James M. Vose

Vegetation dynamics after a prescribed fire in the southern Appalachians

Abstract In April 1995, the USDA Forest Service conducted a prescribed burn along with a south-facing slope of southern Appalachian watershed, Nantahala National Forest, western NC. Fire had been excluded for over 70 years and the purpose of the burn was to create a mosaic of fire intensities to restore a degraded pine/hardwood community and to stimulate forage production and promote oak regeneration along a hillslope gradient. Permanent plots were sampled at three locations along a gradient from 1500 to 1700xa0m. Plot locations corresponded to three community types: mesic, near-riparian cove (low slope); dry, mixed-oak (mid slope); and xeric, pine/hardwood (ridge). Before burning (1994–1995) and post-burn (summer, 1995 and summer, 1996) vegetation measurements were used to determine the effects of fire on the mortality and regeneration of overstory trees, understory shrubs, and herbaceous species. After the burn, mortality was highest (31%) at the ridge location, substantially reducing overstory (from 26.84 pre-burn to 19.05xa0m2xa0ha−1 post-burn) and understory shrub (from 6.52 pre-burn to 0.37xa0m2xa0ha−1 post-burn) basal area. At the mid-slope position, mortality was only 3%, and no mortality occurred at the low slope. Not surprisingly, percent mortality corresponded to the level of fire intensity. Basal area of Kalmia latifolia, Gaylussacia baccata, and Vaccinium spp. were substantially reduced after the fire, but density increased due to prolific sprouting. The prescribed fire had varying effects on species richness and diversity across the hillslope gradient. On the ridge, diversity was significantly increased in the understory and herb-layer, but decreased in the overstory. On the mid slope, no change was observed in the overstory, but diversity significantly decreased in the understory. On the low slope, no change was observed in the overstory or understory.

TSUGA CANADENSIS (L.) CARR. MORTALITY WILL IMPACT HYDROLOGIC PROCESSES IN SOUTHERN APPALACHIAN FOREST ECOSYSTEMS

Eastern hemlock (Tsuga canadensis (L.) Carr.) is one of the principal riparian and cove canopy species in the southern Appalachian Mountains. Throughout its range, eastern hemlock is facing potential widespread mortality from the hemlock woolly adelgid (HWA). If HWA-induced eastern hemlock mortality alters hydrologic function, land managers will be challenged to develop management strategies that restore function or mitigate impacts. To estimate the impact that the loss of this forest species will have on the hydrologic budget, we quantified and modeled transpiration over a range of tree sizes and environmental conditions. We used heat dissipation probes, leaf-level gas-exchange measurements, allometric scaling, and time series modeling techniques to quantify whole-tree and leaf-level transpiration (E(L)) of eastern hemlock. We monitored trees ranging from 9.5 to 67.5 cm in diameter along a riparian corridor in western North Carolina, USA during 2004 and 2005. Maximum rates of daily tree water use varied by diameter and height, with large trees transpiring a maximum of 178-186 kg H2O x tree(-1) x d(-1). Values of E(L) could be predicted from current and lagged environmental variables. We forecasted eastern hemlock E(L) for inventoried stands and estimated a mean annual transpiration rate of 63.3 mm/yr for the hemlock component, with 50% being transpired in the winter and spring. In typical southern Appalachian stands, eastern hemlock mortality would thus reduce annual stand-level transpiration by approximately 10% and reduce winter and spring stand-level transpiration by approximately 30%. Eastern hemlock in the southern Appalachians has two distinct ecohydrological roles: an evergreen tree that maintains year-round transpiration rates and a riparian tree that has high transpiration rates in the spring. No other native evergreen in the southern Appalachians will likely fill the ecohydrological role of eastern hemlock if widespread mortality occurs. With the loss of this species, we predict persistent increases in discharge, decreases in the diurnal amplitude of streamflow, and increases in the width of the variable source area.

Long-term patterns in vegetation-site relationships in a southern Appalachian forest'

depth, soil clay content, depth of A-horizon, potential solar radiation, and mean temperature during the growing season. Fifty percent of the variation in the vegetation distribution was explained by the site variables used in the canonical correspondence analysis. Soil organic matter, terrain shape, and elevation were the variables most strongly related to vegetation distribution. Species associated with convex terrain (upper slopes and ridges), such as Pinus rigida, Quercus coccinea, and Quercus velutina, decreased in abundance from the 1970s to the 1990s; species associated with soils having high organic matter content and deep A-horizons, such as Liriodendron tulipifera, Rhododendron maximum, and Tsuga canadensis increased in abundance. Individual species responded differently to site gradients. For example, Acer rubrum, Quercus prinus, Oxydendrum arboreum, and Nyssa sylvatica were located in the center of the ordination space (i.e., their occurrence was not related to any of the site variables), which suggests that these species are habitat generalists.

Nitrogen trace gas emissions from a riparian ecosystem in southern Appalachia

In this paper, we present two years of seasonal nitric oxide (NO), ammonia (NH3), and nitrous oxide (N2O) trace gas fluxes measured in a recovering riparian zone with cattle excluded and adjacent riparian zone grazed by cattle. In the recovering riparian zone, average NO, NH3, and N2O fluxes were 5.8, 2.0, and 76.7 ng N m(-2) S(-1) (1.83, 0.63, and 24.19 kg N ha(-1) y(-1)), respectively. Fluxes in the grazed riparian zone were larger, especially for NO and NH3, measuring 9.1, 4.3, and 77.6 ng N m(-2) S(-1) (2.87, 1.35, and 24.50 kg N ha(-1) y(-1)) for NO, NH3, and N2O, respectively. On average, N2O accounted for greater than 85% of total trace gas flux in both the recovering and grazed riparian zones, though N2O fluxes were highly variable temporally. In the recovering riparian zone, variability in seasonal average fluxes was explained by variability in soil nitrogen (N) concentrations. Nitric oxide flux was positively correlated with soil ammonium (NH4+) concentration, while N2O flux was positively correlated with soil nitrate (NO3-) concentration. Ammonia flux was positively correlated with the ratio of NH4+ to NO3-. In the grazed riparian zone, average NH3 and N2O fluxes were not correlated with soil temperature, N concentrations, or moisture. This was likely due to high variability in soil microsite conditions related to cattle effects such as compaction and N input. Nitric oxide flux in the grazed riparian zone was positively correlated with soil temperature and NO3- concentration. Restoration appeared to significantly affect NO flux, which increased approximately 600% during the first year following restoration and decreased during the second year to levels encountered at the onset of restoration. By comparing the ratio of total trace gas flux to soil N concentration, we show that the restored riparian zone is likely more efficient than the grazed riparian zone at diverting upper-soil N from the receiving stream to the atmosphere. This is likely due to the recovery of microbiological communities following changes in soil physical characteristics.

Nitrogen deposition and cycling across an elevation and vegetation gradient in southern Appalachian forests

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 12.4 kg ha−1 yr−1. In all sites canopies retained inorganic N and lost organic N; net canopy retention varied among vegetation types. The high elevation site had the greatest litterfall N, soil N transformations, soil solution N, and greater stream N exports (0.60 kg ha−1 yr−1). Low elevation sites had lower litterfall N, soil N transformations, and soil solution N. Low stream N exports (0.14 kg ha−1 yr−1) suggested N limitation. Multivariate analyses showed that abiotic variables account for up to 63% of the variation in biotic site characteristics.

Fuel consumption and fire characteristics during understory burning in a mixed white pine-hardwood stand in the Southern Appalachians.

We characterized fire behavior and fuel consumption resulting from an understory prescribed burn in a mixed eastern white pine-hardwood stand in the Southern Appalachians. Three stands were used for the treatment. Flame lengths, ranging from 0.3 to 1.5 meters (m) for backing fires and from 1.2 to 4.5 m for head fires, reached maximum heights where evergreen understory was found. Rates of spread ranged from 1.8 to 3.0 m per minute for head fires and 0.3 m per minute for backing fires. Fire intensity, measured with ceramic tiles painted with heat-sensitive paint, varied across stands. Mean peak flame temperature ranged from 129 to 290 °C. Pre-burn mass totals were similar among stands, except for stand 1, which had substantially greater humus mass than the other stands. Consumption of litter and humus layers in the forest floor was positively correlated with flame temperature. Small wood (<8 centimeters diameter) consumption was not correlated with temperature. Over all stands, 50 percent of the mass in small wood and litter was lost during burning, and 20 percent of the humus layer was consumed. The losses in the humus layer represented about 40 percent more humus mass consumption than would have occurred in a fell-and-burn treatment. The humus layer is an important nutrient reservoir for plant growth. Maintaining this layer through careful selection of burning conditions will minimize losses during burning and maintain long-term site productivity.

Imidacloprid movement in soils and impacts on soil microarthropods in southern Appalachian eastern hemlock stands.

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 in two sites at Coweeta Hydrologic Laboratory in western North Carolina. Soils differed in profile depth, total carbon and nitrogen content, and effective cation exchange capacity. We injected imidacloprid 5 cm into mineral soil, 1.5 m from infested trees, using a Kioritz soil injector. We tracked the horizontal and vertical movement of imidacloprid by collecting soil solution and soil samples at 1 m, 2 m, and at the drip line from each tree periodically for 1 yr. Soil solution was collected 20 cm below the surface and just above the saprolite, and acetonitrile-extractable imidacloprid was determined through the profile. Soil solution and extractable imidacloprid concentrations were determined by high-performance liquid chromatography. Soil solution and extractable imidacloprid concentrations were greater in the site with greater soil organic matter. Imidacloprid moved vertically and horizontally in both sites; concentrations generally declined downward in the soil profile, but preferential flow paths allowed rapid vertical movement. Horizontal movement was limited, and imidacloprid did not move to the tree drip line. We found a negative relationship between adsorbed imidacloprid concentrations and soil microarthropod populations largely in the low-organic-matter site; however, population counts were similar to other studies at Coweeta.

Terrain and Landform Influence on Tsuga canadensis (L.) Carriere (Eastern Hemlock) Distribution in the Southern Appalachian Mountains

Abstract We examined the relationships between hemlock distribution and abundance and terrain attributes for the Coweeta Basin in the southern Appalachian Mountains. Field measurements were combined with GIS mapping methods to develop predictive models of abundance and distribution of Tsuga canadensis (L.) Carrière (eastern hemlock) and evaluate the co-occurrence of Rhododendron maximum L. (rosebay) and Kalmia latifolia L. (mountain laurel). Terrain variables were derived from USGS DEM 30-meter digital maps. Elevation, slope, aspect, terrain shape index, landform, and distance from stream were calculated from field measurements and the digital data. Terrain attributes such as elevation (r2 u200a=u200a 0.97, p < 0.0001), distance to stream (r2 u200a=u200a 0.94, p < 0.0001), and terrain shape index (r2 u200a=u200a 0.61, p u200a=u200a 0.0015) were good predictors of T. canadensis abundance. Terrain shape index explained 56% of the variation in R. maximum percent aerial cover (r2 u200a=u200a 0.56, p u200a=u200a 0.005). In the Coweeta Basin, T. canadensis was distributed as few, large trees mostly concentrated in near-stream locations, and it was closely associated with R. maximum. Tsuga canadensis mortality due to Adelges tsugae Annand (hemlock wooly adelgid) will result in a minor decrease in basin-wide basal area, but will substantially reduce near-stream basal area, and will also remove the largest trees in near-stream environments. In similar landscapes across the southern Appalachians, where T. canadensis co-occurs with R. maximum, riparian shading will likely remain unchanged.

Watershed-scale responses to ozone events in a Pinus strobus L. plantation

High O3 levels during the 1984 growing season in the southern Appalachian Mountains caused extensive damage to a 28 yr old white pine plantation on a 13.4 ha watershed at the Coweeta Hydrologic Laboratory. Ozone stress effects included premature senescence and loss of foliage, stimulation of pine seedling germination, reduced basal area increment, and small but measurable increases in NO3−-N and K+ concentrations in stream water. There were no observable effects of O3 damage on nutrient concentrations of stemwood and foliage but net nutrient accumulation was reduced due to lower stemwood production. Ozone injury did not predispose trees to root pathogens or bark beetle infestations.

A Soil Temperature Model for Closed Canopied Forest Stands

We developed a soil temperature model to predict hourly temperatures at the litter-soil interface and at soil depths of 0.10 m, 0.20 m, and 1.25 m in hardwood forest stands with closed canopies. The model, which was written in BASIC on a microcomputer, uses a numerical solution for the partial differential heat-flow equation. Litter-soil interface temperature was predicted from air temperature by multiple regression. Daily temperatures at the litter-soil interface and in the soil were predicted for 2-month periods in summer, late fall, and early spring. Predictions were most accurate in summer and late fall but tended to be high in late spring. Results were in general agreement with measured values when approximate soil thermal characteristics were used. Predicted values were within 1 to 3 °C of measured soil temperatures. Soil temperature prediction could be improved by using actual thermal characteristics; however, sensitivity analyses indicate that only large variations in thermal characteristics significantly affect soil temperature predictions. The model predicts soil temperatures within the bounds (±5 °C) of most microbial activity assays and should be suitable for regulating rate functions in nutrient and carbon cycling models.