We must all be vigilant about the way that we specify and buy trees if we are to try to stem the influx of unwanted immigrants in the form of tree pests and diseases.
The London i-tree study published in late 2015 provided a detailed snapshot
of London’s tree population and assessed it as having an asset value of some £6.2 billion.
Few can have visited London without being inspired and sometimes awed by the London plane population. Its significance to London cannot be missed. The replacement cost, even were this possible, of London plane, should the species be lost, would be over £350 million.
Yet hovering in France is a fungal disease called Ceratocystis fimbriata x platani which is lethal to London plane. Where it is present, the only recourse is to fell and not replant London plane.
Individual specimens of Asian longhorn beetle have occasionally been found from time to time in the UK, but 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 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 known host species list is extensive and is worth listing here: Acer spp, Aesculus spp, Albizia spp, Alnus spp, Betula spp, Carpinus spp, Cercidiphyllum japonicum, Corylus spp, Fagus spp, Fraxinus spp, Koelreutaria paniculata,Platanus spp, Populus spp, Prunus spp, Robinia pseudoacacia, Salix spp, Sophora spp, Sorbus spp, Quercus palustris, Quercus rubra, Ulmus spp. The London i-tree report estimates that Asian longhorn beetle could impact on some 3.8 million trees which represent 31% of the total population. Replacing these trees, were it possible, would cost in excess of £23 billion.
Unlike the two cases described above, and there are many more including emerald ash borer, pine processionary moth and Xylela fastidiosa, oak processionary moth is already present in the UK. First identified in west London in 2006, it cannot be eradicated and containment is the only available strategy now open to tree managers. The caterpillar is a defoliator but the real threat is to human health. Hairs, which are produced at the third instar stage of the caterpillar’s development, 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.
The story of ash dieback is well known and does not need to be repeated here but it is undeniably present in the UK. The full and potentially devastating implications of its introduction have yet to manifest themselves.
What can be stated with certainty is that both oak processionary moth and ash dieback have been imported to the UK on trees planted directly into the UK landscape. The introduction of oak processionary moth can be traced back to a single tree planted in west London. This means that someone, an individual or group of individuals, is responsible, although probably inadvertently.
The number of trees, particularly large trees, imported for prestigious landscape schemes and planted directly into the UK landscape, is unknown with government figures not revealing any detail, but it 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 is largely ignored.
‘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.’
These words were written in the first draft of BS 8545 Trees: From Nursery to Independence in the Landscape which went out to consultation in late 2013. The standard was published in 2014 but the above paragraphs were absent. No new standard published in the EU can be seen to inhibit free trade. Yet the problems associated with imported pest and diseases have, if anything, grown, since the above paragraphs were written.
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.
There has been little focus on the end user, the person who actually specifies and or buys the trees, but it is here that the choice is made, whether to use imported tree stock transplanted directly into the UK landscape, and perhaps where the greatest impact on bio-security can be made. Surely it is up to every user of trees to look in the mirror and consider their own working practices with regard to biosecurity and then act in a responsible fashion?
So what can the individual purchasing plant material do?
A first stage would be to avoid going directly to a nursery outside 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:
- Do you have a written biosecurity policy?
- How long have the trees been at your nursery?
- What pest and disease control programmes have been applied while the trees have been at 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?
To provide the range of species, cultivars, and size of trees required by the UK landscape market, trees will always have to be imported and it is unrealistic to suggest otherwise. But how many trees are bought from UK nurseries which have been at that nursery no longer than it takes to transfer the trees from one lorry to another?
It is not the importing of trees that is the problem. It is the importing of trees which are then planted directly into the UK landscape and so provide the route for imported pests and diseases to escape and then potentially devastate particular species within that landscape. Surely it is not too much to ask, given the implications of carrying on with a ‘business as usual’ approach, that the paragraph which did not appear in BS 8545 be implemented on a voluntary basis, by individual businesses, as part of a defined bio-security policy document.
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. The choice really is yours.
Keith Sacre is sales director of Barcham Trees. He is also vice chairman of the Arboricultural Association and a trustee of the Trees and Design Action Group.