Comparison of Mycorrhizal Colonization between Urban and Rural Environments

Human populations continue to grow and with it the urbanization of natural environments. There are numerous implications associated with this transformation, the greatest being the destruction of the ecosystem. It affects the air, the land, and all the organisms residing in the area. Characteristics of urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> areas are buildings, square miles of concrete, and artificial mixes of vegetation<!–[if supportFields]> XE “vegetation” <![endif]–><!–[if supportFields]><![endif]–>. Usually, the introduction of exotic species takes over the remaining patches of habitat for native species. The exposed soil also undergoes drastic changes including lack of aeration, higher pH, and appearance of pollutants. Bainard and Klironomos (2010) focus on one of the effects urbanization produces on the mycorrhizal colonization of 26 tree species It compares the colonization of the tree species between urban and rural forests in Ontario, Canada<!–[if supportFields]> XE “Canada” <![endif]–><!–[if supportFields]><![endif]–>; and it seeks to expand the literature on the effect of urbanization on mycorrhizal fungi. —Daniella Barraza
Bainard, L., Klironomos, J., 2010. The mycorrhizal status and colonization of 26 tree species growing in urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> and rural environments. Mycorrhiza 21, 91–96.

Bainard & Klironomos identified 26 tree species, found in both urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> and rural environments, as their focus for mycorrhizal colonization. The rural area consisted of forests in southern Ontario and the urban area consisted of parks, streetscapes, and residential areas in southern Ontario. The species belonged to the genera Acer, Aesculus, Betula, Cercis, Fraxinus, Gleditsia, Juglans, Juniperus, Populus, Prunus, Quercus, Robinia, and Thuja. In urban and rural environments, five different locations were chosen for each of the tree species. The locations were chosen at 5 km intervals. Besides their location, another difference between tree species is that most trees found in the urban environment were grown in nurseries before being transplanted into their current location. At each location, three trees were chosen to represent their species, and all the chosen trees were mature at 20–25 years of age. For analysis, a soil core was obtained from beneath the trees. The soil cores were collected between May 26 and June 21 to keep seasonal changes at a minimum. From the soil cores, tree roots were examined for ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi colonization. To determine EM fungi colonization, root tips with mycorrhizal structures and a hartig net were counted and percent EM colonization was calculated. A hartig net is the hyphal network for nutrient exchange and its presence is to ensure that the symbiotic relationship between tree and fungi is active. To determine AM fungi colonization, roots were observed under a microscope since they are smaller than EM fungi. Percent AM fungi colonization was also calculated. Statistical analyses were computed to find out the insignificant and significant difference between colonization in urban and rural areas.
In both areas, all of the tree species were colonized by AM fungi and only seven were colonized by EM fungi. These seven species were also colonized by AM fungi and this relationship is called the tripartite association. Across the board, the species showed lower AM and tripartite association colonization in urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> areas. For AM fungi, the range for percent colonization was broad from 2.4 % to 53.5 %. For EM fungi, the range for percent colonization was from 14.4 % to 50.8 %. In tripartite associations, the colonization of AM fungi was the lowest. This result accords with the trees’ stages of growth and fungal colonization.  During the early stages of growth, the tree is colonized by AM fungi, but as it matures, the EM fungi become more prominent. As mentioned, the studied trees were all mature trees.
However, there is one exception to lower fungal colonization in urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> areas. Populus deltoides<!–[if supportFields]> XE “Populus deltoides” <![endif]–><!–[if supportFields]><![endif]–> with a tripartite association had a significantly higher AM fungi colonization in urban areas. When the data collected from each tree were averaged, AM colonization was significantly lower in urban areas; in tripartite association, AM colonization was not significantly different in urban areas.
The reasons for lower colonization could be discovered in the composition of urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> soils, they’re pH, presence of pollutants and nutrients, and lack of aeration. Another reason can be attributed to the density of tree species in urban areas. There are fewer hosts for fungi to colonize since the trees are relegated to certain areas in the city. Finally, in disturbed soils, such as urban soils, fungi have more competition from other plant species, thereby reducing the fungal infectivity of the soil. However, not all disturbed soils result in lower fungal infectivity; some disturbances like clear-cutting have no effect on fungi colonization.
It is not known what the causes are for lower fungal colonization in urban<!–[if supportFields]> XE “urban” <![endif]–><!–[if supportFields]><![endif]–> areas, and it is not known what negative effects can occur. A suggestion for possible negative effects is inoculating the trees with mycorrhizal fungi. Although definitely more research needs to be done since trees in urban areas seem to be doing fine, and there is enough mycorrhizal fungi colonization to form structures for symbiosis.

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