J. D. Knoepp

J. D. Knoepp
US Forest Service | FS · Coweeta Hydrologic Laboratory

PhD, University of Missouri-Columbia

About

74
Publications
8,118
Reads
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4,370
Citations
Introduction
Additional affiliations
August 1989 - present
US Forest Service
Position
  • Research Soil Scientist
August 1989 - June 2015
US Forest Service
Position
  • Research Soil Scientist

Publications

Publications (74)
Article
Full-text available
The critical role streamside riparian zones play in mitigating the movement of nitrogen (N) and other elements from terrestrial to aquatic ecosystems could be threatened by residential development in the southern Appalachian Mountains. Many studies have investigated the influence of agriculture on N loading to streams but less is known about the im...
Article
In the past decade, significant increases in surface water dissolved organic carbon (DOC) have been reported for large aquatic ecosystems of the Northern Hemisphere and have been attributed variously to global warming, altered hydrologic conditions, and atmospheric deposition, among other factors. We analyzed a 25-yr DOC record (1988-2012) availabl...
Article
Full-text available
In this study, we document a functional regime shift in stream inorganic nitrogen (N) processing indicated by a major change in N export from a forested watershed. Evidence from 36 years of data following experimental clearcut logging at Coweeta Hydrologic Laboratory, NC, suggests that forest disturbance in this area can cause elevation of dissolve...
Article
Full-text available
Soil and plant stable N isotope ratios (δ15N) are influenced by atmospheric N2 inputs and processes that regulate organic matter (OM) transformation and N cycling. The resulting δ15N patterns may be useful for discerning ecosystem differences in N cycling. We studied two ecosystems, longleaf pine (Pinus palustris Mill.)-wiregrass (Aristida stricta...
Article
Biogeochemical cycles are inherently linked through the stoichiometric demands of the organisms that cycle the elements. Landscape disturbance can alter element availability and thus the rates of biogeochemical cycling. Nitrification is a fundamental biogeochemical process positively related to plant productivity and nitrogen loss from soils to aqu...
Article
Uncertainty in the estimation of hydrologic export of solutes has never been fully evaluated at the scale of a small‐watershed ecosystem. We used data from the Gomadansan Experimental Forest, Japan, Hubbard Brook Experimental Forest, USA, and Coweeta Hydrologic Laboratory, USA, to evaluate many sources of uncertainty, including the precision and ac...
Article
Full-text available
The importance of the herbaceous layer in regulating ecosystem processes in deciduous forests is generally unknown. We use a manipulative study in a rich, mesophytic cove forest in the southern Appalachians to test the following hypotheses: (i) the herbaceous functional group (HFG) in meso-phytic coves accelerates carbon and nutrient cycling , (ii)...
Article
This chapter provides a synthesis of the more than 30 years of research on Watershed 7 (WS 7) at Coweeta Hydrologic Laboratory. It argues that findings from WS 7 provide important information on the management of southern Appalachians mixed-hardwood forests. The WS 7 study provides an opportunity to conduct detailed research on the effectiveness of...
Article
In 1977, Watershed 7 (WS 7) at the Coweeta Hydrologic Laboratory was clearcut and logged using a mobile cable system that could access logs up to 300 m from a road and suspend the logs completely above the ground for transport to the logging deck. Watershed (WS) 2, a 12.6-ha watershed adjacent to WS 7, served as the experimental control. This chapt...
Article
This chapter examines changes in nutrient availability in Watershed 7 (WS 7) immediately following and for 22 years after harvest. Researchers measured several indices of nutrient availability and nutrient cycling rates including total soil C and N, soil extractable cations, extractable NO3 and NH4, net soil N transformations, soil solution NO3 con...
Conference Paper
In the past decade, significant increases in surface water dissolved organic carbon (DOC) have been reported for large aquatic ecosystems of the Northern Hemisphere and have been attributed variously to global warming, recovery from acid rain, or altered hydrologic conditions. Here, we analyzed one of the longest DOC records (1988-2012) available f...
Article
Because elevated N loading can impair both terrestrial and aquatic ecosystems, understanding the abiotic and biotic controls over retention and export of dissolved inorganic N (DI N) is crucial. Long-term research has been conducted on experimental watersheds at two U.S. Forest Service experimental forests in the Appalachian region: Fernow Experime...
Article
Full-text available
The high level of spatial and temporal heterogeneity in soil N cycling processes hinders our ability to develop an ecosystem-wide understanding of this cycle. This study examined how incorporating an intensive assessment of spatial variability for soil moisture, C, nutrients, and soil texture can better explain ecosystem N cycling at the plot scale...
Article
Background and aims Climate change alters regional plant species distributions, creating new combinations of litter species and soil communities. Biogeographic patterns in microbial communities relate to dissimilarity in microbial community function, meaning novel litters to communities may decompose differently than predicted from their chemical c...
Conference Paper
Background/Question/Methods Few studies have examined how insect outbreaks affect landscape-level hydrologic and biogeochemical processes. Here, we report the hydrologic and biogeochemical effects of the invasive, exotic Hemlock Woolly Adelgid (Adelges tsugae Annand; HWA) in a headwater catchment in the southern Appalachian mountains. The study a...
Article
Full-text available
To examine whether stream nitrogen concentrations in forested reference catchments have changed over time and if patterns were consistent across the USA, we synthesized up to 44 yr of data collected from 22 catchments at seven USDA Forest Service Experimental Forests. Trends in stream nitrogen presented high spatial variability both among catchment...
Article
Full-text available
The impacts of exotic insects and pathogens on forest ecosystems are increasingly recognized, yet the factors influencing the magnitude of effects remain poorly understood. Eastern hemlock (Tsuga canadensis) exerts strong control on nitrogen (N) dynamics, and its loss due to infestation by the hemlock woolly adelgid (Adelges tsugae) is expected to...
Conference Paper
Background/Question/Methods Land use practices and natural disturbances in forested headwater catchments affect stream water chemistry with the potential to influence downstream water quality. Long term data from US Forest Service Experimental Forests and Ranges offer invaluable information to measure the effects of those perturbations and provid...
Conference Paper
Background/Question/Methods Intrinsic soil properties, as well as the microbial community therein, have been identified as potential mechanisms driving nitrification rates across landscapes. Recent literature indicates that a range of local soil properties including moisture, temperature, texture and pH drive differences in nitrogen (N) export am...
Conference Paper
Background/Question/Methods The high level of spatial and temporal heterogeneity in nitrogen cycling processes hinders our ability to develop an ecosystem-wide understanding of this cycle. Determining nitrogen cycling “hotspots” has proved to be very complicated. In the southern Appalachians nitrogen cycling varies along elevation gradients. This...
Conference Paper
Background/Question/Methods We chose a rich, mesophytic cove forest in the Coweeta Basin, western North Carolina to evaluate the role of a diverse herbaceous layer in ecosystem function. We hypothesized that the experimental removal of a functional group of species would alter ecosystem processes. In summer 1998, we established 6 replicates of he...
Article
In the Southern Appalachian Mountains of eastern USA, pine-hardwood ecosystems have been severely impacted by the interactions of past land use, fire exclusion, drought, and southern pine beetle (SPB, Dendroctonus frontalis). We examined the effects of restoration treatments: burn only (BURN); cut + burn on dry sites (DC + B); cut + burn on sub-mes...
Chapter
Many rural areas of the United States are undergoing increasing development pressure from surrounding metropolitan areas, a phenomenon described as 'exurbanization'. This study examines how stream chemistry in a traditionally rural area of the southern Appalachian Mountains is being influenced by changes in both land cover and land use. Results ill...
Article
Full-text available
Aims Wilderness and other natural areas are threatened by large-scale disturbances (e.g., wildfire), air pollution, climate change, exotic diseases or pests, and a combination of these stress factors (i.e., stress complexes). Linville Gorge Wilderness (LGW) is one example of a high elevation wilderness in the southern Appalachian region that has be...
Article
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Imidacloprid is a systemic insecticide effective in controlling the exotic pest (hemlock woolly adelgid) in eastern hemlock () trees. Concerns over imidacloprid impacts on nontarget species have limited its application in southern Appalachian ecosystems. We quantified the movement and adsorption of imidacloprid in forest soils after soil injection...
Article
Full-text available
Understanding the main and interactive effects of chronically altered resource availability and disturbance on phosphorus (P) availability is increasingly important in light of the rapid pace at which human activities are altering these processes and potentially introducing P limitation. We measured P pools and fluxes in eighteen mixed forest stand...
Conference Paper
USFS Experimental Forests and Ranges have been collecting stream hydrologic and chemistry data in reference and disturbed forested basins across the country for several decades. These sites are located across a gradient of climatic conditions and are uniquely positioned to examine stream chemistry trends during the last decades and to study water q...
Conference Paper
Full-text available
The US Environmental Protection Agency (EPA) and state agencies are attempting to develop criteria that will help monitor and sustain delivery of clean water and protect aquatic ecosystems. However, the factors that control stream nitrogen export are not well enough understood to meet this challenge. A number of approaches to generate and assess nu...
Article
Full-text available
Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in head...
Article
Eastern hemlock [Tsuga canadensis (L.) Carriere] trees serve an important ecological role in riparian ecosystems in the southern Appalachians. Significant hemlock mortality is occurring due to infestation by the hemlock woolly adelgid (HWA) (Adelges tsugae Annand), a non-native invasive pest. Our objective was to quantify the impacts of HWA and hem...
Conference Paper
Background/Question/Methods: The high level of heterogeneity in nitrogen cycling hinders our ability to develop an ecosystem- wide understanding of these processes. In southern Appalachia, processes determining nitrogen cycling (redox, C/N dynamics) are highly dependent on elevation. In this study we focused on unraveling spatial and temporal pat...
Conference Paper
Background/Question/Methods Under predictive climate change models, species’ ranges are expected to move poleward in latitude and upward in elevation with warming. Understanding the implications of this movement for biogeochemical cycles will be necessary to accurately predict global change impacts on forested ecosystems. Given the possibility th...
Conference Paper
Background/Question/Methods Under predictive climate change models, species’ ranges are expected to move poleward in latitude and upward in elevation with warming.As tree species move upslope, the leaf litter produced may present a novel litter source to the present microbial community. Given the possibility that biogeographic patterns in microbi...
Conference Paper
Background/Question/Methods Despite the implementation of various biological and chemical controls, eastern hemlock trees (Tsuga canadensis (L.) Carr.) are declining throughout much of the eastern US due to hemlock woolly adelgid (HWA). The decline is especially rapid in the southern Appalachian region of the US. We used a combination of intensive...
Article
Full-text available
We characterized structural and functional attributes along hillslope gradients in headwater catchments. We endeavored to identify parameters that described significant transitions along the hillslope. On each of four catchments, we installed eight 50 m transects perpendicular to the stream. Structural attributes included woody and herbaceous veget...
Article
There is little understanding about the role eastern US forested headwater riparian areas play in protecting aquatic habitats and water quality from impacts of side slope forest harvest. To better understand this important riparian area function, we selected three sites from management units with a 2-age regeneration silvicultural prescription loca...
Article
Full-text available
We examined nutrient cycling responses to prescribed fire on three sub-mesic, mixed-oak sites located in the Blue Ridge Physiographic province of the southern Appalachian Mountains: Alarka Laurel Branch (AL), Robin Branch (RB), and Roach Mill Branch (RM). Each study site was located within a sub-watershed that drained a first order stream. Our obje...
Article
Full-text available
Establishment of riparian buffers is an effective method for reducing nutrient input to streams. However, the underlying biogeochemical processes are not fully understood. The objective of this 4-yr study was to examine the effects of riparian zone restoration on soil N cycling mechanisms in a mountain pasture previously degraded by cattle. Soil in...
Conference Paper
Both land management and natural disturbance can dramatically affect the water quality of headwater streams and downstream rivers. For decades, researchers have studied stream hydrology and solute chemistry in disturbed and undisturbed watersheds at Experimental Forests - a continental-scale research platform that spans gradients of precipitation,...
Conference Paper
Changes in land use and management as well as natural disturbances can dramatically affect water quality in some but not all headwater streams and downstream rivers. For decades, researchers have studied stream hydrology and solute chemistry in disturbed and undisturbed watersheds at Experimental Forests - a continental-scale research platform that...
Article
Full-text available
We predicted the eff ects of sulfate (SO 4) deposition on wilderness areas designated as Class I air quality areas in western North Carolina using a nutrient cycling model (NuCM). We used three S deposition simulations: current, 50% decrease, and 100% increase. We measured vegetation, forest fl oor, and root biomass and collected soil, soil solutio...
Article
Full-text available
We studied nitrogen (N) cycling pools and processes across vegetation and elevation gradients in the southern Appalachian Mountains in SE USA. Measurements included bulk deposition input, watershed export, throughfall fluxes, litterfall, soil N pools and processes, and soil solution N. N deposition increased with elevation and ranged from 9.5 to 12...
Article
Summary Long-term measurements of soil nitrogen (N) transformations along an environmental gradient within the Coweeta Hydrologic Laboratory basin in western North Carolina showed a strong seasonal pattern and suggested that vegetation community type- through its influence on soil properties-was an important regulating factor. Our objective was to...
Article
Soil nutrient concentrations decreased in an aggrading southern Appalachian forest over a 20-year period. Construction of nutrient budgets showed significant nutrient sequestration aboveground including increased forest floor mass. We hypothesized that the changes in forest floor mass resulted from decreased litter decomposition rates because of de...
Article
In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-h...
Article
We evaluated the effects of three regeneration harvest methods on plant diversity and soil resource availability in mixed-hardwood ecosystems. The study area is in the Wine Spring Creek watershed on the Nantahala National Forest of the Southern Appalachian Mountains in western North Carolina. The regeneration treatments were: an irregular, two-aged...
Article
A five-year study of DOC concentration in a stream draining a clear-cut catchment at Coweeta Hydrologic Laboratory reported lower DOC concentration and export than in a reference stream and predicted that DOC concentration would increase until it was indistinguishable from the reference stream. We now have a 25-year record of DOC concentrations to...
Article
Full-text available
This state-of-knowledge review about the effects of fire on soils and water can assist land and fire managers with information on the physical, chemical, and biological effects of fire needed to successfully conduct ecosystem management, and effectively inform others about the role and impacts of wildland fire. Chapter topics include the soil resou...
Article
Full-text available
The mixed oak–pine ecosystems in the southern Appalachians are in decline because of a combination of drought and southern pine-beetle infestation. A commonly applied prescription for restoration of these degraded sites has been to fell all vegetation, allow it to dry, and conduct a site-preparation burn. However, there is little information on the...
Article
Understory prescribed burning is being suggested as a viable management tool for restoring degraded oak–pine forest communities in the southern Appalachians yet information is lacking on how this will affect ecosystem processes. Our objectives in this study were to evaluate the watershed scale effects of understory burning on total aboveground biom...
Conference Paper
Full-text available
Information is lacking on ecosystem effects of understory burning in mesic mixed-oak (Quercus spp.) forests of the southern Appalachians. Native Americans used periodic fires in these forests for driving game and opening the forest. In April 1998, we conducted a low- to moderate-intensity fire in a cove-hardwood forest in the Nantahala National For...
Conference Paper
Full-text available
Southern Appalachian forests are undergoing considerable change due to altered disturbance regimes. For example, fire exclusion has had a major impact on the structure and function of pine-hardwood ecosystems. Recently, fire has been prescribed for a variety of applications: 1) stand-replacement in the form of a mimicked wildfire, 2) site-preparati...
Article
Full-text available
Due to the importance of N in forest productivity ecosystem and nutrient cycling research often includes measurement of soil N transformation rates as indices of potential availability and ecosystem losses of N. We examined the feasibility of using soil temperature and moisture content to predict soil N mineralization rates (Nmin) at the Coweeta Hy...
Article
The effects of harvest intensity (sawlog, SAW; whole tree, WTH; and complete tree, CTH) on biomass and soil C were studied in four forested sites in the southeastern US (mixed deciduous forests at Oak Ridge, TN and Coweeta, NC; Pinus taeda at Clemson, SC: and P. eliottii at Bradford, FL). In general, harvesting had no lasting effects on soil C. How...
Article
Soil quality indices can help ensure that site productivity and soil function are maintained. Biological indices yield evidence of how a soil functions and interacts with the plants, animals and climate that comprise an ecosystem. Soil scientists can identify and quantify both chemical and biological soil-quality indicators for ecosystems with a si...
Article
Pine–hardwood ecosystems in the southern Appalachians are in serious decline due to fire exclusion and insect infestations. Fire has been advanced as a tool to restore these ecosystems, yet there are few studies evaluating overall ecosystem effects. Our objectives were to evaluate the effects of stand restoration burning on forest floor nitrogen (N...
Article
Full-text available
We measured nitrogen (N) transformation rates for six years to examine temporal variation across the vegetation and elevation gradient that exists within the Coweeta Hydrologic Laboratory. Net N mineralization and nitrification rates were measured using 28-day in situ closed core incubations. Incubations were conducted at various intervals, ranging...
Article
There is increasing concern about the effects of nutrient removal associated with various forest harvesting practices on long-term site productivity. We measured exchangeable soil cation concentration responses to a commercial clearcut sawlog harvest in mixed hardwoods on a 59-ha watershed in the southern Appalachians. Soils were sampled 17 months...
Article
Full-text available
Changes in surface soil C and N can result from forest management practices and may provide an index of impacts on long-term site productivity. Soil C and N were measured over time for five watersheds in the southern Appalachians: two aggrading hardwood forests, one south- and one north-facing, undisturbed since the 1920s; a white pine (Pinus sfrob...
Article
Full-text available
The existence of several different methods for measuring net N mineralization and nitrification rates and indexing N availability has raised questions about the comparability of these methods. The authors compared in situ covered cores, in situ buried bags, aerobic laboratory incubations, and tension lysimetry on control and treated plots of a pres...
Article
Full-text available
Assessing potential long-term forest productivity requires identifi- cation of the processes regulating chemical changes in forest soils. We resampled the litter layer and upper two mineral soil horizons, A and AB/BA, in two aggrading southern Appalachian watersheds 20 yr after an earlier sampling. Soils from a mixed-hardwood watershed exhibited a...
Article
Full-text available
Three paired watersheds treated with a fell and burn prescription were studied to determine the effects on soil, soil water, and stream water. Soil nitrification and mineralization were measured by in situ closed-core incubation. Soil water was collected with porous cup lysimeters placed at 30 and 60 cm depths, and water samples were collected from...