Figure 1: One of the tree lines we studied, on a beautiful day.
The invasive European shrub known as Buckthorn, has been recently identified as detrimental to North American ecosystems (Mascoro, 2007). There are two types, Common buckthorn (Rhamnus cathartica) and Glossy buckthorn (Fragula alnus) (See figure 2). In the context of our research we use the umbrella term “Buckthorn”. Our research was conducted at the Morgan Arboretum. It is one of the largest green spaces on the island of Montreal, located on the western tip of the island. The Arboretum is a curated space, offering unique opportunities to study Buckthorn in an area with high tree diversity. Little work concerning Buckthorn has been conducted in this area, presenting new avenues of discovery for management.
Invasive plants are introduced in either an intentional or unintentional manner. There is strong evidence suggesting that Buckthorn was introduced to North America from Europe intentionally due to its ornamental and medicinal properties (Kurylo, 2012).
Buckthorn introduction most likely occurred in the 1800s in the United States. For the following century, Buckthorn gained popularity as an ornamental “hedge” shrub and vast amounts of seed were sold throughout North America (Kurylo, 2012). Only in the 21st century have the negative impacts of its aggressive invasion been noted.
Fig 2: Mature Common Buckthorn (Rhamnus cathartica) on the left and infant glossy buckthorn (Frangula alnus) on the right.
The Common Buckthorn is a deciduous shrub with branches growing nearly opposite from each other. These branches frequently converge into short thorns in R. cathartica. While in F. alnus terminal thorns will not be present. In both, the foliage is ovular (Kurylo et al, 2007). However, the leaf veins and arrangement differ between species. R cathartica has 4-8 lateral veins that all go to the leaf tip, while F. alnus has more common lateral leaf veins. Also, F. alnus’ leaves are mildly shiny and wavy (NCC, 2016).
The distinctive dark grey bark present in both species will commonly have a exposed yellow layer, making it easily misidentified as a native cherry. In the springtime, this shrub blossoms miniscule, honey-scented flowers. Purple-black berries ripen in September and October, and stay well into the winter. The unhabituated Buckthorn will maintain green foliage well into the winter (Knight et al, 2007), making winter identification easy.
Buckthorn prefers soils of high alkalinity (Kurylo et al, 2007). These are the types of soil the species thrived in Central Europe before it made its way over to North America.
Buckthorn thrives in two different conditions: shady understory and open conditions (Knight et al, 2007). Buckthorn is shade tolerant, which mean it can still seed and survive in low-light levels. However, its survival is not nearly as great as in open conditions which consists of more light (Kurylo et al, 2007). Buckthorn takes advantage of open canopies, forest edges and disturbed areas within the forest and grow.
Buckthorn will deposit seeds and grow beneath its mature ‘trees’ (Knight et al, 2007). Due to a preference for disturbed habitats, Buckthorn forms dense thickets on edge habitats such as field treelines. Reproduction occurs via animal-aided seed dispersal (birds and rodents) (Knight et al, 2007).
In North America, Buckthorn can: be harmful to agriculture (Ontario government 2012); inhibit tree regeneration, and decrease herbaceous ground cover (Knight et al, 2007). Buckthorn hosts two main agricultural pests, Oat rust (pathogenic fungus) and the soybean aphid (insect), these affect both quality and yield of nearby crops.
Buckthorn escaped its natural predators when it invaded North America, notably insect specialized predation (Knight et al, 2007). Shade tolerance lets Buckthorn to thrive even when the canopy in closed, allowing the plant to dominate the understory and overstory of edge habitats (Knight et al, 2007). The leaf litter cover is greatly reduced under Buckthorn, an association with earthworms is assumed to be the cause (Knight et al, 2007).
Buckthorn leaves contribute to soil nitrogen concentration (Knight et al, 2007). Buckthorn exudates an allelopathic compound called emodin which has several effect on it environment, and may deter insects and herbivores. The allelopathic effect of emodin on nearby plants and soil microorganisms seems to be compensated by the increased soil nitrogen (Knight et al, 2007). The natural work of succession may be the best way to control Buckthorn abundance (Cunard, 2009).
Buckthorn’s ecological effects, its invasive nature, and that its abundance remains relatively unstudied in Quebec were the primary motivations for our study. We sought to answer the question:
“How does Buckthorn abundance vary across edge habitats in the Morgan Arboretum?”
Figure 3: The forests, plantations, and collections within the Morgan Arboretum with our study areas indicated.
The first step in our research was identifying the locations to study. Because Buckthorn is so apt for both shady understory and open conditions it can be highly abundant along and within the treeline of open fields. We selected 4 fields within the Arboretum, and sampled 4 consecutive 5m*5m quadrats along the center of each side of each treeline. The quadrats spanned 5m away from the field into the treeline. Our independent variables were the cardinal direction that each treeline faced, and the dominant tree species of each quadrat (both noted on site, also referenced from figure 3). The direction that each treeline faced was considered because it has a considerable effect on solar exposure.
The abundance of Buckthorn in each quadrat was measured by counting the number of individual stems. Stems that protruded from others, stems without foliage, and/or stems that did not have the characteristic woodiness of Buckthorn were not counted. Notes were made of major features in the quadrats and tree lines.
Figure 4: Two of our group members, Filip and Joseph, during one of our field excursions. Joseph is confirming his identification of a Buckthorn specimen, Filip was inspecting the quadrats to ensure that they were formed by straight lines.
Our study can contribute to the understanding of Buckthorn abundance in Quebec, and help understanding how varying tree species and the ‘direction-faced’ along a tree line can affect Buckthorn abundance. If replicated over a significant time period, our data could help the application of management techniques by the arboretum staff, as well as help to narrate the natural history of the Morgan Arboretum.
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