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BIOSECURITY, RESILIENCE AND THE TREE NURSERY DILEMA

By Keith Sacre

2020-04-27

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.