Mycorrhizae, both ecto- and endo-mycorrhizal, are absolutely critical for the survival of trees of all ages and species, and exist within the soil wherever there are trees (and other plants). By enabling a tree to uptake additional nutrients (nitrogen, phosophorus, etc) and water, in addition to providing connective networks linking many trees together (and allowing them to trade resources), overall tree health is greatly improved by their presence. Of course, different tree species have different types of associations with different mycorrhizae, and therefore a healthy diversity and abundance of mycorrhizal fungi within a soil environment can support a diversity and abundance of tree species, and vice versa.
In the natural woodland setting, such mycorrhizae and trees naturally operate together, in a sort of successional symbiosis, though in a landscape altered by man such a symbiosis can be aggressively severed. Soil disturbance, pollution, and removal, in addition to the clearance of vegetation, can bring with it a mycorrhizal armageddon, and one can surely not therefore expect trees to exist in the same state as they would in an undisturbed and far more natural environment. Perhaps this is one reason why urban trees can be observed to very much struggle, and notably newly-planted trees put into an already much-altered landscape, and the point of this post is to explore one study that investigated whether mycorrhizae were indeed in less abundant in the urban environment of Ontario, Canada.
The study looked at a total of 26 tree species, including Acer platanoides (Norway maple), Juglans nigra (black walnut), Quercus palustris (pin oak), and Robinia pseudoacacia (black locust), and assessed, in both rural and urban locations, the amount of association (relative to total root mass) each tree species had with mycorrhizal fungi. For every single of the 26 tree species, ten separate locations were identified – five in urban settings (streets, urban parks) and five in rural ones (including forests) – and soil samples were taken for assessment in a laboratory. For the urban trees, it was largely accepted that they were all transplanted nursery stock. Both ectomycorrhizal and arbuscular (a sub-set of the endomycorrhizae) fungi were studied.
Following the examination of soil samples in the lab, it was found that all 26 tree species (in both the urban and rural setting) had associations with arbuscular fungi, whilst 7 also had associations with ectomycorrhizal fungi (and these seven had lower colonisation rates of arbuscular fungi, as a result). However, this is essentially where the similarities end, as trees in rural settings had, by-and-large, a much greater association with mycorrhizae than their urban counterparts. In fact, mycorrhizal associations were 37% and 33% lower (for arbuscular and ectomycorrhizal fungi, respectively) in urban areas when compared to rural areas. We can however see that some trees species had higher colonisation rates of mycorrhizae in the urban location, though this does not extend to many of the tree species studied, and never were these differences statistically significant. Conversely, many were statistically different in favour of rural locations.
Evidently, this has implications for urban trees. Without doubt, mycorrhizal associations do occur, though simply not to the same levels, and thus urban trees do not receive the same benefits that rural trees do. Therefore, there may very well be a problem with the infectivity of mycorrhizal fungi within urban settings, and the authors state this may be for a couple of reasons: soil compaction, nutrient content, pH, and pollution. A build-up of aluminium ions in urban soils, for instance, may mean that the pH of the soil is too low for effective associations between tree roots and mycorrhizae, and the poorly aerated conditions around many well-used areas (including construction sites) is also likely to have a very adverse effect.
On a landscaping level, problems may also arise with there being a general lack of habitat for the mycorrhizal fungi themselves. Because mycorrhizae largely rely upon trees to either complete, or fully support, their life cycle and existence, the lack of tree species that can provide habitat for these fungi may very well be an issue. Particularly where monocultures are present, soils may be lacking in mycorrhizal diversity, and therefore if other tree species are planted into the monoculture then they may very likely not have available to them those fungal symbionts they require. Even if urban trees are present in the right amounts to support a wide range of mycorrhizae, the stress the trees are experiencing due to other adverse biotic and abiotic factors may limit their ability to provide the nutrients the mycorrhizal fungi require, and therefore there may not be scope for a significant association akin to that of rural locations.
This study only goes to demonstrate just another angle in a chapter of a book (that is growing into an encyclopedia of massive proportions) relating to why urban trees will struggle. How can this issue be fixed? I’m sure that can be debated to the ends of the earth. What must be said is that it’s certainly worth considering for when planting tree species and preparing ground for urban developments, amongst the array of other things also worth considering. It makes you wonder how urban trees survive at all, as explained quite brilliantly in this article.
Source: Bainard, L., Klironomos, J., & Gordon, A. (2011) The mycorrhizal status and colonization of 26 tree species growing in urban and rural environments. Mycorrhiza. 21 (2). p91-96.
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Arboriculture 
[…] damage, changing pH levels due to pollution, and so on), trees may markedly suffer. In fact, I have looked at a paper quite recently on such a lack of mycorrhizal symbiosis with fine root hairs in urban […]
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