The importance of shelf fungi in the St-Lawrence lowlands
Polypores (also known as Shelf fungi and Bracket fungi) are found across North America, anywhere woody plants are present (Gilbertson, 1980). They usually grow on fallen logs, stumps, dead branches and even living trees whose bark has been breached and begins to decay (Roberts and Evans, 2011). They have a wood-like, almost leathery texture, and produce spores under their caps. Most of them are wood-rotting and therefore inflict serious damage to their hosts (K. McKnight and V. McKnight, 1987). Polypores that degrade hardwood are a problem for the lumber industry, as they lead to economic losses. However, bracket fungi are of great importance to the St-Lawrence lowlands ecosystems. Indeed, they recycle carbon by degrading cellulose from the tree bark and returning it to the atmosphere. They also allow old trees to weaken and fall to the ground, where they decompose and become important components of the soil. This allows young healthy trees to take their place, as part of the forest regeneration process. Moreover, polypores are crucial to wildlife, as they provide habitat for various species. Birds nest in the cavities they form and arthropods as well as small amphibians find shelter within their fruiting bodies (Gilbertson, 1980). Shelf fungi are very diverse, and we observed many genus during our research project in the Morgan Arboretum. Turkey Tail, Redbelt, Hen-of-the-Wood and Dryad Saddle are among the most abundant and noticeable types of fungi we encountered (K. McKnight and V. McKnight, 1987).
Vulnerablity of trees to environmental factors
Forest composition fluctuates in response to changes in climate as well as different nonnative biotic stressors such as diseases, pests and invasive plants. The association of two or more of these factors can exacerbate population declines in some tree species as well as hinder the growth of others (Fisichelli et al., 2014).
Certain trees exhibit a higher vulnerability to these ecological strains than others. A particularly striking example is the American beech tree, Fagus grandifolia, which is plagued by beech bark disease. This blight is caused by beech scale, Cryptococcus fagisuga, which attacks bark and renders it vulnerable to fungi infestation. The invasion of the wood by fungi of the genus Nectria is deadly to the tree (Houston and O’Brien. 1983).
The disease has severely impacted the composition of North American hardwood forests, where the beech tree is a founding species. The decline in beech tree populations is detrimental to certain species and therefore alters biodiversity (Cale et al., 2013).
The effects of shelf fungi on trees
Trees, regardless of the species, are vulnerable to fungal diseases when their protective outer bark layer is breached. The wound may form due to insect pests who consume the bark or use stylets to access nutrients deep within (for example, sugar maple sap), or animals who scratch the surface, peck it or feed off of it. Humans tend to carve their initials into bark for amusement purposes or expose the bark when trimming the tree and breaking off branches. Once the bark is damaged, the fungus’s spores have an entryway into the woody internal flesh and begin to thrive (Fogal 2006).
As it is extremely pervasive, it is impossible to clear a tree of a fungal infection. This is due to fungi’s composition of filamentous fibers called mycelium, which provide strength and stability. Mycelium hook onto the fibers of the wood and feed off it. By secreting enzymes, they break down wood fibers into cellulose and lignin components, which results in the degradation of the timber. From the wound, the fungus spreads internally and rots the inside of the tree. New cracks on the outside of the tree appear and allow entry for more fungi of the same or different species. The dead tree may remain standing or fall, but either way, the fungus will continue to thrive and work as a community to decompose it until there is nothing left but decayed matter (Ross D.R., n.d)
Within the Morgan Arboretum, we conducted half of our research in the sugar maple stands and half within a mixed beech and red maple forest (as there are no pure beech stands in the Arboretum). In total, we located 40 trees of the American beech and sugar maple species that bore shelf fungi. Our research spanned three weeks, during which we made two trips to the field. On Monday October 5th, we studied 20 standing or fallen sugar maple trees for shelf fungal growth. On Monday October 19th, we inspected shelf fungus specimens on 20 American beech trees.
Upon venturing off the path, all five of us walked in different directions to allow an unbiased selection of our sample trees and to increase the variety of our outcomes. Every time we located an infected tree or log, we tied a pink marker around it to avoid repetition. Then, in an Excel spreadsheet on our portable tablet, we recorded the species, color, average diameter, and abundance of the fungi. We also assessed the tree’s health through the following criteria: approximate percentage of leaves, bark health (presence and abundance of cracks, peeling, scars, rot, etc), general health of the tree (fallen, standing, or diseased), and presence of beech bark disease.
A selection of fungi guides, including A Field Guide to Mushrooms: North America by Kent H. McKnight and Vera B. McKnight and The Book of Fungi by Peter Roberts and Shelley Evans, were of great help in identifying species of shelf fungi found in the Morgan Arboretum. The team’s field knowledge, coupled with our photographic evidence, enabled us to discern the color, shape, texture, size, and distribution of the shelf fungi. Referring to the literature, armed with our data, we then successfully classified the specimens. However, identifying fungi is no easy task due to its vast diversity and many resemblances between certain species of shelf fungi like Turkey Tail and the Multicolor Gill Polypore. Photographing the fungi from many angles, including the underside, gives a whole new perspective on each fungi and helps in the identification of the species.
Our analysis of the data collected reveals interesting trends between different varieties of shelf fungi and their impact and relationship with the sugar maple and American beech. We determined that there is a greater diversity of shelf fungi on American beech trees as compared to sugar maple trees. This raises questions about how fungi affect the health of trees, given that the American beech trees in the Morgan Arboretum are generally in poorer health than the sugar maple trees.
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Houston D and O’Brien J. 1983. Beech Bark Disease. U.S. Department of Agriculture Forest Service. (Forest Insect and Disease Leaflet 75); [accessed 26 Oct 2015]. http:// http://www.na.fs.fed.us/spfo/pubs/fidls/beechbark/fidl-beech.htm
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