Rhododendron Maximum Research Articles

Temperature dependence of leaf breakdown in streams differs between organismal groups and leaf species.

Increased temperatures are altering rates of organic matter (OM) breakdown in stream ecosystems with implications for carbon (C) cycling in the face of global change. The metabolic theory of ecology (MTE) provides a framework for predicting temperature effects on OM breakdown, but differences in the temperature dependence of breakdown driven by different organismal groups (i.e., microorganisms vs. invertebrate detritivores) and litter species remain unresolved. Over two years, we conducted 12 60-day leaf litterbag incubations in 20 headwater streams in the southern Appalachian Mountains (USA). We compared temperature dependence (as activation energy, Ea) between microbial and detritivore-mediated breakdown, and between a highly recalcitrant (Rhododendron maximum) and a relatively labile (Acer rubrum) leaf species. Detritivore-mediated breakdown had a higher Ea than microbial breakdown for both leaf species (Rhododendron: 1.48 > 0.56 eV; Acer: 0.97 > 0.29 eV), and Rhododendron breakdown had a higher Ea than Acer breakdown for both organismal groups. Similarly, the Ea of total (coarse-mesh) Rhododendron breakdown was higher than the Ea of total Acer breakdown (0.89 > 0.52 eV). These effects for total breakdown were large, implying that the number of days to 95% mass loss would decline by 40% for Rhododendron and 26% for Acer between 12°C (our mean temperature value) and 16°C (+4°C, reflecting projected increases in global surface temperature due to climate change). Despite patterns in Ea, overall breakdown rates were higher for microbes than detritivores, and for Acer than Rhododendron over most of our temperature gradient. Additionally, the Ea for a subset of the microbial breakdown data declined from 0.40 to 0.22 eV when fungal biomass was included as a model predictor, highlighting the key role of fungi in determining the temperature dependence of litter breakdown. Our results imply that, as streams warm, routing of leaf litter C to detritivore-mediated fates will increase faster than predicted by previous studies and MTE, especially for labile litter. As temperatures rise, earlier depletion of autumn-shed, labile leaf litter combined with rapid breakdown rates of recalcitrant litter could exacerbate seasonal resource limitation and alter carbon storage and transport dynamics in temperate headwater stream networks.

Leaf Discoloration in Rhododendron Species Exposed to Phytophthora Cinnamomi Corresponds with Future Mortality

Abstract Phytophthora cinnamomi, which causes the disease root rot, is an oomycete pathogen that is damaging to woody plants, including many horticulturally important groups, such as Rhododendron. Infecting the root of plants, Phytophthora cinnamomi inhibits water uptake, leading to root damage, wilting, and increased rates of plant mortality. Some observations suggest that P. cinnamomi infection corresponds to changes in leaf coloration, though whether this indicates a plant stress response or plant damage is generally unknown. We used leaf color analysis to test for differences in leaf discoloration between plants inoculated with the pathogen and control plants. We demonstrate a significant link between leaf discoloration in Rhododendron species and Phytophthora cinnamomi inoculation. This method was most useful when mortality was not exceptionally high, and analyzers must consider mortality as well as leaf damage in quantifying effects of the pathogen. Plants with leaf discoloration were 3.3 times more likely to die 2 weeks from our leaf census than plants with no leaf discoloration (P =0.005). This method is particularly inexpensive to implement, making it a valuable alternative to multi-spectral or hyperspectral imaging, especially in contexts such as horticulture and citizen science, where the high speed and low-cost nature of this technique might prove valuable. Species used in this study: root rot disease pathogen (Phytophthora cinnamomi Rands); Rhododendron atlanticum (Ashe) Rehder; Rhododendron brachycarpum D.Don ex G.Don; Rhododendron kiusianum Makino; Rhododendron maximum L.; Rhododendron minus Michx.; Rhododendron calendulaceum (Michx.) Torr.; Rhododendron kaempferi Planch.; Rhododendron keiskei Miq. Chemicals used in this study: Fosal Select Aliette/aluminum phosphite.

Effects of Rhododendron maximum on the understory herb Cardamine diphylla (Brassicaceae) and associated soils in winter1

Effects of Rhododendron maximum on the understory herb Cardamine diphylla (Brassicaceae) and associated soils in winter1

Infrared thermography of in situ natural freezing and mechanism of winter-thermonasty in Rhododendron maximum.

Evergreen leaves of Rhododendron species inhabiting temperate/montane climates are typically exposed to both high radiation and freezing temperatures during winter when photosynthetic biochemistry is severely inhibited. Cold-induced "thermonasty," that is, lamina rolling and petiole curling, can reduce the amount of leaf area exposed to solar radiation and has been associated with photoprotection in overwintering rhododendrons. The present study was conducted on natural, mature plantings of a cold-hardy and large-leaved thermonastic North American species (Rhododendron maximum) during winter freezes. Infrared thermography was used to determine initial sites of ice formation, patterns of ice propagation, and dynamics of the freezing process in leaves to understand the temporal and mechanistic relationship between freezing and thermonasty. Results indicated that ice formation in whole plants is initiated in the stem, predominantly in the upper portions, and propagates in both directions from the original site. Ice formation in leaves initially occurred in the vascular tissue of the midrib and then propagated into other portions of the vascular system/venation. Ice was never observed to initiate or propagate into palisade, spongy mesophyll, or epidermal tissues. These observations, together with the leaf- and petiole-histology, and a simulation of the rolling effect of dehydrated leaves using a cellulose-based, paper-bilayer system, suggest that thermonasty occurs due to anisotropic contraction of cell wall cellulose fibers of adaxial versus abaxial surface as the cells lose water to ice present in vascular tissues.

Differences in respiration rates and abrasion losses may muddle attribution of breakdown to macroinvertebrates versus microbes in litterbag experiments

AbstractLeaf breakdown is an important process in forested headwater streams. A common method used to quantify the role of macroinvertebrate and microbial communities in leaf litter breakdown involves using paired mesh bags that either allow or exclude macroinvertebrate access to leaves. We examined common assumptions of the paired litterbag method to test (1) whether mesh size alters microbial respiration and (2) whether the effects of abrasive flows (e.g., from water and sediment) differ between coarse‐ and fine‐mesh litterbags. We measured rates of microbial respiration on Acer rubrum and Rhododendron maximum leaves incubated in coarse‐ and fine‐mesh litterbags. We also measured rates of abrasion using aerated concrete blocks in pairs of coarse‐ and fine‐mesh bags in ten streams across a gradient of discharge. We found that rates of microbial respiration on Acer rubrum leaves conditioned in fine‐mesh bags were 65% greater than the rates of respiration in paired coarse‐mesh bags, but respiration rates on Rhododendron maximum were similar in coarse‐ and fine‐mesh bags. Abrasion was, on average, 56% greater in coarse‐mesh than paired fine‐mesh bags, and these effects were greater in streams with higher discharge. These results suggest that more caution is required when attributing the difference in leaf breakdown between coarse‐ and fine‐mesh bags to macroinvertebrates. Because the effect of mesh size on microbial respiration of Acer leaves and abrasion are opposite in direction, the effect that dominates and creates bias likely depends on both environmental context and experimental design.

Characteristics of Dry-Mesic Old-Growth Oak Forests in the Eastern United States

Dry-mesic old-growth oak forests are widely distributed remnants across the eastern U.S. and are expected to increase in number and extent as second-growth forests mature. In this study, we synthesize published and unpublished information to better define the species, structure and extent of these forests. Mean site tree density for trees ≥10 cm dbh ranged from 341–620 trees ha−1. In the eastern part of the region, most stand basal areas were >23 m2 ha−1, compared to ≤23 m2 ha−1 in the westernmost stands. Overall, woody species diversity was relatively low compared to old-growth oak forests on moister sites, with tree species per forest ranging from 5–18. The most common species among the stands were white oak (Quercus alba), northern red oak (Quercus rubra), and black oak (Quercus velutina). Shrub and vine species per forest ranged from 1–10, with common species or genera including Virginia creeper (Parthenocissus quinquefolia), poison ivy (Toxicodendron radicans), Vaccinium spp., and grapevines (Vitis spp.). Within the southern Appalachian Mountains, rosebay rhododendron (Rhododendron maximum) and mountain laurel (Kalmia latifolia L.) were common. Herbaceous species per stand ranged from 4–51, with the highest richness occurring in a southern Appalachian oak-hickory forest. The maximum within-stand age of the large trees ranged from 170 to over 365 years. The mean density of standing dead trees ≥10 cm dbh ranged from 31–78 ha−1 and the volume of coarse woody debris ≥10 cm in diameter averaged 52 m3 ha−1. We more fully describe the characteristics of these forests and fill gaps in the collective knowledge of this increasingly important forest type. However, over the past 20 years, there has been scant research on these forests, and older research studies have used a variety of research plots and methods. A uniform approach to surveying these sites is needed to gain a better understanding of these forests before we are faced with caring for an increase in old-growth forest areas.

Fuel and vegetation changes in southwestern, unburned portions of Great Smoky Mountains National Park, USA, 2003–2019

Overstory basal area, ericaceous shrub cover (Kalmia latifolia L. and Rhododendron maximum L.), and fuels (i.e., woody fuel loads and depths and O Horizon thickness) were assessed within Great Smoky Mountains National Park, USA, in 2003 − 2004. Due to recent wildfire activity within the southern Appalachian Mountain region (including Great Smoky Mountains National Park), the potential spread and expansion of ericaceous shrubs, and the impacts of the hemlock woolly adelgid (Adelges tsugae Annand) on eastern hemlock (Tsuga canadensis (L.) Carrière), these same ecosystem components were again assessed in 2019. Elevation and moisture regime (xeric, intermediate, and mesic) were included in this assessment as potential influential factors. An evaluation of repeated measurements from 40 plots suggested that O Horizon thickness did not change significantly over the 16-year period, but increased as elevation increased, and moisture regime (xeric O Horizon thickness > mesic O Horizon thickness) was a significant, related factor. The sum of 1-, 10-, and 100-h fuel loads (fuels less < 7.6 cm diameter) increased, whereas woody fuel depth decreased over the 16-year period. No significant changes in 1000-h fuel loads (> 7.6 cm diameter), total woody fuel loads, ericaceous shrub cover, total basal area, or live T. canadensis basal area were observed. Live T. canadensis basal area decreased with increasing elevation. Dead, standing T. canadensis basal area increased from 2003–2019, and that increase was most pronounced as elevation increased on xeric and intermediate sites. Overall, we found that: 1. hypothesized increases in total woody fuel loads and ericaceous shrub cover were not present; and 2. elevation and moisture regime were most related to observed changes in vegetation and fuel condition.

Have stream diatom assemblages changed a decade after the loss of a foundation riparian tree species in a headwater Appalachian watershed?

Terrestrial and stream ecosystems are interconnected, with changes in one system cascading to affect the other. Tree mortality related to nonnative pest infestation has increased, yet we do not understand the effects that riparian tree species losses will have on the structural characteristics of stream ecosystems. We compared riparian canopy cover, algal standing crop, and diatom assemblage composition in 8 southern Appalachian headwater stream reaches within the Coweeta Hydrologic Laboratory in North Carolina, USA, before and after a massive die-off of riparian eastern hemlock (Tsuga canadensis [L.] Carr.) related to the introduction of hemlock woolly adelgid (Adelges tsugae Annand), an invasive insect. We analyzed historical preserved algal samples collected prior to hemlock die-off, in both summer (September 2005) and winter (January 2006), and used multivariate analyses to compare them to samples collected in the same 8 reaches and seasons 13 y following hemlock die-off (2018–2019). We predicted that increased canopy openness following hemlock death would change light availability and lead to higher standing crop and increases in the relative biovolume of high-profile diatoms. Contrary to our expectations, we found little difference in canopy cover and lower algal standing crop 13 y post-hemlock die-off, likely due to increased basal area of the understory shrub, Rhododendron maximum L., that replaced shade cover formerly provided by hemlock. Diatom assemblages were dominated by acidophilic taxa (e.g., Eunotia spp.) across all stream reaches before and after hemlock die-off. We observed some shifts in diatom assemblage composition, but the relative biovolume of a majority of the top 20 most abundant taxa, including Eunotia incisa W. Sm. ex W. Greg., Nupela lapidosa (Krasske) Lange-Bert., and Meridion alansmithii Brant (an endemic taxon), remained relatively unchanged 13 y after hemlock mortality. Diatom assemblages differed post-hemlock die-off in summer but not winter due to a decrease in the relative biovolume of Eunotia metamondon complex and an increase in the relative biovolume of Eunotia rhomboidea complex. Our study highlights the relative stability of diatom assemblage composition in southern Appalachian streams and the importance of long-term studies following a major riparian disturbance.

Larger hardwood trees benefit from removing Rhododendron maximum following Tsuga canadensis mortality

Forest disturbance to overstory vegetation can result in increased abundance of understory vegetation, potentially affecting various ecosystem processes. Habitat expansion of a native evergreen woody shrub, Rhododendron maximum (rosebay rhododendron), in southern Appalachian Mountain riparian forests of the eastern United States has occurred following the mortality of Castanea dentata (American chesnut) in the early 20th century and more recently Tsuga canadensis (Eastern hemlock). Increased abundance of R. maximum has been associated with reduced tree seedling recruitment and survival, as well as shorter overstory tree height compared to where R. maximum understory was not present. As part of a larger investigation into the removal of R. maximum to restore vegetation structure and composition, this study examined the effects of removing R. maximum understory (by cutting) and soil O-horizon (by prescribed fire) on microclimate, whole tree and plot-level transpiration during 2016 and 2017 growing seasons. We found increased soil moisture in the wetter year (2017) and greater variation in light transmission to the forest floor after the treatment. Larger trees were able to increase their water use in the drier year (2016), resulting in similar plot-level transpiration of treatment and reference plots. Removal of R. maximum understory is not expected to significantly alter streamflow or stream chemistry during the growing season; however, it can help restore forest structure through improving tree seedling survival and recruitment as well as increasing growth of established trees.

Effects ofRhododendronremoval and prescribed fire on bees and plants in the southern Appalachians

Rhododendron maximum is an evergreen shrub native to the Appalachian Mountains of North America that has expanded in recent decades due to past disturbances and land management. The purpose of this study was to explore how bees and plants were affected by the experimental removal of R. maximum followed by a prescribed fire in one watershed compared to a neighboring reference watershed. Bees and plants were sampled for three years in both watersheds. Comparisons were based on the rarefaction and extrapolation sampling curves of Hill numbers as well as multivariate methods to assess effects on community composition. Bee richness, Shannon's diversity, and Simpson's diversity did not differ between watersheds in the year after removal but were all significantly higher in the removal watershed in year two, following the prescribed fire. Bee Shannon's diversity and Simpson's diversity, but not richness, remained significantly higher in the removal watershed in the third year. Similar but weaker patterns were observed for plants. Comparisons of community composition found significant differences for bees in the second and third year and significant differences for plants in all three years. For both groups, significant indicator taxa were mostly associated with the removal watershed. Because bees appeared to respond more strongly to the prescribed fire than to the removal of R. maximum and these benefits weakened considerably one year after the fire, clearing R. maximum does not appear to dramatically improve pollinator habitat in the southern Appalachians. This conclusion is underscored by the fact that about one quarter of the bee species in our study area were observed visiting R. maximum flowers. The creation of open areas with wildflowers may be a better way to benefit bees in this region judging from the high diversity of bees captured in the small roadside clearings in this study.

Predicted Spatial Distribution of the Eastern Spotted Skunk (Spilogale putorius) in Virginia Using Detection and Non-Detection Records

The geographic distribution of a species is a fundamental component in understanding its ecology and is necessary for forming effective conservation plans. For rare and elusive species of conservation concern, accurate maps of predicted occurrence are particularly problematic and often highly subjective. Spilogale putorius (Eastern Spotted Skunk) populations have experienced large declines since the 1940s. Their elusive behavior and perceived rarity result in low detection probability when using conventional methods for sampling small mammals. Low detection probability often causes uncertainty as to where Eastern Spotted Skunks could be a management concern. We modeled the distribution of predicted occurrence of Eastern Spotted Skunks using verifiable occurrence and non-detection records obtained throughout Virginia from 2010 to 2020. Occurrence data consisted of trapping records reported to the Virginia Department of Wildlife Resources, incidental photo-verified reports of sightings and road-killed animals, and remote-camera detections. Non-detections were presumed at baited remote-camera locations following intense survey efforts. We fit predicted occurrence models using generalized linear modeling in an information-theoretic framework using the package ‘stats’ in Program R. Our results incidated a greater probability of presence from the Blue Ridge westward, increasing with slope steepness along northeastern- to southeastern-facing slopes and decreasing with slope steepness along southeastern- to southwestern-facing slopes. Emergent rock outcrops prominent along northeastern slopes offer ample protective rocky cover, whereas mixed Quercus spp. (oak), Kalmia latifolia (Mountain Laurel), and Rhododendron maximum (Rosebay Rhododendron) forest communities along southern-facing slopes provide suitable areas of cover, both of which are critical for spotted skunk survival and reproductive success. Our analysis provides insight into the relationships between landscape features and Eastern Spotted Skunk distributions across Virginia. Understanding these relationships is critical for the effective management and conservation of this vulnerable species.

Effects of Rhododendron removal on soil bacterial and fungal communities in southern Appalachian forests

Rhododendron maximum, a native ericaceous evergreen shrub, is expanding in forests of the southern Appalachian region following eastern hemlock (Tsuga canadensis) mortality due to hemlock woolly adelgid (Adelges tsugae) infestations. The goal of our study was to examine soil microbial community responses to experimental R. maximum removal treatments. The experiment was implemented as a 2 × 2 factorial design, including two R. maximum canopy removal levels (cut vs. not cut) combined with two forest floor removal levels (burned vs. not burned). These treatments were designed as potential management strategies to facilitate hardwood tree establishment in forests that have experienced T. canadensis declines. We sampled soils after removals and characterized bacterial and fungal communities using amplicon sequencing. Shrub removal did not affect bacterial or fungal α diversity but did affect both bacterial and fungal community composition. Relative abundances of bacterial phyla and fungal classes exhibited no differences among R. maximum removal treatments. However, specific bacterial and fungal taxa that were responsive to R. maximum removal (i.e., differentially abundant sequences) did exhibit clear patterns at high taxonomic levels. Specifically, taxa that responded negatively to R. maximum removal were found primarily in two bacterial phyla (Proteobacteria and Bacteroidetes) and one fungal class (Archaeorhizomycetes) while positive responders were clustered in several other bacterial phyla (e.g., Actinobacteria, Planctomycetes, Cyanobacteria). Fungal functional guilds also responded to R. maximum removal, including negative responses of ericoid mycorrhizae and positive responses of arbuscular mycorrhizae and wood saprotrophs. Effects of R. maximum removal on soil microbial communities were minor overall, but clear effects on some key functional groups were evident (i.e., mycorrhizal fungi), suggesting that microbial responses to R. maximum removal may influence recovery of forests in the southern Appalachian region.

Do crayfish affect stream ecosystem response to riparian vegetation removal?

Abstract Riparian vegetation management alters stream basal resources, but stream ecosystem responses partly depend on top‐down interactions with in‐stream consumers. Large‐bodied omnivores can exert particularly strong influences on stream benthic environments through consumption of food resources and physical disturbance of the benthos. Trophic dynamics studies conducted within the context of reach‐scale riparian vegetation manipulations can provide insights into the interactions and relative importance of top‐down and bottom‐up controls that determine ecosystem response to riparian change. Here, we examine how top‐down control by native crayfish omnivores (Cambarus bartonii) interacts with abiotic conditions created by reach‐scale removal of riparian rhododendron (Rhododendron maximum) in the southern Appalachian Mountains of the U.S.A. We conducted 32‐day trophic experiments by nesting five pairs of electrified (crayfish excluded) and non‐electrified (crayfish access) plots within each of two 300‐m stream reaches (one control and one rhododendron‐removed) for 1 year pre‐removal and 2 years post‐removal. Algal growth responded positively to the reduced canopy cover (post‐rhododendron removal) only under low flow conditions combined with the absence of top‐down control by crayfish. Leaf decomposition rates were reduced by c. 40% in the absence of crayfish, but higher inputs of rhododendron leaf litter during the summer following rhododendron removal reduced the effect of crayfish presence on decomposition. Riparian rhododendron removal also significantly increased benthic sediment and fine benthic organic matter, but crayfish exclusion did not affect these stream properties. Potential long‐term reductions in crayfish abundance could reduce the top‐down effects of crayfish and ultimately lead to higher algal growth and reduced leaf decomposition rates in streams where rhododendron is managed through removal.

Removing riparian Rhododendron maximum in post-Tsuga canadensis riparian forests does not degrade water quality in southern Appalachian streams

In the past century, the evergreen woody shrub, Rhododendron maximum, has experienced habitat expansion following foundational tree species die-off in eastern US deciduous forests. Rhododendron can potentially alter stream chemistry, temperature, trophic dynamics, and in-stream decomposition rates, given its dominance in riparian areas. Here we conducted two operational-scale (3 ha) riparian treatments that removed rhododendron through cutting alone (CR, canopy removal), or removing both the rhododendron canopy and forest floor using cutting and prescribed fire (CFFR, canopy and forest floor removal). We expected that rhododendron shrub removal, with or without soil organic horizon removal, would increase soil nutrient availability and subsequently alter stream pH, acid neutralizing capacity (ANC), inorganic nitrogen (NO3-N, NH4-N), total dissolved inorganic nitrogen, dissolved organic carbon (DOC), calcium (Ca), potassium (K), and magnesium (Mg). We hypothesized that responses would occur more quickly in the CFFR treatment. Treatments reduced shrub-, but not tree basal area. Treatments lowered soil N, but not C. Stream chemistry responses to treatments varied between CR and CFFR and were transient, generally with pH, N, and some cations declining, and aluminum (Al) and DOC showing a pulse increase. By removing rhododendron, the remaining deciduous trees likely accelerated N uptake as soil moisture availability increased. This could partially explain why we observed lower than expected stream nutrients (NO3-N, Ca, and Mg) after treatments. Initial rhododendron slash on the forest floor coupled with incomplete consumption of the O-horizon on the CFFR treatment likely elevated DOC in the upper soil horizons and mobilized Al. From a management perspective, using these treatments to restore structure and function to riparian forests in the wake of eastern hemlock mortality, with or without fire, would most likely not result in short-term diminished water quality that is common when overstory trees are harvested and may even lower stream NO3-N concentrations long term.

Plant pathogenic fungi are harbored as endophytes in Rhododendron spp. native to the Eastern U.S.A.

Invasive plant pathogens cause a substantial loss of yearly revenue and often lead to environmental change. The movement of plants is considered to be the main avenue for introduction of invasive plant pathogens. Government agencies attempt to reduce the risk of introductions by legislation and inspections. However, inspectors may not consider that plant pathogens can infect plants as endophytes that are not considered hosts without showing symptoms. This study surveyed endophytes in rhododendron (Rhododendron maximum and Rhododendron catawbiense) plants in four native stands in the Eastern U.S.A. We isolated associated endophytes from the plant leaves and identified them using a molecular sequencing method. Fifteen (21%) of the identified samples matched with a known plant pathogen not reported to be pathogenic on rhododendron. We demonstrated pathogenicity of selected rhododendron endophytes on red oak, flowering dogwood and lupine. This study is important because it highlights that present detection methods and techniques may not be adequate to detect the potential importation and spread of invasive species. • Over 600 fungal endophyte isolates were collected from 2 Rhododendron spp. • 210 samples were analyzed by sequencing; 70 matched to sequences in GenBank. • 21% matched with a fungal pathogen not reported on rhododendron. • We demonstrated that plant pathogens could harbor as endophytes in non-hosts.

Do southern Appalachian Mountain summer stream temperatures respond to removal of understory rhododendron thickets?

AbstractDense understory thickets of the native evergreen shrub Rhododendron maximum expanded initially following elimination of American chestnut by the chestnut blight, and later in response to loss of the eastern hemlock due to hemlock woolly adelgid invasion. Rhododendron thickets often blanket streams and their riparian zones, creating cool, low‐light microclimates. To determine the effect of such understory thickets on summer stream temperatures, we removed riparian rhododendron understory on 300 m reaches of two southern Appalachian Mountain headwater streams, while leaving two 300 m reference reaches undisturbed. Overhead canopy was left intact in all four streams, but all streams were selected to have a significant component of dead or dying eastern hemlock in the overstory, creating time‐varying canopy gaps throughout the reach. We continuously monitored temperatures upstream, within and downstream of treatment and reference reaches. Temperatures were monitored in all four streams in the summer before treatments were imposed (2014), and for two summers following treatment (2015, 2016). Temperatures varied significantly across and within streams prior to treatment and across years for the reference streams. After rhododendron removal, increases in summer stream temperatures were observed at some locations within the treatment reaches, but these increases did not persist downstream and varied by watershed, sensor, and year. Significant increases in daily maxima in treatment reaches ranged from 0.9 to 2.6°C. Overhead canopy provided enough shade to prevent rhododendron removal from increasing summer temperatures to levels deleterious to native cold‐water fauna (average summer temperatures remained below 16°C), and local temperature effects were not persistent.

Nesting Ecology of a Range-Edge Population of Veeries in the Southern Appalachian Mountains

Very little is known about the nesting ecology of Catharus fuscescens (Veery) breeding at a low-latitude range limit in the southern Appalachians. Between 2017 and 2018, we monitored 34 Veery nests in the mountains of North Carolina. Veery nests were generally placed low to the ground in a variety of substrates, predominantly Rhododendron maximum (Rhododendron) and Gaylussacia spp. (huckleberry); however, estimates of daily nest survival did not vary by the nest-site characteristics we measured. The daily nest survival estimate of 0.925 and estimated overall success rate of 13% are lower than most reports from farther north in the Veery's range, but the reasons for the low nest survival at the southern edge are unclear. This study provides the first report of the nesting ecology of Veeries in the southern Appalachians, and our work suggests the need to determine why Veeries in this region are declining and moving upslope.

Woody Regeneration Response to Overstory Mortality Caused by the Hemlock Woolly Adelgid (Adelges tsugae) in the Southern Appalachian Mountains

Adelges tsugae Annand (hemlock woolly adelgid, HWA, an invasive insect native to Japan), which causes defoliation and death of Tsuga canadensis (L.) Carrière (eastern hemlock), was introduced to the United States in the early 1950s and has spread throughout much of the range of T. canadensis causing widespread mortality. In 2016 and 2017, we resampled long-term vegetation monitoring plots across five forest types (ecogroups) within Great Smoky Mountains National Park that contained T. canadensis in 2003 (prior to the spread of HWA within the park) to examine changes in the species composition and diversity of the regeneration layer. We hypothesized that compositional changes in the seedling and sapling strata would be driven primarily by the pre-HWA importance value of T. canadensis and relative dominance of Rhododendron maximum L. (rosebay rhododendron), and that species diversity metrics would differ across year depending on ecogroup and the relative dominance of R. maximum. Non-metric multi-dimensional scaling (NMDS) revealed that the seedling and sapling strata of plots with greater pre-HWA importance of T. canadensis and lower basal area of R. maximum generally exhibited greater compositional change between 2003 and 2017. Topo-edaphic variables were also significant in both NMDS ordinations and were associated with the distribution of multiple hardwood species. Species richness, evenness, and Shannon-Wiener diversity varied with strata following the loss of T. canadensis, with the degree and direction of change varying with the dominance of R. maximum.

Lack of forest tree seedling recruitment and enhanced tree and shrub growth characterizes post-Tsuga canadensis mortality forests in the southern Appalachians

The loss of Tsuga canadensis from invasion by hemlock woolly adelgid (Adelges tsugae, HWA) has altered ecosystem structure and function in forests across the eastern United States. In southern Appalachian forests, T. canadensis co-occurred with hardwood species and an evergreen Rhododendron maximum shrub layer in riparian and cove positions. In this region, HWA infestation was detected in 2003, with mortality reaching 97% by 2014. In this study we examined responses of light, soil moisture, tree seedling density, and overstory and understory vegetation growth from 2004 to 2014 following HWA infestation and mortality of T. canadensis. We hypothesized that seedling recruitment and vegetation growth would continue to increase over time as observed with initial trends reported through 2009, and that species that associate with ectomycorrhizal (ECM) fungi may benefit more than those that do not due to the pulse of organic material in these stands from dead T. canadensis trees. Light transmission measured at 1 m above the ground increased from 2006 to 2009, but gradually decreased from 2015 to 2017. Basal area of overstory non-Tsuga trees increased only marginally, and there was no recruitment of tree seedlings to the overstory size class, even though seedling density of deciduous species increased initially. Increased basal area and stem density of R. maximum may explain the light and seedling responses, as this species can inhibit tree seedling recruitment by limiting light and nutrients. Overstory species with the highest basal area increment (BAI) in the post-T. canadensis stands were Pinus rigida, Betula lenta and Quercus coccinea, which all associate with ECM fungi. However, not all ECM tree species grew significantly more following T. canadensis mortality compared to pre-mortality growth rates—only those ECM species that had high growth rates prior to mortality did. After a decade, growth of both overstory trees and R. maximum has not compensated for the loss of T. canadensis. Active management of R. maximum, which may involve the removal of the evergreen shrub and soil organic layer, may be required to allow for diverse tree seedling recruitment; and subsequently, restore riparian forest structure, diversity, and function.

Soil microbial response to Rhododendron understory removal in southern Appalachian forests: Effects on extracellular enzymes

Rhododendron maximum is a native evergreen shrub that has expanded in Appalachian forests following declines of american chestnut (Castanea dentata) and eastern hemlock (Tsuga canadensis). R. maximum is of concern to forest managers because it suppresses hardwood tree establishment by limiting light and soil nutrient availability. We are testing R. maximum removal as a management strategy to promote recovery of Appalachian forests. We hypothesized that R. maximum removal would increase soil nitrogen (N) availability, resulting in increased microbial C-demand (i.e. increased C-acquiring enzyme activity) and a shift towards bacterial-dominated microbial communities. R. maximum removal treatments were applied in a 2 × 2 factorial design, with two R. maximum canopy removal levels (removed vs not) combined with two O-horizon removal levels (burned vs unburned). Following removals, we sampled soils and found that dissolved organic carbon (DOC), N (TDN, NO3, NH4), and microbial biomass all increased with R. maximum canopy + O-horizon removal. Additionally, we observed increases in C-acquisition enzymes involved in degrading cellulose (β-glucosidase) and hemicellulose (β-xylosidase) with canopy + O-horizon removal. We did not see treatment effects on bacterial dominance, though F:B ratios from all treatments increased from spring to summer. Our results show that R. maximum removal stimulates microbial activity by increasing soil C and N availability, which may influence recovery of forests in the Appalachian region.