HEMLOCK HEALTH MONITORING PLOT

The Lingering Hemlock Project focuses on locating hemlocks that are still surviving among the dead to find and amplify any traces of genetic resistance they may have to hemlock woolly adelgid (HWA) and/or elongate hemlock scale (EHS). To know whether trees are lingering, at least 80% of the trees around them are dead. That leaves those of us in areas where hemlock mortality has not yet reached 80% (primarily northeastern Northeastern North America), with a different task than searching for the lingering hemlocks specifically.

In locations where hemlock mortality has not yet reached 80%, project partners will set up “hemlock health monitoring plots,” in which hemlocks will be monitored for decline alongside other environmental and site characteristics. Monitoring hemlock decline over time and space in this way will allow us to better understand patterns of hemlock mortality across the landscape. We will be able to understand HWA’s spread and severity across its leading edge and in doing, catch 80% hemlock mortality right as it occurs. Once hemlock mortality reaches 80% in these plots, we will know it is time to pivot to our lingering hemlock search protocol (see our Lingering Hemlock Search page for more information), both within the hemlock health monitoring plots and in the surrounding areas. This guide, published by the Ecological Research Institute, provides good information about how to choose to set up a health monitoring plot or search for lingering hemlocks in your area.Why is it helpful to assess hemlock health before it’s possible to look for lingering hemlocks? Why can’t we just wait until more hemlocks have died before focusing on these geographic areas?

1. There is currently no unified way that hemlock mortality is being tracked across eastern North America. Forest workers communicate with each other across various states, but the information that is shared can be spotty and sparse. Creating a network of hemlock health monitoring plots across eastern North America can help us share information on hemlock mortality in a unified and organized manner.
2. It is unlikely that we will find perfectly pristine hemlocks among 80% mortality. A far more likely scenario is that a few hemlocks will die more slowly than those around them. However, in this scenario, the remaining hemlocks may still die quickly after 80% mortality is reached. Establishing hemlock health monitoring plots allows us to find and preserve any genetic material from these slightly more resistant hemlock trees rapidly once 80% mortality is reached. Waiting too long could result in losing the genetic material and potential traces of genetic resistance these trees hold.

Read through this page to gain a sense of the process of setting up and monitoring a hemlock health monitoring plot. Once you are ready to move forward with plot establishment and monitoring, you can proceed to our Resources page for datasheets and step-by-step instructions.

Selecting a Site for Hemlock Health Monitoring Plot(s)

Once you determine that a hemlock health monitoring plot is best for your site, you may establish multiple plots in the same general area if you are interested and have the capacity. This can be a good way to provide data on patterns in hemlock decline across varied topography and forest types. If you choose to create multiple plots, there are two main rules to bear in mind: there should be no more than three plots to a site, and the plots should be separated by at least 50 meters or 150 feet. 

Plot Characteristics

Once you have selected the area where you will put your plot or plots, you can begin to narrow down a more exact location for each plot. A plot should contain about 20-40 living hemlock trees that are close enough in proximity to be monitored in one visit. You may choose not to select every tree in the area of your plot to be monitored each year, especially if your plot is located in a dense hemlock grove. However, as you select trees to include in your plot by tagging and monitoring them, your selection should not be biased by tree health or size. Do not specifically choose to include all the healthiest trees or largest trees to tag and monitor. Instead, the trees you choose should be representative of the general forest condition in the area – if the hemlock stand contains diverse size classes, be sure to tag and monitor trees of varying sizes.

The goal of these plots is to monitor hemlock trees’ natural decline due to HWA and EHS, so be sure to only monitor trees that have not undergone and will not undergo chemical treatment. If a tree survives due to chemical treatment, that is not indicative of any natural genetic resilience that individual possesses. Trees in your plots should not have received chemical treatment in the past 15 years. Similarly, you should be relatively certain that trees in your plot will not receive chemical treatment in the next 15 years.

Plot Design

Forests throughout the range of eastern and Carolina hemlocks vary widely. A hemlock stand in one area may be incredibly dense, while that in another location might be more sparse. There are multiple options for plot design to ensure that you are able to adapt them to the conditions in the unique forest where you will be establishing plots.

Regardless of the plot design, you will have a point or points that “anchor” your plot, which you should mark both physically and with a GPS coordinate. Below are three general possibilities for plot designs:

Fixed-Radius Plot
You may choose to anchor your plot in a center point, which you would mark with a GPS coordinate and something physical in the plot, such as a labeled rebar or flagging tape.  A fixed-radius plot would include all hemlock trees within a predetermined distance from the center point. In a very dense hemlock stand, a fixed radius is likely to be small, say 10 feet, to incorporate the 20-40 living hemlocks that make up a plot. A fixed-radius plot may be a good option for a relatively flat area with dense hemlocks.

Along a Transect or Trail
You could instead choose to anchor your plot to a beginning point, which you would also mark with a GPS coordinate and something physical such as labeled rebar or flagging tape. A transect plot can extend in the direction of your choice until you have incorporated the 20-40 hemlock trees that make up a plot. This may be a good option for an area that is less dense and/or has a steep grade that would make a circular plot more difficult to traverse.

As a Shape (such as a square)
Finally, you could choose to create anchor points at each corner of a rectangular (or other shape) site. As in the other plot designs, these anchor points should be marked and have GPS coordinates taken. This may be a good option in an area with moderate hemlock density or in which a hemlock stand is bordered by other forest types.

Plot Establishment

Once you have selected a good location(s) for site and plot establishment, your next step will be to collect data on the plot boundaries, characteristics such as habitat, soil type, and slope, and the presence of HWA and/or EHS in the plot.

Be sure to mark the plot boundary that is relevant to the plot design you’ve chosen (i.e., the plot center, corners, or ends of a transect) and collect reference GPS coordinates as needed to guide monitors back to the plot in the future. Please take down this information in such a way that a colleague could fill in for you if you are unable to conduct surveys in future years for any reason. You may want to draw a sketch of the plot design and orientation if you believe that doing so would be a helpful way to orient future monitors to the plot. In your first year of data collection, you will need to tag each living hemlock tree (see box for more information) with a diameter at breast height (DBH) over 4” that you plan to monitor in subsequent years. Make sure you have permission to install permanent markers from the site owner/manager before doing this. Only the trees that are tagged upon establishment of the plot will be measured in the future.

Tree Tags
Numbered tree tags are commercially available online (they can be purchased here, among other locations). These are often round, aluminum tags (see bottom left) that can be hammered into a tree using aluminum nails. Be aware that this type of tag is often used to denote other aspects of forest management, such as marking trees which have been treated or trees to be removed. If hemlock trees in or near your stands have been marked for other reasons, you may want to look for tree tags with a unique color or shape (see bottom center) to help you differentiate them from other tree tags nearby. If you do not wish to purchase tree tags, it is also possible to mark trees using tree paint to write numbers onto their bark (see bottom right). If you choose to mark using paint, please remember that you will need to reapply your labels every few years so as not to lose track of your trees. Please use tree marking paint if you choose this marking method, rather than typical commercial spray paint. This is paint specifically formulated for use on trees and will minimize damage to the tree.

Dead Tree Records

After establishing the boundaries of your plot, we ask that you document each dead hemlock tree within the boundaries. This helps us establish the baseline mortality in your plot so that we can be certain when 80% mortality has been reached. In this survey, you will be asked to identify the species of the dead hemlock (in northeastern North America, the only hemlock species is eastern hemlock, Tsuga canadensis–see the map in the next section for the range where both hemlock species occur). Then, you will be asked to measure each dead tree’s DBH, note whether the tree is standing or fallen, and note any sign(s) of decomposition. Signs of decomposition include bark partially or fully fallen off, rotting wood, or the presence of fruiting bodies.

Initial Tree Records and Plot Information

In the initial year when you are establishing your plot, we ask that you collect GPS coordinates for each tree as you place your tags. You will be asked to indicate the species of each hemlock tree you tag. In eastern North America, there are two species of hemlock trees: eastern hemlock (Tsuga canadensis) and Carolina hemlock (Tsuga caroliniana). However, Carolina hemlock has a very limited range, occurring only in Virginia, Tennessee, North and South Carolina, and Georgia (see the range map below).

Therefore, if you are establishing a hemlock health monitoring plot in any state besides these, the only possible hemlock tree species is eastern hemlock (Tsuga canadensis). Besides noting the hemlock tree species, this datasheet will also require that you measure each tagged tree’s DBH, indicate the level of infestation of HWA and EHS, and note the crown health and canopy position of the tree.

Annual Tree Records and Plot Information

In each subsequent year after you have established your plot, it is no longer necessary to collect the GPS coordinates for each tree, as they will not change from year to year. Much of the data collected annually on each tagged tree will be the same as that collected when the plot is initially established: DBH (to monitor the tree’s growth), the level of infestation by HWA and/or EHS (to monitor the spread of each pest in the stand), and crown health and canopy position (to monitor the progression of hemlock mortality in the plot). In subsequent years, there are three additional relevant datapoints. We want to know if you were able to find the tree, if the tree is alive, and whether or not the treatment status of the tree has changed.

There is an additional, optional datasheet you may choose to use to note additional plot information, both in your initial tree records and your subsequent annual surveys. You may use this space to note additional hemlock stressors in the plot, such as wind damage or drought, or to note the presence of predators released for biological control, or to note signs of active forest management. This additional datasheet also asks about forest regeneration (seedlings or saplings in the understory). As hemlock trees decline from HWA and/or EHS, forests in varied habitats may respond in different ways. Some forests may respond with hemlock regeneration, while others may see different trees resprouting in the understory. Forests in more marginal or at-risk habitats may resprout with shrubs or invasive species instead, indicating a loss of forested area altogether.

Filling out this datasheet is optional–it is not required for participation in this program. However, the information collected in the datasheet may be useful for informing forest management in the future of the plot(s). At-risk habitats where regeneration is primarily shrubs and invasive species may require more active forest management as hemlocks are lost from the landscape.

Visit our resources page to access datasheets and step-by-step instructions