Glaze ice is a winter weather phenomenon that happens frequently in southwestern Quebec and along the Great Lakes in Ontario. It refers to ice that creates a smooth, evenly spread coating on surfaces—including trees and other plants—as a result of freezing rain or freezing drizzle.
Most of the time, glaze ice does little damage to forests. Small or weak branches may break under the weight of the ice coating, and narrow trunks may bend or break. Wind intensifies the effect of glaze ice on trees.
Healthy forests can easily recover from this climatic hazard which, on the plus side, relieves them of unproductive dead branches and hastens the fall of dead or very weak trees. However, injuries caused by broken branches also create openings that can allow insects and diseases already present in the forest to attack the trees.
Glaze ice as a disturbance factor
An exceptional episode of glaze ice, such as that resulting from the 1998 ice storm, can take a heavy toll on vast areas of forest. Ice storms are probably the only cause of widespread death in the deciduous forests of southern Quebec.
Glaze ice that reaches 25 mm in thickness or more becomes a significant disturbance. The impacts can last for several years. They include:
- Numerous and serious injuries – Insects and diseases take advantage of the openings caused by injuries to establish themselves, and an epidemic may ensue.
- Weakened trees – Trees expend a great deal of energy to heal their wounds and must tap into their reserves, since the loss of branches and foliage reduces the level of photosynthesis.
- Large gaps created in the canopy – The sudden increase in light penetrating the undergrowth often promotes the regeneration of species that are less desirable ecologically and economically.
- Increased wood residues – Wood residue decomposes very slowly, and changes occur in the microfauna and soil micro-organism populations in response to the new environment and food source.
- Disturbed animal habitats – Some birds, for example, end up seeking other nesting sites (note: the European starling, an invasive species, is one of the few bird species that will move into a forest that has been damaged by glaze ice).
Sensitivity of trees to glaze ice
Trees vary in their ability to withstand the effects of glaze ice. The shape of a tree and the angle at which its branches are attached to the trunk are determining factors.
- A narrow crown is more resistant than a wide crown because it intercepts less snow and ice, which means that the branches have less weight to support.
- An open crown is less vulnerable to glaze ice than a densely branched crown that allows more ice to accumulate. When small branches bend under the weight of the ice, they transfer the weight to larger branches, which are less flexible and more likely to break.
- Branches that grow at a right angle to the trunk generally have less glaze ice adhering to them than angled branches do.
- Deciduous trees usually suffer more damage from glaze ice than conifers do, because conifers’ branches retain less ice. In most cases, however, deciduous trees are better able to recover from glaze ice than conifers are.
Severe glaze ice often favours the establishment of a new understorey stand or new understorey plant communities. Forest clean-up and wood recovery activities must therefore be performed so as not to damage the new stand or drastically modify how the original one evolves.
For example, increased light on the ground in a sugar bush that has been seriously affected by glaze ice can interfere with the regeneration of maples in favour of other less economically valuable species. The management approach in such cases is to keep the trees standing as long as possible to preserve shade conditions on the forest floor.
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