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The terrestrial invader hemlock woolly adelgid (Adelges tsugae) decimates eastern hemlock (Tsuga canadensis) stands that dominate riparian zones of Appalachian forest streams. However, the ecological consequences for linked aquatic-terrestrial ecosystems remain unknown. We measured stream-riparian trophic linkages at 21 sites of Ohio, West Virginia, and Virginia representing a chronosequence of T. canadensis decline. We measured reciprocal fluxes of basal resources (periphyton, terrestrial detritus), emerging aquatic insect flux rate and community composition, riparian orb-weaving spider density, and estimated spider trophic position and reliance on aquatically-derived energy using stable isotopes (13C, 15N, 2H) and Bayesian mixing models. Stream periphyton biomass was greater at uninvaded reference sites than at invaded sites and composition of the terrestrial-to-stream detritus flux changed with T. canadensis decline. Emergent aquatic insect community composition was partly explained by hemlock decline status, but the relative abundance of functional feeding groups was not. Riparian orb-weaving spider densities were highest at sites with severe hemlock decline (F = 4.27, p = 0.022), but were not linked to insect emergence flux rates (p > 0.10). Both trophic position (x¯ = 2.4) and relative reliance on aquatically-derived energy (x¯ = 83%) were comparable among spider families (Tetragnathidae, Araneidae, Pisauridae) and site decline status. Although spider δ13C signatures were unrelated to those of the most numerous emergent insect families, δ15N signatures of Araneidae and Pisauridae tracked emergent insect δ15N (r2 = 0.42 and 0.78, respectively), suggesting a trophic linkage. Overall, the ecological consequences of this invader were clearest at lower trophic levels, with more nuanced impacts on riparian spiders.

Range expansion by native and exotic species will continue to be a major component of global change. Anticipating the potential effects of changes in species distributions requires models capable of forecasting population spread across realistic, heterogeneous landscapes and subject to spatiotemporal variability in habitat suitability. Several decades of theory and model development, as well as increased computing power and availability of fine-resolution GIS data, now make such models possible. Still unanswered, however, is the question of how well this new generation of dynamic models will anticipate range expansion. Here we develop a spatially explicit stochastic model that combines dynamic dispersal and population processes with fine-resolution maps characterizing spatiotemporal heterogeneity in climate and habitat to model range expansion of the hemlock woolly adelgid (HWA; Adelges tsugae ). We parameterize this model using multiyear data sets describing population and dispersal dynamics of HWA and apply it to eastern North America over a 57-year period (1951-2008). To evaluate the model, the observed pattern of spread of HWA during this same period was compared to model predictions. Our model predicts considerable heterogeneity in the risk of HWA invasion across space and through time, and it suggests that spatiotemporal variation in winter temperature, rather than hemlock abundance, exerts a primary control on the spread of HWA. Although the simulations generally matched the observed current extent of the invasion of HWA and patterns of anisotropic spread, it did not correctly predict when HWA was observed to arrive in different geographic regions. We attribute differences between the modeled and observed dynamics to an inability to capture the timing and direction of long-distance dispersal events that substantially affected the ensuing pattern of spread.

The abundance of eastern hemlock ( Tsuga canadensis ) in eastern US forests has declined since the 1950s owing to the introduction of the non-native insect, hemlock woolly adelgid (HWA, Adelges tsugae ). In southern New England, eastern hemlock is being replaced by the deciduous tree species, black birch ( Betula lenta ). To date there is little understanding of whether hemlock loss will fundamentally alter ecosystem C balance and component fluxes. In this study, we use a comparative approach to study potential changes in C fluxes and N cycling associated with HWA-induced hemlock decline and replacement. The stands include primary- and secondary-growth hemlock forests (>230 and 132 years old, respectively), recently disturbed stands (5 and 18 years old) that now have rapidly growing black birch saplings, and a mature black birch stand of age similar to the second-growth hemlock stand. We found that aboveground net primary production was higher in the aggrading black birch stand and significantly so at 18-years post-HWA compared to the secondary-hemlock stand it would likely replace. Rapid forest regrowth was accompanied by significantly higher rates of N uptake from the soil but also higher N-use efficiency because most of the N taken up from the soil was allocated to the production of wood with a high C-to-N ratio. In contrast to patterns of aboveground production, the rate of soil respiration was lowest in the young stands and not significantly different from the second-growth hemlock stand, suggesting little net effect of stand replacement on soil C efflux. The leaf litter decomposition study showed that black birch litter decomposed more rapidly than hemlock litter but that there was no effect of stand type on the rate of decomposition. Analyses of extracellular microbial exoenzyme activity painted a more nuanced pattern of variation among stands, with fine root biomass the only weakly explanatory variable. In combination with our prior work on C stocks, these results suggest that forests affected by HWA in southern New England will remain a sink for atmospheric CO 2 despite reorganization of stand structure and species composition.

How, where, and why carbon (C) moves into and out of an ecosystem through time are long-standing questions in biogeochemistry. Here, we bring together hundreds of thousands of C-cycle observations at the Harvard Forest in central Massachusetts, USA, a mid-latitude landscape dominated by 80–120-yr-old closed-canopy forests. These data answered four questions: (1) where and how much C is presently stored in dominant forest types; (2) what are current rates of C accrual and loss; (3) what biotic and abiotic factors contribute to variability in these rates; and (4) how has climate change affected the forest's C cycle? Harvard Forest is an active C sink resulting from forest regrowth following land abandonment. Soil and tree biomass comprise nearly equal portions of existing C stocks. Net primary production (NPP) averaged 680–750 g C.m⁻².yr⁻¹; belowground NPP contributed 38–47% of the total, but with large uncertainty. Mineral soil C measured in the same inventory plots in 1992 and 2013 was too heterogeneous to detect change in soil-C pools; however, radiocarbon data suggest a small but persistent sink of 10–30 g C.m⁻².yr⁻¹. Net ecosystem production (NEP) in hardwood stands averaged ~300 g C.m⁻².yr⁻¹. NEP in hemlock-dominated forests averaged ~450 g C.m⁻².yr⁻¹ until infestation by the hemlock woolly adelgid turned these stands into a net C source. Since 2000, NPP has increased by 26%. For the period 1992–2015, NEP increased 93%. The increase in mean annual temperature and growing season length alone accounted for ~30% of the increase in productivity. Interannual variations in GPP and NEP were also correlated with increases in red oak biomass, forest leaf area, and canopy-scale light-use efficiency. Compared to long-term global change experiments at the Harvard Forest, the C sink in regrowing biomass equaled or exceeded C cycle modifications imposed by soil warming, N saturation, and hemlock removal. Results of this synthesis and comparison to simulation models suggest that forests across the region are likely to accrue C for decades to come but may be disrupted if the frequency or severity of biotic and abiotic disturbances increases.

Hemlock woolly adelgid (HWA), Adelges tsugae Annand, threatens hemlock forests throughout eastern North America. Management efforts focus on early detection of HWA to ensure rapid management responses to control and stop the spread of this pest. This study's goal was to identify an affordable, efficient trap to aid with airborne environmental DNA (eDNA) sampling approaches as an early monitoring tool for HWA. We initially compared HWA detection success between a standard sticky trap, commonly used for HWA monitoring, and trap designs potentially compatible with eDNA protocols (i.e., passive trap, funnel trap, and motorized trap). Passive, funnel, and motorized traps' estimated capture success probabilities compared to sticky traps were 0.87, 0.8, and 0.4, respectively. A secondary evaluation of a modified version of the motorized trap further assessed trap performance and determined the number of traps needed in a set area to efficiently detect HWA. By modifying the original motorized trap design, its estimated capture success probability increased to 0.67 compared to a sticky trap. Overall, the cumulative capture success over the 16-week sampling period for the motorized trap was 94% and 99% for the sticky trap. The number of traps did impact capture success, and trap elevation and distance to infested hemlocks influenced the number of adelgids captured per trap. As eDNA-based monitoring approaches continue to become incorporated into invasive species surveying, further refinement with these types of traps can be useful as an additional tool in the manager's toolbox.

The hemlock woolly adelgid, Adelges tsugae Annand (Hemiptera: Adelgidae), has distinct native and invasive populations in Canada. On the country’s west coast, the adelgid is a native insect that feeds on western hemlock, Tsuga heterophylla (Rafinesque-Schmaltz) Sargent, and mountain hemlock, Tsuga mertensiana (Bongard) Carrière (Pinaceae). In eastern Canada, the adelgid is an invasive species that attacks and kills eastern hemlock, Tsuga canadensis (Linnaeus) Carrière (Pinaceae). We obtained all Canadian records of A. tsugae in institutional and public databases, developed updated range maps and phenologies for the species in British Columbia and eastern Canada, and developed dispersal estimates for populations in Nova Scotia. In British Columbia, A. tsugae’s observed distribution is centred around the Lower Mainland and on Vancouver Island but with populations in the British Columbia Interior and along the Pacific coast that have been poorly explored. In eastern Canada, the adelgid has invaded southern Nova Scotia, portions of the Niagara region in Ontario as far west as Hamilton, and at least one site on the north shore of Lake Ontario. No populations have been found in New Brunswick, Quebec, or Prince Edward Island, Canada. Finally, we estimated the rate of spread in Nova Scotia at 12.6 ± 8.2 to 20.5 ± 27.21 km/year.

The density of insect herbivores is regulated by top-down factors (e.g., natural enemies), bottom-up effects (e.g., plant defenses against herbivory), or a combination of both. As such, understanding the relative importance of these factors can have important implications for the establishment of effective management options for invasive species. Here, we compared the relative importance of top-down and bottom-up factors on the abundance of hemlock woolly adelgid (HWA), Adelges tsugae. HWA is invasive in eastern North America, but its native range includes the Pacific Northwest of North America where it has co-evolved with western hemlock, Tsuga heterophylla. Eastern hemlock, Tsuga canadensis, can also be found planted in city and park settings in the Pacific Northwest and the presence of both host species allowed us to directly compare the importance of predators (top-down) and host plant resistance (bottom-up) on HWA abundance by placing mesh exclusion bags on branches of both species and monitoring HWA abundance over two years. We found no evidence for bottom-up control of HWA on western hemlock (a native host). HWA established more readily on that species than on eastern hemlock on which it is a major pest in eastern North America. We found strong evidence for top-down control in that both summer and winter-active predators significantly reduced HWA densities on the branches of both tree species where predators were allowed access. These findings support the validity of the biological control program for HWA, the goal of which is to reduce outbreak populations of HWA in eastern North America.

While the direct effects of white-tailed deer ( Odocoileus virginianus ) on vegetation have been intensively studied, less is known about the indirect and interactive effects of herbivory on lower trophic levels, such as soil microbes and their processing of carbon pools. We explored how carbon dynamics shift with release from over-browsing by white-tailed deer in two mature stands of oak and hemlock trees. We measured soil carbon pools (for example, soil organic matter, carbon stocks, litter biomass, and litter stabilization) and fluxes (for example, soil respiration, methane uptake, microbial substrate use, and litter decomposition) using a spatially balanced survey design inside and outside two 24-year-old deer exclosures, one in each forest stand. Soil carbon pools were higher inside the exclosures than in deer-browsed plots in both forest stands, but the effect of deer herbivory on fine-scale spatial patterning of soil carbon pools and mean carbon fluxes varied by forest type. Release from deer herbivory in the oak stand increased the patchiness of soil pools and led to higher litter decomposition, soil respiration, and methane uptake rates. Release from deer herbivory in the hemlock stand did not affect the spatial structure of soil pools, had little effect on methane uptake, and had negative effects on litter decomposition and soil respiration. These differences may be due in part to the interactive effects of two herbivores, deer and the hemlock woolly adelgid ( Adelges tsugae ), that appear to be limiting regeneration and promoting the proliferation of monodominant hay-scented fern ( Dennstaedtia punctilobula ) in the hemlock-dominated stand. Our work suggests that future efforts consider multiple zoogeochemical stressors simultaneously, in addition to variation in environmental templates, to explain uncertainties in carbon pools and fluxes in temperate forested ecosystems.

The study of vegetation community and structural change has been central to ecology for over a century, yet the ways in which disturbances reshape the physical structure of forest canopies remain relatively unknown. Moderate severity disturbances affect different canopy strata and plant species, resulting in variable structural outcomes and ecological consequences. Terrestrial lidar (light detection and ranging) offers an unprecedented view of the interior arrangement and distribution of canopy elements, permitting the derivation of multidimensional measures of canopy structure that describe several canopy structural traits (CSTs) with known links to ecosystem function. We used lidar‐derived CSTs within a machine learning framework to detect and describe the structural changes that result from various disturbance agents, including moderate severity fire, ice storm damage, age‐related senescence, hemlock woolly adelgid, beech bark disease, and chronic acidification. We found that fire and ice storms primarily affected the amount and position of vegetation within canopies, while acidification, senescence, pathogen, and insect infestation altered canopy arrangement and complexity. Only two of the six disturbance agents significantly reduced leaf area, counter to common assumptions regarding many moderate severity disturbances. While findings are limited in their generalizability due to lack of replication among disturbances, they do suggest that the current limitations of standard disturbance detection methods—such as optical‐based remote sensing platforms, which are often above‐canopy perspectives—limit our ability to understand the full ecological and structural impacts of disturbance, and to evaluate the consistency of structural patterns within and among disturbance agents. A more broadly inclusive definition of ecological disturbance that incorporates multiple aspects of canopy structural change may potentially improve the modeling, detection, and prediction of functional implications of moderate severity disturbance as well as broaden our understanding of the ecological impacts of disturbance.

The valid scientific name for the hemlock woolly adelgid is affirmed to be adelges tsugae Annand. A discrepancy between the names adelges funitectus (Dreyfus) and a. tsugae in the historical literature is investigated and resolved. The generic (or subgeneric) name aphrastasia should not be applied to adelges tsugae, as this species is not closely related to adelges pectinatae (Cholodkovsky), the type species of aphrastasia. We propose annandina subgen. nov. for a. tsugae.