(Arboricultural-styled) 'Fact of the Day'

[Kveldssanger]

24/01/16. Fact #136.

 

Within old-growth boreal forests, which are found at the northern-most latitudes of the world, decaying tree trunks are a key micro-habitat. Within the tree trunks, decay is very slow, and the forests therefore have high levels of both standing and fallen deadwood that exist in ideal conditions for long periods of time. In Fennoscandia, around 1,000 species of beetle rely on either decaying wood, or the macro wood-decay fungi themselves (their sporophores), for habitat. However, because many old-growth boreal forests have come under some form of management, which typically sees deadwood cleared, there are perhaps untold extinctions that have taken -and are still taking – place.

 

In this research study, the authors focus on the sporophores of the wood decay fungus Fomitopsis rosea (which induces a brown rot), found in old-growth spruce swamp forests upon stumps and fallen trunks, and assess whether its decline across Finland (due to logging) has lead to a change in population dynamics of insects reliant upon the fungus’ sporophore (once it is partly dead) in the remaining isolated patches of old-growth spruce swamp forest. The isolated patches surveyed included five fragments isolated for between 2-7 years, and an additional ten fragments isolated for between 12-32 years, whilst the control areas were large patches of old-growth forest not isolated due to logging. All sites were however equal, in the sense that they had similar tree species composition, a similar number of dead stumps and fallen trunks, and were of similar age.

 

A group of sporophores on a fallen stem. Source: Mycoweb.

 

At each site, fruiting bodies were located and samples were taken – a total of 251 were taken from control sites, 60 from sites isolated for 2-7 years, and 44 from sites isolated for 12-32 years. These samples were then taken back to the laboratory, where they were placed in cloth-covered plastic boxes in outdoor conditions for just over a year. Every month, the boxes were checked to ascertain whether any insects had emerged from the sporophores, and any that had emerged were taken and stored either in alcohol or as dry samples for identification.

 

From the samples taken, a total of 33 insect species were identified. Many of the species found are classed as rare across Fennoscandia. The most dominant (33%) insects identified were the larvae of the moth Agnathosia mendicella, which eat the fungal tissues, and the parasitic fly Elfia cingulata that specialises in parasitising on the moth larvae. This fly had not, at that time, been recorded in any other fungal species’ sporophore, and nor was it found in any of the sporophores sampled that contained other moth species in place of Agnathosia mendicella.

 

This sporophore is partly dead, and may very well be the type of sporophore that insects utilise. Source: Mycoweb.

 

The presence of the moth Agnathosia mendicella was most abundant in the control groups that were not isolated, and the parasitic fly Elfia cingulata fared similarly – as did the abundance of the fungus Fomitopsis rosea. However, Elfia cingulata was not found at all in patches isolated for more than 12 years, and the more isolated patches of 2-7 years that contained the fungal sporophores were host to fewer Agnathosia mendicella and Elfia cingulata. In fact, the presence of the moth Agnathosia mendicella was significantly lower in the isolated old-growth fragments, as was the presence of Fomitopsis rosea in patches isolated for 12-32 years – particularly when the forest fragments were small and the decaying trunks were exposed to sunlight (the fungus rarely grows in sun-exposed settings).

 

In light of the data, the authors suggest that fragmentation of old-growth forest, and the amount of time the fragments have been isolated for, is directly related to the declining presence of the Fomitopsis rosea –Agnathosia mendicella – Elfia cingulata trophic relationship. Other insect species observed suggested similarly. Therefore, it is important that not only is habitat fragmentation reversed over time, but patches of old-growth forest are allowed to persist or increase in size. Currently, the isolated fragments simply cannot provide the right conditions for such niche and specialised species, from the fungus itself all the way up the trophic levels to insect parasitoids. Changes in forestry practice are thus necessary, else local extinctions of niche ecosystems (not just those relating to Fomitopsis rosea) may more frequently occur.

 

Source: Komonen, A., Penttilä, R., Lindgren, M., & Hanski, I. (2000) Forest fragmentation truncates a food chain based on an old-growth forest bracket fungus. Oikos. 90 (1). p119-126.

[Kveldssanger]

25/01/16. Fact #137.

 

The Victorians certainly loved their gardens, and also their exotic trees – Cedars of Lebanon (Cedrus libani) could be seen in such abundance that “the traveller could scarcely pass a hundred yards down portions of the western roads [in London] without coming upon fresh specimens or groups of them.” Many also didn’t like the copper beech (Fagus sylvatica Atropurpurea), and by 1890 its planting had almost ceased. If we see a very mature copper beech therefore, perhaps it pre-dates this time.

 

However, this was not the full extent of the Victorian era in terms of arboriculture. They also liked their rock gardens, complete with exotic and pyramidal conifers, mountain ash, silver birch, rhododendrons, gorse, and broom, as did they like a lovely ornate stumpery or rootery. A stumpery was a collection of (usually hardwood) stumps, with ferns, mosses, and lichens growing upon them for ornamental purposes (and probably also fungi), whilst the rootery was a collection of upside-down mature tree stems with their ivy-draped (or any other climbing plant species) roots up in the air.

 

A stumpery would almost certainly be very good, ecologically-speaking – particularly if the stumps used were from large trees. Their provision as deadwood habitat for fungi, and insects associated with such fungal presence, is just one dynamic of how they may have been highly beneficial. Stumperies actually became very popular in Victorian gardens, following the first one being created at Biddulph Grange.

 

The original Victorian stumpery at Biddulph Grange. Source: Parks & Gardens UK.

 

Source: Johnston, M. (2015) Trees in Towns and Cities – A History of British Urban Arboriculture. UK: Windgather Press.

[David Humphries]

Stumperies are certainly in vogue with Gardners both large and small scale.

 

Chelsea flower show had a small urban garden example last year created by Capel Manor College

 

Ickworth National Trust is another fine example on a larger scale.

 

Our own at work is developing nicely with interesting fungi popping up on them.

 

http://arbtalk.co.uk/forum/ecology/58756-stumpery.html

 

 

.

[Kveldssanger]

Oh wow, that's really neat! I had never considered it something that was done, until reading the book I referenced. I wonder what individual came up with the idea, as it's rather clever.

[Kveldssanger]

26/01/15. Fact #138.

 

There are likely very few – if any – trees of any respectable size or age that don’t have at least small pockets of decay. In our urban trees, such areas of decay may be even more common, given how they are prone to a much greater amount of foot and vehicular traffic passing within close vicinity of their presence, as well as sometimes being pruned at (sometimes regular) intervals. Perhaps, decay in urban trees is even more important (in terms of its impacts beyond that of pure economics) than decay in rural or woodland trees, because of the more significant target zones. Despite this, little research has been done into the average amount of decay an urban tree may have, and how often decay will occur within its structure. The authors of this study seek to remedy that, by providing a foundation on which further research can be done.

 

This study sees most attention drawn towards the genus Acer (maples), whose species grace the streets of New York cities in great numbers. Acer platanoides, Acer rubrum, and Acer saccharinum are but three species of maple commonly found, and amongst other maple species they account for as much as 50% of all street trees. Perhaps their abundance is, in part, due to their selection following the removal of Ulmus americana after major outbreaks of Dutch elm disease in the 1930s. Many of the maples are also mature, and therefore the authors note that ascertaining extent and frequency of decay within individuals can be achieved with relative success, whilst being very important in terms of health and safety. To determine decay extent and gather data, the authors of this study used a resistograph, a sounding mallet, and undertook a visual inspection of the trees.

 

All trees within this study were over 30.5cm in DBH (thus, they could be considered mature), and were situated within the New York cities of Albany, Buffalo, Rochester, and Syracuse. Because all four cities had (mostly) complete records of their tree populations, identifying trees with a diameter of over 12in (30.5cm) was swiftly achieved, and from the pool of trees (67,000) that were within the criteria a total of 480 were randomly chosen from each of the four cities – of the 480 in each city, at least 90 were of the species Acer platanoides, Acer saccharinum, and Acer saccharum (other species included – but were not limited to – Acer rubrum, Fraxinus pennsylvanica, Platanus x acerifolia, Quercus rubra, and Tilia cordata). All trees were also split up into DBH classes of 30.5–45.7 cm (12-18 in), 45.7–61 cm (18–24 in), 61–76.2 cm (24–30 in), and greater than 76.2 cm (30 in).

 

For each individual tree, three resistograph measurements were taken (at the height of where decay was considered to be present, following sounding hammer application around the circumference of the tree and visual inspection – if no decay indicators were present, readings were taken at the DBH height; and never above 3.1m up the stem). Each measurement went to a depth of 38cm, so the authors did note that the much larger trees would not see an entire cross-section ‘sampled’, but instead perhaps only around half (which may have caused readings to not be as accurate when ascertaining decay extent). However, the resistograph is a good tool for assessing internal wood properties at a given point, and therefore it was determined that the resistograph would be used and, after a drop in wood resistance of 13mm or greater when in operation, it was assumed that decay was present within the tree being assessed. If decay was present on the outside of the tree, because the bark was dead or sapwood rot was present, but the inner core remained sound, the outer ‘shell’ was marked as zero (to factor into the calculations for t/R).

 

Fewer sugar maples (Acer saccharum) were sampled in Albany as the city has far fewer street trees of this species.

 

In relation to the decay frequency, the city of Syracuse had the highest rate at 61.2% of trees having decay (though across all four cities, the average was 58%), whilst sugar maples (Acer saccharum) were most frequently observed to have decay within (at 63% of all trees). Individuals with a DBH of 61-76.2cm (24–30in) were most likely to have decay, out of all the DBH classes assessed. As for decay severity, only 3.2% of the trees assessed had severe decay (where the sound wall thickness, based on Matthecks’ t/R formula, was from 0.1-0.3), though the range was from 1.5-4.5% across the four cities (and not significantly different). Silver maples (Acer saccharinum) were most often found to have severe decay, with 5.3% of those surveyed found to have a sound wall thickness of below 0.3, whereas sugar maples (Acer saccharum) were least likely at only 1.8% (therefore, there was a significant difference in terms of severe decay frequency between species). Additionally, severe decay was most frequency in trees with a DBH of 76.2cm and above, at nearly 7% – the next highest class range was 61-76.2cm, at around 3.5%.

 

Decay incidence by species.

 

The percentage of each group that was shown to have significant decay (t/R = less than 0.3).

 

Curiously, this means that whilst sugar maples (Acer saccharum) are most often going to harbour decay, they are the least likely of the species surveyed to suffer from significant decay. However, the authors note that silver maple (Acer saccharinum) is a species very prone to decay, and therefore it had been actively removed in the recent past by urban foresters prior to this study. Thus, it’s low ranking for decay frequency is perhaps skewed by past management practices, though ranking highest of the species in terms of decay severity, it is perhaps still evident at how poor of a compartmentaliser the species is. Despite this, all four cities had very few significantly decayed trees, though did have over half of the tree population suffering from some form of decay.

 

With regards to what this means for management practices, even though the frequency of significantly decayed trees was shown to be low from the sample, this may still equate to over 2,000 individuals across the four cities (of which most are of very significant size – over 76.2cm in diameter). This is certainly an important statistic from a health and safety perspective, as it means that there are many areas where there is significant risk to people and property. Therefore, it is imperative that management practices have the identification of decay extent as a top priority, and particularly for much larger trees. The research also shows that many trees do suffer from some degree of decay, and therefore establishing the causes of this, and what can be done to reduce the frequency of decay within urban trees, is required.

 

Source: Luley, C., Nowak, D., & Greenfield, E. (2009) Frequency and severity of trunk decay in street tree maples in four New York cities. Journal of Arboriculture. 35 (2). p94-99.

[Kveldssanger]

27/01/16. Fact #139.

 

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.

 

The significance between growth of each tree species compared to the variables assessed. For a better view of this table, please access the article (linked at the end of this post).

 

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.

 

A statistical analysis of the frequency of each tree species and its growth rate in each area. For a larger version, please access the article (at the bottom of this post).

 

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.

 

Irradiation levels per species and growth rate in commercial zones.

 

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.

 

These urban trees (Populus x canadensis, Populus nigra ‘Italica’, and x cuprocpyaris leylandii) have plenty of light, and also a fair amount of space. They are located within a busy industrial area near to where I work.

 

Source: Jutras, P., Prasher, S., & Mehuys, G. (2010) Appraisal of key biotic parameters affecting street tree growth. Journal of Arboriculture. 36 (1). p1-10.

[Kveldssanger]

Figured half of my forum posts are probably from this thread! Hah.

[Kveldssanger]

28/01/16. Fact #140.

 

The manner in which a tree is planted will have an impact upon the likelihood of that tree surviving beyond not only the first few years, but also into maturity. For amenity trees, this may be a particular problem, as trees planted incorrectly will not provide the amount of value intended, and they may even detract from the environment in an aesthetic sense. A particular importance with regards to transplanting of a tree into the landscape is that of planting depth. In essence, the root collar should not be planted too far above or below grade. In the article that is featured within this post, the authors undertook five different experiments on five different tree species, and assessed how they fared in response to different planting depths (and other variables, which varied between studies).

Study #1: Planting depth and soil amendment effects on Quercus virginiana

 

The oaks in this study were planted at 7.6cm above grade, at grade, and 7.6cm below grade, and various soil amendments were made to individual specimens (native soil, and soil incorporated with sand, peat, clay, or sandy topsoil). From these variables, tree growth and physiology were assessed. Results from the study concluded that oaks grew best when planted at grade, and where the soil was amended with an application of sand.

 

Study #2: Planting depth and soil amendment effects on Taxodium distichum

 

The variables for this study were identical to those from the study on Quercus virginiana, and much akin to that study, Taxodium distichum specimens grew best when planted at grade and with soil containing sand. When planted above grade, the specimens has reduced coarse root growth, and also produced numerous roots that may potentially act as girdling roots later in life. Similarly, trees planted below grade had reduced stem water potentials, and greater levels of mortality. Across all soil types however, adverse impacts of planting below grade were observed, though the severity varied with soil composition (clay soils lead to the greatest impact, however).

 

Study #3: Planting depth during container growth and subsequent landscape establishment on Ulmus parvifolia

 

This study investigated how planting depth of individuals during container production (at 5cm above grade, at grade, and 5cm below grade), when planted into 10.8L containers and then later into 36.6L, impacted upon landscape establishment. During the first transplanting phase, where individuals were placed into 10.8L containers, those planted at grade were observed to grow most, whilst where planted below grade growth was stunted. Following the second transplanting phase into 36.6L containers, individuals planted above grade both times were observed to suffer the most in terms of growth, though responses by all trees was generally varied. When transplanted out into the field however, as long as extreme variations from grade level were avoided, there was no observed significant difference in growth rates. This suggests that the species fares differently during container production than it does out in the landscape.

 

Study #4: Planting depth and irrigation extent effects on Platanus occidentalis

 

The depth individuals were planted at (7.6cm below grade, at grade, and 7.6cm above grade), in addition to the amount of water received via irrigation (0, 1, 2, or 4 spray stakes, each applying 0.42L per minute), were the variables assessed in this study. Results outlined that planting depth was a significant determining factor for tree survival, though irrigation was not (which suggests the adverse impacts of planting depth were not influenced by soil moisture levels). Individuals at 7.6cm below grade, even after just one year, had significantly increased mortality rates. When planted at grade, and in a sandy loam, the best growth was observed.

 

Study #5: Planting depth and season of transplanting effects on Taxodium distichum

 

Individual specimens in this study were planted either during spring or autumn (fall), and at grades 7.6cm below, level, or 7.6cm above. Depth was shown to be a significant factor in determining growth (with trees planted at or 7.6cm below being taller and thicker in the stem), whilst transplanting season was not.

 

Conclusions

 

From the studies mentioned in brief during this report by the authors, it can be seen how planting depths impacts different species in different ways. Curiously however, the authors note that ecotypes may have an effect upon establishment success rates, with seeds sourced from trees growing in the environment in which they are intended to grow offering a potentially higher rate of success than those sourced from environments of other kinds. It is doubtful that this relates to planting depth, though certainly is food for thought with regards to selecting individuals to plant in harsh urban environments.

 

These two alders, due to improper planting (they were planted in their non-degradable bags) and aftercare (no watering regime), have died during their first season.

 

Source: Bryan, D., Arnold, M., Volder, A., Watson, W., Lombardini, L., Sloan, J., & Cartmill, A. (2009) Overview of selected studies on the influence of planting depth on landscape establishment of container-grown trees. In Watson, G., Costello, L., Scharenbroch, B., & Gilman, E. (eds.) The Landscape Below Ground III. USA: International Society of Arboriculture.

[Kveldssanger]

30/01/16. Fact #141.

 

Having recently shared a study that looked at the urban forest and bird species diversity, I thought we could use the momentum from that article and build upon it by looking at bats and the urban forest. The bird species diversity article highlighted the criticality of good landscape connectivity within urban environments (the opposite of habitat fragmentation), and the authors of this study suggest much the same from the very beginning – alongside fragmentation (usually coupled with urban sprawl / urbanisation) comes a decline in species diversity within urban areas, and it can be anticipated that bats are no exception to this.

 

The authors undertook this study in Vitoria, a city in the southeastern part of Brazil, with an intent of assessing whether wooded streets were used by bats (this would allow the authors to conclude whether urban street trees are beneficial in terms of connectivity for bats). Vitoria, home to 1.8 million people, expanded significantly during the 1940s and onwards, following industrial advancements. Such expansion lead to the destruction of the surrounding environment (including the Atlantic Forest) and its ecosystems, and the increased pollution levels have further taxed remaining vestiges. However, the city is still marked as a site important for biodiversity conservation, likely because of its eight municipal parks acting as habitat for a great variety of wildlife to the now (locally) fragmented Atlantic Forest.

 

This study chose to look at three of those eight parks for the presence of bats: (1) Pedra da Cebola Municipal Park, with 100,005 square metres of land that once acted as a transition between coastal shrubland at the Atlantic Forest; (2) Horto de Maruipe Municipal Park, with 60,000 square metres of land and covered by native Atlantic Forest vegetation (though with many introduced species of wildlife), and; (3) Fazendinha Municipal Park, with 22,653 square metres that – like Pedra da Cebola Municipal Park – was a transition zone between the coast and the Atlantic Forest. Three wooded streets, and three non-wooded streets, were also surveyed for bat presence. By surveying the parks and the streets, comparisons could be drawn between the two, and there would be scope to determine whether bats remain isolated to the parks. Sampling for the bats (with the use of mist nets) took place over the course of a year, with visits taking place for three days each month.

 

In total, 172 individuals were captured a total of 174 times. Most were found within the municipal parks, followed by the wooded streets, and then the non-wooded streets. However, the vast majority were only found within the urban parks (both in terms of species diversity and species abundance), with wooded and non-wooded streets not displaying a marked difference between one another. The authors do note that limited samplings perhaps lead to this, and suggest that more sampling visits to all areas would potentially show more individuals of more species using all three environments (only 10 species were recorded, whilst the region has 36 bat species – the authors do suggest that Vitoria may not harbour all species, however). The table below breaks down species presence by environment.

 

The number of individuals of each bat species recorded during the study period of one year.

 

In light of their research, the authors in fact suggest that wooded streets are not particularly important for bat species, though they do recognise that increasing sampling visits and having sampling methods utilise ultrasonic devices may have improved the results obtained. For example, Noctilo sp. bats were observed, though not recorded with the netting method.

 

The fact that Artibeus lituratus was the most frequently observed one across all three environments, the authors allege, is due to its opportunistic behaviour. It has previously been recorded using urban environments, and may have a tolerance to urbanisation as a result (unlike other species). However, other research papers have suggested more species of bat (some found in this study) do use urban areas for feeding, particularly where lights attract insects (insectivorous bat species will benefit most for this).

 

Artibeus lituratus (giant fruit-eating bat).

 

So what does this study mean? As sampling was admittedly quite limited, perhaps it’s not a foregone conclusion that bats don’t use street trees as vehicles to move between larger park areas (or for other reasons), though it does suggest that maybe the urban forest is not structurally supportive of many bat species. Could more be done in the planning stages of development, or even following development, to attract more bats to the urban streets of Vitoria? The authors suggest that using native trees in place of exotic ones may be a good starting point.

 

Source: Oprea, M., Mendes, P., Vieira, T., & Ditchfield, A. (2009) Do wooded streets provide connectivity for bats in an urban landscape?. Biodiversity and Conservation. 18 (9). p2361-2371.

[Kveldssanger]

31/01/16. Fact #142.

 

We could discuss this topic to the ends of the earth, outlining both positives and negatives of tree presence within urban environments, though there is a mounting pile of evidence to suggest that trees really do have a (largely) beneficial impact upon humans in terms of health and well-being. However, most studies have looked at trees on a macro-environmental scale (such as exposure to general green spaces), and not looked at the linkages between individual trees and the impacts such trees have upon residents in the close locale. Therefore, the authors of this study sought to change this, by investigating the health benefits of trees to a “low granularity”, and instead of assessing such ‘greenery’ from the ground, they related their study to tree canopy sizes only – it was considered (by the authors) that tree canopy is the most important of all the ‘green’ available within urban areas. Thus, the study was planned, and the city of Toronto, Canada, was chosen as the location for the project.

 

The authors particularly sought to assess the relationship between street tree canopy density and its impacts upon: (1) overall health ‘perception’ (this was chosen as subjective perceptions of personal health has been found to influence mortality rates, and is considered a strong indicator of actual health of an individual); (2) cardio-metabolic conditions (including high blood pressure, obesity, high blood glucose, high cholesterol, and diabetes), and; (3) mental health problems (depression, anxiety, and addiction). Similarly, the benefits of trees in parks was also assessed, enabling for comparisons to be drawn between trees in differing locations.

 

In terms of the results found, I have segmented the three aforementioned categories into different sections. The manner in which the results section was written was rather disjointed by tables and seemingly tangential (but still contextually beneficial) points, though I pulled out the most important parts and included them below.

One of Toronto’s residential streets, which in this case appears to have a good number of small, medium, and large trees.

 

Health perception

 

It was found that people who live on streets where there are more trees, and / or trees with larger canopies, report better health perceptions (after accounting for demographic factors: age, income, etc) – for every 400 square centimetre increase in ‘treed area’ per one metre squared of neighbourhood area, it was shown that health perceptions would rise by around 1% (around an extra 10 trees per block, of which there are 25 blocks per district area). In fact, this incremental increase in health perception as a result of there being more trees, or larger trees, was on par with an income increase of $10,200 per year per household, or an individual being 7 years younger. However, the authors note that trees alone do not majorly influence health perception (they only do so by a few percent) – there are many others factors that will impact upon an individual’s view of themselves.

 

Cardio-metabolic conditions

 

Where trees are present within streets, the authors found that fewer cardio-metabolic issues were present in individuals. For every 11 trees within a city block, the occurrence of such health issues fell by around 0.5%, and is as impactful as an average increase of $20,200 in household income per year, or being 1.4 years younger. Again, tree presence isn’t the sole driver behind better cardio-metabolic health, though certainly plays a role (albeit, perhaps a rather small one – a few percent).

 

Mental health

 

The authors found no statistical significance of tree presence upon the mental health of individuals, so will be undertaking further investigations to further analyse the relationship.

Concluding remarks

 

In light of the survey data, the authors conclude that an increase in tree presence along urban streets will have a beneficial impact upon an individual’s health perception, and to their cardio-metabolic health. With regards to their comparisons to annual income increases, the authors make an interesting point in that a household earning $10,200 more than another household, whilst on paper would be ‘better off’ in terms of health perception, may not actually be so – if the other family has over 10 additional trees in their block compared to the block where the higher income household is, the lower-income household may perceive themselves as healthier than the higher income household.

 

Toronto has around 28% tree canopy cover, and in this image we can see a heavily-treed parkland area with a building-clad backdrop.

 

Therefore, as Toronto’s street tree density is on average between 0.2%-20.5% (including parks, it is around 28%), if tree planting rates increase (by as little as 10 more per block) – and canopy cover subsequently increases – then the health of residents will improve. However, the authors do note that Canada has a nationalised healthcare system, and therefore other countries where no such healthcare programme exists then the average income of a household may hold more weighting. This means that an increase in 10 trees per block in another country may equate to less of an increase in comparison to household income increase – 10 more trees may only equate to a $4,000 increase in household income, for example.

 

Curiously, the improved health of residents did not rank so significantly for trees within green spaces. This means, so the authors hypothesise, that it is the trees that are situated directly within the locality of a household (and within view) that will have the most beneficial impact on health, though the authors recognise that the other benefits of the street trees (such as reducing air pollution) will play a role. However, parks are still beneficial – that is not being disputed. The results simply underline the necessity for street trees – we cannot rely on parks exclusively to bolster the health of a populace.

 

It is also suggested that once tree density becomes significant (perhaps once over 20%) that the health benefits of trees becomes less significant (though are still evident). Additionally, an area with many evergreen trees does not mean residents who may view them have a better perception of their health – a great variety of trees, both deciduous and coniferous, are required to improve health perceptions of individuals impacted by their presence. However, residents living within areas occupied by many evergreen trees do report a better cardio-metabolic status.

 

Source: Kardan, O., Gozdyra, P., Misic, B., Moola, F., Palmer, L., Paus, T., & Berman, M. (2015) Neighborhood greenspace and health in a large urban center. Scientific Reports. 5 (11610). p1-14.