Structure Condition Services Stressors
Crown dieback is assessed by visually inspecting the leaves and crown of trees. When trees experience stress, they begin to reduce resources to the outermost leaves and branches, and these areas will die. This fine-scale measurement allows for a more detailed assessment of the tree's overall health and vigor and is an early indication of declining health. While dieback can vary from year to year based on weather or insects, trends over time can indicate more subtle issues in overall health. Canopy dieback is estimated as the percentage of missing foliage from the upper and outer areas of a tree’s crown. A higher score means that dieback is staying low over time.
The current score highlights the trend that crown dieback is increasing in Vermont's forests, and has been since data collection began. An average of 10% dieback is not indicative of tree decline, but average dieback ratings greater than 15% are cause for concern.
Josh Halman, Forest Health Specialist; Vermont Department of Forests, Parks and Recreation
The score is calculated using a target value and the historical range of the the entire long-term dataset. The higher the score, the closer this year's value is to the target.
Once the score is computed for each year, the trend in scores over time is calculated. If the trend is significantly positive or negative, the long-term trend is marked as increasing or decreasing respectively.
Distance between target and maximum (scaled 1-5)
|Directionality of scores||
Lower values in the data are better
|Minimum value used in scoring||
|Maximum value used in scoring||
Data maximum + 10% of the range
Crown dieback is a visual assessment of the percentage of the tree’s crown that has recent fine twig mortality. Crown dieback has been assessed annually on Vermont Forest Health Monitoring1 (FHM, a collaboration between FEMC and VT FPR) and North American Maple Project (NAMP) plots2. For the FHM program, plots have been assessed annually since 1994, but between 2008 and 2013, plots were assessed on a 3-year rotation. To generate values for the missing years of 2009 and 2012, we used the average dieback from 2008-2010 and 2011-2013, respectively. FHM dieback scores were averaged per plot per year of survey data for dominant and codominant trees of all species. NAMP plots have assessed on an annual basis, beginning in 1988. While maple species are the focus of the NAMP project, co-occurring species have also been assessed. To approach the unequal species composition on NAMP plots, we selected only those plots where dominant and codominant trees were less than 80% sugar maple. We averaged dieback within species in a plot prior to averaging across plots. We then computed the mean crown dieback for all species across all plots per year in NAMP. From these two data sources, we computed an overall mean crown dieback for all species for each year by taking the mean of the NAMP and FHM yearly dieback averages. For crown dieback, the target was set as the long-term mean. We computed the current year score as the distance between the target and the upper scoring bounds (maximum value in the dataset plus 10% of the range), scaled to be between 1 and 5. Values below the target were scored as 5.
The average crown dieback of trees in Vermont's forest provides us information on overall forest health.
Damages to forests occur from insects, diseases, weather events, animals, and human impacts.
Forest growth provides information on how much biomass Vermont's trees add annually.
Higher values of canopy density indicate a more lush, green, and productive forest.
Mapped forest mortality is an assessment of the total area of current-year tree mortality across the landscape.
The proportion of trees with damage and decay provides information on the condition and the potential timber quality of Vermont's trees.
Individual tree mortality is a natural and common event, but changes to the baseline rate can signify worsening environmental conditions for trees.