Lured by the opportunity of a trip billed for those with “The Explorers Mindset,” intrepid tree officers and commercial arborists made the short journey to central region of Holland in June 2023. 

Our host was Henry Kuppen, a Tree Ambassador with decades of entrepreneurial, public policy and research experience who has also collaborated on many UK projects. As with many Arb Association promoted events, the mix of people from varying backgrounds lead to frequent off-plan stops to see cool trees and thorough discussions lasting well into the night. Our destination is at the same latitude as London (51.4° N), and has similar canopy cover of about 20% and high urban population densities as in England, this trip was an excellent opportunity to learn how our neighbours respond to many of the same tree issues we face. Of course, the first noted difference was a sign at Schiphol Amsterdam airport: 'Welcome Below Sea level'.

Focusing on ash dieback, oak processionary moth and sooty bark disease, we saw that landscape-scale changes in tree composition are occurring. A central theme soon emerged: the arb industry is risk based but it should be more biodiversity driven. This is not a simple dichotomy of minimising liability vs conserving dendro-microhabitats. Rather, a paradigm shift is finally underway as arborists educate the public, decision makers and other industries about Tree Time, reversing 'plant blindness' and highlighting our interdependency on ecologically managed treescapes to tackle climate change and pollution and meet social needs. Diversifying tree planting with a variety of native and exotic tree species, rewilding urban and rural areas, better managing soils, growing flower-rich grasslands, and leaving more standing and lying deadwood are just some examples of easy-to-achieve biodiversity boosts. 

The list of mortal tree woes and tree related risks could be longer by including emerald ash borer, conifer red band needle blight and many more. But when pests directly impact on human health, the stakes are even higher. Here we briefly present some of the innovations and nature-based solutions we encountered.

Beech-lined bicycle avenue. (Kevyn Wightman)

Classic and New Street Plantings

Tree-lined avenues, including those exclusively for bicycles, were plentiful and picturesque. Unfortunately, sometimes historically motivated but even in the recent decades, they were mostly planted with a single species. There is a Dutch fascination with a certain romantic landscape ideal: ash-, lime-, oak-, or poplarlined avenues or wind breaks on field edges. Although beautiful in the paints of Dutch old masters - and equally Van Gogh’s willow tree paintings - these tree monocultures can also succumb more readily to pests and diseases. Housing estates were planted with only a handful of species (or with nursery stock of limited genotypic diversity) and the repercussions are now obvious.

Street tree planting in Ede. (Oliver Stutter)

Since the mid-1980s urban tree pit designs using Amsterdam Sands have been evaluated and promoted across Europe and biochar is now promoted as valuable practice. We admired a well-planned and maintained street planting in the town of Ede. The trees clearly were part of a green infrastructure project for improved storm water infiltration. The mixed underplantings were not only attractive but also bee friendly. In the last decade there has been more awareness for 'wild verges' (aka 'no mow verges'), uncut grass circles at the base of trees, and grassy meadows within parks. This is developing rapidly as the norm in The Netherlands for wildlife, and obviously helps avoid the huge scrouge of mower damage. In the UK, these ideas are only now gaining national traction and public acceptance as people embrace untidy nature' as a force for good.

Kentucky coffee tree, Gymnocladus dioicus, with an excellent crown display, in Ebben Nursery at dusk. (Oliver Stutter)

We visited Ebben Tree Nursery -- the largest in Holland with over 500 ha of trees that are delivered to 28 countries. Years ago, they introduced multi-stemmed trees which are frankly much more interesting than the traditional lollipop on a trunk. Moreover, the varied canopy sizes and shapes increase people’s interactions and ultimately appreciation of trees. This matters especially in urban settings where the main contact people have to nature may just be a single tree or two. This year Ebben has tens of thousands of these appealing trees in all kinds of species. We really appreciated the great BBQ on the rooftop garden with Ebben staff. During the twilight tour with Marko Mouwen, sales manager, the array of tree species and trained crown shapes was inspiring.


Ash dieback

Originating in Asia now well established in Holland and the UK, Hymenoscyphus fraxineus, is easily recognised as crown vitality declines. Roel Geerts, an ash die back (ADB) expert, helped develop the Dutch ADB national protocol with the University of Wageningen. Trend analyses are based on surveys of 13,000 trees between 2018 and 2022 using the following factors: amount of decay in young shoots, degree of net defoliation loss (light, moderate or severe) and size of dead branches (none, 10cm). For Wendy Batenburg (PhD and with Terra Nostra like Henry), the trend analysis is sobering: disease severity is rising exponentially. The percentage of trees affected by ADB is now at 60% in Groningen Region and similar in other parts of The Netherlands. Since 2018 there has been an increase in defoliation, dieback and the diameter of dead branches. Currently 64% of infected trees are producing dead branches and 20% of them need to be pruned.

The short wood fibres of ash equate to particularly brittle limbs prone to snap outs. Trees in high target areas are a concern - and climbing is not an option for severely affected trees. The costs for private landowners, housing associations and municipalities mount as many trees require pruning, felling and replacing. In The Netherlands, central government does not offer grants to manage ADB, although in the UK there are limited funds for some costs including restocking. 


Left picture: Five-year-old Manchurian ash, Fraxinus mandshurica, exibits exceptional growth on a naturalised verge. (Kevyn Wightman) 

Right picture: An ADB-infected twig. (Kevyn Wightman)

Some ash genotypes will exhibit strong genetic tolerance and resiliency, but their analysis also showed that susceptibility to and recovery from ADB is affected by environmental and site conditions. As the climate extremes of drought, high temperatures and other pest outbreaks become more frequent, the decline in tree condition especially in urban areas may accelerate.


Oak processionary moth

Originating from central and southern Europe, Thaumetopoea processionea L. lays eggs that can survive temperatures below -18°C but the larvae in nests will not survive above 32°C. Oak processionary moth (OPM) caterpillars feed on all different Quercus species causing defoliation in some cases and can lead to lower crown vitality in heavily infested areas, but their impact on human health is the main reason for managing them. OPM has a lifecycle of one year and eggs overwinter in the upper crown. Larvae emerge in the early spring as tree leaves start to grow. During the fourth larval stage (about 45 days after hatching), they develop urticating hairs, and the first nests become visible. They continue to feed in the tree canopy, forming processions as they inch their way along the trunk. Occasional soil nests, often wrapped in the grass at the base of a tree, can lead to a different life cycle which disrupts management practices. The Dutch national OPM protocol classifies the nests by size: ping-pong ball, tennis ball, football, or blanket - the latter contains thousands of caterpillars and can be a daunting sight especially if your garden is nearby. 

A voluntary network of OPM specialists, Kenniscentrum Eikenprocessierups, was established in 2012 to better understand OPM life cycles, food preferences, management options and predator behaviours. Henry Kuppen has built a screened insect station -a greenhouse with young oak trees to record OPM behaviour. The information is analysed in real time, showing when populations are surging, but it also helps predict outbreaks over time. Kenniscentrum Eikenprocessierups informs the general public about OPM through various e-media like Nature Today. Now 33,000 people regularly consult the information generated and central government provides some funding for this research. Online maps show affected trees and treatment methods employed across the country. Municipalities also survey oaks and use signs and red warning tape on individual trees as vivid warnings.

In 2019, there was a peak OPM outbreak and people had to curtail their garden activities and take precautions in infested areas. Populations vary from year to year and may be linked to weather, management and population dynamics. As with all integrated pest management approaches, the goal is to understand the life cycles of the pest and its enemies in order to control and balance because they will never be completely eradicated.

Football-size OPM nest. (Henry Kuppen)

Some municipalities chose to remove nests mechanically with the operative on a cherry-picker using a vacuum and, of course, PPE including a helmet with a respirator. This is costly, not least due to the disposal of the nests in special incinerators reaching at least 600°C. Using a blow torch to kill nests is never recommended because it is not hot enough and only blows the irritating hairs around, even if the caterpillars die.

The least preferred control method is spraying with the naturally occurring soil-based bacteria Bacillus thuringiensis or with a nematode solution. The main problem is these are not selective and can affect innocuous Lepidoptera. The government will probably ban these methods in the future.

Natural predators include the egg-parasitic Chalcid wasps (Anastatus bifasciatus) and Carcelia iliaca and Tachinid 'sleeping' flies. Both the wasps and flies rely on flower-rich grasslands and have a preference for cow parsley and ground elder flower nectar as a late-summer food source. These of course, are the characteristic microflora of oak forests and can be easily managed as no-mow verges. A great finding this year at the insect station was that the Carcelia hatched exactly at the same day as OPM, showing that predator/pest relationships are dynamic. 


Left picture: Tiny white eggs of parasitic Tachnidae on an OPM caterpillar. (Henry Kuppen)

Right picture: OPM egg plaque on a young twig. (Simon Rogers)

We watched an OPM caterpillar being taunted by a fly as it deposited one single egg on one single hair. In fact, most OPM nests contain these parasites and leaving the nests in situ helps increase the predator populations. Providing nest boxes and water sources for bats and birds such as nuthatches, blue tits, great tits and wood peckers has also shown positive results. These are also easy projects to involve residents and help them connect to nature.

A group of researchers is working with Terra Nostra to develop a unique OPM mating disruption system with trials both in The Netherlands and with collaborators in UK and in Germany. Synthetic disrupting male OPM pheromone paint balls made with a plant wax are shot into trees using a 'phero-launcher', aka paintball gun. After one application in the first year, 85% fewer male moths were present. This approach is different than just using conventional pheromone traps because traps attract male moths for monitoring purpose. Mating disruption results in fewer fertilised females, which should lead to fewer fertilised eggs. 


Sooty Bark Disease

We spent a morning in Flevoland, an area 3 meters below sea level which was taken from the sea and reclaimed by dyke building. Over 4,000 ha were reforested in the 1950s in even-aged monocultures including cherry, poplar, ash, and sycamore. New plantings are mixed species but mostly natural succession is the driver as the woodlands mature.  

Vincent Troost of Staatsbosbeheer, the Dutch Forestry Commission, manages the forest which is dedicated equally to recreation, wood industry and ecology - including rewilding. It is no small task to achieve these far-reaching aims. Some areas are completely fenced off and diversion signs abound, even for the ubiquitous bicyclists.

While the woodland is developing well, especially as many good soils were planted, a lethal fungus is active here and across The Netherlands, Belgium, Germany and the UK. As noted in Henry's summer 2021 article for the Arb Magazine, 'Sooty bark disease can no longer be ignored'.” Cryptostroma corticale is a fungal disease that affects 14 species of Acer by entering the cell wall structures and growing from inside the stem. Due to its latent presence, the fungus is successful when an Acer is in decline because of drought stress. It spreads enormous amounts of spores which easily enter trees wounded by pruning or storm damage.


Left picture: Sycamore, Acer pseudoplatanus, dead after just one year form sooty bark disease. (Kevyn Wightman)

Right picture: White poplar, only 50 years old growing on land reclaimed from the sea. (Oliver Stutter)

Sooty bark disease (SBD), also known as bark stripper lung disease is characterised by black spots that easily rub onto your hands. It was probably introduced to Europe from North America by imports during the Second World War. Affected areas should not be touched without gloves and face mask because it is a respiratory tract irritant causing chronic pneumonitis, mainly in tree workers. 

It can sometimes be confused with other fungi which cause black discolouration on the trunk such as strip canker on beech Eutypa spinosa or Fusarium or Phytophtera. Black bleeding or cankers is not typically associated with SBD.

If the disease is suspected, lab testing of a sample is the best way to diagnose its presence and determine the scale of probable  tree mortality and associated costs. Vincent fells infected trees which have 60% or less live crown. The Dutch national protocol recommends that infected trees are not climbed and that cut logs remain in situ for three years when practical. When the risk of spreading is high, under strictly controlled conditions, tree workers with specialist PPE including respirators, fell and chip infected trees using chippers with sprayers and P3 filters. In remote areas burning is allowed when permitted by local authorities. 


Picture left: Van Gogh is omni-present - even on the cows at our farm guesthouse. (Kevyn Wightman)

Picture right: Pleached silver maple trees with dancing wooden shoes. (Oliver Stutter) 

Back on this side of the Channel (UK), SBD has recently been found on Jersey and in London at Dulwich and Chiselhurst Common in Bromley. Due to its latent presence within the tree, outbreaks are triggered by dry hot conditions. The disease rapidly affects Acer trees that are already in decline due to drought stress.



This study tour was a refreshing opportunity to meet new people and share ideas, as well as several good meals. We take to heart what the 19th-century American writer, entrepreneur and pillar of American literature Mark Twain said, “Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime”.

We send a shout-out to colleagues who have the explorer’s mindset: Make sure you bok onto the next tour in 2024!



This article is been written by Kevyn Wightman and Henry Kuppen and published in ARB magazine Winter 2023 edition of the Arboricultural Association, United Kingdom.

Kevyn Wightman, PhD, Tree Officer with the London Borough of Bromley, has extensive research and training experience in Central American reforestation.

Henry Kuppen, Tree Ambassador and former director of Terra Nostra, Knowlegde Center for Trees and Solis, works closely with UK colleagues on OPM. He has presented at Arboricultural Association conferences and taken part in podcasts on pleached trees and much more. 

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