Experimentally testing the role of foundation species in forests: the Harvard Forest Hemlock Removal Experiment

Contrasting range changes and drivers of four forest foundation species under future climate change in China

Forest foundation species, vital for shaping community structure and dynamics through non-trophic level interactions, are key to forest succession and sustainability. Despite their ecological importance, the habitat ranges of these species in China and their responses to future climate change remain unclear. Our study employed the optimal MaxEnt model to assess the range shifts and their essential drivers of four typical forest foundation species from three climatic zones in China under climate scenarios, including Acer tegmentosum, Acer pseudo-sieboldianum (temperate zone), Quercus glandulifera (subtropical zone), and Ficus hispida (tropical zone). The optimal MaxEnt model exhibited high evaluation indices (AUC values > 0.90) for the four foundation species, indicating excellent predictive performance. Currently, we observed that A. tegmentosum and A. pseudo-sieboldianum are predominantly inhabited temperate forest areas in northeastern China, Q. glandulifera is primarily concentrated in subtropical forests in southeastern China, and F. hispida is mainly distributed across the tropical forests in southern China. Climate factors, particularly temperature, emerged as the primary environmental factors influencing the potential range of forest foundation species. Moreover, precipitation strongly influenced the potential range of A. tegmentosum and A. pseudo-sieboldianum, while elevation exhibited a greater impact on the range of Q. glandulifera and F. hispida. Under future climate scenarios, suitable areas for A. tegmentosum and A. pseudo-sieboldianum tend to expand southward, F. hispida tends to expand northward, while Q. glandulifera exhibited a tendency to contract towards the center. This study advances our understanding of the spatial and temporal dynamics of forest foundation species in China under climate change, providing critical insights for conservation efforts and sustainable forest management practices.

Multi-biomarker approach reveals the effects of heavy metal bioaccumulation in the foundation species Prosopis laevigata (Fabaceae)

Mining is a major economic activity in many developing countries. However, it disturbs the environment, producing enormous quantities of waste, known as mine tailings, which can have deleterious environmental impact, due to their high heavy metals (HM) content. Often, foundation species that establish on mine tailings are good candidates to study the effects of HM bioaccumulation at different levels of biological organization. Prosopis laevigata is considered a HM hyperaccumulator which presents attributes of a foundation species (FS) and establishes naturally on mine tailings. We evaluated the bioaccumulation of Cu, Pb, and Zn in P. laevigata foliar tissue, the leaf micro- and macro-morphological characters, DNA damage, and population genetic effects. In total, 80 P. laevigata individuals (20/site) belonging to four populations: The individuals from both sites (exposed and reference) bioaccumulated HMs (Pb > Cu > Zn). However, in the exposed individuals, Pb and Cu bioaccumulation was significantly higher. Also, a significant effect of macro- and micro-morphological characters was registered, showing significantly lower values in individuals from the exposed sites. In addition, we found significant differences in genotoxic damage in P. laevigata individuals, between the exposed and reference sites. In contrast, for the micro-morphological characters, none of the analyzed metals had any influence. P. laevigata did not show significant differences in the genetic structure and diversity between exposed and reference populations. However, four haplotypes and four private alleles were found in the exposed populations. Since P. laevigata is a species that establishes naturally in polluted sites and bioaccumulates HM in its foliar tissues, the resulting genetic, individual and population effects have not been severe enough to show detrimental effects; hence, P. laevigata can be a useful tool in phytoremediation strategies for soils polluted with Pb and Cu, maintaining its important ecological functions.

Logging response alters trajectories of reorganization after loss of a foundation tree species

Forest insect outbreaks cause large changes in ecosystem structure, composition, and function. Humans often respond to insect outbreaks by conducting salvage logging, which can amplify the immediate effects, but it is unclear whether logging will result in lasting differences in forest structure and dynamics when compared with forests affected only by insect outbreaks. We used 15 years of data from an experimental removal of Tsuga canadensis (L.) Carr. (Eastern hemlock), a foundation tree species within eastern North American forests, and contrasted the rate, magnitude, and persistence of response trajectories between girdling (emulating mortality from insect outbreak) and timber harvest treatments. Girdling and logging were equally likely to lead to large changes in forest structure and dynamics, but logging resulted in faster rates of change. Understory light increases and community composition changes were larger and more rapid in the logged plots. Tree seedling and understory vegetation abundance increased more in the girdled plots; this likely occurred because seedlings grew rapidly into the sapling- and tree-size classes after logging and quickly shaded out plants on the forest floor. Downed deadwood pools increased more after logging but standing deadwood pools increased dramatically after girdling. Understory light levels remained elevated for a longer time after girdling. Perhaps because the window of opportunity for understory species to establish was longer in the girdled plots, total species richness increased more in the girdled than logged plots. Despite the potential for greater diversity in the girdled plots, Betula lenta L. (black birch) was the most abundant tree species recruited into the sapling- and tree-size classes in both the girdled and logged plots and is poised to dominate the new forest canopy. The largest difference between the girdling and logging treatments-deadwood structure and quantity-will persist and continue to bolster aboveground carbon storage and structural and habitat diversity in the girdled plots. Human responses to insect outbreaks hasten forest reorganization and remove structural resources that may further alter forest response to ongoing climate stress and future disturbances.

Measuring change in biological communities: multivariate analysis approaches for temporal datasets with low sample size

Effective and robust ways to describe, quantify, analyse, and test for change in the structure of biological communities over time are essential if ecological research is to contribute substantively towards understanding and managing responses to ongoing environmental changes. Structural changes reflect population dynamics, changes in biomass and relative abundances of taxa, and colonisation and extinction events observed in samples collected through time. Most previous studies of temporal changes in the multivariate datasets that characterise biological communities are based on short time series that are not amenable to data-hungry methods such as multivariate generalised linear models. Here, we present a roadmap for the analysis of temporal change in short-time-series, multivariate, ecological datasets. We discuss appropriate methods and important considerations for using them such as sample size, assumptions, and statistical power. We illustrate these methods with four case-studies analysed using the R data analysis environment.

A Four-Year, Seven-State Reforestation Trial with Eastern Hemlocks (Tsuga canadensis) Resistant to Hemlock Woolly Adelgid (Adelges tsugae)

We conducted over a decade of research into individual eastern hemlock (Tsuga canadensis; hemlock) trees that are potentially resistant to hemlock woolly adelgid (Adelges tsugae; HWA), an invasive xylem-feeding insect that is capable of rapidly killing even mature trees. Following clonal propagation of these individuals, in 2015 we planted size- and age-matched HWA-resistant and HWA-susceptible hemlocks in HWA-infested forest plots in seven states. In 2019, we re-surveyed the plots; 96% of HWA-resistant hemlocks survived compared to 48% of susceptible trees. The surviving HWA-resistant trees were also taller, produced more lateral growth, retained more foliage, and supported lower densities of the elongate hemlock scale Fiorinia externa, another invasive hemlock pest, than the surviving HWA-susceptible trees. Our results suggest that HWA management may benefit from additional research exploring the identification, characterization, and use of HWA-resistant eastern hemlocks in future reforestation efforts.

Communities and Attachment Networks Associated with Primary, Secondary and Alternative Foundation Species; A Case Study of Stressed and Disturbed Stands of Southern Bull Kelp

Southern bull kelps (Durvillaea spp., Fucales) are ‘primary’ foundation species that control community structures and ecosystem functions on temperate wave-exposed rocky reefs. However, these large foundation species are threatened by disturbances and stressors, including invasive species, sedimentation and heatwaves. It is unknown whether ‘alternative’ foundation species can replace lost southern bull kelps and its associated communities and networks. We compared community structure (by quantifying abundances of different species) and attachment-interaction networks (by quantifying which species were attached to other species) among plots dominated by Durvillaea spp. and plots where Durvillaea spp. were lost either through long-term repeated experimental removals or by recent stress from a marine heatwave. Long-term experimental removal plots were dominated by ‘alternative’ foundation species, the canopy-forming Cystophora spp. (Fucales), whereas the recent heatwave stressed plots were dominated by the invasive kelp Undaria pinnatifida (Laminariales). A network analysis of attachment interactions showed that communities differed among plots dominated by either Durvillaea spp., Cystophora spp. or U. pinnatifida, with different relationships between the primary, or alternative, foundation species and attached epiphytic ‘secondary’ foundation species. For example, native Cystophora spp. were more important as hosts for secondary foundation species compared to Durvillaea spp. and U. pinnatifida. Instead, Durvillaea spp. facilitated encrusting algae, which in turn provided habitat for gastropods. We conclude that (a) repeated disturbances and strong stressors can reveal ecological differences between primary and alternative foundation species, (b) analyses of abundances and attachment-networks are supplementary methods to identify linkages between primary, alternative and secondary foundation species, and (c) interspersed habitats dominated by different types of foundation species increase system-level biodiversity by supporting different species-abundance patterns and species-attachment networks.

Dynamic Responses of Ground-Dwelling Invertebrate Communities to Disturbance in Forest Ecosystems

In forest ecosystems, natural and anthropogenic disturbances alter canopy structure, understory vegetation, amount of woody debris, and the properties of litter and soil layers. The magnitude of these environmental changes is context-dependent and determined by the properties of the disturbance, such as the frequency, intensity, duration, and extent. Therefore, disturbances can dynamically impact forest communities over time, including populations of ground-dwelling invertebrates that regulate key ecosystem processes. We propose conceptual models that describe the dynamic temporal effects of canopy gap formation and coarse woody debris accumulation following disturbances caused by invasive insects, wind, and salvage logging, and their impacts on ground-dwelling invertebrate communities. Within this framework, predictions are generated, literature on ground-dwelling invertebrate communities is synthesized, and pertinent knowledge gaps identified.

Do Terrestrial Salamanders Indicate Ecosystem Changes in New England Forests?

Long-term ecological research (LTER) and monitoring programs accrue invaluable ecological data that inform policy and improve decisions that enable adaptation to and mitigation of environmental changes. There is great interest in identifying ecological indicators that can be monitored easily and effectively to yield reliable data about environmental changes in forested ecosystems. However, the selection, use, and validity of ecological indicators to monitor in LTER programs remain challenging tasks for ecologists and conservation biologists. Across the eastern United States of America, the foundation tree species eastern hemlock (Tsuga canadensis (L.) Carrière) is declining and dying from irruptions of a non-native insect, the hemlock woolly adelgid (Adelges tsugae Annand). We use data from the Harvard Forest LTER site’s Hemlock Removal Experiment together with information from other eastern US LTER sites to show that plethodontid salamanders can be reliable indicators of ongoing ecological changes in forested ecosystems in the eastern USA. These salamanders are abundant, they have a history of demographic stability, are both predators and prey, and can be sampled and monitored simply and cost-effectively. At the Harvard Forest LTER, red-backed salamanders (Plethodon cinereus Green) were strong indicators of intact forests dominated by eastern hemlock (Tsuga canadensis); their high site fidelity and habitat specificity yielded an indicator value (robust Dufrêne and Legendre’s “IndVal”) for this species of 0.99. Eastern red-spotted newts (Notopthalmus viridescens viridescens Rafinesque) were better indicators of nearby stands made up of young, mixed hardwood species, such as those which replace hemlock stands following adelgid infestation. At the Hubbard Brook and Coweeta LTER sites, plethodontid salamanders were associated with intact riparian habitats, which may also be dominated by eastern hemlock. We conclude that plethodontid salamanders satisfy most criteria for reliable ecological indicators of environmental changes in eastern US forests.

The Past, Present, and Future of the Hemlock Woolly Adelgid (Adelges tsugae) and Its Ecological Interactions with Eastern Hemlock (Tsuga canadensis) Forests

The nonnative hemlock woolly adelgid is steadily killing eastern hemlock trees in many parts of eastern North America. We summarize impacts of the adelgid on these forest foundation species; review previous models and analyses of adelgid spread dynamics; and examine how previous forecasts of adelgid spread and ecosystem dynamics compare with current conditions. The adelgid has reset successional sequences, homogenized biological diversity at landscape scales, altered hydrological dynamics, and changed forest stands from carbon sinks into carbon sources. A new model better predicts spread of the adelgid in the south and west of the range of hemlock, but still under-predicts its spread in the north and east. Whether these underpredictions result from inadequately modeling accelerating climate change or accounting for people inadvertently moving the adelgid into new locales needs further study. Ecosystem models of adelgid-driven hemlock dynamics have consistently forecast that forest carbon stocks will be little affected by the shift from hemlock to early-successional mixed hardwood stands, but these forecasts have assumed that the intermediate stages will remain carbon sinks. New forecasting models of adelgid-driven hemlock decline should account for observed abrupt changes in carbon flux and ongoing and accelerating human-driven land-use and climatic changes.

Loss of foundation species: disturbance frequency outweighs severity in structuring kelp forest communities

Disturbances often cause the disproportionate loss of foundation species but understanding how the frequency and severity of disturbance to such organisms influence biological communities remains unresolved. This gap in knowledge exists in part because of the rarity of ecologically meaningful studies capable of disentangling different elements of disturbance. Hence, we carried out a long-term (9 yr), large-scale (2,000 m2 plots), spatially replicated (4 sites) field experiment in which we manipulated disturbance to a globally distributed marine foundation species, the giant kelp Macrocystis pyrifera, and tracked community responses over time. To distinguish the effects of disturbance frequency and severity on the biodiversity and composition of temperate rocky reef communities, we simulated the repeated loss of giant kelp from destructive winter waves across a background of natural variation in disturbance. By following the response of over 200 taxa from the surrounding community, we discovered that the frequency of disturbance to giant kelp changed the biomass, diversity, and composition of community guilds in a manner commensurate with their dependence on the physical (i.e., benthic light and space), trophic (i.e., living and detrital biomass), and habitat (i.e., biogenic structure) resources mediated by this foundation species. Annual winter disturbance to giant kelp reduced living and detrital giant kelp biomass by 57% and 40%, respectively, enhanced bottom light by 22%, and halved the seafloor area covered by giant kelp holdfasts. Concomitantly, the biomass of understory algae and epilithic sessile invertebrates more than doubled, while the biomass of rock-boring clams, mobile invertebrates, and fishes decreased 30-61%. Frequent loss of giant kelp boosted understory algal richness by 82% and lowered sessile invertebrate richness by 13% but did not affect the biodiversity of mobile fauna. In contrast to changes driven by disturbance frequency, interannual variation in the severity of disturbance to giant kelp had weaker, less consistent effects, causing only modest changes in assemblages of sessile invertebrates, mobile invertebrate herbivores, and fishes. Our results broaden the foundation species concept by demonstrating that repeated disturbance to a dominant habitat-forming species can outweigh the influence of less frequent but severe disturbances for the surrounding community.

Giant kelp, Macrocystis pyrifera, increases faunal diversity through physical engineering

Foundation species define the ecosystems they live in, but ecologists have often characterized dominant plants as foundational without supporting evidence. Giant kelp has long been considered a marine foundation species due to its complex structure and high productivity; however, there is little quantitative evidence to evaluate this. Here, we apply structural equation modelling to a 15-year time series of reef community data to evaluate how giant kelp affects the reef community. Although species richness was positively associated with giant kelp biomass, most direct paths did not involve giant kelp. Instead, the foundational qualities of giant kelp were driven mostly by indirect effects attributed to its dominant physical structure and associated engineering influence on the ecosystem, rather than by its use as food by invertebrates and fishes. Giant kelp structure has indirect effects because it shades out understorey algae that compete with sessile invertebrates. When released from competition, sessile species in turn increase the diversity of mobile predators. Sea urchin grazing effects could have been misinterpreted as kelp effects, because sea urchins can overgraze giant kelp, understorey algae and sessile invertebrates alike. Our results confirm the high diversity and biomass associated with kelp forests, but highlight how species interactions and habitat attributes can be misconstrued as direct consequences of a foundation species like giant kelp.

Does genomic variation in a foundation species predict arthropod community structure in a riparian forest?

Understanding how genetic variation within a foundation species determines the structure of associated communities and ecosystem processes has been an emerging frontier in ecology. Previous studies in common gardens identified close links between intraspecific variation and multispecies community structure, and these findings are now being evaluated directly in the complex natural ecosystem. In this study, we examined to what extent genomic variation in a foundation tree species explains the structure of associated arthropod communities in the field, comparing with spatial, temporal and environmental factors. In a continuous mixed forest, arthropods were surveyed on 85 mature alders (Alnus hirsuta) in 2 years. Moreover, we estimated Nei's genetic distance among the alders based on 1,077 single nucleotide polymorphisms obtained from restricted-site-associated DNA sequencing of the alders' genome. In both years, we detected significant correlations between genetic distance and dissimilarity of arthropod communities. A generalized dissimilarity modelling indicated that the genetic distance of alder populations was the most important predictor to explain the variance of arthropod communities. Among arthropod functional groups, carnivores were consistently correlated with genetic distance of the foundation species in both years. Furthermore, the extent of year-to-year changes in arthropod communities was more similar between more genetically closed alder populations. This study demonstrates that the genetic similarity rule would be primarily prominent in community assembly of plant-associated arthropods under temporally and spatially variable environments in the field.

Community assessment techniques and the implications for rarefaction and extrapolation with Hill numbers

Diversity estimates play a key role in ecological assessments. Species richness and abundance are commonly used to generate complex diversity indices that are dependent on the quality of these estimates. As such, there is a long-standing interest in the development of monitoring techniques, their ability to adequately assess species diversity, and the implications for generated indices. To determine the ability of substratum community assessment methods to capture species diversity, we evaluated four methods: photo quadrat, point intercept, random subsampling, and full quadrat assessments. Species density, abundance, richness, Shannon diversity, and Simpson diversity were then calculated for each method. We then conducted a method validation at a subset of locations to serve as an indication for how well each method captured the totality of the diversity present. Density, richness, Shannon diversity, and Simpson diversity estimates varied between methods, despite assessments occurring at the same locations, with photo quadrats detecting the lowest estimates and full quadrat assessments the highest. Abundance estimates were consistent among methods. Sample-based rarefaction and extrapolation curves indicated that differences between Hill numbers (richness, Shannon diversity, and Simpson diversity) were significant in the majority of cases, and coverage-based rarefaction and extrapolation curves confirmed that these dissimilarities were due to differences between the methods, not the sample completeness. Method validation highlighted the inability of the tested methods to capture the totality of the diversity present, while further supporting the notion of extrapolating abundances. Our results highlight the need for consistency across research methods, the advantages of utilizing multiple diversity indices, and potential concerns and considerations when comparing data from multiple sources.

Coastal regime shifts: rapid responses of coastal wetlands to changes in mangrove cover

Global changes are causing broad-scale shifts in vegetation communities worldwide, including coastal habitats where the borders between mangroves and salt marsh are in flux. Coastal habitats provide numerous ecosystem services of high economic value, but the consequences of variation in mangrove cover are poorly known. We experimentally manipulated mangrove cover in large plots to test a set of linked hypotheses regarding the effects of changes in mangrove cover. We found that changes in mangrove cover had strong effects on microclimate, plant community, sediment accretion, soil organic content, and bird abundance within 2 yr. At higher mangrove cover, wind speed declined and light interception by vegetation increased. Air and soil temperatures had hump-shaped relationships with mangrove cover. The cover of salt marsh plants decreased at higher mangrove cover. Wrack cover, the distance that wrack was distributed from the water's edge, and sediment accretion decreased at higher mangrove cover. Soil organic content increased with mangrove cover. Wading bird abundance decreased at higher mangrove cover. Many of these relationships were non-linear, with the greatest effects when mangrove cover varied from zero to intermediate values, and lesser effects when mangrove cover varied from intermediate to high values. Temporal and spatial variation in measured variables often peaked at intermediate mangrove cover, with ecological consequences that are largely unexplored. Because different processes varied in different ways with mangrove cover, the "optimum" cover of mangroves from a societal point of view will depend on which ecosystem services are most desired.

Unveiling the species-rank abundance distribution by generalizing the Good-Turing sample coverage theory

Based on a sample of individuals, we focus on inferring the vector of species relative abundance of an entire assemblage and propose a novel estimator of the complete species-rank abundance distribution (RAD). Nearly all previous estimators of the RAD use the conventional "plug-in" estimator Pi (sample relative abundance) of the true relative abundance pi of species i. Because most biodiversity samples are incomplete, the plug-in estimators are applied only to the subset of species that are detected in the sample. Using the concept of sample coverage and its generalization, we propose a new statistical framework to estimate the complete RAD by separately adjusting the sample relative abundances for the set of species detected in the sample and estimating the relative abundances for the set of species undetected in the sample but inferred to be present in the assemblage. We first show that P, is a positively biased estimator of pi for species detected in the sample, and that the degree of bias increases with increasing relative rarity of each species. We next derive a method to adjust the sample relative abundance to reduce the positive bias inherent in j. The adjustment method provides a nonparametric resolution to the longstanding challenge of characterizing the relationship between the true relative abundance in the entire assemblage and the observed relative abundance in a sample. Finally, we propose a method to estimate the true relative abundances of the undetected species based on a lower bound of the number of undetected species. We then combine the adjusted RAD for the detected species and the estimated RAD for the undetected species to obtain the complete RAD estimator. Simulation results show that the proposed RAD curve can unveil the true RAD and is more accurate than the empirical RAD. We also extend our method to incidence data. Our formulas and estimators are illustrated using empirical data sets from surveys of forest spiders (for abundance data) and soil ciliates (for incidence data). The proposed RAD estimator is also applicable to estimating various diversity measures and should be widely useful to analyses of biodiversity and community structure.

Scymnus camptodromus (Coleoptera: Coccinellidae) Larval Development and Predation of Hemlock Woolly Adelgid (Hemiptera: Adelgidae)

Development time and prey consumption of Scymnus (Neopullus) camptodromus Yu and Liu (Coleoptera: Coccinellidae) larvae by instar, strain, and temperature were evaluated. S. camptodromus, a specialist predator of hemlock woolly adelgid Adelges tsugae (Annand) (Hemiptera: Adelgidae), was brought to the United States from China as a potential biological control agent for A. tsugae. This beetle has been approved for removal from quarantine but has not yet been field released. We observed that temperature had significant effects on the predator's life history. The larvae tended to develop faster and consume more eggs of A. tsugae per day as rearing temperature increased. Mean egg consumption per day of A. tsugae was less at 15°C than at 20°C. However, as larvae took longer to develop at the lower temperature, the total number of eggs consumed per instar during larval development did not differ significantly between the two temperatures. The lower temperature threshold for predator larval development was estimated to be 5°C, which closely matches the developmental threshold of A. tsugae progrediens. Accumulated degree-days for 50% of the predator neonates to reach adulthood was estimated to be 424. Although temperature had a significant effect on larval development and predation, it did not impact survival, size, or sex ratio of the predator at 15 and 20°C. Furthermore, no remarkable distinctions were observed among different geographical populations of the predator.

Experiments Are Revealing a Foundation Species: A Case Study of Eastern Hemlock (Tsuga canadensis)

Foundation species are species that create and define particular ecosystems; control in large measure the distribution and abundance of associated flora and fauna; and modulate core ecosystem processes, such as energy flux and biogeochemical cycles. However, whether a particular species plays a foundational role in a system is not simply asserted. Rather, it is a hypothesis to be tested, and such tests are best done with large-scale, long-term manipulative experiments. The utility of such experiments is illustrated through a review of the Harvard Forest Hemlock Removal Experiment (HF-HeRE), a multidecadal, multihectare experiment designed to test the foundational role of eastern hemlock, Tsuga canadensis, in eastern North American forests. Experimental removal of T. canadensis has revealed that after 10 years, this species has pronounced, long-term effects on associated flora and fauna, but shorter-term effects on energy flux and nutrient cycles. We hypothesize that on century-long scales, slower changes in soil microbial associates will further alter ecosystem processes in T. canadensis stands. HF-HeRE may indeed continue for >100 years, but at such time scales, episodic disturbances and changes in regional climate and land cover can be expected to interact in novel ways with these forests and their foundation species.

Foundation species loss affects vegetation structure more than ecosystem function in a northeastern USA forest

Loss of foundation tree species rapidly alters ecological processes in forested ecosystems. Tsuga canadensis, an hypothesized foundation species of eastern North American forests, is declining throughout much of its range due to infestation by the nonnative insect Adelges tsugae and by removal through pre-emptive salvage logging. In replicate 0.81-ha plots, T. canadensis was cut and removed, or killed in place by girdling to simulate adelgid damage. Control plots included undisturbed hemlock and mid-successional hardwood stands that represent expected forest composition in 50–100 years. Vegetation richness, understory vegetation cover, soil carbon flux, and nitrogen cycling were measured for two years prior to, and five years following, application of experimental treatments. Litterfall and coarse woody debris (CWD), including snags, stumps, and fallen logs and branches, have been measured since treatments were applied. Overstory basal area was reduced 60%–70% in girdled and logged plots. Mean cover and richness did not change in hardwood or hemlock control plots but increased rapidly in girdled and logged plots. Following logging, litterfall immediately decreased then slowly increased, whereas in girdled plots, there was a short pulse of hemlock litterfall as trees died. CWD volume remained relatively constant throughout but was 3–4× higher in logged plots. Logging and girdling resulted in small, short-term changes in ecosystem dynamics due to rapid regrowth of vegetation but in general, interannual variability exceeded differences among treatments. Soil carbon flux in girdled plots showed the strongest response: 35% lower than controls after three years and slowly increasing thereafter. Ammonium availability increased immediately after logging and two years after girdling, due to increased light and soil temperatures and nutrient pulses from leaf-fall and reduced uptake following tree death. The results from this study illuminate ecological processes underlying patterns observed consistently in region-wide studies of adelgid-infested hemlock stands. Mechanisms of T. canadensis loss determine rates, magnitudes, and trajectories of ecological changes in hemlock forests. Logging causes abrupt, large changes in vegetation structure whereas girdling (and by inference, A. tsugae) causes sustained, smaller changes. Ecosystem processes depend more on vegetation cover per se than on species composition. We conclude that the loss of this late-successional foundation species will have long-lasting impacts on forest structure but subtle impacts on ecosystem function.