Chelcy F. Miniat

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 catchments 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.

Predictive models for radial sap flux variation in coniferous, diffuse-porous and ring-porous temperate trees

Accurately scaling sap flux observations to tree or stand levels requires accounting for variation in sap flux between wood types and by depth into the tree. However, existing models for radial variation in axial sap flux are rarely used because they are difficult to implement, there is uncertainty about their predictive ability and calibration measurements are often unavailable. Here we compare different models with a diverse sap flux data set to test the hypotheses that radial profiles differ by wood type and tree size. We show that radial variation in sap flux is dependent on wood type but independent of tree size for a range of temperate trees. The best-fitting model predicted out-of-sample sap flux observations and independent estimates of sapwood area with small errors, suggesting robustness in the new settings. We develop a method for predicting whole-tree water use with this model and include computer code for simple implementation in other studies.

Simulating vegetation controls on hurricane-induced shallow landslides with a distributed ecohydrological model

The spatial distribution of shallow landslides in steep forested mountains is strongly controlled by aboveground and belowground biomass, including the distribution of root cohesion. While remote sensing of aboveground canopy properties is relatively advanced, estimating the spatial distribution of root cohesion at the forest landscape scale remains challenging. We utilize canopy height information estimated using lidar (light detecting and ranging) technology as a tool to produce a spatially distributed root cohesion model for landslide hazard prediction. We characterize spatial patterns of total belowground biomass based on the empirically derived allometric relationship developed from soil pit measurements in the Coweeta Hydrologic Laboratory, North Carolina. The vertical distribution of roots and tensile strength were sampled at soil pits allowing us to directly relate canopy height to root cohesion and use this model within a distributed ecohydrological modeling framework, providing transient estimates of runoff, subsurface flow, soil moisture, and pore pressures. We tested our model in mountainous southern Appalachian catchments that experienced a number of landslides during the 2004 hurricane season. Slope stability estimates under the assumption of spatially uniform root cohesion significantly underpredicted both the total number of landslides and the number of “false positives,” unfailed areas of the landscape that were predicted to fail. When we incorporate spatially distributed root cohesion, the accuracy of the slope stability forecast improves dramatically. With the growing availability of lidar data that can be used to infer belowground information, these methods may provide a wider utility for improving landslide hazard prediction and forecasting.

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.

Cold air drainage flows subsidize montane valley ecosystem productivity

In mountainous areas, cold air drainage from high to low elevations has pronounced effects on local temperature, which is a critical driver of many ecosystem processes, including carbon uptake and storage. Here, we leverage new approaches for interpreting ecosystem carbon flux observations in complex terrain to quantify the links between macro-climate condition, drainage flows, local microclimate, and ecosystem carbon cycling in a southern Appalachian valley. Data from multiple long-running climate stations and multiple eddy covariance flux towers are combined with simple models for ecosystem carbon fluxes. We show that cold air drainage into the valley suppresses local temperature by several degrees at night and for several hours before and after sunset, leading to reductions in growing season respiration on the order of ~8%. As a result, we estimate that drainage flows increase growing season and annual net carbon uptake in the valley by >10% and >15%, respectively, via effects on microclimate that are not be adequately represented in regional- and global-scale terrestrial ecosystem models. Analyses driven by chamber-based estimates of soil and plant respiration reveal cold air drainage effects on ecosystem respiration are dominated by reductions to the respiration of aboveground biomass. We further show that cold air drainage proceeds more readily when cloud cover and humidity are low, resulting in the greatest enhancements to net carbon uptake in the valley under clear, cloud-free (i.e., drought-like) conditions. This is a counterintuitive result that is neither observed nor predicted outside of the valley, where nocturnal temperature and respiration increase during dry periods. This result should motivate efforts to explore how topographic flows may buffer eco-physiological processes from macroscale climate change.

Streamflow response to increasing precipitation extremes altered by forest management

Increases in extreme precipitation events of floods and droughts are expected to occur worldwide. The increase in extreme events will result in changes in streamflow that are expected to affect water availability for human consumption and aquatic ecosystem function. We present an analysis that may greatly improve current streamflow models by quantifying the impact of the interaction between forest management and precipitation. We use daily long-term data from paired watersheds that have undergone forest harvest or species conversion. We find that interactive effects of climate change, represented by changes in observed precipitation trends, and forest management regime, significantly alter expected streamflow most often during extreme events, ranging from a decrease of 59% to an increase of 40% in streamflow, depending upon management. Our results suggest that vegetation might be managed to compensate for hydrologic responses due to climate change to help mitigate effects of extreme changes in precipitation.

Frequency and magnitude of selected historical landslide events in the southern Appalachian Highlands of North Carolina and Virginia: relationships to rainfall, geological and ecohydrological controls, and effects

Landsliding is a recurring process in the southern Appalachian Highlands (SAH) region of the Central Hardwood Region. Debris flows, dominant among landslide processes in the SAH, are triggered when rainfall increases pore-water pressures in steep, soil-mantled slopes. Storms that trigger hundreds of debris flows occur about every 9 years and those that generate thousands occur about every 25 years. Rainfall from cyclonic storms triggered hundreds to thousands of debris flows in 1916, 1940, 1969, 1977, 1985, and 2004. Debris flows have caused loss of life and property, and severely affected forest lands by altering forest structure and disrupting aquatic ecosystems. Forests on mountain slopes are critical in mitigating the impacts of recurring landslide events. Forest cover is an important stabilizing factor on hillslopes by intercepting precipitation, increasing evapotranspiration, and reinforcing roots. Precipitation and hillslope-scale landforms have a controlling effect on soil moisture, root strength, and debris flow hazards. Anthropogenic influences have increased the frequency of mass wasting for a given storm event above historical natural levels through changes in vegetation and disturbances on mountain slopes. Climate change that results in increased occurrences of high intensity rainfall through more frequent storms, or higher intensity storms, would also be expected to increase the frequency of debris flows and other forms of mass-wasting in the SAH. The interdisciplinary technical and scientific capacity exists to investigate, analyze, identify and delineate landslide prone areas of the landscape with increasing reliability.

Topography may mitigate drought effects on vegetation along a hillslope gradient

Ecohydrology 2017;e1825. https://doi.org/10.1002/eco.1825 Abstract Topography may mitigate drought effects on vegetation along a hillslope gradient through redistribution of soil moisture. We examined the interaction of topography, climate, soil moisture, and transpiration in a low‐elevation, mixed‐hardwood forest in the southern Appalachian Mountains. The effects of meteorological variation (wet and dry years) and topographic position (upslope and cove) were tested on daily soil moisture amplitude and recession and plot and species‐specific transpiration. Trees in the cove plot were 17% taller and had 45% greater sapwood area than those in the upslope plot. Lower rates of soil moisture recession following rainfall events were observed at the cove plot compared to the upper plot. Greater daily soil moisture amplitude and plot transpiration, even in dry years, suggest that lower slope positions may have been buffered against moderate drought. We also observed similar transpiration in Quercus spp., Carya spp., and Liriodendron tulipifera in the cove plot between dry and wet years. Plot transpiration was reduced by 51% in the dry year in the upslope plot only, and transpiration by individual species in the plot reflected this pattern, suggesting water stress in dry years may be exacerbated by topography. With drought predicted to increase for these systems, the different drought responses of species, in addition to topographic effects, may lead to complex shifts in species composition.

Projected Changes in Future Climate

Temperature in the United States has warmed over the past 100 years, with high rates of warming in Alaska (∼4.5 °C) and the West (∼1.5 °C), whereas precipitation has increased in the East and South and decreased in the Southwest. Global climate models project a steady increase in future temperature through the end of the twenty-first century. Compared to 1971 through 2000, average annual air temperature will likely increase from 0.8 to 1.9 °C by 2050, from 1.4 to 3.1 °C by 2070, and from 2.5 to 5.3 °C by 2099, where the range is bounded by the B2 (low) and A2 (high) greenhouse gas emission scenarios. Temperature increases will be higher in northern and interior areas of the United States, especially during the winter, and extreme droughts are expected to increase. Changes in precipitation are expected to be small (higher in some regions, lower in others), although potential changes in timing and spatial distribution of extreme precipitation events may occur. Sea level may rise by as much as 2 m, affecting coastal forests and human communities. Most climate models project similar climatic trends until around 2050, but diverge considerably after that. Users of climate information often represent future climate with a range of output from different climate models and emission scenarios. Given that greenhouse gas emissions will likely increase unabated for at least the next few decades, using a high emission scenario will provide a more accurate future climate for forest management and planning.

Total C and N Pools and Fluxes Vary with Time, Soil Temperature, and Moisture Along an Elevation, Precipitation, and Vegetation Gradient in Southern Appalachian Forests

The interactions of terrestrial C pools and fluxes with spatial and temporal variation in climate are not well understood. We conducted this study in the southern Appalachian Mountains where complex topography provides variability in temperature, precipitation, and forest communities. In 1990, we established five large plots across an elevation gradient allowing us to study the regulation of C and N pools and cycling by temperature and water, in reference watersheds in Coweeta Hydrologic Laboratory, a USDA Forest Service Experimental Forest, in western NC, USA. Communities included mixed-oak pine, mixed-oak, cove hardwood, and northern hardwood. We examined 20-year changes in overstory productivity and biomass, leaf litterfall C and N fluxes, and total C and N pools in organic and surface mineral soil horizons, and coarse wood, and relationships with growing season soil temperature and precipitation. Productivity increased over time and with precipitation. Litterfall C and N flux increased over time and with increasing temperature and precipitation, respectively. Organic horizon C and N did not change over time and were not correlated to litterfall inputs. Mineral soil C and N did not change over time, and the negative effect of temperature on soil pools was evident across the gradient. Our data show that increasing temperature and variability in precipitation will result in altered aboveground productivity. Variation in surface soil C and N is related to topographic variation in temperature which is confounded with vegetation community. Data suggest that climatic changes will result in altered aboveground and soil C and N sequestration and fluxes.