Back to index

BIOSECURITY, RESILIENCE AND THE TREE NURSERY DILEMMA

By Keith Sacre

2020-02-24

Issues around biosecurity are high on the agenda today, and rightly so.  The introduction of a new pathogen to a population can be devastating.  The UK tree population faces increased threats from a number of pests and diseases present overseas.  One of the dominant strategies for ensuring resilience within the tree population is species diversity.  This creates challenges within the tree nursery sector.  In this article, I am going to explore why an increased demand for native trees is making this more difficult, and some of the alternative approaches which may prove equally effective.

If a new pest or disease is introduced to an area and finds a suitable host tree, it can quickly spread and become established.  If a tree population contains a higher proportion of related specimens, e.g. Acer or Prunus, management can become more challenging.  If the pathogen attack results in felling of the host tree, then a population of trees can be decimated.  Ideally, no more that 10% of a tree population should be of a single species.

This creates a dilemma for the tree nursery. There is increased pressure to import trees to meet increased demand and then for greater biosecurity control on imported trees.  At the same time, there is increased pressure for diversity in the tree population. Reliance on native species does not enable the diversity required and it is not possible to grow the range trees in the UK to achieve true diversity which, it can be argued, fuels imports which, very often will be planted directly into the UK landscape.

The London i-tree study published in late 2015 provided a detailed snapshot of London’s tree population.   The study assessed London’s trees to have an asset value of some £6.2 billion. Few can have visited London without being inspired and sometimes left in awe by the London Plane population, some of which are significant specimens. It is difficult to emphasise the significance to London of this species. The replacement cost of London Plane should the species be lost would be over £350 million, not including the loss of maturity. Yet, hovering in France is a fungal disease called Ceratocystis fimbriata x platani which is lethal to London Plane. Where present the only recourse is to fell, and not to replant with London Plane.

The Asian Longhorn beetle causes devastation where it becomes established. Individual specimens of Asian longhorn beetle have occasionally been found from time to time in the UK.  However, in March 2012, a breeding population was confirmed by Forest Research scientists in the Paddock Wood area near Maidstone in Kent. More than 4700 potential host trees were surveyed and 2166 of these host trees were removed. A total of 66 infested trees were detected, of which only 24 were found by visual inspection, the remaining 42 only being detected after they were felled. Fortunately, this outbreak was detected before the 2012 adult beetle emergence period, which provided time to inspect and deal with infested trees. The London i-tree report estimates that Asian long horn beetle could impact on some 3.8 million trees which represent 31% of the total population. Replacing these trees, were it possible, would cost more than £23 million (that is just supplying replacement standard trees).

I recently visited Russia and my trip took me to the front line of the progress of Emerald Ash Borer. I have seen the aftermath of attacks in the USA but witnessing it as it develops was a real eye opener. I was in a place called Voronezh, which is approximately four hours flying time by plane from London Heathrow. The immediacy of it all was compounded by the presence of a lorry in the city centre with ‘from Preston’ emblazoned on its side.

These two case studies concern pests that have yet to become established in the UK.  Oak processionary moth is, sadly, established in the UK. First identified in West London in 2006, it cannot now be eradicated and containment is the only available strategy. The caterpillar is a defoliator, so a problem for host trees.  However, the real threat is to human health from hairs, produced at the third instar stage of the caterpillar’s development.  These contain an urticating, or irritating, substance called thaumetopoein. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems.  It is not pleasant and results in victims needing hospital treatment.

The story of Ash dieback is well known.  Its impact has become more widespread in the last year or so as more and more Ash trees have succumbed, declining and then dying. What can be stated with certainty is that both Oak Processionary Moth and Ash die back have been imported in to the UK on trees important and then planted directly into the UK landscape. The introduction of Oak Processionary Moth can be traced back to one tree planted in West London which means that someone, an individual or group of individuals, is directly responsible.  This would have been accidental, but the pest is here regardless. 

Xylella fastidiosa is present in mainland Europe. It has a host list of over 150 different species of plants including Olive, and many shrubs including Lavender. Many trees are affected including Plane, Cherry and Maple. Its introduction into the UK is potentially catastrophic.  It could decimate the population of London Plane. The current DEFRA risk register has over 980 separate entries. Many of these entries do not relate to trees, but the message is clear or should be by now. The exact number of trees, particularly larger specimens imported for prestigious landscape schemes, and planted directly into the UK landscape is unknown.  Import data does not reveal any detail, but the number is considerable and can be verified by a walk around almost any new development in the UK. 

It can therefore be stated that Bio-security should be a priority when trees are imported into the UK. As shown above the implications and costs are dramatic but experience suggests that the question of bio security has been largely ignored by practitioners.  Yet from those same practitioners comes the call for more diversity. The demand for a greater number of species to select from which, of course issues a challenge to the nursery industry. This challenge unfolds in many ways.

One of the principle discussion points when considering biosecurity is, ‘why not use more native species and less imported exotics?’ This seems to be a great solution,  but sadly, it does not stand up to scrutiny if the desired outcome is population diversity. There are just 41 listed species which are native to the UK.  Several of these are shrubs and a significant percentage of the tree species are smaller will never make outstanding large specimens in the urban environment.

At Barcham Trees we list over 350 species and cultivars. Still there is a pressure for us to extend that list in the name of diversity. There has been increased demand for Celtis australis and Celtis occidentalis.  Specifiers are looking more favourably at Styphnolobium japonicum. Lagerstroemia indica is becoming a popular choice for urban centres in the south to give just a few examples. These are difficult trees to get started in the UK and to be supplied at an economic price, need to be imported. This presents the dilemma: how can the very justifiable demands for increased biosecurity be reconciled with the demand for diversity? I shall explore this in part 2.

In part one, I explored the threat of introducing tree pests and diseases to the UK poses to the tree population here, and the limitations of using species variation as a main strategy.  In part two, I shall explore the role of biosecurity, the practical challenges it presents and some basic steps buyers can apply to protect UK trees from this threat.

In 2013, I was part of the team drafting the new BS 8545 Trees: From Nursery to Independence in the Landscape.  The words in bold text below were written in the first draft of this document, which went out to consultation in late 2013. The standard was published in 2014 but the above paragraphs were absent in order for it to be applied to the EU. No new standard published in the EU can be seen to inhibit free trade. Many reading this article will be sick to death of me regurgitating this story and the paragraphs highlighted. Yet the problems associated with imported pest and diseases has, if anything, grown, since the above paragraphs were written

Bio-security is an important consideration. To minimize the risk of pests and or diseases being imported directly into the UK, all young trees produced abroad but purchased for transplanting should spend at least one full growing season on a UK nursery and be subjected to a full pest and disease control programme.

 Evidence of this control programme, together with a comprehensive audit trail of when imported trees were received and how long they have been on the nursery, should be obtained from the supplying nursery. The audit trail should extend beyond the nursery after dispatch, allowing for full recall in the event that any pest and/or disease problems manifest themselves in the landscape.’

There seems to be a clamour for action by well- meaning politicians, usually too late, and unidentified, anonymous others from government, who jump onto emotional bandwagons fuelled by often inaccurate and hysterical reporting in the media. Action is called for from hastily gathered teams of advisors and experts who are often divorced from what is happening on the ground. Resources limit what government agencies can achieve, protected zone status has some impact, plant passports are effective but can be doctored and the sheer volume of imported trees coming into the UK is actually unknown.  Individual nurseries in most cases carry on with business, largely, as usual.

Barcham Trees have issued a biosecurity document and the Arboricultural Association have produced a biosecurity statement coupled with a series of videos and biosecurity was the topic of the 2017 AA conference in Exeter with a screened presentation from HRH The Prince of Wales.

But there has been little focus on the end user, the person who specifies and or buys trees.  It is here that the choice is made: which species to use and whether to use directly imported tree stock and plant it directly into the UK landscape.  For all the efforts to grow good stock and a diverse range, poor biosecurity can compromise the process.

So, what can the individual who is purchasing plant material do?

A first stage would be to avoid going directly to a nursery outside of the UK for supply of trees that are going to be planted directly into the UK landscape. A second stage would be to ask a UK nursery supplying trees the following questions and make certain decisions about procurement.

• Have you got a written biosecurity policy?
• How long have the trees been on your nursery
• What pest and disease control programmes have been applied while the trees have been on the nursery?
• Is there a complete documented audit trail which confirms the above and confirms that all legislative plant health requirements have been met?
• Does this audit trail apply to whole batches of trees which may have already been shipped out for planting into the UK landscape where the subsequent emergence of a particular pest and /or disease is identified? Can the whole batch be recalled or destroyed?
• Can I come and see and tag the trees I am ordering at your nursery?
• Do you, as a nursery, make any other provision to ensure that the trees supplied are in optimum physiological health prior to despatch?
• Understand and use the advantages of contract growing if a wider palette is required. Give the nursery time to import and grow the trees that will achieve the diversity required.

It is not the importing of trees which is a problem; it is the importing of trees which are then planted directly into the UK landscape providing the route for imported pest and diseases to escape and then potentially devastate species within that landscape. Surely it is not too much to ask given the implications of carrying on with a ‘business as usual’ approach to ask that the paragraph which did not appear in BS 8545 2014 be implemented on a voluntary basis, by individual businesses, as part of a defined biosecurity policy document. Also, surely, it is not impossible for the nursery to work with the specifier or buyer to produce the ever-increasing range of species required in a bio-secure way

It is salutary to think that an individual or small group of individuals are directly responsible for the importation of Oak processionary moth and the problems associated with it. Will it happen again with Plane Wilt, Asian longhorn beetle or any of the other pests/diseases not yet present in the UK? Possibly it will but would you want it to be you or a group you were associated with to be the ones responsible?

Just think of London without its Plane trees and the choice really is yours. Surely a question each of us should ask is ‘What can I do and how can I modify my current working practices?’

This is an edited version of an article which previously appeared in the ProLandscaper magazine.

Keith Sacre 27.09.18

Other Images