The American Beech Tree
The American Beech, by its scientific name Fagus grandifolia, is a deciduous tree native from Nova Scotia. F. grandifolia holds an important role in forestry since its nuts constitute an element of the diet of several species, including humans. Furthermore, due to its hard, heavy and strong wood, Beech trees are a material of choice. Therefore, it also has an economic importance (Tubbs & Houston, 2001)
In 1849 a severe disease affecting the American Beech appeared in Europe named Beech bark disease (BBD). Around 1890 BBD was accidentally introduced in Nova Scotia, spreading over most of the North America. BBD, which is now a big threat to North American forests, is the result of an interaction between an insect and a fungus.
The beech scale insects can be recognized by the white wax that they produce for protection, which appear as small white points. They live on Beech bark and feed with their stylet which penetrate the bark and give them access to the sap, creating billions of tiny holes. These holes then stay open due to a substance produced by the scales.
This large amount of tiny holes constitutes a good environment for the fungi to live, which arrive on the bark as spores transported by wind or rain. The fungi could be Neonectria faginata or Neonectria ditissima, both easily recognizable by their small rounded shape and red color. They attack the bark by creating lemon shaped cankers (around two centimeters in diameter) in each infested wound.
The disease weakens the wood which leads to difficulties in water circulations and it makes the trunk more likely to break by wind pressure (Lavallé & Laflamme, 2010). Thus, a Beech with a severe infection could present a lower amount of smaller leaves and a large number of cankers infested with little red bubbles on its bark. In most cases, BBD leads to tree death.
Edge effect can be described as the change in ecosystem structure occurring at the boundary between two habitats. The most pronounced edge effects in forests such as the Morgan Arboretum are caused by trails. They fragment the ecosystem in several sections and can ultimately change it by altering several different factors such as soil, wildlife and plants (Ballantyne, 2014). These factors are impacted not only by the trails themselves but also the increased human traffic. Some commonly researched problems are the introduction of foreign species in these new habitats (Indiana University, 2011) and compaction and erosion of the soil by increased human activity in the forest (e.g. joggers, domestic animals or cars). Other problems include these same activities interfering with the natural habitat of flora and fauna. Included in this, trail networks with high human usage can increase the spread insects and spores through physical and wind dispersal. These are all relevant when looking at the edge effect at the Morgan Arboretum.
These factors could all potentially contribute to changing the surrounding forest. Knowing that trails can cause such edge effects, we wished to study how trails affected BBD and more specifically, if BBD was most severe along trails.
The other factor considered in our research question is the influence of tree diversity on the severity of BBD. Tree diversity is influenced by both the number of different species present and the number of trees growing of each species. A forest or tree stand (an area sharing common borders, age and tree diversity) is considered to be Beech dominant if it is composed of around 50% Beech trees or more (McCullough et al., 2005). Previous studies have shown that the age and density of the stand, as well as the tree sizes and the diversity of tree species composing it will have an effect on the severity of the disease. Tree mortality worsens in older stands with numerous mature beech trees. (Houston, 1998).
Having trees of other species surrounding these beech trees can serve them as a buffer against stressors (Grondin & DesRochers, 2013). When managing a stand, either threatened by the incoming of the beech scale or already affected by it, specialists recommend increasing tree diversity which may help in reducing the spread and reproduction of the scale insect. (McCullough et al., 2005). Also, reducing the number of older large beech trees will “lessen the impact of the disease once it reaches the stand” (Grondin & DesRochers, 2013).
Tree diversity has been found to buffer the spread of BBD in highly diverse forests. To assess whether this was applicable in the Arboretum and, if so, how edge effects from trails could alter this buffer, we chose to sample in forest stands with different levels of diversity.
What is the impact of edge effects produced by Arboretum trails on the distribution of BBD by comparison of three different forest stands: Beech, Beech/Red Maple and Hemlock/Beech/Red Maple?
In order to assess the impact of edge effect on the distribution of BBD, we chose three forest types to study based on their increasing amount of diversity: Beech, Beech/Red Maple and Hemlock/Beech/Maple. This allowed us to assess any correlation between edge effect and diversity (i.e. if diversity lessens the spread of BBD). Within each forest type, we sampled beech within quadrats measuring 10mx10m along transects at a 90 degree angle from the trail. Quadrats within transects were 20m apart, the first quadrat located trailside. In addition, transects were separated by 10m to cover more forest area and make sure no trees were counted twice. Overall, 9 quadrats were sampled in each area. For every Beech tree we measured diameter at breast height (DBH) in centimeters, and the severity of: scales, cankers, fungi, and bark loss. The severity of each characteristic was rated on a scale of 0 to 2, 0 being absent/non=severe and 2 being present/severe. From this, a total score was calculated for each tree indicating BBD severity on a scale of 0-8. In this way, each quadrat could be analyzed for its average BBD severity. This allows us to more accurately depict whether or not the edge effects from the trail are positively affecting distribution of the BBD (i.e. helping to spread the disease).
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