Hemlock Removal Experiment - Vegetation Response

• Investigators: Marco Albani, Audrey Barker Plotkin, Aaron Ellison, David Foster, David Orwig
• Contact: Audrey Barker Plotkin
• Start date: 2003-06-01
• End date: ongoing
• Location: Simes Tract (Harvard Forest)
• Latitude: +42.47 to +42.48
• Longitude: -72.22 to -72.21
• Elevation: 200 to 240 meters
• Taxa: Tsuga canadensis (eastern hemlock)
• Keywords: forest dynamics, girdling, hemlock, hemlock woolly adelgid, logging, tree maps
• Release date: 2005
• EML version: knb-lter-hfr.106.2
• Revisions: data updated 2006-02-21
• Abstract:

  Hemlock decline in New England is caused by direct and indirect effects of invasion of the hemlock woolly adelgid. Direct damage from the insect is causing gradual mortality of hemlock, and widespread harvesting of hemlock in advance of mortality creates a contrasting disturbance. Although both processes affect thousands of acres of forest annually we have only a limited understanding of their effects on forest ecosystem function and productivity and the nature of the subsequent forest community. We anticipate that harvesting will yield different consequences than gradual mortality from the insect. Therefore we have designed an experiment to simulate the impact of both in order to contrast them. To simulate some of the effects of the adelgid (e.g., progressive mortality, retention of the wood on the site) we are girdling all hemlocks in a hemlock-dominated stand. In the adjacent area we are conducting a commercial harvesting of hemlock. Results from both experimental treatments will be compared to the changes observed in forests that are being infested by the adelgid, and can also be included in integrated analyses of a suite of large experiments that form a core component of the Harvard Forest LTER program.

• Methods:

  Plot Design - General Layout

  Intensive study plots will be 30 x 30 m lying in the center of 90 x 90 m treatment units. There will be two replicates of four treatments: control, hardwood control, commercial harvesting and girdling. Intensive measurements will focus on the central plots, but the buffers and outlying areas will provide additional area for other studies and manipulations including transplants. Eventually the control will become a HWA plot and that the effects of HWA will overlay all treatments.

  Control (Plots 3 and 6)

  No manipulation other than the standard suite of low intrusiveness sampling.

  Hardwood Control (Plots 7 and 8)

  Intended to simulate a likely future forest condition after HWA kills all hemlock in a stand and deciduous trees grow into the site. No manipulation other than the standard suite of low intrusiveness sampling.

  Commerical Logging (Plots 2 and 4)

  Intended to simulate an intensive commercial logging operation with the intent of removing hemlock and other commercially valuable material. This would include the removal of larger hardwoods and pine for saw logs as well as the removal of smaller stems that a logger might take in order to: improve future stand quality, facilitate skidding and general operation, or initiate a new cohort of sprouts. The result would be a substantial removal of basal area, leaving only the next generation of well-formed hardwoods and pine such as a landowner would leave expecting to return in 30 years. Commercial logging equipment would be used with the expectation that there would be soil disturbance, damage to some residual material, etc. Slash would be scattered/clumped across the area. Cordwood (i.e. smaller diameter boles and branches) would be removed. The intent is to generate an extreme response that is within the range of those seen on real commercial cuts. This treatment was implemented in January to April 2005.

  In plot 2, a hemlock-oak-pine stand, 66% of the live basal area was marked for removal. All hemlock greater than or equal to 8 inches diameter was marked and smaller hemlock were cut by the logger's discretion to facilitate the harvest. Most maple and birch greater than or equal to 6 inches diameter was marked, as was about half the basal area of white pine, red oak and white oak. The better-quality oaks and pine were left for seed and value growth.

  In plot 4, a hemlock-dominated stand with some birch and oak, 72% of the live basal area was marked for removal. All hemlock greater than or equal to 8 inches diameter was marked and smaller hemlock were cut by the logger's discretion to facilitate the harvest. Most maple and birch greater than or equal to 6 inches diameter was marked. All of the white pine, most of the red oak, and about half of the white oak basal area were left for seed and value growth.

  Girdling (Plots 1 and 5, pilot plot 9)

  Intended to simulate some of the characteristics of HWA, but differing in key ways. The treatment is to girdle all (and only) hemlock, with the intent of killing these over a period of weeks to months. The result would be standing dead hemlock that gradually lose their leaves, twigs, fine branches, larger branches, and then fall apart in place. Thus the change in overstory and microenvironmental conditions would be gradual (i.e., over 1-2 years) and much less severe than that of the harvesting. Important characteristics of HWA would be missing, most notably the very lengthy period of decline during which the plant is undergoing physiological stress and metabolic imbalance that may induce biogeochemical and microbial changes on the site. No mechanical site disturbance, removal of material, etc. This treatment was implemented in late May – early June 2005. Hemlocks were girdled in a pilot plot (plot 9) in July 2004. For larger trees, two parallel strips about 10 cm apart were cut with a chainsaw about 1 inch deeper than the bark around each stem. For smaller trees, a 5-10 cm strip of bark was peeled off with a knife or small saw.

  Vegetation Response

  Overstory: Trees greater than or equal to 5cm (species, diameter at breast height, crown class, XY coordinates within plot); entire plot; includes dead trees if solid and taller than about 5m (ongoing). A manual autolevel/stadia rod is used to map trees within each plot. Tree heights and crown dimensions were recorded for a subset of trees in each core area in Summer 2004.

  Saplings: Trees greater than 1.3m tall but less than 5cm dbh are tallied by species in the core 30m x 30m area of each plot (Fall 2004 to Spring 2005).

  Understory vegetation: percent cover of herbaceous and shrub vegetation (vascular); percent cover and density of tree seedlings; organic layer depth; 10, 1m2 plots within the core area of each plot (late August 2003, late June 2004, early September 2005, ongoing).

  Understory Sampling

  In 2003, we set up two transects running north-south through the center 30m x 30m of each plot at the Simes Tract of Harvard Forest for understory vegetation sampling. Five 1 m2 subplots were spaced evenly along each transect. We labeled the NW corner, unless otherwise marked. Transects are marked with string and yellow tent stakes, and plots are marked with orange flags.

  We estimated percent cover for herbs, shrubs, and seedlings to the nearest one percent. We counted the number of seedlings by species. A seedling is a tree less than 1.3m tall. If rocks, stumps, slash, standing tree bases or coarse woody debris covered more than 5% of the plot, their percent cover was recorded to the nearest 5%. In addition, the depth of the organic soil layer was measured to the nearest 0.5 cm at the NW corner of each subplot (August 2003 only). We searched each plot and compiled a species list for the entire 30m x 30m area.

  Saplings were tallied by species across the interior 30m x 30m plot. A sapling is a tree greater than 1.3m tall but less than 5cm in diameter. They were noted live or dead. Saplings were tallied between January and May 2005 (all before 2005 growing season).

• Use:

  This dataset is released to the public and may be freely downloaded. 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. For more information on LTER Network data access and use policies, please see: http://www.lternet.edu/data/netpolicy.html.

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