Person: Orwig, David
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Orwig
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Orwig, David
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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 Nonnative forest insects and pathogens in the United States: Impacts and policy options(Wiley-Blackwell, 2016) Lovett, Gary M.; Weiss, Marissa; Liebhold, Andrew M.; Holmes, Thomas P.; Leung, Brian; Lambert, Kathy Fallon; Orwig, David; Campbell, Faith T.; Rosenthal, Jonathan; McCullough, Deborah G.; Wildova, Radka; Ayres, Matthew P.; Canham, Charles D.; Foster, David; LaDeau, Shannon L.; Weldy, TroyWe review and synthesize information on invasions of nonnative forest insects and diseases in the United States, including their ecological and economic impacts, pathways of arrival, distribution within the United States, and policy options for reducing future invasions. Nonnative insects have accumulated in United States forests at a rate of ~2.5 per yr over the last 150 yr. Currently the two major pathways of introduction are importation of live plants and wood packing material such as pallets and crates. Introduced insects and diseases occur in forests and cities throughout the United States, and the problem is particularly severe in the Northeast and Upper Midwest. Nonnative forest pests are the only disturbance agent that has effectively eliminated entire tree species or genera from United States forests within decades. The resulting shift in forest structure and species composition alters ecosystem functions such as productivity, nutrient cycling, and wildlife habitat. In urban and suburban areas, loss of trees from streets, yards, and parks affects aesthetics, property values, shading, stormwater runoff, and human health. The economic damage from nonnative pests is not yet fully known, but is likely in the billions of dollars per year, with the majority of this economic burden borne by municipalities and residential property owners. Current policies for preventing introductions are having positive effects but are insufficient to reduce the influx of pests in the face of burgeoning global trade. Options are available to strengthen the defenses against pest arrival and establishment, including measures taken in the exporting country prior to shipment, measures to ensure clean shipments of plants and wood products, inspections at ports of entry, and post-entry measures such as quarantines, surveillance, and eradication programs. Improved data collection procedures for inspections, greater data accessibility, and better reporting would support better evaluation of policy effectiveness. Lack of additional action places the nation, local municipalities, and property owners at high risk of further damaging and costly invasions. Adopting stronger policies to reduce establishments of new forest insects and diseases would shift the major costs of control to the source and alleviate the economic burden now borne by homeowners and municipalities.Publication Variations in Old-Growth Structure and Definitions: Forest Dynamics on Wachusett Mountain, Massachusetts(Wiley-Blackwell, 2001) Orwig, David; Cogbill, Charles; Foster, David; O, John F.One of the largest old-growth forests in southern New England was recently ‘‘discovered’’ on the exposed upper slopes of Wachusett Mountain, Massachusetts, a heavily used recreational area located ,80 km (,50 miles) from Boston. We analyzed historical records, dendroecological data, vegetation, and coarse woody debris to characterize the dynamics and development of four major old-growth stands on the mountain and explored the potential factors enabling these forests to survive and elude recognition as old-growth forests for .150 yr. Historical data suggest that most of the area’s forests were composed of a mixture of Quercus rubra and northern hardwood species. Species abundances and recruitment dynamics in the four stands exhibit highly variable spatial and temporal patterns across sites that differ in aspect and exposure. Three uneven-aged hardwood stands contain Quercus rubra in the largest size classes, various amounts of Fagus grandifolia, Acer, and Betula species in the middle size classes, and dense thickets of several shrub species in the small size classes. Several of a set of randomly aged individuals of Q. rubra, B. lenta, and B. alleghaniensis are at or very near the maximum longevity known for these species. A Tsuga canadensis stand contains unimodal size and age distributions, with trees ,60- cm dbh and 100–300 yr old. Quercus rubra recruitment occurred on all sites from the 1600s through the early 1800s, when it dropped precipitously on most sites and was replaced by either Tsuga or Acer and Betula species. These recruitment changes were apparently driven by changes in disturbance regime over the last two centuries, from fire in the early record to hurricanes (1815 and 1938), and more recently, frequent wind, ice, and snow damage but no fire. Asynchronous tree-ring releases and suppression and relatively low amounts of coarse woody debris corroborate this interpretation. Chronic canopy damage produced short-statured and unusually gnarled trees, which gave the forest unusual resistance to severe winds from hurricanes, discouraged logging, and prevented the recognition of the forest’s old-growth status despite its heavy recreational use.Publication Loss of Foundation Species: Consequences for the Structure and Dynamics of Forested Ecosystems(Wiley-Blackwell, 2005) Ellison, Aaron; Bank, Michael; Clinton, Barton D.; Colburn Mirkovic, Elizabeth; Elliott, Katherine; Ford, Chelcy R.; Foster, David; Kloeppel, Brian D.; Knoepp, Jennifer D.; Lovett, Gary M.; Mohan, Jacqueline; Orwig, David; Rodenhouse, Nicholas L.; Sobczak, William V.; Stinson, Kristina A.; Stone, Jeffrey K.; Swan, Christopher M.; Thompson, Jill; Holle, Betsy Von; Webster, Jackson R.In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation treespecieschangesthelocalenvironmentonwhichavarietyofotherspeciesdepend; howthisdisruptsfun- damental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynam- ics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small pertur- bations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, antic- ipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.Publication A foundation tree at the precipice: Tsuga canadensis health after the arrival of Adelges tsugae in central New England(Wiley-Blackwell, 2012) Orwig, David; Thompson, Jonathan; Povak, Nicholas; Manner, Megan; Niebyl, Donald; Foster, DavidHemlock (Tsuga canadensis) plays a unique role in Eastern forests, producing distinctive biogeochemical, habitat, and microclimatic conditions and yet has begun a potentially irreversible decline due to the invasive hemlock woolly adelgid (Adelges tsugae; HWA) that causes foliar damage, crown loss, and mortality of host trees. Understanding the regional, landscape, site, and stand factors influencing HWA spread and impact is critical for predicting future landscape dynamics and directing effective management. Using aerial photographs, we documented hemlock distribution throughout central Massachusetts and subsampled 123 stands to examine the spatial pattern of HWA and its impact on tree vigor and mortality since its arrival in 1989. In the study region, over 86,000 ha of hemlock forest were mapped in 5,127 stands. White pine (Pinus strobus), red oak (Quercus rubra), red maple (Acer rubrum), and black birch (Betula lenta) were common overstory associates. Hemlock abundance increased from south to north, commonly on western and northwestern slopes. Average stand size was 55 ha, overstory basal area ranged from 23 to 55 m2 ha1 and overstory stem densities averaged 993 ha1. By 2004, 40% of sampled stands were infested, but most stands remained in good health overall; only 8 stands contained high HWA densities and only two had lost .50% overstory hemlock. Out of fifteen stand and landscape predictor variables examined, only latitude and winter climate variables were related to HWA density. Cold temperatures appear to be slowing the spread and impact of HWA at its northern extent as HWA infestation intensity and hemlock mortality and vigor were significantly correlated with average minimum winter temperature. Contrary to predictions, there was no regional increase in hemlock harvesting. The results suggest that regional HWA-hemlock dynamics are currently being shaped more by climate than by a combination of landscape and social factors. The persistence and migration of HWA continues to pose a significant threat regionally, especially in the northern portion of the study area, where hemlock dominates many forests.Publication The Influence of Successional Processes and Disturbance on the structure of Tsuga canadensis Forests(Wiley-Blackwell, 2008) D, Anthony W.; Orwig, David; Foster, DavidOld-growth forests are valuable sources of ecological, conservation, and management information, yet these ecosystems have received little study in New England, due in large part to their regional scarcity. To increase our understanding of the structures and processes common in these rare forests, we studied the abundance of downed coarse woody debris (CWD) and snags and live-tree size-class distributions in 16 old-growth hemlock forests in western Massachusetts. Old-growth stands were compared with eight adjacent secondgrowth hemlock forests to gain a better understanding of the structural differences between these two classes of forests resulting from contrasting histories. In addition, we used standlevel dendroecological reconstructions to investigate the linkages between disturbance history and old-growth forest structure using an information–theoretic model selection framework. Old-growth stands exhibit a much higher degree of structural complexity than secondgrowth forests. In particular, old-growth stands had larger overstory trees and greater volumes of downed coarse woody debris (135.2 vs. 33.2 m3 /ha) and snags (21.2 vs. 10.7 m3 /ha). Secondgrowth stands were characterized by either skewed unimodal or reverse-J shaped diameter distributions, while old-growth forests contained bell-shaped, skewed unimodal, rotated sigmoid, and reverse J-shaped distributions. The variation in structural attributes among oldgrowth stands, particularly the abundance of downed CWD, was closely related to disturbance history. In particular, old-growth stands experiencing moderate levels of canopy disturbance during the last century (1930s and 1980s) had greater accumulations of CWD, highlighting the importance of gap-scale disturbances in shaping the long-term development and structural characteristics of old-growth forests. These findings are important for the development of natural disturbance-based silvicultural systems that may be used to restore important forest characteristics lacking in New England second-growth stands by integrating structural legacies of disturbance (e.g., downed CWD) and resultant tree-size distribution patterns. This silvicultural approach would emulate the often episodic nature of CWD recruitment within old-growth forests.Publication Understory Vegetation in Old-Growth and Second-Growth Tsuga Canadensis Forests in Western Massachusetts(Elsevier, 2009) D’Amato, Anthony W.; Orwig, David; Foster, DavidWe compared the understory communities (herbs, shrubs, and tree seedlings and saplings) of old-growth and second-growth eastern hemlock forests (Tsuga canadensis) in western Massachusetts, USA. Second-growth hemlock forests originated following clear-cut logging in the late 1800s and were 108–136 years old at the time of sampling. Old-growth hemlock forests contained total ground cover of herbaceous and shrub species that was approximately 4 times greater than in second-growth forests (4.02 ± 0.41%/m^2 versus 1.06 ± 0.47%/m^2) and supported greater overall species richness and diversity. In addition, seedling and sapling densities were greater in old-growth stands compared to second-growth stands and the composition of these layers was positively correlated with overstory species composition (Mantel tests, r > 0.26, P < 0.05) highlighting the strong positive neighborhood effects in these systems. Ordination of study site understory species composition identified a strong gradient in community composition from second-growth to old-growth stands. Vector overlays of environmental and forest structural variables indicated that these gradients were related to differences in overstory tree density, nitrogen availability, and coarse woody debris characteristics among hemlock stands. These relationships suggest that differences in resource availability (e.g., light, moisture, and nutrients) and microhabitat heterogeneity between old-growth and second-growth stands were likely driving these compositional patterns. Interestingly, several common forest understory plants, including Aralia nudicaulis, Dryopteris intermedia, and Viburnum alnifolium, were significant indicator species for old-growth hemlock stands, highlighting the lasting legacy of past land use on the reestablishment and growth of these common species within second-growth areas. The return of old-growth understory conditions to these second-growth areas will largely be dependent on disturbance and self-thinning mediated changes in overstory structure, resource availability, and microhabitat heterogeneity.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.