here is nothing quite like a well formed, majestic, dark green sugar maple tree standing proudly in a forest, field or village. It shows strength. It speaks of resiliency. It has history. When you hear “sugar maple tree health,” this is the image that comes to mind.

Vermont’s forests are valuable ecologically, economically, and socially. Covering nearly 80 percent of the state, our ever-evolving woodlands provide jobs, stability to the landscape, wildlife habitats, biological diversity, clear water, scenic vistas, and diverse recreational opportunities. These values are common ground for Vermonters, and make monitoring and protecting the health of our forests a priority.

Assessing Sugar Maple Health

The health of sugar maple trees, and forest health in general, involves a complex set of conditions and factors, which often have an effect on one another. Rarely, can a tree's poor health be traced to a single factor; normally a tree's health problems result from a number of causes.

Forest scientists use a number of measures to help measure tree health, such as:

Growing conditions
Sugar maple trees grow best on deep, moist, fertile, well-drained soils. Likewise, these trees prefer cool, moist, climates that receive around 40 inches of rain during the growing season and where average temperatures range from 0° F in January to 60° F in July. In New England, sugar maple trees have an elevation limit of around 2500 feet. They are tolerant of shade, and in many recent studies, have been shown to be tolerant of ozone and acid deposition pollution. Although sugar maples will reproduce and grow on less than optimal sites, they will be less vigorous and shorter lived, and may be more sensitive to stress from insect, diseases, weather or other factors.

Structure, sustainability, and diversity of forests
Diversity in tree size and species tend to promote a stable forest for the long-term. A natural healthy forest, is a dynamic system in which new trees are sprouting each year, and older or less vigorous trees are dying. Weather events such as high winds, lightning and ice storms may create natural openings in forests. These openings are a fantastic way for new tree species to take hold: Trees that require more sunlight than is typical in the understory of dense forests. The abundance of sugar maple trees of different ages, tree growth rates, and the mix of sugar maple and other species, are all measures of forest health and sustainability. In Vermont, these health factors are measured periodically as part of the Forest Inventory and Assessment Program.

A woodlot tapped for maple syrup production is called a sugarbush. Healthy trees are essential for good sap production. Sugar maple is usually the dominant species, since it is desirable to thin out non-maples to benefit trees that can be tapped. However, creating a monoculture of sugar maple can be detrimental to the long-term health of the forest, should a pest which attacks sugar maple become established and begin injuring or killing trees. Other sugarbush management guidelines should also be incorporated into the operation to maintain tree health. See Sugarbush Management: A Guide to Maintaining Tree Health, for more information.

Reproduction and Leaf Development
Long strands of yellow-green sugar maple flowers are produced early in the spring, before leaf buds open. Trees begin reproducing at about 50 years of age, with older trees bearing large quantities of seed periodically. Bud development in the spring begins in late-April to early-May, and generally full leaf-out has occurred by early June. Each tree is unique in the timing of this process. Monitoring of spring bud development can provide another indicator of tree health, and is a process that can be altered when trees are under stress. Yearly examination of bud expansion is part of Vermont's state monitoring program.

Insects and Diseases
If you look closely at a sugar maple leaf you will generally find some sort of insect feeding or symptom of disease present. Their presence and abundance ebbs and flows according to natural population cycles. Insect and disease outbreaks depend on weather conditions that enhance or deter the organisms, and/or the health of trees themselves (stressed trees often lack chemical mechanisms to fight off insects and diseases). Keeping track of what insects and diseases are in abundance each year, correctly diagnosing the organisms involved, and making management recommendations to landowners are part of the Vermont Forest Health Protection program.

Some of the major sugar maple insects that affect tree health include: Forest Tent Caterpillar, Sugar Maple Borer, Maple Leaf Cutter, Pear Thrips, Saddled Prominent and Bruce Spanworm. Sugar maple diseases include: Anthracnose, Eutypella Canker, Nectria Canker, Armillaria Root Rot and Sapstreak. Vermont's, Forest Health Conditions Report, summarizes the current status of many of these pests.

Fall Foliage
The brilliant display of orange, red and yellow fall colors make sugar maples special. Add sunlight to the mix, and they appear to glow. Our hillsides become an artist’s palette with a variety of colors and hues. The science behind this art involves leaf chemistry, day length, and fall weather conditions. Each year’s foliage season varies. Along with color qualities, timing and duration of fall foliage are additional indications of healthy or stressed forests. Trees or branches under stress can change color earlier than normal. Drought or damage from insects or diseases can cause early fall color, or in extreme cases, can cause leaves to turn brown rather than normal fall color. Fall foliage monitoring is part of Vermont's Forest Health Monitoring Program.

Determining Tree Health
There are a few measures of tree health that can be used as guidelines to determine, with some certainty, that a tree is healthy. Many of these indicators of tree health are used in statewide forest health monitoring programs. Remember that a healthy tree can become unhealthy, and unhealthy trees can improve.

A healthy tree has …

• a live top (live crown) that is greater than one-third the total tree height,
• newly dead branches that make up less than 15% of the total crown,
• less than 15% of the tree crown with missing branches,
• foliage that is dense throughout the live portion of the crown,
• a tree trunk with open wounds that are less than one-third the diameter of the tree, no wounds in contact with the ground, and no wounds that have soft, punky wood (an indication of decay).

For more information:

Health of Sugar Maple in Vermont-2003: a 2-page public leaflet summarizing tree health, current health problems and news on sugar maple trees.

Vermont Monitoring Cooperative: Ongoing forest monitoring including sugar maple trees. VMC is a partnership program of long-term data monitoring and research on forest ecosystems with studies at 2 locations in Vermont, Mount Mansfield and the Lye Brook Wilderness Area.