Tree health in London – a Forestry Commission event

I had the pleasure of attending the tree health event at Lesnes Abbey Wood, which sits within the outer skirts of London, on Thursday 9th March 2017. Hosted by the Forestry Commission, the purpose of the day was to provide the attendees with some information on the risks that the trees of London may soon be – or already are – subject to. In all, it was a very enjoyable day, and I share below the key findings from two of the earlier speakers.

Talk 1: The Tree Health Resilience Plan – a strategic approach to tree health

Presented by Andrea Deol, who works for DEFRA in their Tree Health Policy Team, this presentation discussed the approach currently being synthesised and undertaken by plant health authorities in Great Britain, with regards to safeguarding tree health against exotic pests.

Andrea first drew our attention to four recent reports released since 2013, with all pertain to tree health to some appreciable degree. Chronologically, these are:

From these reports, in addition to more recent and otherwise perhaps undocumented decisions made, DEFRA’s currently policy extends to 2020 – after which time, the policy will undoubtedly be extensively reviewed. This current policy has three principal arms, as regards to the means of safeguarding Great Britain against pests and diseases of trees. These three aspects of protection are pre-border, at the border and within mainland Great Britain.

Certainly, work before the border is very important, if we are to safeguard our shores from undesirable pests and diseases. For this reason, DEFRA take plant health screening very seriously and the Plant Health Risk Register is reviewed on a monthly basis by governmental ministers. Of course, for organisms that make their way to the border, it is then down to robust border checks to identify the organism, quarantine it and subsequently destroy it. If this fails, then DEFRA’s aim is to identify an outbreak and then reduce the impact of the outbreak by containing it swiftly and effectively. Identification measures principally arise through aerial and ground-based inspections, for which entire Forestry Commission teams also exist. Aerial inspection are, for example, utilised effectively as the means of identifying Phytopthora outbreaks in larch, where crown dieback is a main symptom of infection. For this reason, a formal contingency planning document was very recently released (in February 2017), which outlines how the government would respond to any outbreak event that impacts upon plants and trees in England – and bees. This document is entitled General Contingency Plan for Plant and Bee Health in England.

From this document, DEFRA’s objective is to be better protected against pathogenic organisms that impact upon plants and trees and possess a strong response and associated recovery capability to such potential outbreaks. Their associated vision is, with reference to trees, to build a treescape which is resilient and provides an array of valuable ecosystem services. Granted, such an objective and vision demands an intricate understanding of the threats facing Great Britain’s (specifically England) trees, a robust management team to manage outbreak events, otherwise healthy tree populations free of significant biotic and abiotic stress and also a good amount of species and genetic diversity. In this sense, monocultures are wholly bad and should be avoided, which includes along streets. In fact, clonally-propagated trees (such as London plane and fastigiated varieties of trees, which are so commonly found in urban environments) are a big no-no, as they are utterly diabolical in terms of their genetic diversity (or lack of!).

More on the topic of building a resilient treescape, an outline of what constitutes resilience was detailed. Specifically, this refers to a three-step process: recovery, adaptation and resistance. Beginning with recovery, this entails:

  • the formation of robust contingency plans should an organism arrive on these shores
  • the creation of solid re-planting programmes
  • the adoption and utilisation of good arboricultural and silvicultural practices
  • the responsible surveillance of at-risk areas
  • the synthesis of local, regional and national plans detailing all of the above

As for recovery, this involves:

  • the careful selection of trees being re-planted so that there is a solid diversity between and within the species (i.e. genetic diversity)
  • the sourcing of tree stock with provenance in mind, whereby local sources of seed are to be preferred over foreign imports
  • the increase of total tree cover within Great Britain, which sits at around 10% at present (of which 14% of trees are outside woodlands of over 2ha in size)

Lastly, adaptation demands:

  • the active diversification of tree populations
  • the design of planting schemes at the landscape-scale, which will demand many bodies liaise with one another over on choices and the location of such plantings
  • the effective management of deer populations
  • the allowance for trees to naturally reproduce via sexual means, so that resilience can emerge as a natural consequence of reproductive biology and not through clonal nursery propagation (which carries its own major risks)

Finally, it was stated that DEFRA is currently assessing its priorities, with regards to tree health. Indeed, a formal publication date of this plan is due to be some time during autumn of 2017, though there will be a public consultation prior to that document being formally published. Thus, it may be pertinent to monitor DEFRA’s activity, in anticipation of this draft report to be sent for consultation.

Talk 2: The Arboricultural Association’s Biosecurity Statement

Presented by Keith Sacre of Barcham Trees PLC, the talk began by giving some context to the situation in Great Britain, with an obvious focus on London. This context was sobering, though the risks are very much real and thus must be treated with the respect they deserve.

For example, the potential emergence of the Asian longhorn beetle into London would cause untold damage to a potentially massive number of tree species. Because of its diverse host range and the warmer climate found within the urban zone (courtesy of the urban heat island effect), its virulence is perhaps more marked than if an outbreak occurred in a rural area (as it did in Kent during 2012, where sycamore – an ideal host tree – was the only significant host for the 563 beetles found in the outbreak). Therefore, when noting for London’s very diverse tree population, up to 31% of all of London’s trees could be impacted by this insect pest – a number that equates to 3,800,000 trees, which would have a replacement cost of a staggering £23,000,000,000.

Furthermore, ash dieback, which is presenting itself as a very widespread threat to ash in Great Britain, would – in London alone – kill a possible 374,195 ash trees. Not accounting for the ecosystem services ash provide and looking again solely at replacement costs, it is projected that it would cost £447,345,251 to replace the dead ash. As regards to plane wilt, which currently plagues parts of continental Europe, because London has a huge number of plane trees (9% of the total canopy cover in Inner London!) its emergence and impact would lead to 121,000 plane trees being felled / dying and an attributed replacement cost of £351,623,660.

Speaking more macrocosmically, Keith then alluded to whether it is wise to rely upon governmental institutions for tree health safeguarding. Indeed, as would be expected by such a rhetorical question, his answer was that it is probably not, for the response is generally too slow and reactive. Curiously, the working party for the British Standard 8545:2014 – Trees: from nursery to independence in the landscape – Recommendations, which relates to managing and planting nursery trees in the landscape, did try to put into the document a stipulation that any imported tree stock should always be quarantined before being dispatched to the buyer. However, and quite unfortunately – though not unexpectedly, due to current policy – the pursuit and sustenance of free trade trumps biosecurity, thereby meaning that the quarantine measure had to be revised significantly, else it would effectively be promoting something other than free trade.

On that note, Keith drew us back to the AA’s biosecurity policy, where he remarked that the fact that so many organisations from across the industry have endorsed the policy as being very positive, though did lament the lack of top-down communication from the board level to staff within such endorsing companies (as some individuals didn’t know their organisation had endorsed the document).

As an aside, an important piece of progress in biosecurity within Great Britain was the recent position change of the Royal Horticultural Society on tree imports for the Chelsea Flower Show. Importantly, they are now mandating all of the trees being imported are quarantined before being dispatched to the buyer for use at the show. Certainly, this is great news, though Keith did also detail the concerns that are so very evident regarding the lack of an integrated management plan that spans across land ownerships and incorporates different land owners. Specifically, he noted that whilst Royal Botanic Gardens Kew are managing their oak processionary moth issue effectively, a neighbouring landowner could do nothing whatsoever with their oak trees and thus put Kew at a yearly risk of reinfection. The means of solving such a problem were not addressed, though for the sake of limited time availability I can understand why.

Tree health in London – a Forestry Commission event

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