Urban street trees must cope adequately with numerous adverse factors, in order for maturity to be reached. But a few of the stressors our urban trees face include: (1) high ambient temperatures in the summer bringing about water deficits that impact upon efficient photosynthate production; (2) reduced moisture availability as a result of restricted rooting space and an impermeable surface layer; (3) mechanical damage associated with mowing operations and road vehicle collisions; (4) pollution of various forms within both the air and the soil; (5) deficiencies of necessary nutrients within the soil, and; (6) a lack of sufficient solar irradiation (principally because of the shade cast by buildings). In light (no pun intended) of the aforementioned factors, and the ones not eludicated to, research must be undertaken in order to ascertain exactly how each factor may impact upon the ability for an urban tree to survive. This study seeks to achieve exactly that, by looking at the urban trees of Montreal, Canada.
The city of Montreal has 4,460km of road, streets, and boulevards, with a total of over 240,000 public trees. These highway networks are situated amongst downtown, residential, institutional, and commercial areas, and therefore the authors decided to analyse trees from all four areas in order to have a suitable range for the study – though they split them into street types of (1) intensive commercial, (2) commercial, (3) institutional, (4) intensive residential, and (5) residential. In the five catergories, locations were selected based on the height of the buildings surrounding the trees, the orientation of the buildings compared to the trees, the rate at which highways are used, size of tree pits, and street width.
In total, 1,532 trees were surveyed. The species assessed were representative of 75% of Montreal’s trees, and were: Acer platanoides, Acer saccharinum, Celtis occidentalis, Fraxinus pennsylvanica, Gleditsia triacanthos, Tilia cordata, and Ulmus pumila. Each tree had data captured including DBH, crown diameter, height, crown volume, annual DBH increment, and annual height increment. Similarly, many abiotic factors were measured, including street type (the five categories were outlined above), distance from tree to the closest building, volume of the tree pit, soil penetration resistance, and the type of ground cover. Soil samples were also taken and analysed for theit nutrient content. From the above perameters (and those not mentioned), the table below outlines the significance of each perameter for each species. Emboldened statistics, which are for a probability of 0.02 or less, indicate the most significant influencing factors for each species.
From the above data, we can see how different species have different variables that have significant influences upon their growth. For example, iron availability in the soil is significantly related to the growth of Celtis occidentalis, whilst it is not for Acer saccharinum. Similarly, Acer saccharinum growth is not significantly impacted by the volume of the tree pit, whilst it is for Gleditsia triacanthos. It is likely that species-specific traits govern this, as Acer saccharinum is known to tolerate encorachment into its root zone very well.
The authors note that it is interesting how the presence of a metal grate atop the surface of the ground is a significant factor in effective growth of nearly all tree species. It is almost certain that this is because the grate stops soil compaction from manifesting as a result of traffic (be it on foot, or vehicular). However, it is solar irradiation that is the most significant determining factor for tree growth, suggesting that the most important thing an urban tree needs is light (which may not always be provided in urban areas).
Location of urban trees also appears to be a significant factor for all species studied, and the below table breaks down growth rates for each species for every urban zone – though the authors outline that annual DBH increments for zones 1 and 2 were 0.53cm and 0.78cm respectively, whilst for zones, 3, 4 and 5, increments were 1.18cm, 1.03cm, and 1.02cm respectively (suggesting institutional locations provided for best radial growth of trunks). The authors also make note of stressed trees being most present within commercial zones (1 and 2), meaning 82% of the poorly-growing trees were located within these two sectors alone. Conversely, non-commercial zones were home to the greatest number of fast-growing trees, indicating better vitality. It is suggested that the reasons for this difference may be that residential and institutional areas are more open, and therefore there are greater levels of sun exposure for the trees (which was seen as the most significant factor impacting upon tree growth). Respectively, commercial and non-commercial zones receive, on average, 205-480 hours and 1,495 hours of solar irradiation during the growing season – a stark contrast.
In fact, when looking at total solar irradiation hours across the growing season for each species in commercial zones, it is highly evident exactly how significant a factor it is for tree growth. The below table does a very simple job of explaining the significance, though also shows how different species’ growth rates fare differently at different total irradiaton hours. For example, Celtis occidentalis grows well only at very high levels of irradiation exposure, whereas Fraxinus peensylvanica can tolerate around 200 hours less across a growing season.
The closeness of a tree to a street also appears to ensure the tree has a greater level of sun exposure, though if the width of the verge is narrow then growth may markedly suffer as a result. This may be because a narrow verge restricts the ability for the tree to grow radially, though also means the tree is more likely to be pruned because of its proximity to the highway. Additionally, wide streets was also a marked factor for influencing tree growth, and this may likely be because wide streets are usually busier (hence they have more lanes), and thus the amount of pollution emanating from the vehicles is greater – as is there a greater chance of de-icing salts being used. Such pollutants have adverse effects on trees, stifling tree growth at higher rates and at greater frequencies.
For a better understanding of exactly how each factor impacts upon tree growth, I highly suggest you visit the article page and have a read for yourself. It’s certainly a very information-dense piece (though the first table in this post explains it all very well). However, I hope that this post goes some way to outlining the important factors that shape a tree’s future, and hope that such information is of use to tree officers and urban foresters in towns and cities across the world.
Source: Jutras, P., Prasher, S., & Mehuys, G. (2010) Appraisal of key biotic parameters affecting street tree growth. Journal of Arboriculture. 36 (1). p1-10.
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