Coweeta Hydrologic Laboratory Research Articles

Herbivore‐induced shifts in carbon and nitrogen allocation in red oak seedlings

* A dual-isotope, microcosm experiment was conducted with Quercus rubra (red oak) seedlings to test the hypothesis that foliar herbivory would increase belowground carbon allocation (BCA), carbon (C) rhizodeposition and nitrogen (N) uptake. Plant BCA links soil ecosystems to aboveground processes and can be affected by insect herbivores, though the extent of herbivore influences on BCA is not well understood in woody plants. * Microcosms containing 2-yr-old Q. rubra seedlings and soil collected from the Coweeta Hydrologic Laboratory (NC, USA) were subjected to herbivory or left as undamaged controls. All microcosms were then injected with 15N-glycine and pulsed with 13CO2. * Contrary to our hypothesis, herbivore damage reduced BCA to fine roots by 63% and correspondingly increased allocation of new C to foliage. However, 13C recoveries in soil pools were similar between treatments, suggesting that exudation of C from roots is an actively regulated component of BCA. Herbivore damage also reduced N allocation to fine roots by 39%, apparently in favor of storage in taproot and stem tissues. * Oak seedlings respond to moderate insect herbivore damage with a complex suite of allocation shifts that may simultaneously increase foliar C, maintain C rhizodeposition and N assimilation, and shift N resources to storage.

Seasonal dynamics of ectomycorrhizal fungus assemblages on oak seedlings in the southeastern Appalachian Mountains

The potential for seasonal dynamics in ectomycorrhizal (EM) fungal assemblages has important implications for the ecology of both the host trees and the fungal associates. We compared EM fungus distributions on root systems of out-planted oak seedlings at two sites in mixed southeastern Appalachian Mountain forests at the Coweeta Hydrologic Laboratory in North Carolina, from samples taken in mid-July and early September. Species level EM fungus type specificity, and identification in some cases, was enabled by direct sequencing of the mycobionts from the seedling roots. Seventy-four EM fungal ITS types were documented, most of which occurred only in the midsummer or early-fall samples, respectively. Cenococcum geophilum (morphotyped) was ubiquitously present and accounted for the majority of root tips sampled. Abundance and relative frequency of types other than C. geophilum were significantly higher in the July samples, while C. geophilum was significantly more frequent and abundant in September. Several generalistic dominants were found fairly equally at both sites and on both sample dates. Other taxa with relatively high frequency were recovered from both sites and tree seedling species, but were reliable indicators occurring primarily in the July sample (e.g., Laccaria cf laccata). Notable shifts in mycobiont dominance were apparent in relation to sample date, including increases in Cortinarius spp. richness, decreases in Thelephoraceae richness, and the disappearance of Amanita spp. types in the early fall compared to midsummer samples. However, diversity and rarity were high and differences in overall community composition (other than C. geophilum) by season were not significant based on multi-response permutation procedures. Although these results based on a single growing season are preliminary, changes in abundance and frequency, detection of significant indicator species, and the apparent systematic affinities of shifting EM types support the potential for seasonal variability in EM associations in this system.

Invasive woolly adelgid appears to drive seasonal hemlock and carcass inputs to a detritus-based stream

Key words: eastern hemlock (Tsuga canadensis)eastern U. S. forestsinvasive specieslife cyclelitterfallstreamswoolly adelgid (Adelges tsugae)

Insect herbivores and their frass affect Quercus rubra leaf quality and initial stages of subsequent litter decomposition

Defoliation‐induced changes in plant foliage are ubiquitous, though factors mediating induction and the extent of their influence on ecosystem processes such as leaf litter decomposition are poorly understood. Soil nitrogen (N) availability, which can be affected by insect herbivore frass (feces), influences phytochemical induction. We conducted experiments to test the hypotheses that insect frass deposition would (1) reduce phytochemical induction following herbivory and (2) increase the decomposition and nutrient release of the subsequent leaf litter. During the 2002 growing season, 80 Quercus rubra saplings were subjected to a factorial experiment with herbivore and frass manipulations. Leaf samples were collected throughout the growing season to measure the effects of frass deposition on phytochemical induction. In live foliage, herbivore damage increased tannin concentrations early, reduced foliar N concentrations throughout the growing season, and lowered lignin concentrations in the late season. Frass deposition apparently reduced leaf lignin concentrations, but otherwise did not influence leaf chemistry. Following natural senescence, litter samples from the treatment groups were decomposed in replicated litterbags for 18 months at the Coweeta Hydrologic Laboratory, NC. In the dead litter samples, initial tannin concentrations were lower in the herbivore damage group and higher in the frass addition group relative to their respective controls. Tannin and N release rates in the first nine months of decomposition were also affected by both damage and frass. However, decomposition rates did not differ among treatment groups. Thus, nutrient dynamics important for some ecosystem processes may be independent from the physical loss of litter mass. Overall, while lingering effects of damage and even frass deposition can therefore carry over and affect ecosystem processes during decomposition, their effects appear short lived relative to abiotic forces that tend to homogenize the decomposition process.

Solute geochemical mass-balances and mineral weathering rates in small watersheds: Methodology, recent advances, and future directions

Solute-based geochemical mass balance methods are commonly used in small-watershed studies to estimate rates of a variety of geochemical processes at the Earth’s surface, including primary-mineral weathering and soil formation, and the quantitative contribution of these elemental transfer processes to cation budgets, nutrient cycling, and landscape susceptibility to acid deposition. Weathering rates of individual minerals in watershed mass-balance studies are determined by solving a system of simultaneous linear geochemical mass-balance equations with constant (stoichiometric) coefficients. These equations relate the measured net fluxes to the (known) stoichiometries and (unknown) rates of weathering reactions for multiple minerals in the weathering profiles. Solving the system of equations requires petrologic, mineralogic, hydrologic, botanical, and aqueous geochemical data. The number of mineral-weathering rates that can be determined is limited by the number of elements for which solute mass-balance equations can be written. In addition to calculating mineral weathering rates, elemental transfer into or out of the biomass may also be calculated. Elemental uptake by aggrading forest vegetation can act as an intrawatershed sink for at least some mineral-derived cations, producing mineral weathering rates higher than would be estimated from solute fluxes alone; similarly, element release from decaying forest biomass can result in higher solute fluxes than are produced by weathering alone. The mathematics of, significant contributions from, role of biomass in, and recent advances in, watershed geochemical mass-balance methods are discussed using examples from the Appalachian headwaters watersheds of the Coweeta Hydrologic Laboratory in the southern Blue Ridge Physiographic Province of North Carolina, USA.

Regulation of nitrogen mineralization and nitrification in Southern Appalachian ecosystems: Separating the relative importance of biotic vs. abiotic controls

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 determine the relative effects of biotic vs. abiotic factors on soil N transformations. During the 1999 and 2000 growing seasons we transplanted soil cores from each of the five gradient plots to all other gradient plots for their 28-day in situ incubation. N mineralization and nitrification rates in soils from the northern hardwood (NH) site were significantly increased when soils were transplanted to warmer sites. N mineralization rates also increased in transplanted soil from the dry mixed-oak/pine site to a wetter site. Multiple regression analysis of N mineralization from all five sites found that biotic (total soil N and C:N ratios) and climatic factors (moisture and temperature) regulate N mineralization. Regression analyses of individual sites showed that N mineralization rates responded to variation in temperature and moisture at only the high elevation northern hardwood site and moisture alone on the dry warm mixed-oak/pine site. N mineralization was unrelated to temperature or moisture at any of the other sites. Results indicate that soil properties plus climatic conditions affect soil N transformations along the environmental gradient at Coweeta. Environmental controls were significant only at the extreme sites; i.e., at the wettest and warmest sites and soils with highest and lowest C and N contents. The high degree of temperature sensitivity for the northern hardwood soils indicates potentially large responses to climatic change at these sites.

Effects of understory prescribed burning on shortleaf pine (Pinus echinata Mill.)/mixed-hardwood forests

Abstract Elliott, K. J. and J. M. Vose (Coweeta Hydrologic Laboratory, Southern Research Station, USDA Forest Service, Otto, NC 28763). Effects of prescribed burning on shortleaf pine (Pinus echinata Mill.)/mixed-hardwood forests. J. Torrey Bot. Soc. 132: 236–251. 2005.—We examined the effects of a single dormant season fire on overstory and understory species diversity and composition and tree seedling regeneration patterns the first and second years following a prescribed burn in the Conasauga River Watershed of southeastern Tennessee and northern Georgia. We asked: Can a single dormant season fire initiate a trajectory of overstory and understory species change consistent with restoring Pinus echinata/mixed-oak/bluestem (Andropogon gyrans and Schizachyrium scoparium)-grass community types? Six sub-watersheds (similar in vegetation, soil type, stream size, and disturbance history) were located within the Conasauga River Watershed; four of the sites were burned in March 2001, and two sites were designate...

Allanite and epidote weathering at the Coweeta Hydrologic Laboratory, western North Carolina, U.S.A.

Allanite and epidote occur in the parent rocks of weathered regolith at the Coweeta Hydrologic Laboratory in North Carolina and exhibit different responses to weathering. Petrographically, epidote and allanite are identical at Coweeta, and only with additional analytical techniques (e.g., EDS or LA-ICP-MS) can the two be distinguished. Allanite is more abundant in unweathered bedrock but weathers readily below the weathering front. In contrast, the much less abundant epidote persists into the solum with only minimal evidence of weathering. The rapid dissolution of allanite is likely influenced by its metamict nature. Both epidote and allanite at Coweeta dissolve by interface-controlled dissolution kinetics, evidenced by etch pits on grain surfaces. Etch pits appear to be either large “negative crystals,” or small, shallow, and elongated. The incipient stage of allanite weathering is characterized by Al mobility and Fe precipitation as goethite. During the initial stage of allanite weathering, carbonate precipitates, but with progressive weathering the carbonate is dissolved. Based on electron microprobe analyses and point-count data of the Ca-bearing phases in the Coweeta bedrock, accessory (<1%) allanite contains a minimum of approximately 25% of the total bedrock Ca by volume, whereas garnet and plagioclase contain 5% and 70%, respectively. Although allanite and epidote are Ca-hosts, only allanite is present in significant quantities, and is weathering sufficiently rapidly, to serve as an important source of Ca in pore and stream waters at Coweeta. Allanite weathering should, therefore, be evaluated in weathering studies of crystalline silicate bedrock, especially where other lines of evidence indicate the need to invoke additional Ca sources.

Rates and time scales of clay-mineral formation by weathering in saprolitic regoliths of the southern Appalachians from geochemical mass balance

Rates of clay formation in three watersheds located at the Coweeta Hydrologic Laboratory, western North Carolina, have been determined from solute flux-based mass balance methods. A system of mass balance equations with enough equations and unknowns to allow calculation of secondary-mineral formation rates as well as the more commonly determined primary-mineral dissolution rates was achieved by including rare earth elements (REE) in the mass balance. Rates of clay-mineral formation determined by mass balance methods have been used to calculate the time needed for a 5% (50 g kg−1) change in relative clay abundance in the saprolite at Coweeta; this corresponds to the “response time” of the clay mineral to, for example, a change in climate. The 5% change in relative clay abundance is the smallest change that can generally be detected using X-ray diffraction (XRD). Response times range from tens of thousands to hundreds of thousands of years. Extrapolating the Coweeta clay formation rates to other southern Appalachian regoliths, the time required to form measured clay abundances (“production times”) in eastern Blue Ridge and Inner Piedmont regolith have been calculated. The production times of clay-mineral assemblages range from 2 k.y. to 2 m.y., with mean values ranging from 50 k.y. to 1 m.y. The results of this study are consistent with the arguments of [Thiry (2000)][1] that the best resolution of the paleoclimatic record in marine clay-rich sediments and mudrocks is ∼1 or 2 m.y. [1]: #ref-107

INSECT CANOPY HERBIVORY AND FRASS DEPOSITION AFFECT SOIL NUTRIENT DYNAMICS AND EXPORT IN OAK MESOCOSMS

Increased nitrogen (N) mobilization and export from terrestrial forest ecosystems following canopy herbivory have been well documented, though the mechanism behind the loss is not clear. Because carbon (C) and N dynamics are closely linked, herbivore activity may also affect C distribution. We initiated a replicated mini-ecosystem experiment to test the hypothesis that insect frass (feces) influences soil C and N dynamics following insect defoliation. One hundred and sixty red oak (Quercus rubra) saplings were transplanted to seven-gallon (26.5-L) pots with soil and litter from the Coweeta Hydrologic Laboratory (CWT) (Otto, North Carolina, USA) and overwintered in experimental pot stands. During the 2002 growing season, trees were subjected to a 3 × 2 factorial experimental design with three damage groups (herbivore, mechanical, “undamaged”) and two frass depositions (frass, no frass). Frass deposition increased soil total C, total N, and the soil NH4+ pool. Leachate NO3− export also increased following frass additions. We suggest that herbivore frass mobilizes sufficient C and N to affect soil pools and N export, though abiotic factors may influence the ultimate fate of the nutrients in frass. In addition, herbivory increased soil respiration and decreased total soil N relative to “undamaged” controls independent of frass deposition. While we discuss four possible mechanisms for this observation, we hypothesize that the increased soil respiration results from enhanced root-exudate C and subsequent microbial oxidation. This mechanism has implications for C sequestration and N retention in forest soils. In addition, the effects of mechanical damage consistently did not match those of real herbivory, suggesting that differential responses of Q. rubra to damage types also may affect soil nutrient dynamics. Our results demonstrate that the feeding activity of insect herbivores can have direct and indirect effects on the cycling of C and N within the season of defoliation.

Effects of nutrient enrichment on the decomposition of wood and associated microbial activity in streams

Summary1. We determined the effects of nutrient enrichment on wood decomposition rates and microbial activity during a 3‐year study in two headwater streams at Coweeta Hydrologic Laboratory, NC, U.S.A. After a 1‐year pretreatment period, one of the streams was continuously enriched with inorganic nutrients (nitrogen and phosphorus) for 2 years while the other stream served as a reference. We determined the effects of enrichment on both wood veneers and sticks, which have similar carbon quality but differ in physical characteristics (e.g. surface area to volume ratios, presence of bark) that potentially affect microbial colonisation and activity.2. Oak wood veneers (0.5 mm thick) were placed in streams monthly and allowed to decompose for approximately 90 days. Nutrient addition stimulated ash‐free dry mass loss and increased mean nitrogen content, fungal biomass and microbial respiration on veneers in the treatment stream compared with the reference. The magnitude of the response to enrichment was great, with mass loss 6.1 times, and per cent N, fungal biomass and microbial respiration approximately four times greater in the treatment versus reference stream.3. Decomposition rate and nitrogen content of maple sticks (ca. 1–2 cm diameter) also increased; however, the effect was less pronounced than for veneers. Wood response overall was greater than that determined for leaves in a comparable study, supporting the hypothesis that response to enrichment may be greater for lower quality organic matter (high C : N) than for higher quality (low C : N) substrates.4. Our results show that moderate nutrient enrichment can profoundly affect decomposition rate and microbial activity on wood in streams. Thus, the timing and availability of wood that provides retention, structure, attachment sites and food in stream ecosystems may be affected by nutrient concentrations raised by human activities.

Evergreen Understory Dynamics in Coweeta Forest, North Carolina

A number of studies have elucidated the distributional patterns of various components of Southern Appalachian forests. The evergreen understory here is composed largely of a dominant ericaceous shrub, Rhododendron maximum L., which is believed to be expanding and inhibiting the development of other species with consequent impacts on overall forest structure and composition. We use a GIS and logistic regression to examine this less-studied forest element in the Coweeta Hydrological Laboratory, North Carolina, over a 17-year period to determine whether expansion is occurring and, if so, whether it can be predicted based on terrain characteristics. We examine two adjacent, physically similar basins with differing degrees of experimental manipulation in the 20th century in an attempt to examine the role of environmental and historical factors in determining spatial patterns of persistence, expansion, and decline of the evergreen understory. Results indicate that significant expansion of the evergreen understory occurred in both basins during the period 1976-1993, and that stream proximity, topographic setting, and elevation are related to patterns of evergreen-understory dynamics. Patterns differ between the two basins, suggesting that disturbance and differing land-use histories are also influential. Predictive power of models based on terrain factors alone ranges from <10% to >50%.

Effects of whole-stream nutrient enrichment on the concentration and abundance of aquatic hyphomycete conidia in transport

The concentrations and relative abundances of aquatic hyphomycete conidia in water were followed during a three-year study in two headwater streams at Coweeta Hydrologic Laboratory, North Carolina, using the membrane-filtration technique. After a one-year pretreatment period, one of the streams was enriched continuously with inorganic nutrients (N+P) for two years while the other stream served as the reference. This ecosystem-level nutrient manipulation resulted in concentrations of aquatic hyphomycete conidia in the water of the treated stream that were 4.5–6.9 times higher than the concentrations observed during the pretreatment period and in the reference stream. Nutrient enrichment led to an increase in the number of fungal species detected on each sampling date. Changes in dominance patterns and relative abundances of individual species also were detected after treatment. Nutrient addition stimulates the reproductive activity of aquatic hyphomycetes, their colonization success and fungal-mediated leaf-litter decomposition. Such changes in the activity of the fungal community might affect higher trophic levels in lotic ecosystems.

Effects of Whole-Stream Nutrient Enrichment on the Concentration and Abundance of Aquatic Hyphomycete Conidia in Transport

The concentrations and relative abundances of aquatic hyphomycete conidia in water were followed during a three-year study in two headwater streams at Coweeta Hydrologic Laboratory, North Carolina, using the membrane-filtration technique. After a one-year pretreatment period, one of the streams was enriched continuously with inorganic nutrients (N+P) for two years while the other stream served as the reference. This ecosystem-level nutrient manipulation resulted in concentrations of aquatic hyphomycete conidia in the water of the treated stream that were 4.5–6.9 times higher than the concentrations observed during the pretreatment period and in the reference stream. Nutrient enrichment led to an increase in the number of fungal species detected on each sampling date. Changes in dominance patterns and relative abundances of individual species also were detected after treatment. Nutrient addition stimulates the reproductive activity of aquatic hyphomycetes, their colonization success and fungal-mediated leaf-litter decomposition. Such changes in the activity of the fungal community might affect higher trophic levels in lotic ecosystems.

Chemical weathering indices applied to weathering profiles developed on heterogeneous felsic metamorphic parent rocks

Chemical weathering indices are commonly used for characterizing weathering profiles by incorporating bulk major element oxide chemistry into a single metric for each sample. Generally, on homogeneous parent rocks, weathering indices change systematically with depth. However, the weathering of heterogeneous metamorphic rocks confounds the relationship between weathering index and depth. In this paper, we evaluate previously defined chemical weathering indices for their suitability in characterizing weathering profiles developed on felsic, heterogeneous, metasedimentary bedrock from Coweeta Hydrologic Laboratory, North Carolina, USA. Among the weathering indices evaluated here, the Weathering Index of Parker (WIP) is the most appropriate for application to weathering profiles on heterogeneous (and homogeneous) parent rock. Because the WIP includes only the highly mobile alkali and alkaline earth elements in its formulation, it yields values that differ greatly from those of the parent rock. In addition, the WIP allows for aluminum mobility, unlike other weathering indices. These characteristics combine to make the Weathering Index of Parker the most applicable index for studying the weathering of heterogeneous metasedimentary rocks. However, the WIP should be applied judiciously, as alkali and alkaline earth metals may be readily depleted during weathering. In addition to reflecting weathering, the Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), Plagioclase Index of Alteration (PIA) and Vogt's Residual Index (V) may be sensitive to subtle geochemical changes such as hydrothermal alteration along a fault and/or alteration at the water table. Weathering indices that include iron in their calculations were not included in this study, because they do not distinguish between ferric and ferrous iron. The oxidation state and distribution of iron in a weathering profile is influenced by the proportion of oxidation states in the pre-weathered metamorphic rock and interaction with groundwater during weathering.

Predicting daily flows in ungauged catchments: model regionalization from catchment descriptors at the Coweeta Hydrologic Laboratory, North Carolina

AbstractRegionalization approaches to daily streamflow prediction are investigated for 13 catchments in the Coweeta Hydrologic Laboratory using a conceptual rainfall‐runoff model of low complexity (six parameters). Model parameters are considered to represent the dynamic response characteristics (DRCs) of a catchment. It is demonstrated that all catchments within the region cannot be assumed to have a similar hydrological behaviour, and thence a regionalization approach considering differences in physical catchment descriptors (PCDs) is required. Such a regionalization approach can be regarded as a top‐down method, in the sense that factors controlling parameter variability are identified first within the entire region under study, and then such information is exploited to predict runoff in a smaller sub‐region. Regionalization results reveal that consideration of interrelations between dependent variables, which here are the parameters of the rainfall‐runoff model, can improve performance of regression as a regionalization method. Breaking the parameter correlation structure inherent in the model, and exploiting merely relationships between model parameters and PCDs (no matter how weakly related they are), can result in a significant decrease in regionalization performance. Also, high significance of regression between values of PCDs and DRCs does not guarantee a set of parameters with a good predictive power. When there is a reason to believe that, in the sense of hydrological behaviour, a gauged catchment resembles the ungauged catchment, then it may be worthwhile to adopt the entire set of calibrated parameters from the gauged catchment instead of deriving quantitative relationships between catchment descriptors and model parameters. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Changes in stream nitrate concentrations due to land management practices, ecological succession, and climate: Developing a systems approach to integrated catchment response

This study uses time series analysis to examine long‐term stream water nitrate concentration records from a pair of forested catchments at the Coweeta Hydrologic Laboratory, North Carolina, USA. Monthly average concentrations were available from 1970 through 1997 for two forested catchments, one of which was clear‐felled in 1977 and the other maintained as a control. The time series were decomposed into their trend and annual cycle before modeling as an autoregressive (AR) process. AR models were calculated for both an expanding and a shifting window so that prefelling could be directly compared with the effects of tree clearance. In comparison with flow records for both of the catchments, transfer function‐noise models were calculated on a moving window basis, and the impulse functions were derived. Analysis shows that both catchments show an annual memory effect but that the clear‐felled catchment shows, in addition, a 6‐month memory effect. The annual effect in the control catchment responds to drought conditions while in the felled catchment, it reflects the change in vegetation. The 6‐month effect in the felled catchment responds to drought conditions independent of both the annual effect and of logging operations. The control catchment shows no significant impulse function with respect to flow, while for the felled catchment a distinct impulsivity develops over time subsequent to logging and coincident with the onset of nitrogen saturation. By examining the nature of the nitrate export, rather than solely the levels of export, a systems approach can be taken to understanding catchment behavior. Such an approach shows that the catchment is in metastable equilibrium with respect to its hydrological pathways and nitrogen reserves but in dynamic equilibrium with respect to long‐term temperature change. The onset of nitrogen saturation may represent irreversible changes in the catchment behavior, and impulsivity, with respect to streamflow, represents a new indicator of N‐saturated conditions.

Hillslope Nutrient Dynamics Following Upland Riparian Vegetation Disturbance

We investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K+, Na+, Ca2+, Mg2+, NO3 −-N, NH4 +-N, PO4 3−-P, and SO4 2− were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO3 −-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO4 2−, Ca2+, and Mg2+. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO3 −-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO3 −-N increased four fold, and streamwater NO3 −-N doubled. Significant changes also occurred in soilwater pH, DOC, SO4 2−, Ca2+, and Mg2+. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO3 −-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams.

Effects of endemic densities of canopy herbivores on nutrient dynamics along a gradient in elevation in the southern Appalachians

In southern Appalachian forests, outbreaks of insect herbivores have been shown repeatedly to increase the availability of nutrients in soil and the export of nitrate in forest streams. The mechanisms underlying herbivore-induced changes in nutrient dynamics include inputs of insect frass (feces) and modification of precipitation as it passes through the forest canopy (throughfall). Here, we consider the effects of endemic (non-outbreak) populations of insect herbivores on soil processes in the southern Appalachians. We measured inputs of frass and throughfall at three elevations at the Coweeta Hydrologic Laboratory, North Carolina. We also measured soil nutrient availability and soil respiration. Inputs of total frass, frass nitrogen and frass carbon exhibited early- and late-season peaks, with those peaks occurring earlier at low elevation where leaf flush begins first. The C:N ratio of frass generally increased over time at all elevations, presumably reflecting seasonal declines in foliar nitrogen. Nitrate in throughfall generally increased over time, whereas throughfall phosphate declined and throughfall ammonium remained relatively constant. Relationships among frass deposition and throughfall nutrients varied with elevation. At low elevation, frass nitrogen was strongly correlated with throughfall nitrate, but this relationship was absent at mid and high elevation. The relationships between frass deposition and throughfall ammonium were inconsistent among elevations. The availabilities of nitrate and ammonium in soil were both related to frass deposition. For example, frass deposition in May explained about 62 % of the variance in soil nitrate availability. Soil respiration exhibited summer maxima at all elevations and was related primarily to soil temperature. There was also a weak positive relationship between the C:N ratio of frass and soil respiration. Overall, we suggest that endemic densities of canopy herbivores can influence forest soil processes, but that the relationships exhibit pronounced spatial and temporal variability.

Response of soil invertebrates to forest canopy inputs along a productivity gradient

Previous studies have suggested that herbivory in forest canopies can influence forest floor processes such as nutrient cycling and decomposition. We studied the response of litter decomposition to a moisture/productivity gradient with manipulations of the effects of canopy herbivory at the Coweeta Hydrologic Laboratory, North Carolina. Litterbags containing Quercus rubra L. and Acer rubrum L. litter were placed at three elevations along the gradient and sampled monthly for two years. Microarthropods, nematodes, and litter mass loss responses to the productivity gradient were measured. The relative abundance of Collembola and three suborders of mites (Oribatida, Mesostigmata and Prostigmata) was compared across the gradient. Mass loss was greater at the middle and high elevation sites in both years and was correlated with increased numbers of oribatid mites per gram of litter. The abundance of all the above microarthropods (of which oribatids were the most common) was also greater on the middle and high elevation sites and greater on two- year -old litter than on one- year- old litter. Nematode densities were also greater on the older litter. The herbivore inputs study, simulating the effects of canopy herbivory, included frass additions, throughfall additions, greenfall exclusion, total litter exclusion, and controls. Experimental additions of frass to plots on the low and middle elevation sites led to an increase in Collembola abundance in litterbags from those plots. Plots with frass and artificial throughfall additions also showed increased numbers of fungal feeding and bacterial feeding nematodes in some months. Numbers of oribatid and prostimatid mites were reduced in litter exclusion plots. Results from these studies suggest not only significant influences of elevation on litter decomposition and soil fauna abundance but direct links between canopy herbivory and responses in population densities of forest floor biota.