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Harvard Forest Data Archive

HF081

Landscape Response to Hemlock Woolly Adelgid in Southern New England 1997-2011

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Data

Overview

  • Lead: David Orwig, David Foster
  • Investigators: David Mausel
  • Contact: David Orwig
  • Start date: 1997
  • End date: 2011
  • Status: completed
  • Location: Central Connecticut, Central Massachusetts
  • Latitude: +41.28 to +42.73
  • Longitude: -72.73 to -72.25
  • Elevation:
  • Taxa: Adelges tsugae (hemlock woolly adelgid), Betula lenta (black birch), Tsuga canadensis (eastern hemlock)
  • Release date: 2017
  • Revisions:
  • EML file: knb-lter-hfr.81.20
  • DOI: digital object identifier
  • Related links:
  • Study type: long-term measurement
  • Research topic: conservation and management; ecological informatics and modelling; invasive plants, pests and pathogens; large experiments and permanent plot studies; physiological ecology, population dynamics and species interactions; regional studies
  • LTER core area: disturbance
  • Keywords: hemlock, hemlock woolly adelgid, landscape, mortality, timber harvest
  • Abstract:

    The hemlock woolly adelgid (Adelges tsugae Annand; HWA), a small, aphid-like insect native to Japan, is currently migrating northward through eastern North America and threatens to eliminate eastern hemlock (Tsuga canadensis (L.) Carriere), one of the most abundant, long-lived shade tolerant species, across its range. In addition, a second invasive pest, the elongate hemlock scale (Fiorinia externa; EHS), often co-occurs with HWA on hemlock with unknown long-term consequences. The major objectives of this study were: (1) To characterize the pre-HWA distribution, composition, and structure of hemlock stands; (2) to characterize the spatial patterns of damage generated by HWA across southern New England since the time of its arrival in 1985; (3) to examine environmental and stand factors that are associated with declines in crown vigor and mortality of hemlock; (4) examine the dynamics of HWA and EHS in hemlock stands over time; and (5) assess whether there is a difference in the response of these insects to abiotic conditions (winter temperature, summer temperature, summer precipitation), and whether the distribution and abundance of each insect species is dependent on biotic interactions with the co-occurring insect species.

    Connecticut Stands (originally sampled in 1997 and 1998)

    Aerial photographs, photographic overlays, and extensive field study were used to map and develop GIS overlays of almost 6000 hemlock stands in a 4900 km2 study region extending from Long Island Sound northward to the Massachusetts border and including the Connecticut River Valley in Connecticut, USA. Intensive sampling of a random selection of 114 hemlock stands across the study area was used to document patterns of hemlock infestation, vigor, and mortality in relation to stand and site characteristics. Mantel tests were utilized to assess the relative importance of environmental and stand variables in controlling the intensity of HWA infestation and damage. Most stands were located along ridge tops, steep hillsides, and narrow valleys. Hemlock importance values ranged from 22 to 96% and stand densities varied from 300 to 1450 stems ha-1. Adelgid presence and adelgid-induced hemlock mortality were found in 88% and 74% of the sampled forests, respectively. Approximately 25% of stands were logged recently, ranging in intensity from partial hemlock cutting to large clearcuts. A geographical trend in reduced HWA infestation intensity and tree mortality and enhanced crown vigor of overstory and understory hemlock occurs from south to north, coincident with the temporal colonization pattern of HWA. Mantel analyses indicated that patterns of HWA infestation, hemlock mortality, and crown vigor were most strongly correlated with latitude. Mortality was also weakly related to aspect and stand size. Average mortality was highest on western aspects but exceeded 20% on most slopes. Remaining trees averaged over 50% foliar loss, with no significant difference among aspects. Results suggest that as HWA becomes abundant, stands on xeric aspects succumb rapidly, but that stand and landscape variables such as overstory composition and structure, slope, and elevation, exert little control over susceptibility or eventual mortality. Ultimately, duration of infestation controls the intensity of hemlock decline and mortality. Over 4,290 ha of hemlock forest were eliminated by logging or HWA just within the southern part of our transect since the mid-1980s, and we predict continued HWA infestation will lead to unprecedented hemlock loss throughout the northeastern U.S., regardless of site conditions or location.

    Massachusetts Stands (originally sampled in 2002-2004)

    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 55m2 ha-1 and overstory stem densities averaged 993 ha-1. 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.

    Insect dynamics over time

    We created a multivariate spatio-temporal model of the joint abundance of HWA and EHS, which share a host plant. Data on abundance of both insects were examined from the time the stand was initially sampled and again in 2005, 2007, 2009, and 2011. The model revealed differences in the way each species responded to weather extremes, and indicated a commensalism in which high abundance of HWA increased the probability of subsequent infestation by EHS– but not vice versa. This study provides evidence that incorporating biotic interactions into a species distribution model indicates whether a species’ distribution and abundance is dependent on other species in the community, and importantly, improves estimation of the abiotic niche for species whose abundance depends on interactions with other species in the community.

  • Methods:

    Hemlock stand sampling

    Aerial photographs and extensive field study were used to map and develop GIS overlays of 6000+ hemlock stands in a 7500 km2 study region extending from Long Island Sound northward to the Massachusetts border and including the Connecticut River Valley. During the summers of 1997-1998 in Connecticut and 2002 – 2004 in Massachusetts, vegetation was sampled in one fixed-area (20 x 20 m) plot and 5 to 10 variable-radius plots located every 30 to 50 m along a transect oriented through the long dimension of the forest in each stand. In fixed-area plots, all trees (stems at least 8 cm diameter breast height (dbh)) were tallied by species and dbh, and assigned a canopy position based on a visual estimation of the amount of intercepted light received by the tree crown. Hemlocks that died within the previous 2 to 4 years, which were identified by extensive retention of fine twigs in the crown, were tallied to determine species composition prior to HWA infestation. Crown vigor classes were assigned to each hemlock tree based on the amount of retained foliage: 1 = 76 - 100%; 2 = 51 - 75%; 3 = 26 - 50%; 4 = 1 - 25%; 5 = dead. Presence and intensity of infestation by HWA, EHS, and hemlock borer were estimated from the number of egg sacs present on branches from several trees in each plot and categorized as: 0 = absent; 1 = low density (1-10 insects/m branch); 2 = moderate density (11-100 insects/m branch); or 3 = heavy density (>100 insects/m branch).

    All saplings (less than 8 cm dbh and over 1.4 m tall) were tallied by species. Overstory species composition, degree of hemlock mortality, and basal area were also assessed within variable radius cruise plots using the Bitterlich method with a 5 or 10 basal area factor gauge. Slope, aspect, elevation, and depth of the soil organic horizon to the nearest 0.5 cm were recorded at each sampling point. A relative importance value was calculated for each overstory species in every stand by summing the relative basal area derived from the variable radius sampling and the relative density derived from the fixed-area plots. Values for hemlock included both live and recently dead trees to represent "pre-HWA" importance.

    Insect sampling and related climate data

    A subset of the hemlock stands sampled in central Connecticut and Massachusetts (n = 142) were intensively sampled for insect densities over time and climatic variables were obtained for each forest. In the initial year of sampling (1997-98 in Connecticut and 2002-2004 in Massachusetts), each stand was given an ordinal score for each insect species (hemlock woolly adelgid and elongate hemlock scale) representing the average infestation level of the stand based on thorough examination of trees as follows: 0 = 0 insects per meter of branch; 1 = 1-10 insects per meter; 2 = 11 – 100 insects per meter; 3 = >100 insects per meter. In subsequent years (2005, 2007, 2009, 2011), 50 trees were haphazardly selected in each stand for observation, and the density of each insect species on each tree was assigned to one of the above categories. Individual trees could not be marked. Fewer than 50 trees were sampled per stand in some highly-damaged stands, and stands impacted by logging or development during the study period were not sampled post-disturbance. Daily temperature (maximum/minimum) and precipitation data were obtained for each stand from 1996 to 2011 from the online PRISM Gridded Climate database (PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu, created 26 Mar 2015) by interpolating 4km2 resolution climate data at the centroid of each eastern hemlock stand using values from surrounding grid cell centers and inverse-distance squared weighting.

  • Use:

    This dataset is released to the public under Creative Commons license CC BY (Attribution). Please keep the designated contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement.

  • Citation:

    Orwig D, Foster D. 2017. Landscape Response to Hemlock Woolly Adelgid in Southern New England 1997-2011. Harvard Forest Data Archive: HF081.

Detailed Metadata

hf081-01: Connecticut data

  1. stand: 4-5 letter site abbrev., arranged N to S
  2. year: year of sampling
  3. aspect: direction slope faces in degrees (unit: degree / missing value: NA)
  4. slope: percent slope (unit: number / missing value: NA)
  5. elevation: elevation of plot in m a.s.l. estimated from topo. maps (unit: meter / missing value: NA)
  6. latitude: latitude of stand derived from arcview (unit: degree / missing value: NA)
  7. longitude: longitude of stand derived from arcview (unit: degree / missing value: NA)
  8. humus: depth of soil organic matter from quantitative plot (unit: centimeter / missing value: NA)
  9. hem.dbh: average hemlock dbh from quantitative plot (unit: centimeter / missing value: NA)
  10. hard.dbh: average hardwood dbh from quantitative plot (unit: centimeter / missing value: NA)
  11. qba: stand basal area derived from quantitative plot (unit: meterSquaredPerHectare / missing value: NA)
  12. cba: average stand basal area derived from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  13. hem.cc: average hemlock crown class derived from quantitative plot
    • 1: overtopped tree
    • 2: intermediate canopy position
    • 3: codominant canopy position
    • 4: dominant canopy position
  14. hard.cc: average hardwood crown class derived from quantitative plot
    • 1: overtopped tree
    • 2: intermediate canopy position
    • 3: codominant canopy position
    • 4: dominant canopy position
  15. hem.vigor: average crown vigor of hemlock trees
    • 1: healthy; no visible foliar loss
    • 2: healthy; 1 – 25% foliar loss
    • 3: declining; 26-50% foliar loss
    • 4: declining; 51-75% foliar loss
    • 5: declining; 76-99% foliar loss
    • 6: dead
  16. hem.bad: average amount of dead hemlock basal area from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  17. hem.d.per: percent of dead overstory hemlock in quantitative plots (unit: number / missing value: NA)
  18. hem.dead: density of dead overstory hemlock stems in quantitative plots (unit: numberPerHectare / missing value: NA)
  19. live.hem: density of live hemlock overstory stems in quantitative plots (unit: numberPerHectare / missing value: NA)
  20. hem.dens: density of hemlock overstory stems in quantitative plots (unit: numberPerHectare / missing value: NA)
  21. stem.dens: total overstory stem density in quantitative plots (unit: numberPerHectare / missing value: NA)
  22. hwa.dens: average density of HWA observed on hemlock branches
    • 0: absent
    • 1: low density
    • 2: moderate density
    • 3: heavy density, or HWA egg sacs at the base of most to all needles or many scale insects on most to all needles
  23. scale.dens: average density of hemlock scale insects observed on hemlock branches
    • 0: absent
    • 1: low density
    • 2: moderate density
    • 3: heavy density, or HWA egg sacs at the base of most to all needles or many scale insects on most to all needles
  24. hem.sapl: total density of live hemlock saplings within quantitative plots (unit: numberPerHectare / missing value: NA)
  25. hem.sapd: percent of dead hemlock saplings within quantitative plots (unit: number / missing value: NA)
  26. hem.iv: importance value derived by summing basal area from cruise plots and density from quantitative plots (unit: number / missing value: NA)
  27. size: stand size (unit: hectare / missing value: NA)

hf081-02: Massachusetts data

  1. stand: town name from USGS topo maps
  2. year: year when stand was sampled (2002-2004)
  3. map.code: map delineation of hemlock
    • A: at least 50% hemlock
    • B: 1- 50% hemlock
  4. aspect: direction slope faces in degrees, derived from cruise and fixed-area, quantitative plots (unit: degree / missing value: NA)
  5. slope: percent slope averaged from cruise and quantitative plots (unit: number / missing value: NA)
  6. long: longitude of stand derived from Arcview (unit: degree / missing value: NA)
  7. lat: latitude of stand derived from Arcview (unit: degree / missing value: NA)
  8. elevation: elevation of plot in meters a.s.l. estimated from topo maps (unit: meter / missing value: NA)
  9. area: stand size (unit: hectare / missing value: NA)
  10. humus: depth of soil organic matter from quantitative plot (unit: centimeter / missing value: NA)
  11. ba: average stand basal area derived from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  12. hem.ba: average hemlock basal area from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  13. hard.ba: average hardwood basal area from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  14. live.ba: average live basal area from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  15. dead.hem.ba: average dead hemlock basal area from cruise plots (unit: meterSquaredPerHectare / missing value: NA)
  16. hem.mort: average hemlock mortality from cruise plots (%) (unit: number / missing value: NA)
  17. hem.dbh: average hemlock dbh from quantitative plot (unit: centimeter / missing value: NA)
  18. hard.dbh: average hardwood dbh from quantitative plot (unit: centimeter / missing value: NA)
  19. qba: stand basal area derived from quantitative plot (unit: meterSquaredPerHectare / missing value: NA)
  20. hem.cc: average hemlock crown class derived from quantitative plot
    • 1: overtopped tree
    • 2: intermediate canopy position
    • 3: codominant canopy position
    • 4: dominant canopy position
  21. hard.cc: average hardwood crown class derived from quantitative plot
    • 1: overtopped tree
    • 2: intermediate canopy position
    • 3: codominant canopy position
    • 4: dominant canopy position
  22. hem.vigor: average crown loss of hemlock trees
    • 1: healthy; no visible foliar loss
    • 2: healthy; 1-25% foliar loss
    • 3: declining; 26-50% foliar loss
    • 4: declining; 51-75% foliar loss
    • 5: declining; 76-99% foliar loss
    • 6: dead
  23. hem.iv: importance value derived by summing relative basal area from cruise plots and relative density from quantitative plots (unit: number / missing value: NA)
  24. hem.den: total hemlock overstory density in quantitative plots (unit: numberPerHectare / missing value: NA)
  25. dead.hem.den: dead hemlock density in quantitative plots (unit: numberPerHectare / missing value: NA)
  26. tree.den: total overstory stem density in quantitative plots (unit: numberPerHectare / missing value: NA)
  27. sap.den: total sapling stem density in quantitative plots (unit: numberPerHectare / missing value: NA)
  28. live.sap.den: live sapling density in quantitative plots (unit: numberPerHectare / missing value: NA)
  29. live.hem.sap.den: live hemlock sapling density in quantitative plots (unit: numberPerHectare / missing value: NA)
  30. dead.hem.sap.den: dead hemlock sapling density in quantitative plots (unit: numberPerHectare / missing value: NA)
  31. acer.sap: sapling density of maple species (unit: numberPerHectare / missing value: NA)
  32. bele.sap: sapling density of black birch (unit: numberPerHectare / missing value: NA)
  33. quru.sap: sapling density of red oak (unit: numberPerHectare / missing value: NA)
  34. pist.sap: sapling density of white pine (unit: numberPerHectare / missing value: NA)
  35. hwa.density: average density of HWA observed on hemlock branches
    • 0: absent
    • 1: low density
    • 2: moderate density
    • 3: heavy density, HWA egg sacs at base of most to all needles, or many scale insects on most to all needles
  36. scale.density: average density of elongate hemlock scale observed on hemlock branches
    • 0: absent
    • 1: low density
    • 2: moderate density
    • 3: heavy density, HWA egg sacs at base of most to all needles, or many scale insects on most to all needles
  37. borer.density: average density of hemlock borer observed on hemlock branches
    • 0: absent
    • 1: low density
    • 2: moderate density
    • 3: heavy density, HWA egg sacs at base of most to all needles, or many scale insects on most to all needles
  38. logged: presence of any past logging activity
    • 0: no
    • 1: yes
  39. hem.logged: evidence of past hemlock logging
    • 0: no
    • 1: 1-10 years ago
    • 2: more than 10 years ago
  40. under.rich: number of understory species tallied in quantitative plots (herb, shrub, and seedlings) (unit: number / missing value: NA)

hf081-03: stand location

  1. state: state location of hemlock stand
    • CT: Connecticut
    • MA: Massachusetts
  2. stand: eastern hemlock stand identification - 3 letter town abbreviation (CT) or town name (MA) and stand number
  3. long: longitude (unit: degree / missing value: NA)
  4. lat: latitude (unit: degree / missing value: NA)

hf081-04: climate

  1. state: state location of hemlock stand
    • CT: Connecticut
    • MA: Massachusetts
  2. stand: eastern hemlock stand identification - 3 letter town abbreviation (CT) or town name (MA) and stand number
  3. year: year of sampling
  4. sample.size: number of trees in the stand for which abundance was recorded in the year of sampling (unit: number / missing value: NA)
  5. mins.yr: minimum winter temperature in the preceding winter (unit: celsius / missing value: NA)
  6. winter.tmean.yr: mean winter temperature Dec 1 through Mar 31 in the winter preceding sampling (unit: celsius / missing value: NA)
  7. precip.yr: precipitation sum April 1 – Sept 30 of the sampling year (unit: millimeter / missing value: NA)
  8. max.yr: maximum temperature April 1 through Sept 31 of the sampling year (unit: celsius / missing value: NA)

hf081-05: abundance

  1. stand: eastern hemlock stand identification - 3 letter town abbreviation (CT) or town name (MA) and stand number
  2. year: year of sampling
  3. tree: tree number (individual trees were not marked, so e.g. tree #3 is the third tree observed in a stand in a year, but tree #3 is not the same individual year after year)
  4. hwa: hemlock woolly adelgid density per meter of branch on an ordinal scale
    • 0: 0 insects per meter of branch
    • 1: 1-10 insects per meter
    • 2: 11 – 100 insects per meter
    • 3: greater than 100 insects per meter
  5. ehs: elongate hemlock scale density per meter of branch on an ordinal scale
    • 0: 0 insects per meter of branch
    • 1: 1-10 insects per meter
    • 2: 11 – 100 insects per meter
    • 3: greater than 100 insects per meter