Tree roots beneath paved and compacted surfaces

A few months ago, a few paving slabs in a town centre were lifted up in order for the rooting environment beneath to be inspected. What was seen was certainly incredible, though of course not unexpected. The highly compacted surface just beneath the tiles and lack of any sort of planting pit has lead to these plane trees (Platanus x hispanica) being near-exclusively comprised of fine and fibrous roots. No anchorage roots were identified in the slabs lifted up, and all were within around 2m of the tree. Additionally, the fact none of these plane trees have significantly uplifted any of the slabs – even right around their base – is telling. Usually, plane roots with poor pit sizes will tear absolutely everything apart (from paths to roadways), though not here.

Granted, the planes (of which there are about 25) all shelter one another, and the tall buildings on the eastern and western sides also shelter them very significantly. Therefore, perhaps anchorage root growth is not a major requirement, though the abundance of roots directly beneath the slabs was interesting. I suspect that the moisture held just below the surface has attracted the tree roots, as even during a very dry period (when these slabs were lifted up) the sand was very damp. Frankly, it is amazing at how these planes exhibit so signs of stress during the summer, even where their rooting environment is so incredibly harsh.

Lessons to take from this? Have sizeable planting pits, and install underground stormwater retention and irrigation systems directing water into the planting pits (particularly necessary during drought periods).

We can see how close by a plane tree is to this area exposed. A plane tree was perhaps even closer to my rear, as were they to my left and right.
The soil is incredibly sandy and made up of a huge network of branching roots.
Another look at the rooting system and its sandy environment. The sand was very damp, and the moisture was likely retained due to the slabs above.
A look at how many roots there were under just one of the three slabs lifted up.
Tree roots beneath paved and compacted surfaces

What factors significantly influence urban tree growth rates?

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 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.

Ti discuss this post, please either leave a comment below or post over on Arbtalk.

What factors significantly influence urban tree growth rates?

The jelly-ear fungus (Auricularia auricula-judae)

Many of you have probably seen this aplenty, usually on stumps or fallen stems and large branches. A coloniser of dead wood, this fungus induces a white rot that delignifies its substrate. It is typically found on hardwood trees, with elder (Sambucus nigra) being a particularly common host, though I have to say that I have seen it on an array of species of deciduous broadleaved trees – Acer pseudoplatanus, Quercus robur, and Robinia pseudoacacia to name three. I find the morphology of this fungus very interesting, as well as its plentiful production of sporophores. Not one that you see in isolation, very often.

As with all saprophytes, it will mineralise (liberate) nutrients locked away with wood, ensuring that a supply of nutrients is put back into the soil for living plants to utilise.

A close-up shot of a few sporophores, which have colonised a Robinia pseudoacacia stump.
The fungus has colonised and is fruiting on exposed wood, free from bark. This is a common theme I have notice with this fungus, in the sense that it doesn’t appear to regularly fruit upon bark-covered substrate.
The jelly-ear fungus (Auricularia auricula-judae)

A great standing (almost) dead oak with deadwood beneath

Old parks are great for mature and veteran trees, both living and dead. Where trees have died, have become unsafe, or even if there is a general lack of deadwood habitat, a tree may be monolithed or left standing with its principal structure left in tact. This is the case with this oak, which for whatever reason has ‘suffered’ (wrong word!) this fate.

In this instance, I am glad to see that the deadwood has not been removed from beneath the tree. In many cases, the standing deadwood will remain but the removed material is transported off site. Ecologically, this is not as desirable as if the deadwood remains beneath the tree. Not only can the nutrients be mineralised back into the soil from where the nutrients were taken up from, but there’s a greater variety of deadwood habitats within such a small space, which may provide habitat for a greater number of species (or simply support more of the same species, as there is a great mass of deadwood). In the second image, we can see how fungi have colonised the fallen branch wood and are releasing nutrients back into the soil.

A rich variety of habitats are provided by this oak, from the fallen branch wood to the standing trunk. At the base, the exposed heartwood may also provide for a niche habitat. Its exposed setting also ensures any insects inside are incubated by the wood, as it warms during the day when the sun shines down upon the structure.
Trametes species have colonised this log, and are releasing essential nutrients back into the soil for use by other plants.
A great standing (almost) dead oak with deadwood beneath

Decay in New York’s street trees – how often and how bad?

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.

To discuss this post, please either post your comment below or head on over to Arbtalk.

Decay in New York’s street trees – how often and how bad?

Victorian Britain and the advent of the ‘stumpery’

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.

To discuss this, please leave a comment below or head over to the discussion thread on Arbtalk.

Victorian Britain and the advent of the ‘stumpery’

Old-growth boreal forests and niche wood-decay ecosystems

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 roseaAgnathosia 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.

This post may either be discussed here within the comment section, or over on Arbtalk.

Old-growth boreal forests and niche wood-decay ecosystems

Fungus spotlight: Piptoporus quercinus (oak polypore)

You may have either not heard of the oak polypore (syn: Buglossoporus pulvinus), or never come across it in the field. That would be because it is very rare in the UK. So rare, in fact, that it is a protected species under Schedule 8 of the Wildlife and Contryside Act 1981 in the UK.

This fungus is principally found in the southern half of England, where it can be seen upon mature and veteran oaks in old growth woodlands and pastures where exposed heartwood is present. However, research suggests that the fungus may be so infrequent that it has suffered from genetic drift (as a result of inbreeding) – only four mating alleles were found across six study sites where the fungus exists. This may be as a result of either the loss of old growth woodland stands and veteran oaks within wood pasture, or because the fungus is a very poor competitor when paired with other fungi – it grows slowly and in narrow range of environmental conditions, and simply is not aggressive enough in its competitive ability when other fungi are present.

Therefore, if we are to assist with the conservation of this rare fungus, we need to be conserving mature, veteran, and ancient oaks, perhaps with exposed heartwood, and also retaining older woodland stands. Perhaps, where there is a lack of a mature or maturing oak population, veteranisation techniques may assist with the creation of viable habitat for the fungus. However, given it’s suspected inbreeding and lack of competitive ability, conservation management may be challenging.

Interestingly, the fungus was supposedly found on a living beech tree (Fagus sylvatica) at Woodstock, England, in 1949. This contrasts with other sources, that states it can only be found on Quercus species.

piptoporus quercinus
These two sporophores were found growing on an ancient oak in Richmond Park, during 2008. Source: Overall (2010).


Cartwright, K. (1951) Polyporus quercinus on Fagus sylvatica. Transactions of the British Mycological Society. 34 (4). pp.604-606.

Crockatt, M. (2008) Ecology of the Rare Oak Polypore Piptoporus Quercinus and the Tooth Fungi Hericium Cirrhatum, H. Coralloides, and H. Erinaceus in the UK. Doctor of Philosophy thesis. Cardiff University.

Crockatt, M., Campbell, A., Allum, L., Ainsworth, A., & Boddy, L. (2010) The rare oak polypore Piptoporus quercinus: Population structure, spore germination and growth. Fungal Ecology. 3 (2). p94-106.

Overall, A., 2010. Fungi Royale: Some interesting larger fungi of the Royal Parks-Part 1. Field Mycology. 11 (3). p101-104.

Rogers Mushrooms. (2016) Buglossoporus pulvinus. [Online] Available at: [Accessed: 24th January 2016].

Wald, P., Crockatt, M., Gray, V. and Boddy, L., 2004. Growth and interspecific interactions of the rare oak polypore Piptoporus quercinus. Mycological Research. 108 (2). p189-197.

Fungus spotlight: Piptoporus quercinus (oak polypore)