Southern Appalachian Forests Research Articles

Rhododendron thickets alter N cycling and soil extracellular enzyme activities in southern Appalachian hardwood forests

Summary Rhododendron maximum L., a spreading understory shrub, inhibits overstory regeneration and alters forest community structure in southern Appalachian hardwood forests. Using paired plots and reciprocal litter transplants in forests with and without R. maximum cover, we examined the influence of R. maximum on litter mass and quality, N cycling and soil extracellular enzymes. Standing stocks of soil organic matter, soil N, leaf litter mass and fine root biomass were greater in forests with R. maximum than those without. Tannin extracts from R. maximum foliage, and leaf litter and fine roots collected under R. maximum had a relatively high capacity to precipitate protein compared to extracts from trees. Across the growing season, soil inorganic N availability was generally lower under R. maximum , mostly due to reduced NO 3 − availability. Our data suggest that R. maximum litter alters N cycling through the formation of recalcitrant polyphenol–organic N complexes. Soil extracellular enzymes indicate the potential processing rates of organic substrates. Between forest types, polyphenol oxidase activity was greatest in R. maximum O horizons, regardless of litter type, suggesting that the local microbial community can better degrade and access protein–tannin-complexed N. Protease activity did not differ between forest types, but was greater on R. maximum leaf litter than hardwood leaf litter. The alteration of the N cycle via the formation of polyphenol–organic N complexes may contribute to hardwood seedling suppression, while the enzymatic release of these complexes by ericoid mycorrhizal fungi may increase N acquisition for R. maximum and contribute to its expansion in southern Appalachian forests.

The effect of Appalachian mountaintop mining on interior forest

Southern Appalachian forests are predominantly interior because they are spatially extensive with little disturbance imposed by other uses of the land. Appalachian mountaintop mining increased substantially during the 1990s, posing a threat to the interior character of the forest. We used spatial convolution to identify interior forest at multiple scales on circa 1992 and 2001 land-cover maps of the Southern Appalachians. Our analyses show that interior forest loss was 1.75–5.0 times greater than the direct forest loss attributable to mountaintop mining. Mountaintop mining in the southern Appalachians has reduced forest interior area more extensively than the reduction that would be expected based on changes in overall forest area alone. The loss of Southern Appalachian interior forest is of global significance because of the worldwide rarity of large expanses of temperate deciduous forest.

Response of white-footed mice ( Peromyscus leucopus) to fire and fire surrogate fuel reduction treatments in a southern Appalachian hardwood forest

An experiment conducted as part of the multidisciplinary National Fire and Fire Surrogate Study was designed to determine effects of three fuel reduction techniques on small mammals and habitat structure in the southern Appalachian mountains. Four experimental units, each >14-ha were contained within each of three replicate blocks at the Green River Game Land, Polk County, NC. Treatments were (1) prescribed burning (B); (2) mechanical felling of shrubs and small trees (M); (3) mechanical felling + burning (MB); (4) controls (C). Mechanical understory felling treatments were conducted in winter 2001–2002, and prescribed burning was conducted in March 2003. After treatment, there were fewer live trees, more snags, and greater canopy openness in MB than in other treatments. Leaf litter depth was reduced by burning in both B and MB treatments, and tall shrub cover was reduced in all fuel reduction treatments compared to C. Coarse woody debris pieces and percent cover were similar among treatments and controls. We captured 990 individuals of eight rodent species a total of 2823 times. Because white-footed mice composed >79% of all captures, we focused on this species. Populations in experimental units increased 228% on average between 2001 and 2002, but there was no evidence of an effect of the mechanical treatment. From 2002 to 2003, all units again showed an average increase in relative population size, but increases were greater in MB than in the other treatments. Age structure and male to female ratio were not affected by the fuel reduction treatment. Average adult body weight declined from 2001 to 2002, but less so in M than in units that remained C in both years. The proportion of mice captured near coarse woody debris was similar to the proportion captured in open areas for all treatments, indicating that white-footed mice did not use coarse woody debris preferentially or change their use patterns in response to fuel reduction treatments. Land managers should understand possible effects of different fuel reduction treatments on white-footed mouse populations, as they are an important component of the fauna and food chain of deciduous southern Appalachian forests.

Interactions between past land use, life-history traits and understory spatial heterogeneity

Past land use has contributed to variability in the distribution of herbaceous species by reducing plant abundance and altering species’ chances of recolonizing suitable habitat. Land use may also influence plant heterogeneity by changing environmental conditions within stands. We compared the variability of understory herb abundance in southern Appalachian forests with different land-use histories to examine how past land use influenced plant heterogeneity. The cover of eleven focal species or genera was estimated and mineral soil concentrations were determined during 2001 and 2002 in eight stands that were farmed, logged, or had no disturbance history (reference) in western North Carolina. Analysis of the coefficients of variation revealed that the abundance of understory plants was more heterogeneous in disturbed stands compared with reference stands. However, when nutrient availability differences were accounted for by detrending the plant cover data, understory variability within stands declined, and no differences between disturbed and reference stands could be distinguished. This finding suggests that nutrient availability has important effects on plant heterogeneity, which depend on past land use. Species dispersal, seed size, and phenology also explained variability in the spatial heterogeneity of plants, but generally only before soil nutrient differences were statistically controlled. In addition to demonstrating that past land use has longterm effects on plant heterogeneity, these results indicate that soil nutrients may play different roles in determining vegetation patterns in historically altered and unaltered forests.

Previous land use alters plant allocation and growth in forest herbs

Summary Former human practices can persistently influence forest ecosystems, particularly by altering the distribution and abundance of vegetation. Previous research has focused on the role of colonization success in governing plant community patterns in abandoned forests, but few studies have explored how changes in the performance of adult plants may contribute to changes in plant populations. We examined patterns of biomass allocation and growth of 12 herbaceous plant species in southern Appalachian forest stands that have developed after agricultural abandonment or logging at least 55 years ago, to determine whether plant performance varied with land‐use history. Soil nutrient availability and canopy closure were also investigated. Adult plant biomass allocation varied appreciably among stands with different histories. Herbs in farmed stands generally allocated the most to leaves and the least to stems, while reference stands showed the opposite pattern. Plants in previously farmed sites had the highest rate of growth, although we observed considerable interspecific variation in plant performance. Stem allocation and relative growth rate were positively correlated in reference stands, but not in farmed or logged stands. Similarly, the growth of plants was clearly associated with soil nutrient concentration in reference stands but not in farmed or logged stands. Differences in understorey density and soil nutrient availability may account for the observed patterns. Total herbaceous cover was appreciably lower in farmed and logged stands (58% and 51%) than in reference stands (78%), and soil phosphorus was higher in farmed stands than in logged and reference stands. Thus, competition for light and nutrients may be lower in farmed and logged stands than in reference stands, despite there being no difference in canopy closure with land‐use history. Overall, these results suggest that land‐use history may influence environmental variables in ways that can enhance the performance of some herbaceous species. However, not all species may respond similarly to these changes.

MICROBIAL COMMUNITY VARIATION AND ITS RELATIONSHIP WITH NITROGEN MINERALIZATION IN HISTORICALLY ALTERED FORESTS

Past land use can impart soil legacies that have important implications for ecosystem function. Although these legacies have been linked with microbially mediated processes, little is known about the long-term influence of land use on soil microbial communities themselves. We examined whether historical land use affected soil microbial community composition (lipid profiles) and whether community composition was related to potential net nitrogen (N) mineralization rates in southern Appalachian (USA) forest stands abandoned from agriculture or logging and reforested >50 yr ago. Microbial community composition was determined by a hybrid procedure of phospholipid fatty acid (PLFA) and fatty acid methyl ester (FAME) analysis. We found that community composition varied significantly with past land use. Communities in formerly farmed stands had a higher relative abundance of markers for gram-negative bacteria and a lower abundance of markers for fungi compared with previously logged and reference (i.e., no disturbance history) stands. Potential net N mineralization rates were negatively correlated with fungal and gram-negative bacterial markers in both farmed and reference stands, and fungal abundance and soil bulk density effectively predicted mineralization rates in all stands. Our results indicate that the alteration of microbial communities by historical land use may influence the ecosystem processes they mediate. This is in contrast to typical expectations about microbial community resilience to change. Here, the decrease in fungal abundance observed from disturbance appeared to result in decreased nitrogen mineralization over the long term.

Gap size and succession in cutover southern Appalachian forests: an 18 year study of vegetation dynamics

We used clearcut logging in establishing four replicated sizes of canopy openings (0.016, 0.08, 0.4, and 2.0 ha) in a southern Appalachian hardwood forest in 1981 to examine the long-term effects of disturbance size on plant community structure, biomass accumulation, aboveground net primary productivity (NPP), and mode of recovery. The reestablishment of NPP and biomass following logging was 6–7-fold greater in large than small openings by 17 years. Total biomass in the 2.0 ha openings (127.3 Mg ha−1) recovered 59.5% as NPP (19.7 Mg ha−1 yr−1) reached 225% of precut forest levels. Biomass accumulation was 2.6–3.6-fold greater in interior than edge locations of all but the 0.016 ha gaps. The absence of significant patch size or edge vs. interior differences in tree densities suggests that growth rates of individual trees were enhanced in more insolated microenvironments. Sprouting (86–95% of tree NPP) was much more important than advance regeneration (4–10%) or seedling germination (<2%) during early recovery in all opening sizes. Canopy dominant Quercus and Carya trees exhibited limited sprouting following disturbance. Instead, shade-intolerant Robinia pseudoacacia and Liriodendron tulipifera were major sprouters that used N-fixation (Robinia) and rapid growth (Liriodendron) in attaining 7.4 and 5.9 fold greater biomass accumulation, respectively in 2.0 ha than 0.016 ha opening sizes. Seedling germination and understory production were extensive in all openings following logging, but declined rapidly as the young tree canopy began closing by 4–6 years. The relative importance of shade-intolerant tree biomass approximately doubled over 17 years as shade-tolerant tree seedlings, herbs, and shrubs gradually regained importance under the emerging canopy. Sprouting caused the persistence of a tree species composition in all openings that remained relatively similar to the precut forest. Large disturbances on mountain slopes of the southern Appalachians generally promote sprouting and rapid recovery, whereas small disturbances in low-elevation cove forests lead to a gradual recovery through seedling germination and/or advance regeneration. Continued logging in the southern Appalachians will increase the relative size and frequency of large disturbances, further the importance of sprouting of shade-intolerant species, and lead to more even-aged forest stands throughout the region.

Land Use Legacies and the Future of Southern Appalachia

ABSTRACT Southern Appalachian forests have apparently recovered from extractive land use practices during the 19th and 20th centuries, yet the legacy of this use endures in terrestrial and aquatic systems of the region. The focus on shallow time or the telling of stories about the past circumscribes the ability to anticipate the most likely outcomes of the trajectory of change forecast for the Southeast as the “Old South” continues its transformation into the “New South.” We review land use research of the Coweeta Long Term Ecological Research (LTER) project that addresses the nature and extent of past and present human land use, how land use has affected the structure and function of terrestrial and aquatic communities, and the forces guiding the anticipated trajectory of change. Unlike development in the western or northeastern regions of the United States, the southeastern region has few practical, political, or geographical boundaries to the urban sprawl that is now developing.

Long-term changes in forest floor processes in southern Appalachian forests

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 decreased litter quality. In 1992 and 1993, we repeated a litter decomposition experiment conducted in 1969 and 1970 to test this hypothesis. In addition, we examined microarthropod populations and functional groups as litter decomposed. For four of the five species tested, first-year decomposition rates were about the same in both experiments. Initial litter nutrient concentrations of P were lower in all tree species in the most recent sampling. N, Ca, and Mg concentrations also declined in some species. These declines often resulted in decreased nutrient release rates during decomposition. Microarthropod populations differed significantly among litter species, as well as between years (probably resulting from differences in growing-season rainfall). For some litter species we found significant relationships between microarthropod populations and nutrient concentration (primarily C and N); however, most r 2-values were low. Data suggest that changes in forest floor mass probably resulted from decreased litter quality and that those changes may have an effect on microarthropod populations.

Investigating the Allelopathic Potential of Kalmia latifolia L. (Ericaceae)

Evergreen, understory shrubs, often members of the Ericaceae, have been implicated in the suppression of tree recruitment in many ecosystems. One possible mechanism of this suppression could be an allelopathic interaction between shrubs and seedlings. We tested the allelopathic potential of Kalmia latifolia L., an important component of southern Appalachian forest understories. Aqueous extracts of soil, humus, K. latifolia litter, mixed forest floor litter, and green leaves from K. latifolia did not inhibit germination or initial root elongation of our test species Lactuca sativa L. (lettuce). Root, shoot, and total biomass were reduced in P. rigida seedlings grown in forest floor substrate collected beneath K. latifolia compared to those grown in forest substrate without K. latifolia. While these differences were not significant, they could be exacerbated in the field when seedlings are exposed to multiple stressors (reduced light, water, and nutrient availability). Our findings suggest that allelopathy is not a strong mechanism in the inhibition of canopy tree recruitment by K. latifolia.

Carbon limitation leads to suppression of first year oak seedlings beneath evergreen understory shrubs in Southern Appalachian hardwood forests

Inhibition of canopy tree recruitment beneath thickets of the evergreen shrubs Rhododendron maximum L. and Kalmia latifolia L. has long been observed in Southern Appalachian forests, yet the mechanisms of this process remain unresolved. We present a first-year account of suppression of oak seedlings in relation to Rhododendron and Kalmia basal area, light and resource availability, seedling performance and the rates of seedling damage i.e., herbivory. We found no evidence of first-year seedling suppression or significant resource deficiencies beneath thickets of K. latifolia in mature mixed hardwood stands. Suppression beneath R. maximum was apparent during the first growing season. We found that seedling biomass, light availability prior to canopy closure, and seedling tissue C:N ratios were negatively correlated with R. maximum basal area. Basal area of R. maximum was positively correlated with seedling mortality rates, soil Al, and early-growing season leaf herbivory rates. Seedling growth was positively correlated with light and tissue C:N, while negatively correlated with soil Al. Overall, our results support the inhibition model of shade-mediated carbon limitation beneath dense understory shrubs and indicate the potential importance of herbivory and aluminum toxicity as components of a suppression mechanism beneath R. maximum thickets. We present a causal model of first year inhibition beneath R. maximum in the context of our findings and the results of prior studies.

EFFECTS OF PAST LAND USE ON SPATIAL HETEROGENEITY OF SOIL NUTRIENTS IN SOUTHERN APPALACHIAN FORESTS

We examined patterns of nutrient heterogeneity in the mineral soil (0–15 cm depth) of 13 southern Appalachian forest stands in western North Carolina &gt;60 yr after abandonment from pasture or timber harvest to investigate the long‐term effects of land use on the spatial distribution and supply of soil resources. We measured soil carbon (C), nitrogen (N), acid‐extractable phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg) concentrations and pools, and potential net N mineralization and nitrification rates to evaluate differences in mean values, variance at multiple scales, and fine‐scale spatial structure.While comparisons of averaged values rarely indicated that historical land use had an enduring effect on mineral soil or N cycling, differences in variance and spatial structure suggested that former activities continue to influence nutrient distributions by altering their spatial heterogeneity. Patterns differed by element, but generally variance of soil C, N, and Ca decreased and variance of soil P, K, and Mg increased with intensive past land use. Changes in variance were most conspicuous and consistent locally (&lt;28 m), but C, Ca, P, and Mg also exhibited appreciable differences in variance at coarser scales (&gt;150 m). High variability in soil compaction resulted in some changes in scale‐dependent patterns of nutrient pool variance compared with nutrient concentration variance. It also affected the variance of N cycling rates, such that mass‐based rates varied less and area‐based rates varied more in intensively used areas than in reference stands. Geostatistical analysis suggested that past land use homogenized the spatial structure of soil C, K, and P in former pastures. In contrast, logged stands had highly variable spatial patterning for Ca.These results suggest that land use has persistent, multi‐decadal effects on the spatial heterogeneity of soil resources, which may not be detectable when values are averaged across sites. By interacting with patterns of variability in the plant and heterotrophic biota, differences in nutrient distribution and supply could alter the composition and diversity of forest ecosystems. Scale‐dependent changes in nutrient heterogeneity could also complicate efforts to determine biogeochemical budgets and cycling rates.

Using hyperdocuments to manage scientific knowledge: the prototype Encyclopedia of Southern Appalachian Forest Ecosystems

Despite the overwhelming body of research available on the ecology and management of Southern Appalachian forests, a gap exists between what scientists know and what the management community is able to apply on the ground. Most research knowledge still resides in highly technical, narrowly focused research publications housed in libraries. The internet, combined with increasingly sophisticated hyperdocument authoring systems, makes web-based hyperdocuments a practical and affordable way to manage this scientific knowledge. The USDA Forest Service developed the Encyclopedia of Southern Appalachian Forest Ecosystems (ESAFE; http://www.forestencyclopedia.net), a hyperdocument-based encyclopedia system available on the internet, to address this need for more accessible, understandable, condensed, and synthesized research knowledge. This project aims to synthesize what we know scientifically about the management and ecology of Southern Appalachian forest ecosystems, organize it logically, and make it universally available at no cost to users. ESAFE is composed of original summaries of hundreds of topic areas compiled from over 5000 literature sources by over 15 authors specifically for this purpose. Presently, ESAFE has over 1100 pages of content that includes over 150 tables, 150 figures, 3000 internal hyperlinks, and 1800 external hyperlinks. Unlike most internet-based hyperdocuments, quality control of the encyclopedia is ensured through a complete peer-review process similar to traditional scientific journals. The encyclopedia is built upon a dynamic content management system (CMS), developed using Zope software, that provides a platform for authoring, editing, reviewing, publishing, and updating content directly through the internet. This CMS creates a mechanism for updating the site with peer-review content directly through the internet, so that the knowledge base can be continually updated, expanded, and improved. Using tools like ESAFE, busy forest managers can more easily find answers to questions from their own desks. It has been favorably evaluated by a diverse group of land managers, the general public, and ecosystem scientists. It is also currently being used as a prototype for several other forestry-related hypertext encyclopedias.

Soricid Response to Canopy Gaps Created by Wind Disturbance in the Southern Appalachians

We used drift fences with pitfall traps to compare soricid abundance, richness, and demographic parameters among intact multiple-tree windthrow gaps, salvaged gaps, and mature forest in a xeric southern Appalachian forest type during 1997–1999. We also tested whether capture rates were correlated with rainfall, and whether similar-sized species did not co-occur as predicted by multi-species assemblage rules. We captured six species: Blarina brevicauda (Say) (northern short-tailed shrew), Cryptotis parva (Say) (least shrew), Sorex cinereus Kerr (masked shrew), S. fumeus G.M. Miller (smoky shrew), S. hoyi Baird (pygmy shrew), and S. longirostris Bachman (southeastern shrew). Tree basal area, forest structural features, and arthropod abundance differed among treatments, but species richness and abundance of most shrews did not. Captures during June–October were higher in 1998 than in 1997, although 1998 summer rainfall was low. Rainfall was correlated with shrew activity, but explained little of the variability in capture rates (r2 = 0.05). The sex ratio within each species was similar among gap treatments and controls, but differed from 1:1 for S. hoyi, S. longirostris, and S. fumeus. The soricid assemblage did not conform to multi-species assemblage rules; three small-bodied species co-occurred in similar numbers at all study sites. Our results suggest that forest management that mimics conditions created by multiple windthrows in xeric forest of the southern Appalachians is unlikely to affect shrew communities adversely, at least in the short-term.

Red Spruce Dynamics in an Old Southern Appalachian Forest

BUSING, R.T. (U.S. Geological Survey, 200 SW 35h St., Corvallis, OR 97333). Red spruce dynamics in an old southern Appalachian forest. J. Torrey Bot. Soc. 131: 337-342. 2004.-By the late 1980s the composition and structure of forest stands in the southern Appalachian spruce-fir zone were altered by insect infestations to Fraser fir. The response of red spruce, the sole remaining coniferous forest dominant, to this disturbance was followed over twenty years (1983-2003) in an old spruce-fir forest at Mt. Collins, Great Smoky Mountains National Park. Although diameter growth of canopy red spruce (>30 cm dbh) at six plot sites was considerable (mean 10-yr increment 2.1 cm; 1993-2003), red spruce mortality increased sharply (mean 4% yr-'; 1993-2003). Wind-related mortality of canopy red spruce was substantial after the loss of Fraser fir from the canopy circa 1985 (>70% of the dead spruce had broken or uprooted boles; 1983-2003). Wind damage to red spruce was observed at most plot sites, but it was most pronounced on exposed topographic positions, where canopy gap expansion was extensive. The elevated mortality of red spruce at Mt. Collins was not associated with reduced diameter growth. Altered canopy structure has left large red spruce vulnerable to high winds. With the loss of canopy fir and the subsequent increase in mortality of canopy spruce, total live basal area has declined to about half of its pre-disturbance level.

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%.

Population recovery following differential harvesting of Allium tricoccum Ait. in the southern Appalachians

Over 100 native herbaceous forest perennials are harvested in the United States. Management of these populations for continued survival requires information on the long-term effects of varied harvesting levels on populations of forest perennials. Allium tricoccum Ait. is a species that is harvested throughout its range for its edible bulb. We examined the recovery of A. tricoccum populations over a 5-year period following experimental harvests of different intensity to predict the number of years required to return to pre-harvest levels and to establish guidelines for sustainable harvesting. Our harvest treatments included removal of 0, 25, 50, 75, and 100% of plants within experimental plots. Treatments were repeated at three sites. Population recovery was monitored for 4 years at two sites and 7 years at one site. We found a relationship between the level of harvest and the negative impacts on populations of A. tricoccum, with limited population recovery in the 4 years following harvesting. We projected recovery times as a function of harvest level using a population projection model. Deterministic projections of recovery time ranged from 148 years for a 95% harvest to 2.5 years for a 5% harvest. Incorporating stochasticity into our projections increased the uncertainty in our estimates of recovery time. For example the recovery time from a 5% harvest ranged from 1 to 89 years (95% confidence limit). Based on our results, a 10% harvest once every 10 years would, on average, be a sustainable level of harvest for A. tricoccum in southern Appalachian forests.

Regenerative Response of a Southern Appalachian Forest to Surface Wildfire and Canopy Gap Disturbances

This study examines the independent and combined effects of canopy gaps and fire on tree regeneration in southern Appalachian mixed hardwood stands by analyzing an area in northern Georgia that burned in the fall of 1999. Two hypotheses, the gap-partitioning and density hypotheses, that explain how spatial variation in regeneration following canopy gap disturbance maintains species diversity in mature forests were tested. In the understory, species diversity, density, the mean height of understory stems, and species composition differed significantly between burned and unburned plots. Species diversity and the mean height of understory stems were lower in burned plots, although density was higher in burned plots relative to unburned plots. Shade-intolerant species, Liriodendron tulipifera L., Robinia pseudo-acacia L., and Sassafras albidum (Nuttall) Nees., had much greater abundance in the burned understory, while moderately shade-tolerant species exhibited mixed responses to burning. Neither the gap-partitioning nor the density hypothesis were supported by evidence collected for this research, and the effect of burning did not influence the rejection of these hypotheses.

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.

Effects ofRhododendron maximumThickets on Tree Seed Dispersal, Seedling Morphology, and Survivorship

In the southern Appalachian forests, the regeneration of canopy trees is severely inhibited by Rhododendron maximum L., an evergreen understory shrub producing dense thickets. While light availability is a major cause, other factors may also contribute to the absence of tree seedlings under R. maximum. We examined the effects of R. maximum on several life history stages of tree species, including seed dispersal, seed bank germination, seedling growth, and survivorship. We found no significant effect of R. maximum on seed reaching the forest floor for Acer rubrum, Liriodendron tulipifera, Quercus rubra, Quercus prinus, Carya spp., and Nyssa sylvatica. This indicates that either seed output of maternal trees rooted within the thicket were unaffected by R. maximum or seed dispersal from surrounding areas into thickets compensated for a lower seed production of canopy trees rooted in the thickets. Germination of tree seeds (A. rubrum, L. tulipifera, Q. rubra, and Betula lenta) from the seed bank also was not reduced by leaves and substrates within the thickets. Seedling mortality of all species (Q. rubra, Prunus serotina, and Tsuga canadensis) planted in our experimental plots was up to fivefold higher in thickets of R. maximum compared with those outside the thickets. The order of mortality under the R. maximum thickets, $$Prunus> Quercus> Tsuga$$, was consistent with the shade tolerance ranking of these species. Loss of Tsuga seedlings was attributed to burial by litter rather than shade. Surviving seedlings of Quercus and Prunus in R. maximum thickets were taller than those outside the thickets, but the seedlings in R. maximum thickets produced significantly fewer leaves, smaller total leaf area, leaf mass, and stem mass. Leaf N (%) was significantly higher in Quercus seedlings in R. maximum thickets compared with seedlings outside the thickets. Moreover, no difference was found in leaf N (%) between forest types for Prunus and Tsuga, indicating that seedlings in R. maximum thickets were not N limited. Rather, light limitation, herbivory, and litter fall contributed to the lack of tree regeneration under R. maximum thickets.