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Summer Research Project 2019

• Title: Group Project: Hemlock, Oak, and the Insects of Doom
• Group Project Leader: Audrey Barker Plotkin
• Mentors: Audrey Barker Plotkin; Joseph Elkinton; Jackie Matthes
• Collaborators: Meghan Blumstein; Kristin Godfrey; Danelle Laflower; David Orwig; Valerie Pasquarella; Greta VanScoy
• Project Description:

  Forest insects and pathogens are dominant disturbance agents in many forest ecosystems. Over the past 150 years, global trade has driven an increasing exchange of non-native forest insects and the Northeastern U.S. is an invasion hotspot (Lovett et al. 2016). Outbreaks trigger short-term perturbations in carbon, water, and nutrient cycling, and invasive forest insects’ ability to effectively eliminate entire tree species can trigger state changes in forest composition and structure that lead to persistent alterations in forest function.
  
  In addition to the pressing need to understand and manage these disturbance agents, forest insects present fascinating study systems. Understanding the consequences of losing an individual species and unravelling the multi-trophic interactions among the insects, hosts (trees), insect population controls (insects, diseases, and predators), and secondary mortality agents of the hosts (native insects, diseases, drought) informs both management ecological theory.
  
  This summer we will investigate the consequences of invasive forest insects on two dominant forest trees at the Harvard Forest: hemlock and oak. Hemlock is a foundation species that is often dominant, strongly shapes its environment, and is functionally unique, but currently facing extirpation from the hemlock wooly adelgid (Ellison 2014). In central and southern New England, oaks are the powerhouse of the forest, but are threatened by the canopy defoliator gypsy moth (Eisen and Barker Plotkin 2015), whose appetite can have profound effects on forest productivity.
  
  Students should indicate which sub-project is of most interest: (1) hemlock and the hemlock woolly adelgid; (2) oak and gypsy moth. We will meet every 1-2 weeks as a larger group to hear from collaborators and discuss journal articles on the topic of forest insects and pathogens.
  
  
  PROJECT 1. HEMLOCK AND THE HEMLOCK WOOLLY ADELGID (2 students)
  Primary mentor: Audrey Barker Plotkin. Collaborators: Jackie Hatala Matthes, Dave Orwig)
  
  Eastern hemlock (Tsuga canadensis) forests are declining in abundance as they are colonized by an exotic insect, the hemlock woolly adelgid (HWA, Adelges tsugae). HWA feeds on the tree’s sap, effectively girdling the tree over time. Hemlock is then often replaced by hardwoods, causing an abrupt shift in ecosystem characteristics that is often exacerbated by salvage logging. Using data we collect from a permanent plot and a long-term experiment, we will explore shifts in canopy and understory vegetation species’ composition and structure. These field data will also eventually be key inputs in an ecoinformatic model-data synthesis framework to forecast the future impacts of insects and pathogens on temperate forests (Dietze and Hatala Matthes 2014).
  
  Experimental Plot: The Harvard Forest Hemlock Removal Experiment, initiated in 2003, includes 8 plots – two replicates of each of four treatments: hemlock controls; hemlocks logged and removed from the site; hemlocks girdled and left dead on site; and hardwood controls (Orwig et al. 2013). Logging simulates a common management response to adelgid infestation. The hardwood controls represent the expected future forest conditions after ca. 50 years of succession following hemlock death. The hemlock controls were colonized by the hemlock woolly adelgid in 2010 and their trajectory of decline and regeneration will be compared to the girdling treatment, which simulated the physical, but not chemical or biotic, impacts of the adelgid.
  
  This summer, we will conduct the Year 15 tree census. The objective is to explore the trajectory of change in the forest vegetation by re-measuring all tagged trees in the plots and mapping the trees that have grown into the forest since the last tree census in 2009. In addition, students will assist with an annual survey of tree seedling abundance and understory flora cover and biodiversity.
  
  Permanent Plot: The Hemlock Woodlot is a 2-acre permanent plot that lies at the heart of the Harvard Forest, adjacent to an eddy-flux tower, 35 hectare ForestGEO plot, and paleoecological sites. Initiated in 1990, we census the mapped trees every 5-10 years. We detected hemlock woolly adelgid in the plot in 2009, and tree vigor is declining, with consequences for carbon and water fluxes (Kim et al. 2017). We expect to observe hemlock mortality and changes in the understory vegetation since the last census in 2014.
  
  Audrey Barker Plotkin will lead this project. This is a field-based study, and the two students should expect to spend ~80% of their time in the woods, with the joys and challenges associated with it, including biting insects, ticks, and the typical extremes of New England summer weather. Forest measurements skills and the ability to work with large, long-term datasets will be developed. Tree census, understory, and other data from past years will be available to examine trajectories of vegetation change over time. In addition, there may be opportunities to engage with the ecosystem modeling component of the project.
  
  PROJECT 2: GYPSY MOTH AND OAK (3 students)
  Primary mentors: Audrey Barker Plotkin; Joe Elkinton. Collaborators: Meghan Blumstein, Val Pasquarella, Rich MacLean, Kristin Godfrey
  
  For most of the 20th century, gypsy moth (Lymantria dispar) was the most serious insect threat to forests and shade trees in the northeastern United States, but outbreaks have been sporadic and light since 1989, after the successful establishment of a fungal pathogen, Entomophaga maimaiga. However, in 2015 a surprising new outbreak of gypsy moth began in southern New England. A combination of drought and a failure of E. maimaiga or other organisms to control the gypsy moth may explain the current, ongoing outbreak, in which oak mortality is exacerbated by secondary mortality agents such as native wood-boring insects and fungal diseases. We seek 3 students to help us in a new effort at Harvard Forest to understand the drivers and implications of the current gypsy moth outbreak, and the long-term consequences of defoliation.
  
  Our study sites are primarily located in the Quabbin Watershed Forest, a state-owned watershed protection forest that is actively managed for timber and (along with Harvard Forest) is part of the National Ecological Observatory Network (NEON) Northeastern core study site. Students will collaborate with researchers developing algorithms to estimate gypsy moth defoliation via LANDSAT satellite imagery, foresters, and physiological ecologists studying non-structural carbohydrate reserves in trees on a series of subprojects to explore the following questions:
  
  1) What are the patterns and causes of oak defoliation and mortality?
  2) What are the patterns and causes of gypsy moth mortality?
  
  What is the seasonal pattern of defoliation, and how do field measurements correspond with satellite (LANDSAT) near-real-time imagery? (Audrey Barker Plotkin, Val Pasquarella)
  We will establish a set of permanent plots at the Quabbin Forest and the Harvard Forest, in oak forests with differing intensities of defoliation over the past 3 years. In these plots, we will take weekly ground measurements of defoliation, litterfall, and light levels. The dates of measurement will coincide with the day that LANDSAT8 makes its cyclical passage overhead. We will also conduct a one-time defoliation survey across many sites during the peak of defoliation in early July.
  
  Is carbon starvation a proximal cause of oak mortality? (Audrey Barker Plotkin, Meaghan Blumstein)
  We will collect wood samples from healthy, defoliated, and recently deceased oaks to determine whether non-structural carbohydrate (NSC) stores are drained with defoliation. We may have time to analyze some of the samples, but may simply collect the samples during the field season for future processing.
  
  What secondary mortality agents are killing oaks? (Joe Elkinton)
  Multiple years of defoliation by gypsy moth can kill healthy trees, but often, trees weakened by defoliation die from secondary infections by native insects or diseases. In the early summer (June), we will analyze trees that have died over the winter for cause of death (Armillaria spp fungi or wood boring beetles, mainly two-lined chestnut borer, Agrilus bilineatus) Galleries of A. bilineatus can be detected by peeling bark. In late summer, after trees defoliated by gypsy moth have refoliated, we will note the tell-tale signs of A bilineatus attacks of leaves turning brown in the canopy of the trees. Dying foliage with reach of pole pruners can be dissected. At UMASS we will confirm the identity larvae with DNA analysis .
  
  What is killing gypsy moth larvae during the current outbreak? (Joe Elkinton)
  Many organisms (parasitoids, mammals, viruses, fungi) play a role in controlling gypsy moth populations. Concurrently with the defoliation and light measurements, we will conduct weekly gypsy moth larvae collections in the permanent plots. These larvae will be reared for one week in the lab, and the percent mortality noted at the end of each week. Dead larvae will be autopsied under the microscope to determine the cause of death from the two major pathogens of gypsy moth, Entomophaga maimaiga or LdNPV. In addition, emerging parasitoids of gypsy moths will be identified and recorded.
  
  Do white-footed mice play a role in the current outbreak? (Kristin Godfrey, Audrey Barker Plotkin, Joe Elkinton)
  During non-outbreak years, white-footed mice are a primary control of gypsy moth populations. In turn white-footed mice populations are largely determined by acorn abundance, which fluctuates from year-to-year (Elkinton et al. 1996). During the July peak defoliation survey, we will include visits to NEON plots with 5 years of annual small mammal population data. Although mice are unlikely to make a dent in gypsy moths in outbreak conditions, perhaps fluctuations in mouse populations contributed to how the current outbreak began. We will collaborate with NEON staff to use these data. There may (or may not) be a possibility to assist NEON staff with one of their small mammal surveys.
  
  Students will work on all aspects of this multi-faceted project, although each student will have a primary focus: 1) seasonal patterns of defoliation and light, and NSC sampling; 2) gypsy moth larvae mortality; 3) oak secondary mortality agents. Overall, students should expect to split their time between a long field day once a week, lighter field days, litterfall and wood sample processing, and working with gypsy moth larvae and other insects in the field and lab. Field work is full of marvels, as well as biting insects, ticks, and the typical extremes of New England summer weather. Forest measurements skills, insect rearing and necroscopy, and the ability to work with multi-faceted datasets will be developed.
  

• Readings:

  Dietze, M.C. and J. Hatala Matthes. 2014. A general ecophysiological framework for modelling the impact of pests and pathogens on forest ecosystems. Ecology Letters 17:1418-1426.
  
  Eisen, K. and A. Barker Plotkin 2015. Forty years of forest measurements support steadily increasing aboveground biomass in a maturing, Quercus-dominated northeastern forest. Journal of the Torrey Botanical Society 142:97-112.
  
  Elkinton, J.S., W.M. Healy, J.P. Buonaccorsi, G.H. Boettner, A.M. Hazzard, H.R. Smith, and A.M. Liebhold. 1996. Interactions among gypsy moth, white-footed mice, and acorns. Ecology 77:2332-2342.
  
  Ellison, A.M. 2014. Experiments are revealing a foundation species: a case study of the eastern hemlock (Tsuga canadensis). Advances in Ecology 3:1-16.
  
  Kim, J., Hwang, T., Schaaf, C. L., Orwig, D. A., Boose, E. R., Munger, J. W. 2017. Increased water yield due to the hemlock woolly adelgid infestation in New England. Geophysical Research Letters 44: 2327-2335.
  
  Lovett, G. M., Weiss, M., Liebhold, A. M., Holmes, T. P., Leung, B., Lambert, K. F., Orwig, D. A., Campbell, F. T., Rosenthal, J., McCullough, D. G., Wildova, R., Ayres, M. P., Canham, C. D., Foster, D. R., LaDeau, S. L., Weldy, T. 2016. Nonnative forest insects and pathogens in the United States: Impacts and policy options. Ecological Applications 26: 1437-1455.
  https://harvardforest.fas.harvard.edu/sites/harvardforest.fas.harvard.edu/files/publications/pdfs/Lovett_EcoApp_2016.pdf
  
  Orwig, D.A. A. Barker Plotkin, E. Davidson, H. Lux, K.E. Savage, and A.M. Ellison. 2013. Foundation species loss affects vegetation structure more than ecosystem function in a Northeastern USA forest. PeerJ e41:1-29.
  

• Research Category: Large Experiments and Permanent Plot Studies, Invasive Plants, Pests & Pathogens, Group Projects