Person: Barker-Plotkin, Audrey
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Barker-Plotkin
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Audrey
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Barker-Plotkin, Audrey
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Publication When a foundation crumbles: forecasting forest dynamics following the decline of the foundation species Tsuga canadensis(Wiley-Blackwell, 2017) Case, Bradley S.; Buckley, Hannah L.; Barker-Plotkin, Audrey; Orwig, David; Ellison, AaronIn the forests of northeastern North America, invasive insects and pathogens are causing major declines in some tree species and a subsequent reorganization of associated forest communities. Using observations and experiments to investigate the consequences of such declines are hampered because trees are long-lived. Simulation models can provide a means to forecast possible futures based on different scenarios of tree species decline, death, and removal. Such modeling is particularly urgent for species such as eastern hemlock (Tsuga canadensis), a foundation species in many northeastern forest regions that is declining due to the hemlock woolly adelgid (Adelges tsugae). Here, we used an individual-based forest simulator, SORTIE-ND, to forecast changes in forest communities in Central Massachusetts over the next 200 yr under a range of scenarios: a no-adelgid, status quo scenario; partial resistance of hemlock to the adelgid; adelgid irruption and total hemlock decline over 25 yr, adelgid irruption and salvage logging of hemlock trees; and two scenarios of preemptive logging of hemlock and hemlock/white pine. We applied the model to six study plots comprising a range of initial species mixtures, abundances, and levels of hemlock dominance. Simulations indicated that eastern white pine, and to a lesser extent black birch and American beech, would gain most in relative abundance and basal area following hemlock decline. The relative dominance of these species depended on initial conditions and the amount of hemlock mortality, and their combined effect on neighborhood-scale community dynamics. Simulated outcomes were little different whether hemlock died out gradually due to the adelgid or disappeared rapidly following logging. However, if eastern hemlock were to become partially resistant to the adelgid, hemlock would be able to retain its dominance despite substantial losses of basal area. Our modeling highlights the complexities associated with secondary forest succession due to ongoing hemlock decline and loss. We emphasize the need both for a precautionary approach in deciding between management intervention or simply doing nothing in these declining hemlock forests, and for clear aims and understanding regarding desired community- and ecosystem-level outcomes.Publication Foundation species loss affects vegetation structure more than ecosystem function in a northeastern USA forest(PeerJ, 2013) Orwig, David; Barker-Plotkin, Audrey; Davidson, Eric A.; Lux, Heidi; Savage, Kathleen E.; Ellison, AaronLoss 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.Publication Building a foundation: land-use history and dendrochronology reveal temporal dynamics of a Tsuga canadensis (Pinaceae) forest(The New England Botanical Club, 2014) Ellison, Aaron; Lavine, Michael; Kerson, Peter; Barker-Plotkin, Audrey; Orwig, DavidPublication Foundation Species Loss and Biodiversity of the Herbaceous Layer in New England Forests(MDPI AG, 2015) Ellison, Aaron; Barker-Plotkin, Audrey; Khalid, ShahEastern hemlock (Tsuga canadensis) is a foundation species in eastern North American forests. Because eastern hemlock is a foundation species, it often is assumed that the diversity of associated species is high. However, the herbaceous layer of eastern hemlock stands generally is sparse, species-poor, and lacks unique species or floristic assemblages. The rapidly spreading, nonnative hemlock woolly adelgid (Adelges tusgae) is causing widespread death of eastern hemlock. Loss of individual hemlock trees or whole stands rapidly leads to increases in species richness and cover of shrubs, herbs, graminoids, ferns, and fern-allies. Naively, one could conclude that the loss of eastern hemlock has a net positive effect on biodiversity. What is lost besides hemlock, however, is landscape-scale variability in the structure and composition of the herbaceous layer. In the Harvard Forest Hemlock Removal Experiment, removal of hemlock by either girdling (simulating adelgid infestation) or logging led to a proliferation of early-successional and disturbance-dependent understory species. In other declining hemlock stands, nonnative plant species expand and homogenize the flora. While local richness increases in former eastern hemlock stands, between-site and regional species diversity will be further diminished as this iconic foundation species of eastern North America succumbs to hemlock woolly adelgid.Publication The Relative Contributions of Seed Bank, Seed Rain, and Understory Vegetation Dynamics to the Reorganization of Tsuga Canadensis Forests After Loss due to Logging or Simulated Attack by Adelges tsugae(Canadian Science Publishing, 2012) Farnsworth, Elizabeth Jean; Barker-Plotkin, Audrey; Ellison, AaronProfound changes are occurring in forests as native insects, nonnative insects, or pathogens irrupt on foundation tree species; comprehensive models of vegetation responses are needed to predict future forest composition. We experimentally simulated hemlock woolly adelgid (Adelges tsugae Annand) infestation (by girdling trees) and preemptive logging of eastern hemlock (Tsuga canadensis (L.) Carrière) and compared vegetation dynamics in replicate 90 m × 90 m treatment plots and intact hemlock stands from 2004 to 2010. Using Chao–Sørensen abundance-based similarity indices, we assessed compositional similarities of trees, shrubs, forbs, and graminoids among the seed bank, seed rain, and standing vegetation over time and among treatments. Post-treatment seed rain, similar among treatments, closely reflected canopy tree composition. Species richness of the seed bank was similar in 2004 and 2010. Standing vegetation in the hemlock controls remained dissimilar from the seed bank, reflecting suppressed germination. Recruits from the seed rain and seed bank dominated standing vegetation in the logged treatment, whereas regeneration of vegetation from the seed bank and seed rain was slowed due to shading by dying hemlocks in the girdled treatment. Our approach uniquely integrates multiple regeneration components through time and provides a method for predicting forest dynamics following loss of foundation tree species.Publication Experimentally Testing the Role of Foundation Species in Forests: The Harvard Forest Hemlock Removal Experiment(British Ecological Society, 2010) Ellison, Aaron; Barker-Plotkin, Audrey; Foster, David; Orwig, David1. Problem statement– Foundation species define and structure ecological systems. In forests around the world, foundation tree species are declining due to overexploitation, pests and pathogens. Eastern hemlock (Tsuga canadensis), a foundation tree species in eastern North America, is threatened by an exotic insect, the hemlock woolly adelgid (Adelges tsugae). The loss of hemlock is hypothesized to result in dramatic changes in assemblages of associated species with cascading impacts on food webs and fluxes of energy and nutrients. We describe the setting, design and analytical framework of the Harvard Forest Hemlock Removal Experiment (HF-HeRE), a multi-hectare, long-term experiment that overcomes many of the major logistical and analytical challenges of studying system-wide consequences of foundation species loss. 2. Study design– HF-HeRE is a replicated and blocked Before-After-Control-Impact experiment that includes two hemlock removal treatments: girdling all hemlocks to simulate death by adelgid and logging all hemlocks >20 cm diameter and other merchantable trees to simulate pre-emptive salvage operations. These treatments are paired with two control treatments: hemlock controls that are beginning to be infested in 2010 by the adelgid and hardwood controls that represent future conditions of most hemlock stands in eastern North America. 3. Ongoing measurements and monitoring– Ongoing long-term measurements to quantify the magnitude and direction of forest ecosystem change as hemlock declines include: air and soil temperature, light availability, leaf area and canopy closure; changes in species composition and abundance of the soil seed-bank, understorey vegetation, and soil-dwelling invertebrates; dynamics of coarse woody debris; soil nitrogen availability and net nitrogen mineralization; and soil carbon flux. Short-term or one-time-only measurements include initial tree ages, hemlock-decomposing fungi, wood-boring beetles and throughfall chemistry. Additional within-plot, replicated experiments include effects of ants and litter-dwelling microarthoropods on ecosystem functioning, and responses of salamanders to canopy change. 4. Future directions and collaborations– HF-HeRE is part of an evolving network of retrospective studies, natural experiments, large manipulations and modelling efforts focused on identifying and understanding the role of single foundation species on ecological processes and dynamics. We invite colleagues from around the world who are interested in exploring complementary question.