Conference: Plane speaking

It is unlikely that tourists or Londoners consciously notice this green infrastructure and the benefits provided by these and the many other large trees, including London planes, growing throughout London. Would they notice if they were gone?

This is, in part, why The Conservation Foundation’s David Shreeve welcomed nearly 90 attendees to this conference. Attendees included the Forestry Commission’s Sir Harry Studholme, Mathew Frith of London Wildlife Trust, Mark Lane, head gardener of the Royal Household, London-wide Assembly member, Nicky Gavron, tree professionals, stakeholders and, encouragingly, a handful of members of the public.

The Conservation Foundation – the brainchild of David Shreeve and David Bellamy – grew from a desire to increase public interest in elm trees. They then moved on to yews, the mulberry, and now the London plane.

Matt Brown, editor-at-large of the Londonist, chaired the event. Matt, who explores and writes about all aspects of London life, admitted to knowing little about London’s trees. He said: “Most Londoners would be unable to identify this iconic species, even though it grows in garden squares, avenues and down side streets. The more their story is told, the more recognisable they will become, and this will help with some of the challenges these trees face.”

Professor Chris Baines, independent environmentalist and campaigner, said that London planes are easy to spot: “They look diseased with their bark falling off. The London planes we see today, lining The Mall for example, were planted in streets where they could soak up rainwater and (presumably) nutrients from horse manure.”

Since then, he continued: “Our environment has changed beyond all recognition. Somehow these mature tree roots have survived in streets full of underground services. While many services continue below the root zone, it is during the process of change, installing trenching for services in the same zone that tree roots inhabit, that these trees are particularly threatened.”

London planes are particularly susceptible to drought. Massaria is triggered by drought stress and they respond by shedding branches. A principal source of irrigation, Professor Baines believes, comes from night-time mains leaks.

He said: “Solve the leaks and you take this away. Designing valves that purposefully leak between 12 pm and 8 am is one option – it would reduce water pressure build up while also providing the trees with a reliable water source. At the same time it would remove the need to dig up the streets to find the leaks. It would be great to see Thames Water and tree officers working together on this.”

To manage groundwater and retain large canopied London planes, research, joined-up thinking and partnerships between bodies like the Environment Agency and utility companies are required.

It can be done. Trafalgar Place, a new development in Elephant and Castle, has won awards. Root zones identified and incorporated into designs for service trenching at the planning stage have been successfully retained.

“We need to recognise the value of the leaf volume of these big trees. They cool the environment, reduce storm water run-off and enhance the quality of urban life and public health.”
Paul Wood, author of London’s Street Trees: A Field Guide to the Urban Forest and London is a Forest, gave a brief history of the London plane.

A hybrid of American Sycamore and Oriental plane, they ‘hybridised’ in Spain (or Southern France) and became known as the ‘hybrid plane’. While Oriental planes live over 1,000 years, it is still too soon to estimate the lifespan of these hybrids.

Lining some of Europe’s great boulevards, Paris’s Champs Elysees and Barcelona’s Las Ramblas, early London plantings can be dated to 1689 (Barnes) and 1789 (Berkeley Square). Building the embankment, a cover for Bazalgette’s sewers along the north side of the Thames, and topping it with a boulevard planted up with London planes, assured their popularity in the UK.

From 1871 to 1920, the great planting of London ensued and the species became known as the London plane, accounting for (it is thought) up to 60 per cent of street plantings at its peak. 

Nowadays, many mature street plantings are kept on tight pruning regimes, often as pollards, to retain a relatively small crown size in keeping with their surroundings, to limit their water uptake and reduce subsidence risk.

London planes come in many shapes, sizes and cultivars. Seen frequently in parks, ‘Augustine Henry’ has a classic straight trunk and peeling bark. Rarer varieties include the ‘baobab plane’ (squat trunk) and ‘Weeping plane’, though where they came from and which nurseries supplied them needs research.

Greg Packman is now senior tree inspector for Islington Council. During his four years with the Royal Parks, he made 45,000 inspections for Massaria and developed an allergy to London plane pollen.

London plane currently accounts for four per cent of inner London tree populations. Their leaf area, the bit that provides ecosystem services, accounts for 8.9 per cent. According to a recently published Forest Research paper (Ecosystem services delivery by large stature urban trees, P16), London planes provide the most benefits of any mature species in avoided water run-off and pollution removal, and contribute much to carbon storage and sequestration.

“Bigger trees,” he said, “contribute more to ecosystem services. We need to plant more and retain more of those we have.”

Plane trees are managed through pollarding and cyclical crown reductions, the difference being at which height and which part of the canopy is cut.  Pollarding scales back the size of the canopy to fit within the cityscape, and cycles must be maintained annually or every three, five, 10 or 15 years, depending on the objectives.

“A regular crown-reduction cycle lowers a tree’s demand for water. Where large trees could trigger foreseeable subsidence damage, cyclical pruning enables the tree’s retention.

“Britain’s oldest trees are nearly all pollards, think of the 800-year-old oaks growing in Windsor Great Park. The base is old, but by enacting a crown reduction cycle – pollarding young – regrowth on the batons (knuckles) – you are effectively hitting the reset button. The tree redistributes its carbohydrate deposits to the ‘batons’ enabling vigorous (foliage) regrowth. Using smaller cuts, it is easier for the tree to heal and protect itself from future harm. For risk management, cutting every three to five years is beneficial. Climbers performing aerial inspections will look for (among other things) signs of Massaria (Splanchnonema platani). 

Massaria is a species-specific fungal parasite that causes lesions on the upper branch surface. “London plane bark is typically a light olive khaki colour. Masssaria-infected wood is darker and crumbles when touched. In cities, dropping branches pose a high risk.”

Further threats to the London plane include budget cuts: less money to spend on regular pruning cycles (ideally starting as early as 10 years old and not to be confused with drastic crown reductions) leads to lapsed management and accelerated decline. Climate change-induced drought increases Massaria outbreaks and bad pruning – removing all foliage,  especially the nutrient reserves in the baton buds – creates dysfunctional wood.

Current planting trends, favouring less resilient native tree species, plays a part and, in future, public health could influence alternative species selections. The underside of a London plane leaf traps air-polluting micro-particles. The pollen binds itself to diesel fumes and does not disperse well in an urban environment. Increasingly, people are developing allergies to London plane pollen.
Dr Anna Perez, Forest Research’s head of tree health diagnostic and advisory service, warned of two pests and diseases causing plane fatalities across mainland Europe: “Canker stain of plane (Ceratocystis platani) or plane wilt, and Xylella, affect all species. Both are fatal. Early detection and containment is important.”

Canker stain of plane, introduced from the US to Italy in wood-packaging material during World War II (highlighting the need for biosecurity), is a fungal pathogen that colonises and blocks a tree’s vascular system, causing it to weep and dry out. 

In spring and summer a tree will wilt. If infection occurs at the end of the growing season, the following year a tree may not flush. Where buds do emerge, the leaves will form a sparse chlorotic canopy and die quickly. Inner bark exhibits elongated orange purple streaking with ovals or flame-shaped patterns of bluish black to reddish brown discolouration.

Spreading via root contact (through soil, water, debris), avenue plantings are at risk. Of the Canal du Midi’s 42,000 plane trees, 25,000 have been cut down in 12 years. Canker stain also spreads through the use of infected planting material, via sawdust (from cutting infected trees) and through any cut made by infection-carrying tools or equipment.

Bacterial leaf scorch (Xylella fastidiosa) has been described by the European Commission as one of the most dangerous plant bacteria worldwide. Transmitted by insects, mainly the Meadow Spittlebug (Philaenus spumaris), the pathogen affects more than 500 plants (including crops and trees). Bacteria multiply in trees’ vessels, blocking the water supply and effectively causing drought.

In England, Xylella could additionally be transmitted by the Neophilaenus lineatus (grasses and rushes) or Aphrophora alni (bushes and trees).

In Europe, Xylella has been identified as the underlying cause of numerous diseases in cropping fruit trees. Needing temperatures of 25°C or more to survive, it is the causal agent for citrus variegated chlorosis disease, Pierce’s disease of grape vine, and olive quick decline syndrome. In Italy, a million olive trees are dying.

In America, Xylella has been listed as a major disease of street and landscape trees since 2010.  Planes are a potential host.  The latency period is four to five years and symptoms include leaf scorch: wilting foliage with a marginal halo of scorch around the leaf edge.

“What can we do? Do not bring organic material back from abroad. Disinfect all tools and equipment.”

Barbara Milne, Westminster City Council’s head of arboriculture, is chair of the London Tree Officers Association (LTOA), whose members are on the front line in the battle to protect trees from pests and diseases.

For London plane trees, the LTOA (in partnership with Arboricultural Association and Treework Environmental Practice) has produced a practical guide, Massaria: disease of London plane, which is updated as new information comes to light.

“Since 2013, we have worked with the Forestry Commission annually surveying the capital’s plane trees for CSP and Xylella. It is early warning surveillance, important for maintaining the ‘Protected Zone’ status the UK has for canker stain of plane.”

The LTOA has produced guidance notes on CSP and Xylella biosecurity and how to deal with outbreaks should they occur.

Thanking everyone for coming, David Shreeve closed the afternoon saying that he hopes this conference was just the beginning of a longer conversation.

Q&A WITH ALL PANEL MEMBERS

Q. Is there a conflict between conserving architecture and conserving trees? 

A. Yes. It can be a conflict with canopy size. We try to plant the right tree in the right place, recognising that diversity is also important. Trees growing in conservation areas are recognised as being important to that area. Other Inner City areas, for example Regent Street and Pimlico, have never had trees.
 
Q. I have never seen a sapling or seedling of plane, they are grown from hardwood cuttings. Is the lack of genetic diversity a risk?

A. Planes came from nurseries that no longer exist and there is a limited gene pool. Many have come from Hillier Nurseries, who are always looking for new champion trees to propagate from.
 
Q. Is London plane suitable for planting in a changing climate? Those planted in the former Olympic Park are looking very stressed at the moment. How can we avoid high failure rates?

A. We hear much about planting trees, but not much about their aftercare and watering. The Olympic Park wanted to get trees (not saplings) in the ground in a short period of time, with no provision for aftercare. The smaller the tree planted now, the better it will establish itself in the future, but this takes time. Section 106 developer contributions to the planning system could be used for planting and aftercare (establishment).

Q. Many people object to pollards. How can we change this view?

A. Pollarding is an ancient form of management. By starting the regime early in a tree’s life (from as early as year 10) the trees then get used to them and lead a healthier (and longer) life in the long term.
 
Q. People care about what they know. If they cannot identify a London plane, how can they love them? How can you raise the public’s love and engagement with plane trees?

A. There is a history of tree adoption and community groups water them (Lewisham). We are trying to raise awareness with Citizen Science Projects and to communicate in ways people understand, using media and technology they engage with on a daily basis.
 
Q. Is there any way to incentivise canopy planting?

A. We have public health policies, cool city policies, clean air policies. The missing link is translating that into the size of tree canopies needed: making the connection to the role that big trees play.  Small trees are no substitute for planting trees that will become big and new trees are no substitute for long-established trees.