Tree Health

Determining the number of taps to install in each maple tree depends first on the health of the sugar bush and second, on diameter of each trunk.

Syrup production will remove approximately 10% of the sugar from a healthy mature maple tree, which is not enough to stress trees.

The Importance of Monitoring

Monitoring the health of a sugar bush is best done every time you enter the forest. Observations can be made all season long.  With leaves absent, winter time allows a clear view of the shape and size of tree canopies.  Ideal sugaring trees that have large spreading crowns can be clearly seen.  A sugar bush that is stocked with many healthy trees will support full summer leaf canopies to manufacture large quantities of sugar.

Sugar energy is utilized by the trees during spring and summer for new growth of the tree, a seed crop and root system, for repair of wounds, defense against diseases and insect pests, and an ability to defend against other stressors, such as periods of drought or harsh cold winters.  Sucrose sugar is converted to starch in fall and is stored in plant cells during winter. Stored starch is the important energy reserve that is utilized by the trees between growing season.

Dormant crowns

Trees that lack a large canopy, or have been damaged previously by heavy ice accumulation or wind can be identified this time of year.  Trees damaged by disease or by insect pests can be marked for removal or identified as potential habitat to benefit wildlife, depending on risk to the remaining healthy trees and on the desire of each operator to provide habitat for wildlife. Young understory trees can be managed to eventually take over as the future sugar bush or diverse woodlot.

Landowners can learn to make their own management decisions, or they can hire accredited consultants or supporting industries to assist with the overall management of the sugar bush and woodlot.  The Ontario Woodlot Association provides a useful service for anyone wanting to learn more about all aspects of forest management.

How To Tell If Trees Are Healthy

Researchers have devised a method of determining whether sustainable tapping practices are occurring:

  1. Tap 10 trees
  2. Look at the colour of the wood shavings on the tapping bit for each
  3. Healthy trees = At least 9 of every 10 tap holes drilled have creamy white wood shavings and only 1 shows dark brown wood shavings
  4. Unhealthy trees = less than 9 of every 10 tap holes drilled have creamy white wood shavings

The trees may not be growing enough new sapwood each spring and summer to replace the sapwood that is being removed by tapping. There may be problems with soil nutrition, drainage or pest activity.  Excessive tapping may have occurred in the past.

A diagram showing two sets of 10 wood shavings. In the first set of 10, 9 wood shavings are creamy white and one is dark brown. This means trees are healthy. In the second set of 10, only 7 wood shavings are creamy white. This means trees may not be healthy and excessive tapping may have occurred.
Reference: Dr. Abby van den Berg, Proctor Maple Research Centre, University of Vermont

Common Tree Stressors

If trees seem stressed, reduce tapping. Common contributors to tree stress are listed below.

Excessive tapping

Observe tap holes from previous seasons. A healthy tree will heal over previous tap holes with new sapwood within one or two growing seasons. Small diameter tap holes of 5/16th inch diameter will heal over faster than the larger 7/16th inch diameter tap holes. Trees that require more than three years to heal tap holes may not be in a healthy condition.

Severe defoliation

Maple trees that are under stress due to moderate to severe defoliation by insect pests during the previous spring and summer will benefit from reduced tapping or no tapping during the current year’s sap harvest.

Larvae of forest tent caterpillar, gypsy moth and spring cankerworm may cause severe defoliation and affected trees will benefit from reduced tapping this season. Trees that were severely defoliated will have significantly reduced levels of sucrose sugar for one to two years.  The sugar may best be left for the trees to utilize for their recovery.

Reduced Snowfall

In addition to leaf litter, a layer of snow will act as an insulating cover to protect shallow tree roots from freezing damage. Roots of trees are far less winter-hardy than the above ground tree components.  Unlike the trunk, branches and buds, tree roots don’t become dormant but remain in a state of readiness. 

Where moisture in the soil freezes, any encased roots will remain in a dormant-like condition until the soil thaws. Roots in frozen soil will not die as long as the surrounding temperature remains above – 6 to – 7 ⁰C.  Fracturing and heaving of frozen soil can cause significant damage to fine tree roots during winters that lack snow cover.

Roots can begin to grow whenever the soil temperature rises above zero to +5 ⁰C. 

Rapid Temperature Drop

A fast plunge of ambient temperature, from relatively mild winter temperatures to deeply frozen temperatures that occurs quickly over a few days, will have many operators of commercial tree nurseries, fruit and nut orchards and grape vineyards very nervous about winter freezing injury to trees, shrubs and vines of all ages.

Although dormant, flower buds and vegetative buds can be killed when sudden sharp fluctuations happen.  A gradual drop in temperature over several weeks or months allow trees a chance to adjust to changing ambient conditions.  Sugar maple trees in Ontario are known to be very hardy to winter extremes. 

Drought/low soil moisture

Long droughts in the previous summer suggest that maple trees will not contain as much stored starch and sugar, compared to a summer with normal precipitation. 

Other causes

  • limb breakage (due to wind or heavy ice)
  • nutrient deficiency in soil (e.g. low calcium and magnesium levels in acidified soil on the Canadian Shield)
  • flooding
A group of people under a tall canopy of maple trees. It is summertime and the trees and surroundings are all green.
Viewing an impressively large Carolinian sugar bush in southwestern Ontario during the 2014 maple industry summer conference.  The 2014 growing season provided warm sun and adequate rainfall to enable sugar maple trees to manufacture the 2015 maple syrup crop.  We continue to realize the immense ecological services that are provided to us by having adequate healthy forest cover, in balance with agricultural and urban land.

The written and graphic content in these pages was originally created by Todd Leuty (previous Agroforestry Specialist), and edited by Jenny Liu (Maple, Tree Nut, and Agroforestry Specialist).