How do earthworms affect canopy structure of North American forests?

Canada and the northern reaches of the USA have no indigenous earthworm species (Hale, 2007). Earthworms native to regions that now surround the Great Lakes were made extinct during the last Ice Age. In fact, almost Therefore, the Great Bear Rainforest and other forest expanses have developed without the annelids that Eurasia has relied on. In this way the make-up of the N. American forests is vastly different from their Eurasian cousins, often with increased leaf litter and low-growing plants. However, since the colonisation of N. America by European settlers in the 1600s, earthworms have become entrenched in the continent. There are now 15 species present in Minnesota (Holdsworth et al., 2003), 15 in Vermont (Gorres & Melnichuk, ND), 17 in Southern Ontario (Choi, 2012) all of which originated from Europe or Asia. Whilst all are non-native, not all are invasive and many species dieback during the cold winters, e.g. the Malaysian Blue (Perionyx excavatus), and some are seen as beneficial by farmers and gardeners in agricultural scenarios, e.g. the Crazy Snakeworm (Amynthas agrestis).

Earthworms are known for their properties as decomposers and have been categorised into three types. Epigeic worms (e.g. Amynthas agrestis) are small and live in the leaf litter without burrows, typically being the first to colonise a region. Endogeic worms (e.g. Apporectodea spp.) are unpigmented small worms that live in horizontal borrows in the topsoil layer. Finally, Anecic worms (e.g. Lumbricus terrestris) are the largest of the three types, feed on both soil and litter, and have extensive burrow networks deep into the soil. The worms can be differentiated based on pigmentation as well as size and location. Earthworms aerate soil, accelerate nutrient cycles and increase water storage. These are vital processes for agriculture; however, they can prove disastrous for forest ecosystems.

In N. America, the humus was likely to be a thick mat of undecomposed or partially-decomposed litter that was not significantly incorporated into the mineral soil, known as Mor Humus. This is often found in fire-dependent woodland with most decomposition being accomplished by fungi. It is characterized by the presence of a thick litter layer with thin or no fermentation and humus layers. However, since the invasion of earthworms, the humus layer has changed into a well-decomposed organic matter, mixed deeply into the mineral soil, known as Mull Humus. It is characterized by the presence of a thin litter layer comprised only of remnants from the previous fall and a thick topsoil layer (Ontario Institute of Pedology, 1985).

The changes in soil type can cause greater erosion and runoff, decreasing the stability of the land. The earthworms can even change the soil chemistry, increasing levels of Calcium oxalate seven-fold in some regions (Gorres & Melnichuk, ND). This is particularly problematic for herbaceous vegetation (like shrubs and small plants) which have been removed from many woodlands by the change in soil make-up and chemistry. However, this has been shown to have a knock on effect. Whilst the large canopy trees can remain relatively unaffected by the presence of earthworms, deer and other herbivores will feed on the saplings of canopy species after the removal of the herbaceous vegetation, caused by earthworms. In some cases, the change in the soil chemistry has even allowed other invasive plants to become prevalent in the dying forests of North America (Gorres & Melnichuk, ND).

This dissertation would encompass research and previous studies in order to examine the affects of earthworms on the canopy structure using XXX study sites: White Mountain National Forest (Vermont, USA), Superior National Forest (Minnesota, USA), Algonquin Provincial Park (Ontario, Canada), Forest of Bowland (Lancashire, UK) in order to compare to an earthworm rich environment, and the Great Bear Rainforest (British Columbia, Canada) as a site without earthworms. This study

would involve conversations, presumably using technology such as skype or something similar, with people from institutions such as the University of Vermont, University of Minnesota, and the University of Western Ontario, as well as taking soil samples from the local Forest of Bowland. Photographic evidence would be needed from each of the foreign locations and so universities or researchers would need contacting for this to be achieved. However, with the expanse of current research available into this topic and with researchers like Cindy Hale, Amy Choi, Tara Sackett, Graeme Davis, and Andy Holdsworth, I do believe that a comparative study should be achievable based out of the Lancaster Environment Centre.

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References

Hale C. 2007 Earthworms of the Great Lakes. 2nd Edition. Kollath-Stensaas Publishers

Holdsworth A., Hale C., Frelich L. 2003 http://www.dnr.state.mn.us/invasives/terrestrialanimals/earthworms/index.html [accessed 20/12/2015] University of Minnesota Center for Hardwood Ecology

Gorres J. & Melnichuk R. (No Date Provided) Powerpoint presentation from a lecture given at The University of Vermont. accessed: http://www.vtinvasives.org/sites/default/files/gorres_invasiveearthworms_vermont_0.pdf [Date Accessed: 20/12/2015]

Choi A. 2012 Invasive Earthworm (Oligochaeta: Lumbricidae) Populations in varying Vegetation Types on a Landscape- and Local-scale. Master of Science in Forestry, Faculty of Forestry, University of Toronto

Field Manual for Describing Soils; Ontario Institute of Pedology, 1985