Preparing for the future of London’s urban forest
James Chambers
1. Plane tree with extensive massaria colonisation throughout the crown.
London plane trees are a striking visual component of many streets, parks and garden squares, providing great visual amenity. They also improve air quality, sequester carbon, provide shade and cooling, storm water attenuation, habitat and landscape screening, and they contribute to high property values.
They are outstanding large urban trees, unique in stature and appearance – a common sight in central London and many other cities around the world. As has rightly been said, there really is nothing quite like a plane tree.
The species makes a very significant contribution to ecosystem services in central London, and it could be said that it is over-relied upon. For example, although the capital’s i-Tree report states that London planes constitute a relatively low percentage of the tree population (1.43%), for inner London they are responsible for sequestering 6% of the total carbon captured by trees, more than twice the percentage held in the next largest carbon-sequestering species (sycamore).1
The i-Tree report also confirms that London planes’ contribution to canopy cover across Greater London is very significant. The species’ population is approximately 121,000 (1.43% of all trees), but planes provide 8.9% of the total leaf area – the highest percentage for any species and therefore the most associated benefits.1 The variance between the number of plane trees and their ecosystem service provision is much higher than for any other species included in the report.
The species is also very resilient, with hard, strong wood which is able to withstand very significant pruning and produce healthy new growth the following season. This may have been another factor in its popularity as pollarding and crown reduction management regimes became common in city streets where so many of these trees had been planted.
London planes are also known for being capable of compartmentalising decay well (when compared to other species), which has provided them with a degree of resistance to weakening of structural condition through fungal colonisation.
However, in recent years this ‘bulletproof’ status has been repeatedly challenged by numerous new threats. The potential effects and consequences of three of them are briefly outlined below. Other significant threats are available.
These threats must be taken into account when we consider species choice for urban tree planting across London. The importance of maintaining canopy cover in the urban environment is well understood, and the i-Tree report indicates the need to increase the diversity of large tree species in order to increase the resilience of the tree population as a whole.1 Any of the threats may be exacerbated by secondary issues like powdery mildew and anthracnose, which otherwise may be of little consequence.
How would Londoners react to the sudden loss of entire avenues of large trees across the city? Many well-used streets (e.g. the Embankment) would be dramatically altered, suddenly changed as the tall, mottled stems and green canopies disappeared, leaving grey, exposed buildings and harsh sunlight. If this happened, the tall-treed environment would obviously take decades to restore.
It is clear that the loss of this species would have a significant negative impact on the environment and ecosystem service provision at a time when environmental concerns are understandably a very high priority due to the ever-increasing impacts of climate change.
Massaria Splanchnonema platani
Anyone who has been managing London plane trees over the past 20 years or so will be aware of the problems associated with massaria and the increased likelihood of branch failure to which it contributes.2 This latent, endophytic fungi can rapidly colonise and weaken branches, making branch failure more likely.
Repeated pruning involving the removal of more and more branches obviously has negative impacts both on the structural condition of the affected tree (an increase in wounds and decay entry points) and its photosynthetic capacity. This problem is becoming a significant issue in itself on trees where multiple branches have been removed over the years.
Plane trees have always been considered to be much more resistant to decay from Inonotus hispidus (shaggy bracket) than other tree species, particularly when compared to ash, for example.3 However, the recent development of massaria in the UK has led to the combination of both these fungi presenting an elevated likelihood of branch failure in plane trees. This is because the two types of decay result in damaged wood on the top of the branch (soft-rot from massaria) and hollowing of the centre (white-rot from I. hispidus) which weakens branches significantly more than either fungus would independently, and leads to an increase in branch failures as a result.
In East London there is a popular public park with a great many early-mature plane trees, virtually all of which have been significantly impacted by massaria. (They are apparently in poor growing conditions, shallow rooting in made ground etc.) The elevated risk of harm due to the frequent occupation of the area where the trees are growing has resulted in repeated pruning to remove affected limbs, leaving disfigured crowns with multiple wounds and coalesced decay, and loss of amenity value to some extent. Several of the trees have been removed as a result, and it is inevitable that more will also have to be replaced. It is highly likely that this is the case in other locations too.
Additionally, as local authorities and other landowners have struggled with a lack of resources for tree pruning, there are many locations where the removal of massaria affected branches has not occurred regularly. I have noted several locations where the physiological and structural condition of trees affected by massaria has significantly declined as the fungal colonisation has developed throughout the crown and into main scaffold limbs.
Two of the locations where I’ve observed plane trees which have been significantly affected by massaria but no remedial pruning has been carried out are in East London and Hertfordshire.
The physiological condition of trees in both locations indicates that they are not in ideal growing conditions, and all have been significantly affected by massaria for some years. Over approximately five years, large scaffold limbs have died, with the typical indicators of massaria colonisation (bark loss, cinnamon colouring of branches etc.) having developed throughout the crowns of trees in both locations, resulting in virtually complete crown death in several early-mature trees. Trees in both locations have been ‘topped’ or ‘monolithed’ over the past couple of years (Figs 2 and 3). It looks likely that these trees will require removal and replacement at some point soon, and there are many other locations where a similar situation exists in the plane tree population.
2. An East London location with plane trees ‘topped’ following crown death with massaria. (Bo Chapman)
3. A street in Hertfordshire: plane tree crown death with massaria, and trees being ‘monolithed’ due to safety concerns. Figure 1 shows some of the trees before they were cut. All the other planes along the street are also significantly impacted by massaria.
4. The first Neofusicoccum parvum lesions I observed in 2017.
Plane canker Neofusicoccum parvum
The first cases of Neofusicoccum parvum identified on plane trees in the UK were those I found in central London during 2017 (Fig. 4). These cases were reported and samples sent to Forest Research, who identified the fungal species and found the existing research from elsewhere in Europe.
One of the affected trees was removed a year later due to very significant physiological decline. Interestingly, the neighbouring plane, approximately 3 m from the tree which was removed, was also confirmed to have been colonised by N. parvum, with identical lesions, but it remains in apparently healthy condition to this day. Furthermore, all but two of the plane trees in that location have since produced visually identical lesions on stems and around their crowns.
Because I’ve been aware of this issue, naturally I’ve been looking for N. parvum symptoms whenever I’ve seen plane trees on my travels since 2017. I’ve observed the lesions everywhere I’ve looked in plane trees all around the UK – although to be clear, not in every plane tree by any means.
5. A large plane tree felled with N. parvum. Note the dead cambium and discolouration associated with the early-stage lesion.
6. Lesions appeared in many trees during the winter of 2021/22.
7. Central London. Lesions and crown dieback associated with N. parvum in September 2022 (left) and the same tree in October 2024 (right – image by Colleen O’Sullivan).
What is known about this fungus?
This fungus has a wide range of hosts including grapevines, Araucaria heterophylla, Sequoiadendron giganteum, Eucalyptus species, Fraxinus ornus and an estimated 300 other species so far. It is in the Botryosphaeriaceae family, and causes canker and dieback in affected plants, including plane trees. ‘Fungi in the Botryosphaeriaceae are some of the most widespread tree pathogens across the globe, causing stem cankers, shoot tip and branch dieback, blue stain, fruit rot and whole plant death … [They are] endophytic, only becoming pathogenic when trees are stressed, e.g. by drought, mechanical damage, frost and water-logging.’4
Symptoms initially consist of vertical bark lesions which occasionally exude small quantities of sap. These lesions differ from those caused by Ceratocystis platani (canker stain of plane, see below) as they very quickly stimulate callus growth around the wound. Trees which are further affected may show significant dieback in the crown.
A study in Geneva between 2011 and 2013, investigated the cause of decline in six plane trees in four locations. C. platani was suspected to be the cause. However, that fungus was only found in two of the trees, all of which later died. Several other fungi were isolated in the trees, including N. parvum. Further testing involved deliberate inoculation of saplings with those fungi, and the results of these tests confirmed that N. parvum is pathogenic on plane trees: ‘This survey showed that most cases of plane tree dieback in the Geneva region were not caused by C. platani and that other fungi could be responsible for similar symptoms.’5
Research has continued since the initial discovery in the UK. I’ve sent a considerable number of samples from similar lesions on other plane trees across London to Forest Research over the years. While there have been multiple confirmed cases, the majority of the samples have provided negative results for N. parvum, despite the visual symptoms (lesions) and sampling technique (extraction of discoloured wood from the affected area) being identical.
Worryingly, multiple new lesions believed to be due toN. parvum developed on a great many plane trees across various sites in central and west London during the winter of 2021/22, and were observed during February 2022 by me and several colleagues with whom I had previously discussed this fungus (Figs 6 and 7).
I am aware of two trees that were confirmed to have been colonised by N. parvum which have shown unambiguous crown symptoms. These are both in central London (including the tree removed in 2018), in different locations. The second tree is shown in Figure 7. As the fungus does not appear to cause decay in the short term and this tree did not pose an unacceptable risk of harm to the public, a decision to adopt a ‘wait and see’ approach was considered appropriate. On this occasion the tree began to recover the following spring and happily appeared to have made a full recovery by 2024.
The key issue here, I think, is that N. parvum is a fungus not known to affect plane trees in the UK before 2017, but visual symptoms (vertical lesions) are now present in a very significant proportion of the UK plane tree population. The cause of this development has not been determined, but again it seems likely to be related to climate change, and that factor is highly unlikely to improve in the near future. The potential threat to the longevity of the UK plane tree population from this fungus is unclear, but given the rapid transition from ‘unknown’ to ‘everywhere’ in less than a decade, it would be unwise to ignore it.
8. Plane tree affected by C. platani in Padua, Italy (London Tree Officers Association). A typical canker on a stem (Forest Research). An educational sketch by Lucio Montecchio, University of Padua.
Canker stain of plane Ceratocystis platani
Canker stain of plane is a disease caused by the ascomycete fungus Ceratocystis platani, which causes severe chlorosis and wilting of leaves, staining of xylem vessels and death of affected trees, usually within two to five years (Fig. 8).6, 7
Canker stain spread rapidly through France into Italy and Switzerland, and more recently there have been numerous cases identified in southern France and Greece, resulting in the loss of many trees. It has not yet been identified in the UK, but it has resulted in the felling of thousands of plane trees in Italy and France over the past few years.8
The disease is highly communicable because the spores are quite hardy and can survive and remain active for some time. Trees can become infected by spores entering through bark wounds, pruning cuts, insect or bird damage, roots, tools, scuffs from machinery or equipment, and even via arborists’ ropes and clothing.
It has been determined that the most significant cause of the spread of this disease is human activity.9 C. platani spores are easily transported, which means that a significant possibility of accidental introduction exists, by tree care professionals or even members of the public travelling between countries where the disease is present and the UK.
It logically follows that a significant threat to the UK plane tree population exists, because of the high potential for the accidental introduction of the disease, its virulence and the ease with which it can be spread.
Plane tree clones and variable susceptibility to pathogens
As you are no doubt aware, there are ‘a number’ of different plane clones which were planted in varying numbers in different areas of the UK. These clones are visually similar with only slight differences in either leaf shape or size, bark colour/pattern, trunk taper etc. to distinguish them, which is particularly difficult when we are likely to be faced with only one or two examples for comparison at any one time.
I.M. Chengappa has produced a website which contains lots of useful material about London plane trees, including some information on approximately 40 variations, though some are considered exceptionally rare.10
London planes are hybrids, which are necessarily clones, and this raises concerns about the lack of genetic variability across the population. There is a heightened potential for a particular pathogen to severely impact both clonal variants and the species as a result.
I have also observed evidence of a variance in susceptibility between different plane clones, as Forest Research has previously noted with anthracnose.11 As mentioned above, at the location where I first identified N. parvum, there are 20 plane trees, two of which have slightly different bark and leaf shape to the others. These two are barely affected by either massaria or plane canker, while all the other trees clearly are.
No information on which clones are present at this site is currently available, but this does suggest that further research to both identify plane clones and investigate their susceptibility to various pathogens would be valuable.
8. Plane tree affected by C. platani in Padua, Italy (London Tree Officers Association). A typical canker on a stem (Forest Research). An educational sketch by Lucio Montecchio, University of Padua.
What next?
What can we do to maintain the plane tree population we have and diversify for the future?
Identify, record and monitor plane trees with visual symptoms of N. parvum, send samples to Forest Research (sterilising tools in between samples, of course!) and update your tree inventories etc. This will help keep track of the problem and provide more information about trees which may become more seriously affected.
Develop a better understanding of plane clones. Are you familiar with different clones and can you confidently identify them? This may be useful in selecting resistant clones for planting in some areas. Maybe start a plane tree clone identification working party?
Select alternative large canopy species for planting in order to increase diversity in the urban forest. No one is talking about ‘replacing’ plane trees (there is nothing like a plane!) but if and when they have to be removed, what species might provide similar ecosystem service values in the future? For example, I’ve had to arrange the removal of three large plane trees in central London over the last decade and have replanted with Pinus nigra (black pine), Liriodendron tulipfera (tulip tree) and Zelkova serrata (Japanese zelkova). Conifers are certainly underrepresented in London streets and perhaps their high ecosystem service provision is not properly recognised.12 What alternative tree species do you think would work?
This is an abridged version of my Dip ARB L6 (ABC) Independent Research Project – special thanks to Dave Dowson – TreeLife – for coming out of retirement to mark my long overdue submission!
James Chambers has over 25 years’ experience in arboriculture, from climbing arborist to tree officer, urban forest manager to principal consultant at TMA. A veteran tree specialist qualified to VETcert consulting level, he is also a VETcert trainer for the Ancient Tree Forum (ATF). James volunteers for Forest Research, identifying, collecting and submitting samples of several novel pests and diseases.
References
1 Treeconomics (2015). Valuing London’s Urban Forest: Results of the London i-Tree Eco Project. www.treeconomics.co.uk/wp-content/uploads/2018/08/London-i-Tree-Report.pdf
2 London Tree Officers Association [LTOA](2013). Massaria Disease of Plane: Practical Management Guidance. www.ltoa.org.uk/massaria-disease-of-plane
3 www.tma-fungi.co.uk/44.html
4 Boddy, L. (2021). Fungi and Trees: Their Complex Relationships, p. 106. Arboricultural Association.
5 Pelleteret, P., Crovadore, J., Cochard, B., Pasche, S., Bovigny, P.-Y., Chablais, R., & Lefort, F. (2017). Urban London plane tree dieback linked to fungi in the Botryosphaeriaceae. Urban Forestry & Urban Greening 22: 74–83. www.doi.org/10.1016/j.ufug.2017.01.014
6 Montecchio, L.; 2nd edition revised and updated by J. Parker and N. Fay (2016). Detecting and Identifying Canker Stain of Plane. Treework Environmental Practice, London Tree Officers Association, De Rebus Plantarum/University of Padua.
7 Defra (2020). Supplementary data on Ceratocystis platani. www.planthealthportal.defra.gov.uk/assets/uploads/Summary-assessment-C-platani-2020.pdf
8 France Today (2012). www.francetoday.com/learn/history/trees_in_trouble/
9 Forest Research. Canker stain of plane (Ceratocystis platani). www.bit.ly/49MimNM
10 Chengappa, I.M. Plane trees in London. www.aranya.co.uk/planes/index.html
11 Forest Research (2008). Problems on plane trees. Pathology Advisory Note 7. www.bit.ly/4sN3tn1
12 Hirons, A.D., & Sjöman, H. (2019). Tree Species Selection for Green Infrastructure: A Guide for Specifiers. Trees & Design Action Group. www.tdag.org.uk
All websites accessed 20/1/26.
This article was taken from Issue 212 Spring 2026 of the ARB Magazine, which is available to view free to members by simply logging in to the website and viewing your profile area.