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Beech Bark Disease Severity vs. Tree diversity

The American beech tree (Fagus grandifolia) is a species of canopy tree found in the northeastern U.S and Canada, growing in mixed deciduous, hardwood, and temperate forests (Zhang et al., 2015). In the Morgan Arboretum, which is dominated primary by sugar maple and beech, these beech tree plays a significant role in the ecosystem by providing plenty of shade, leaf litter and sources of food through beechnuts. (Zhang et al., 2015). Zhang elaborates by stating that American beech trees are also capable of reproducing by sprouting clones from their roots, normally as a response to some stress or disturbance. These young beech trees grow very well even under thick canopies, indicating American beech is a shade-tolerant species. Furthermore, this suggests that the number of young beeches will possibly increase when a disturbance is present.

Image 1

Image 1: This is a sample of one of our quadrats, where the beech trees are indicated with green arrows, yellow birch with yellow arrows and maple trees with red arrows.

The spread of Beech Bark Disease (BBD) in North America began in Nova Scotia in 1890 when sailors from Europe began transporting ornamental trees. (Cale et al., 2017).  BBD then quickly spread to other maritime provinces, followed by the Eastern U.S. and Quebec, and in 1999 was confirmed to be in Ontario (McLaughlin & Greifenhagen, 2012). During this invasion, the beech bark disease developed in three stages (Giencke et al. 2014). Giencke further elaborates by describing each stage: The advancing front, which refers to the period during which trees are infected with scale insects but not the fungal pathogen (discussed further on). The killing front, which consists of a high presence of the fungal pathogen resulting in high mortality rates for the beech trees, and the Aftermath forest, or zone, which consists of moderate levels of mortality and fungal pathogen presence. At this point, the scale insect has most likely moved on.

As previously hinted upon, the beech bark disease is a two step complex. The first step involves the presence of the beech scale insect (Cryptococcus fagisuga) which feeds on American and European beech. Cryptococcus is extremely small, about 0.5 – 1.0 mm at adulthood, soft-bodied, and with no wings or legs at maturity (McCullough, Heyd & O’Brien, 2005). The scale insect eats its way through the bark of the beech tree and secretes a white waxy substance which makes the tree vulnerable to the other agents of BBD.

Image 2

Image 2: To the left, the arrows point to the cankers which form when the fungi (Nectria genus) invades the openings created by the scale insects. To the right is the wooly waxy covering that the scale insect secretes.

The first fungal species, native to North America, Nectria galligena, causes some small cankers (holes in the bark). The mechanisms through which this happens is still not well understood (Griffin et al. 2003). The second, and most significant contributor, is the non-native species Nectria faginata (McLaughlin & Greifenhagen, 2012). This fungus kills the bark of the beech tree as well, leaving distinct cankers on the bark. The resulting tree begins losing its twigs and branches, its leaves begin to wilt, and structural breakage as well as eventual death is observed (Griffin et al. 2003).

Image 3

Image 3: An example of a recently infected beech tree. Notice the fruiting bodies of the fungus creates circular shapes which later form cankers and kills the bark.

The research on the spread of and factors influencing BBD has found that only about 1% of beech trees are resistant to the disease and that resistant trees usually occur in groups which are genetically similar – a result of reproduction by sprouting clones (McCullough, Heyd & O’Brien, 2005). Also, the degree of the effect of BBD on a stand is largely based on how much of the stand is composed of beech. If beech is a minor component, it’s likely that the beech trees killed by BBD will be replaced by another species, (McLaughlin & Greifenhagen, 2012) and the other tree species will have more access to light, water, and nutrients (McCullough, Heyd & O’Brien, 2005). In stands where beech makes up roughly half or more of the trees present, management techniques may be necessary to minimize the effects of the disease. For instance, increasing tree diversity may slow the spread of the scale insect (McCullough, Heyd & O’Brien, 2005). A 2015 study of Mont St-Hilaire conducted by Zhang et al found that tree diversity may limit the severity of BBD in that area, but not necessarily the frequency of BBD. That same study also found that habitat disturbance and human activity might lead to higher frequency of BBD, since human activity could help transport the scale insect (ex; through firewood) and injuries to the bark of beech trees (such as people carving initials into the bark, as seen frequently in the arboretum) makes it easier for the fungi to infect the tree. Based on the research we conducted, we decided to investigate the following question:

How does the occurrence and severity of beech bark disease change between pure beech tree stands and mixed/beech tree stands  in the Morgan Arboretum?


In order to measure the severity of BBD, the following scale was created:

Table 1: Scale of the severity of the sampled beech tree with BBD. This table is inspired by the severity scale from Griffin et al.

Severity level Name Description of BBD
1 Uninfected Bark is smooth with no visual appearance of cankers
2 Recently infected Cankers are small and spread out
3 Middle level infected Cankers are larger and closer together, bark has begun peeling in multiple locations
4 Almost completely infected Cankers are very close together throughout the tree. (Tree exhibits a softening of the wood?)
5 Completely infected (Dead) Tree is visibly dead, infection has destroyed majority of bark. Multiple large cankers visible throughout.


Image 4

Image 4: Visual representation of each severity level. The numbers correspond to the severity level numbers on Table 1. Sample 1 is an uninfected beech tree, with no BBD presence while sample 5 beech tree is dead resulting from BBD.

Sampling took place once per week at the Morgan arboretum for three weeks. Each week, 1 area in the pure beech stand and 1 in the mixed stand was chosen by superimposing a grid onto the forest map of the arboretum, where each square represents a possible location to choose from. A random number generator was then used to select the two squares. Each square was a quadrat of size radius=10m. For each tree within the quadrat sampled  we posed the following questions: i) Is the tree a beech? If not, then the type  is determined and marked. ii) Does this beech tree have beech bark disease, and if so, how severe is it? (see table above). Furthermore, each beech tree diameter was measured at DBH (diameter at breast height), and trees which had a diameter smaller than 3cm was excluded from the results because they are too young to draw results from.

We created a video demonstrating how we sampled a tree!

            At the end of the sampling, we counted a total of 233 trees, and obtained an occurrence rate of 71% for pure beech stands and 41.8% for mixed stands. For both stands, we found that trees with a larger diameter had a higher disease severity on average. In terms of severity, on average, the pure beech stands had higher severity levels than the mixed stands which demonstrated lower disease severity. This seems to be a direct relationship to the occurrence rate (or tree diversity). As a side note, we observed that the pure beech stands contained many more young clones next to their infected “parent”, which leads us to believe that a disturbance can lead to the production of a high amount of beech clones.

Image 6

Image 6: Beech Bark Disease research group. From left to right: Richard, Elijah, Bhavisha, Quentin and Elena


Cale, J., Castello, J., Garrison-Johnston, M., Teale, S. (2017) Beech bark disease in North America: Over a century of research revisited. Forest Ecology and Management, 394, 86-103.

Giencke, L.M., Dovčiak, M., Mountrakis, G., Cale, J.A. & Mitchell, M. (2014) Beech bark disease: spatial patterns of thicket formation and disease spread in an aftermath forest in the northeastern United States. NRC Research Press, 44, 1042-1050.

Griffin, Jacob M. Lovett, Gary M. Arthur, Mary A., and Weathers, Kathleen C. (2003) The distribution and severity of beech bark disease in the Catskill Mountains, N.Y. Canadian Journal of Forest Research 33(9): 1754-1760

McCullough, D.G., Heyd, R.L. & O’Brien, J.G. (2005) Biology and management of beech bark disease: Michigan’s newest exotic pest. Michigan State University Extension, E-2746, 1-12.  

McLaughlin J. & Greifenhagen, S. (2012) Beech bark disease in Ontario: A primer and management recommendations. Ontario Forest Research Institute, 71, 1-8.

Zhang, Z., Perez, E.C.V., Chinn, A. & Davies, J. (2015) Tree diversity has limited effects on beech bark disease incidence in American beech population of Mont St-Hilaire. McGill Science Undergraduate Research Journal, 10, 26-30.





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