change is altering biogeochemical, metabolic, and ecological functions in lakes across the globe. Historically, mountain lakes in temperate regions have been unproductive because of brief ice-free seasons, a snowmelt-driven hydrograph, cold temperatures, and steep topography with low vegetation and soil cover. We tested the relative importance of winter and summer weather, watershed characteristics, and water chemistry as drivers of phytoplankton dynamics. Using boosted regression tree models for 28 mountain lakes in Colorado, we examined regional, intraseasonal, and interannual drivers of variability in chlorophyll a as a proxy for lake phytoplankton. Phytoplankton biomass was inversely related to the maximum snow water equivalent (SWE) of the previous winter, as others have found. However, even in years with average SWE, summer precipitation extremes and warming enhanced phytoplankton biomass. Peak seasonal phytoplankton biomass coincided with the warmest water temperatures and lowest nitrogen-to-phosphorus ratios. Although links between snowpack, lake temperature, nutrients, and organic-matter dynamics are increasingly recognized as critical drivers of change in high-elevation lakes, our results highlight the additional influence of summer conditions on lake productivity in response to ongoing changes in climate. Continued changes in the timing, type, and magnitude of precipitation in combination with other globalchange drivers (e.g., nutrient deposition) will affect production in mountain lakes, potentially shifting these historically oligotrophic lakes toward new ecosystem states. Ultimately, a deeper understanding of these drivers and pattern at multiple scales will allow us to anticipate ecological consequences of global change better

The Role of Warm, Dry Summers and Variation in Snowpack on Phytoplankton Dynamics in Mountain Lakes

Climate change is altering biogeochemical, metabolic, and ecological functions in lakes across the globe. Historically, mountain lakes in temperate regions have been unproductive because of brief ice-free seasons, a snowmelt-driven hydrograph, cold temperatures, and steep topography with low vegetation and soil cover. We tested the relative importance of winter and summer weather, watershed characteristics, and water chemistry as drivers of phytoplankton dynamics. Using boosted regression tree models for 28 mountain lakes in Colorado, we examined regional, intraseasonal, and interannual drivers of variability in chlorophyll a as a proxy for lake phytoplankton. Phytoplankton biomass was inversely related to the maximum snow water equivalent (SWE) of the previous winter, as others have found. However, even in years with average SWE, summer precipitation extremes and warming enhanced phytoplankton biomass. Peak seasonal phytoplankton biomass coincided with the warmest water temperatures and lowest nitrogen-to-phosphorus ratios. Although links between snowpack, lake temperature, nutrients, and organic-matter dynamics are increasingly recognized as critical drivers of change in high-elevation lakes, our results highlight the additional influence of summer conditions on lake productivity in response to ongoing changes in climate. Continued changes in the timing, type, and magnitude of precipitation in combination with other globalchange drivers (e.g., nutrient deposition) will affect production in mountain lakes, potentially shifting these historically oligotrophic lakes toward new ecosystem states. Ultimately, a deeper understanding of these drivers and pattern at multiple scales will allow us to anticipate ecological consequences of global change better

Learn from the burn: The High Park Fire 5 years later

It has been 5 years since the High Park Fire burned over 85,000 acres in Northern Colorado, causing extensive property damage, loss of life, and severe impacts to the water quality of the Poudre River. In the fall of 2016, a conference was organized by the USFS Rocky Mountain Research Station and the Coalition for the Poudre River Watershed to discuss what has been learned about our response to the fire. Topics covered (and discussed in this article) included how treatment areas were prioritized; the main effects of the High Park fire; the effectiveness of postfire treatments, and where to find the most recent postfire treatment planning tools online