Soil moisture causes dynamic adjustments to root reinforcement that reduce slope stability

In steep soil-mantled landscapes, the initiation of shallow landslides is strongly controlled by the distribution of vegetation, whose roots reinforce the soil. The magnitude of root reinforcement depends on the number, diameter distribution, orientation and the mechanical properties of roots that cross potential failure planes. Understanding how these properties vary in space and time in forests remains a significant challenge. Here we test the hypothesis that spatio-temporal variations in root reinforcement along a hillslope occur as a function of topographic soil moisture gradients. To test this hypothesis we compared root reinforcement measurements from relatively dry, divergent noses to relatively wet, convergent hollows in the southern Appalachian Mountains, North Carolina, USA. Our initial results showed that root reinforcement decreased in areas of higher soil moisture because the tensile strength of roots decreased. A post-hoc laboratory experiment further demonstrated that root tensile strength decreased as root moisture content increased. This effect is consistent with other experiments on stem woods showing that increased water content in the cell wall decreases tensile strength. Our experimental data demonstrated that roots can adjust to changes in the external root moisture conditions within hours, suggesting that root moisture content will change over the timescale of large storm events (hours-days). We assessed the effects of the dynamic changes in root tensile strength to the magnitude of apparent cohesion within the infinite slope stability model. Slopes can be considerably less stable when precipitation-driven increases in saturated soil depth both increase pore pressures and decrease root reinforcement. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved

Impact of Climate Change on Hydrochemical Processes at Two High-Elevation Forested Watersheds in the Southern Appalachians, United States

Climate change increasingly affects primary productivity and biogeochemical cycles in forest ecosystems at local and global scales. To predict change in vegetation, soil, and hydrologic processes, we applied an integrated biogeochemical model Photosynthesis-EvapoTranspration and BioGeoChemistry (PnET-BGC) to two high-elevation forested watersheds in the southern Appalachians in the US under representative (or radiative) concentration pathway (RCP)4.5 and RCP8.5 scenarios. We investigated seasonal variability of the changes from current (1986–2015) to future climate scenarios (2071–2100) for important biogeochemical processes/states; identified change points for biogeochemical variables from 1931 to 2100 that indicate potential regime shifts; and compared the climate change impacts of a lower-elevation watershed (WS18) with a higher-elevation watershed (WS27) at the Coweeta Hydrologic Laboratory, North Carolina, United States. We find that gross primary productivity (GPP), net primary productivity (NPP), transpiration, nitrogen mineralization, and streamflow are projected to increase, while soil base saturation, and base cation concentration and ANC of streamwater are projected to decrease at the annual scale but with strong seasonal variability under a changing climate, showing the general trend of acidification of soil and streamwater despite an increase in primary productivity. The predicted changes show distinct contrasts between lower and higher elevations. Climate change is predicted to have larger impact on soil processes at the lower elevation watershed and on vegetation processes at the higher elevation watershed. We also detect five change points of the first principal component of 17 key biogeochemical variables simulated with PnET-BGC between 1931 and 2100, with the last change point projected to occur 20 years earlier under RCP8.5 (2059 at WS18 and WS27) than under RCP4.5 (2079 at WS18 and 2074 at WS27) at both watersheds. The change points occurred earlier at WS18 than at WS27 in the 1980s and 2010s but in the future are projected to occur earlier in WS27 (2074) than WS18 (2079) under RCP4.5, implying that changes in biogeochemical cycles in vegetation, soil, and streams may be accelerating at higher-elevation WS27

Streamflow response to increasing precipitation extremes altered by forest management

Published (Publication status

Changes to southern Appalachian water yield and stormflow after loss of a foundation species

ABSTRACT Few studies have examined how insect outbreaks affect landscape-level hydrologic processes. We report the hydrologic effects of the invasive, exotic hemlock woolly adelgid (HWA) in a headwater catchment in the southern Appalachian Mountains. The study watershed experienced complete mortality of an evergreen tree species, Tsuga canadensis (L.) Carr. (eastern hemlock), after infestation was first detected in 2003. Hemlock mortality resulted in a~6% reduction in basal area in the watershed, and this loss was primarily concentrated in riparian zones. We used a paired-watershed approach to quantify changes in water yield and peak stormflow using streamflow data from the infested watershed and a nearby watershed with significantly lower hemlock basal area. We hypothesized that yield would increase shortly after hemlock infestation but decrease over the longer-term. We found that annual yield did not increase significantly in any year after infestation but decreased significantly by 12·0 cm (~8%) in 2010. Monthly yield also decreased after infestation, but changes were limited to the dormant season. The decline in yield is likely to persist as hemlock is replaced by species with higher transpiration rates. Peakflow increased significantly after infestation during the two largest flow events in the post-infestation period. Changes in stormflow during extreme events may have been temporary as another evergreen, Rhododendron maximum, may have mitigated some of the changes after hemlock loss. Thus, streams draining watersheds where eastern hemlock has been lost due to HWA infestation demonstrate permanent reductions in yield and transient increases in peakflow during large-flow events. Published 2014. This article is a U.S. Government work and is in the public domain in the USA

Time lags: insights from the U.S. Long Term Ecological Research Network

Ecosystems across the United States are changing in complex ways that are difficult to predict. Coordinated long-term research and analysis are required to assess how these changes will affect a diverse array of ecosystem services. This paper is part of a series that is a product of a synthesis effort of the U.S. National Science Foundation’s Long Term Ecological Research (LTER) network. This effort revealed that each LTER site had at least one compelling scientific case study about “what their site would look like” in 50 or 100 yr. As the site results were prepared, themes emerged, and the case studies were grouped into separate papers along five themes: state change, connectivity, resilience, time lags, and cascading effects and compiled into this special issue. This paper addresses the time lags theme with five examples from diverse biomes including tundra (Arctic), coastal upwelling (California Current Ecosystem), montane forests (Coweeta), and Everglades freshwater and coastal wetlands (Florida Coastal Everglades) LTER sites. Its objective is to demonstrate the importance of different types of time lags, in different kinds of ecosystems, as drivers of ecosystem structure and function and how these can effectively be addressed with long-term studies. The concept that slow, interactive, compounded changes can have dramatic effects on ecosystem structure, function, services, and future scenarios is apparent in many systems, but they are difficult to quantify and predict. The case studies presented here illustrate the expanding scope of thinking about time lags within the LTER network and beyond. Specifically, they examine what variables are best indicators of lagged changes in arctic tundra, how progressive ocean warming can have profound effects on zooplankton and phytoplankton in waters off the California coast, how a series of species changes over many decades can affect Eastern deciduous forests, and how infrequent, extreme cold spells and storms can have enduring effects on fish populations and wetland vegetation along the Southeast coast and the Gulf of Mexico. The case studies highlight the need for a diverse set of LTER (and other research networks) sites to sort out the multiple components of time lag effects in ecosystems

Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America

Climate warming is projected to affect forest water yields but the effects are expected to vary. We investigated how forest type and age affect water yield resilience to climate warming. To answer this question, we examined the variability in historical water yields at long-term experimental catchments across Canada and the United States over 5-year cool and warm periods. Using the theoretical framework of the Budyko curve, we calculated the effects of climate warming on the annual partitioning of precipitation (P) into evapotranspiration (ET) and water yield. Deviation (d) was defined as a catchment’s change in actual ET divided by P [AET/P; evaporative index (EI)] coincident with a shift from a cool to a warm period – a positive d indicates an upward shift in EI and smaller than expected water yields, and a negative d indicates a downward shift in EI and larger than expected water yields. Elasticity was defined as the ratio of inter annual variation in potential ET divided by P (PET/P; dryness index) to inter annual variation in the EI – high elasticity indicates low d despite large range in drying index (i.e., resilient water yields), low elasticity indicates high d despite small range in drying index (i.e., non-resilient water yields). Although the data needed to fully evaluate ecosystems based on these metrics are limited, we were able to identify some characteristics of response among forest types. Alpine sites showed the greatest sensitivity to climate warming with any warming leading to increased water yields. Conifer forests included catchments with lowest elasticity and stable to larger water yields. Deciduous forests included catchments with intermediate elasticity and stable to smaller water yields. Mixed coniferous/deciduous forests included catchments with highest elasticity and stable water yields. Forest type appeared to influence the resilience of catchment water yields to climate warming, with conifer and deciduous catchments more susceptible to climate warming than the more diverse mixed forest catchments

High elevation watersheds in the southern Appalachians: Indicators of sensitivity to acidic deposition and the potential for restoration through liming

Southern Appalachian high elevation watersheds have deep rocky soils with high organic matter content, different vegetation communities, and receive greater inputs of acidic deposition compared to low elevation sites within the region. Since the implementation of the Clean Air Act Amendment in the 1990s, concentrations of acidic anions in rainfall have declined. However, some high elevation streams continue to show signs of chronic to episodic acidity, where acid neutralizing capacity (ANC) ranges from 0 to 20 µeq L-1. We studied three 3rd order watersheds (North River in Cherokee National Forest, Santeetlah Creek in Nantahala National Forest, and North Fork of the French Broad in Pisgah National Forest) and selected four to six 1st order catchments within each watershed to represent a gradient in elevation (849–1526 m) and a range in acidic stream ANC values (11–50 leq L-1). Our objectives were to (1) identify biotic, physical and chemical catchment parameters that could be used as indices of stream ANC, pH and Ca:Al molar ratios and (2) estimate the lime required to restore catchments from the effects of excess acidity and increase base cation availability. We quantified each catchment’s biotic, physical, and chemical characteristics and collected stream, O-horizon, and mineral soil samples for chemical analysis seasonally for one year. Using repeated measures analysis, we examined variability in stream chemistry and catchment characteristics; we used a nested split-plot design to identify catchment characteristics that were correlated with stream chemistry. Watersheds differed significantly and the catchments sampled provided a wide range of stream chemical, biotic, physical and chemical characteristics. Variability in stream ANC, pH, and Ca:Al molar ratio were significantly correlated with catchment vegetation characteristics (basal area, tree height, and tree diameter) as well as O-horizon nitrogen and aluminum concentrations. Total soil carbon and calcium (an indicator of parent material), were significant covariates for stream ANC, pH and Ca:Al molar ratios. Lime requirement estimates did not differ among watersheds but this data will help select catchments for future restoration and lime application studies. Not surprisingly, this work found many vegetation and chemical characteristics that were useful indicators of stream acidity. However, some expected relationships such as concentrations of mineral soil extractable Ca and SO4 were not significant. This suggests that an extensive test of these indicators across the southern Appalachians will be required to identify high elevation forested catchments that would benefit from restoration activities

COSORE: A community database for continuous soil respiration and other soil‐atmosphere greenhouse gas flux data

Globally, soils store two to three times as much carbon as currently resides in the atmosphere, and it is critical to understand how soil greenhouse gas (GHG) emissions and uptake will respond to ongoing climate change. In particular, the soil‐to‐atmosphere CO2 flux, commonly though imprecisely termed soil respiration (RS), is one of the largest carbon fluxes in the Earth system. An increasing number of high‐frequency RS measurements (typically, from an automated system with hourly sampling) have been made over the last two decades; an increasing number of methane measurements are being made with such systems as well. Such high frequency data are an invaluable resource for understanding GHG fluxes, but lack a central database or repository. Here we describe the lightweight, open‐source COSORE (COntinuous SOil REspiration) database and software, that focuses on automated, continuous and long‐term GHG flux datasets, and is intended to serve as a community resource for earth sciences, climate change syntheses and model evaluation. Contributed datasets are mapped to a single, consistent standard, with metadata on contributors, geographic location, measurement conditions and ancillary data. The design emphasizes the importance of reproducibility, scientific transparency and open access to data. While being oriented towards continuously measured RS, the database design accommodates other soil‐atmosphere measurements (e.g. ecosystem respiration, chamber‐measured net ecosystem exchange, methane fluxes) as well as experimental treatments (heterotrophic only, etc.). We give brief examples of the types of analyses possible using this new community resource and describe its accompanying R software package

Changing forest water yields in response to climate warming: results from long-term experimental watershed sites across North America

Climate warming is projected to affect forest water yields but the effects are expected to vary. We investigated how forest type and age affect water yield resilience to climate warming. To answer this question, we examined the variability in historical water yields at long‐term experimental catchments across Canada and the United States over 5‐year cool and warm periods. Using the theoretical framework of the Budyko curve, we calculated the effects of climate warming on the annual partitioning of precipitation (P) into evapotranspiration (ET) and water yield. Deviation (d) was defined as a catchment's change in actual ET divided by P [AET/P; evaporative index (EI)] coincident with a shift from a cool to a warm period – a positive d indicates an upward shift in EI and smaller than expected water yields, and a negative d indicates a downward shift in EI and larger than expected water yields. Elasticity was defined as the ratio of interannual variation in potential ET divided by P (PET/P; dryness index) to interannual variation in the EI – high elasticity indicates low d despite large range in drying index (i.e., resilient water yields), low elasticity indicates high d despite small range in drying index (i.e., nonresilient water yields). Although the data needed to fully evaluate ecosystems based on these metrics are limited, we were able to identify some characteristics of response among forest types. Alpine sites showed the greatest sensitivity to climate warming with any warming leading to increased water yields. Conifer forests included catchments with lowest elasticity and stable to larger water yields. Deciduous forests included catchments with intermediate elasticity and stable to smaller water yields. Mixed coniferous/deciduous forests included catchments with highest elasticity and stable water yields. Forest type appeared to influence the resilience of catchment water yields to climate warming, with conifer and deciduous catchments more susceptible to climate warming than the more diverse mixed forest catchments