Forest Health: Beech Bark Disease
What is Beech Bark Disease?
Beech Bark Disease (BBD) is a new threat affecting American beech (Fagus grandifolia) in Canada’s hardwood and mixed forests. This disease is the result of an insect-fungus complex caused by a non-native insect, beech scale (Cryptococcus fagisuga), coupled with a Neonectria fungus.
Beech scale comes from Europe and first arrived in Nova Scotia in the 1890s. This insect, and the ensuing disease, is gradually spreading throughout much of the natural range of beech trees and is now found in all the maritime provinces, Quebec, and several eastern US states. In 1999, it was confirmed to be in Ontario.
How do Beech Trees Contract the Disease?
The disease begins with healthy trees being attacked by the scale insect. Infestation is easily detected as the nymphs or crawlers secrete a white, waxy covering to protect their body, giving the tree a woolly or snowy appearance. They pierce the living bark, creating puncture wounds as they feed on the tree’s sap. The feeding holes kill the bark cells causing small cracks to form on its surface.
While the insect does not move far, some are transported by wind, mammals, birds, and the movement of firewood, causing long range dispersal. Usually, the larger trees in a stand are attacked first. Although the scales generally do not kill the tree, infestations reduce tree vigor and growth, and lower the tree’s resistance to fungal infections.
Three to six years after the introduction of beech scale, the causal fungus (Neonectria faginata) enters the tree through the wounds opened by the insects and begins to colonize and destroy the cambial layer. This stage of the disease produces circular or oval shaped cankers, deforming the bark surface.
In fall, small orange-red fruiting bodies appear in and around the canker, releasing minute fungal spores that are dispersed to new sites by wind and splashing rain. Branches above large patches of dead bark often show signs of decline, producing yellowing leaves and reduced canopy. As the cankers become progressively worse, the tree is girdled, resulting in 90% of the trees succumbing to the disease within five to 10 years.
How Does the Disease Progress?
The stages of progression of BBD have three distinct phases, representing different ecosystem states: the advancing front, the killing front, and the aftermath forest.
The advancing front begins when previously unaffected beech trees are colonized by the non-native beech scale and their populations start to increase. During this phase, ecosystem changes are minimal, but damage caused by the feeding scales make the stand susceptible to fungal infestation.
A few years after the insects arrive, the disease progresses into the killing front. At this point, stands of beech trees exhibit high populations of beech scale and the Neonectria fungus infects colonized trees, causing severe cankering. Within five to 10 years of infection, many of the larger beech trees die.
After this first wave of beech mortality, the aftermath forest shows lower but consistent levels of beech scale and Neonectria fungus. In this phase, the remnant large trees continue to gradually decline, and stem breakage is common. Beech now makes up a lower percentage of the overstory and thickets of shade-tolerant saplings sucker from the roots around dead and dying trees. These younger beech trees subsequently become infected with BBD as they age and eventually die.
Ecological, Economic, and Social Impacts
American beech plays an important role in the forests of eastern Canada. BBD causes severe die-back in mature Beech trees, negatively impacting wildlife, biodiversity, and sustainable forests in Ontario.
- The disease typically kills larger trees first, significantly reducing canopy cover and altering the stand structure and forest dynamics.
- Along with oaks, beech trees are valuable nut-producing trees, providing an important source of mast (food) for many forest-dwelling birds and mammals. Therefore, the loss of beech trees would mean a change in food availability for wildlife, including black bears, deer, squirrels, and several bird species. Their demise also results in a loss of habitat for cavity-nesting birds and mammals.
- While not particularly sought after, beech wood does provide valuable lumber, possessing a beautiful, fine grain for flooring and furniture. BBD may reduce the marketability of infected trees, thus negatively impacting the forest industry.
- The loss of these beautiful trees decreases the aesthetic value of our hardwood forests.
Trees that are at Risk
Trees are more at risk of BBD when there is a nutrient imbalance in the forest soils. Drought and other pests can predispose the trees to the disease and weather conditions, such as winter temperatures that remain above -25 °C and continual autumn rainfall can favour the build-up of beech scale populations and the Neonectria fungus.
On a positive note, heavy rains can wash crawlers off the trees and delay infestation, and some trees (about 1%) may exhibit resistance to the scale insect or tolerance to the fungus. Trees less than 30 years old also seem to have a natural immunity to attacks.