Lion’s-tailing and topping trees are common practices in Groningen yet discredited everywhere by tree experts

So why are they still frequently practiced here by our own municipal tree workers especially when such techniques lead to the premature decline of trees?

This is an open letter to the Director of the Stadsbeheer (city management), the Bomenploeg (tree club), and the tree bureaucrats of the municipality of Groningen,

Kristin McGee, Chair Boomwachters Groningen

August 8, 2022

I’ll begin with a question for the director of the Stadsbeheer:

Are lions tailing (hollowing out a tree’s crown) and topping large-stature trees practiced in Groningen and Haren and if so, how often and why?

The Boomwachters Groningen has written multiple times about dangerous pruning techniques such as over-raising, topping, and especially lion’s-tailing trees, or hollowing out the crown of trees. Yet once again, it appears that the contracted companies for tree care in municipality practice these techniques as their standard procedures, even as all accredited tree pruning programs and handbooks reveal the inadequacies of such techniques (see sources below most often used in both Dutch and international training programs).

Lion’s-tailing is the process of removing all of the branches in the inner crown which produces a dangerous tree because it increases weight at the ends of branches, the only place where foliage is allowed to remain. This lack of foliage elsewhere stimulates extra growth at the tips, which become wider and heavier, and therefore more dangerous in high winds. The weight on these branches is redistributed from once having been evenly distributed throughout the tree’s well-balanced crown to the upper ends of the tree where these heavy branches can and do break easily in high winds.

Further, hollowing out a crown will prevent the tree’s foliage and architecture from engaging in ‘mass damping’, a natural and complex process which allows weight during winds to be evenly distributed throughout a tree’s branches and leaves, and therefore the force of that wind is dissipated. In fact, many tree workers will wrongly tell you that hollowing out the inner crown reduces wind pressure by allowing wind to travel through these large gaps, and thus they assume this makes the tree safer and more resistant. The opposite is true and is studied by Dr. Ken James at the University of Melbourne.

For an excellent explanation of the occurrence of ‘mass damping‘ in trees, an important wind dissipating process which is forestalled when trees are lion’s-tailed, see this webinar at the section (at 5:00):

Tree Biology and Pruning by USU Extension Forestry Webinar by Dr. Randy Miller

Lion’s-tailing is almost as bad as topping as it is ultimately limits a tree’s life-giving resources. This is because the tree no longer has the capability to sufficiently feed itself with so much foliage removed. A lion’s-tail tree will suffer from many serious defects including:

  • Structural instability of the tree’s architecture
  • Hunger from a lack of food resources (because there are too few leaves near the trunk to engage in photosynthesis and produce carbohydrates)
  • Sun scald on the trunk from a lack of shade which produces larges cracks
  • Breakage of large, top-heavy branches where extra growth is stimulated to compensate for an over-thinning of the inner crown
  • Lack of nesting places for birds as there are too few intersecting branches for nest building
  • Increased heat because of reduced shade
  • Susceptibility to pests and disease from many pruning wounds
  • Decreased CO2 mitigation
  • Decreased pollution filtration
  • Decreased oxygen production
  • Severely decreased life-expectancy from the tree because of its slow deterioration after over-pruning in the critical inner crown
  • An unattractive bare tree

Meeting requested on-site

We request a meeting with the persons responsible for assigning work to the contracted tree services, to the director of the “stadsbeheer”, to the alderperson for green and infrastructure, and to the head of the bomenploeg (tree club) to consult with us and two of our tree experts (both with PhD in trees, ecology, and urban forestry).

Petition ignored

We represent more than 3,000 residents in Groningen and Haren who have complained about excessive and bad pruning methods since 2016 in our petition. We have also contracted an accredited German tree company to inspect pruning practices and have shared these results with the municipality, but this evaluation and letter has produced no response.

In the requested meeting, we are seeking not simply justification of these substandard practices, but a collaborative approach where new methods are seriously discussed and considered to better protect both our young and more mature trees from the challenges at hand including excessive drought (during which already stressed trees should not be pruned), heat islands, soil compaction, pollution, floods, climate heating, and root mutilation from road and infrastructure projects.

Greatest danger to our urban trees: poor maintenance, toxic masculinity, and bad pruning

But one of the most dangerous foes of our urban trees at this moment are the city’s contracted tree workers who are recklessly pruning our trees. We also seek a respectful demeanor, rather than the dismissal of a group of intelligent, diligent, and knowledgeable women, often pitted against a group of arrogant and stubborn men who care little for considering critiques about their craft. In these gendered scenarios we are often dismissed as overly emotional and irrational, the most common cliche in ‘everyday sexism’ within such dynamics. This is simply not the case. In fact, we are often more informed than the city tree bureaucrats on the latest urban tree science and simultaneously considerably pained by what we must witness with knowledge of the irreversible and bad consequences of these techniques for the present and future.

Recent videographed examples from Haren:

Why Lions Tailing hurts trees (Boomwachters video in Haren, August 2022)

Mature oak trees lion’s tailed in Haren (August 2022, Jachtlaan)

Photographic examples

Here are just a few photographs of the sheer damage and vandalism enacted on our urban trees in the last few years which include topping, lion’s-tailing, and severe crown-raising well above the 4.5 required limit (and even well above the requested 6-meters on street sides). Side-walk clearance is required at 2.5 meters. There is a reason for this limit of 4.5 meters (and not say 8 meters); this is that the tree needs branches in its middle crown for stability and the transference of carbohydrates, water, and minerals from roots to leaves and to the top of the tree.

Oak tree with over-raised crown on Oosterweg in Haren, August 2022
A large portion of the live foliage removed from the middle, already heavily-raised crown, and in the middle of a drought in August, when the tree most needs these resources. These branches were not dead or diseased but healthy and supporting the tree’s food production and stability as well as providing shade from sun on the trunk (which will now suffer from burning and cracking). They were also cooling this hot area and sequestering greenhouse gases and releasing oxygen for us to breathe. Not only are trees good – LEAVES ARE GOOD!
Here is the same oak tree (August 2022) with the middle crown removed (as the lower crown has been removed for decades).
And the poor younger oak next to this tree has already had many training prunes (and wounds) and now even more of the crown is removed with a live crown ratio to 1 to 3, when the desired ratio is just the opposite, 2 to 1, or two thirds of the trunk shall be connected to live foliage and no more than one-third of the trunk shall be bare.
Haren, Oosterweg August 2022.
This younger oak has been raised to well over 5 meters on both sides (the measuring stick is 2 meters), leaving so little of the original crown and resulting in a live crown ratio of about 1 to 3. This tree will never grow old (old in a tree’s terms, although it might survive another 10 years) and clearance was already well-secured on the street side. Training younger trees means that you allow lower branches to grow partially until the tree has produced enough of a larger middle-crown before removing all of the lower crown.
Illustration of live-crown ratio from Gilman’s Illisutrated Guide to Pruning (2011, p. 291)
Lion’s-tailed plane tree in Haren, Kromme Elleboog (this was lions tailed in 2016), but this photo is from August 2022. The entire tree is hollowed out.
Topped poplar trees near the Martini Hospital in 2021. Where to begin in describing the danger and damage enacted by this “technique.” Isn’t it obvious? If not, see sources below and countless tutorials, videos, webinars, flyers, textbooks and articles decrying this practice as the pruning enemy number 1.
In Eelde in a nature reserve, more than 30 topped trees in June of 2022
Another topped oak in Eelde, August 2022 (there were several topped in one small area)
Here is a residential oak pruned by a professional arborist once a year. This high quality pruning reveals a strong crown architecture and healthy foliage with clearance for cars, the house, and for the residents. This tree has been pruned in a sustainable way, which will allow it to grow old (and on the tree’s terms which can be hundreds of years, even in urban environments). WELL DONE WHOEVER YOU ARE!
Haren, August 2022 (Anjerlaan). This tulip tree has been side-topped in the last three-four years, with the entire crown removed on one side, not because these branches were diseased or dead or blocking clearance, just because apparently. Now a large crack in the trunk has begun which reveals the stress on the tree as it fights for water, minerals, and resources. This is not “training” pruning, this is vandalism. This tree will likely be removed within ten years.
A whole street of mature oaks lion’s-tailed in August 2022 in Haren (Kromme Elleboog). Notice the long bare branches with no attached leaves until the now heavy load-bearing ends.
Aug, 2022, Haren.
These brittle branches on the Kromme Elleboog are now a high risk for breakage in high winds because they will stimulate foliage at the ends and weigh down these branches. Mass damping can no longer occur. And you can see that they were well above the contracted 6 meters, high above this passing bus.
The tree’s crown has thinned itself dramatically since it was over-pruned in 2016 (or 2018) (Emmalaan, Haren 2022).
Here the over-pruned hornbeam is seen with a broader perspective. See how the tree’s crown suffers from such large pruning wounds, which have not compartmentalized.
A close up of these same pruning wounds. This picture is only a few years after the pruning in Haren on the Emmalaan. This tree was not in the way nor on a major street. This is cutting for cutting’s sake. Rot and disease are now spreading quickly throughout the trunk.
August, 2022. Here more hollowed out trees by the city tree workers and in the middle of a drought, in August, a period not recommended for pruning mature trees.
This maple near the Onnerweg in Haren was over-raised many times in the last decades and now look at it. There is little crown remaining. These leaves, now absent, once provided so many benefits including food for the tree (through photosynthesis), oxygen for humans, shade, beauty, pollution filtration, and nesting for birds. No bird would now consider such an unstable, hollow tree for nesting. Is this dangerous, top heavy tree the fruits of our tax-paying labors?
More lion’s-tailed oaks in Eelde, on the park side of the street (August 2022)
Haren, 2022. Here are two topped poplars down the way from the fallen topped poplar. They too were not yet showing significant signs of die back, but now they are destined to fall. We understand the risk of co-dominant stems and that is why an elated mail was sent to one of the tree workers after seeing these braces inserted before the upper crown was removed. But only a year later, both trees were topped. Hum let’s do the math here for failure predictions.
Topping is not only for our city tree workers, but also private residents who hire ‘experienced’ gardeners to top their trees and tile over much needed soil for plants and for drainage and insects (Haren 2022).
Here is a hornbeam? (difficult to tell) completely topped for the first time in March 2022 in front of the UMCG hospital, which should be a place for health and recovery. This wasn’t necessary, so why was this done? Perhaps Stedelijk Groen who recommended this pruning can try and justify it. There is no justification.
Here a topped tree in Groningen (which they typically describe as ‘candelaberen’). We call this hat-racking in English because it looks more like a piece of furniture than a tree, but still when you remove the entire crown, this is enormously stressful and damaging for the tree. So begins this tree’s human-induced decline (2022).
March 2022 on the Helperzoom in Groningen. Another day of topping trees in Groningen, perhaps the tree-topping capital of the world. But this technique does produce enormous piles of branches, which can be shredded, sold, and burned in local biomass heaters such as at Kardinge or at the Biotoop. You’ll notice that in Groningen nearly all willows poplars of a certain age, regardless of health, stability, and stature are either topped (as if this makes them safer) or removed. How about looking at individual trees in their environment and making educated choices about how to mitigate risk and protect the tree’s health based upon its unique state and vitality? There are other safer ways to prune a maturing tree, even a poplar with the tendency to drop limbs at a certain point.
A few days later, these same willows sought out by local (confused) birds.
These white willows had been previously topped and a better remedy would have been to cultivate a new leader trunk rather than re-topping every 4/5 years. However, retopping does produce more jobs and is therefore expensive for the municipality.
Let’s see how long these trees last now.
This maple on the Jachtlaan in Haren was severely crown raised and lion’s tailed in 2016 for a road project (as was every maple on this side of the street while many other healthy oaks and maples were just removed). The tree wasn’t in the way and subsidies were used to raise this tree’s crown. Then in May of 2022, the top heavy lion’s tailed branches fell. You can see one broken branch on this now sad looking tree, whose life-span has been severely reduced. This is the opposite of sustainable tree care.
This is just blatant over-raising of the crown on a younger birch tree on the Waterhuizerweg in Haren. ‘Training’ pruning should happen slowly as branches are growing as the lower and middle crown is still needed to provide resources and stability for the tree. The LCR (live crown ratio) is here reversed to what it should be: most textbooks advise a 2 to 1 crown to trunk ratio, this is the opposite, 1 part crown/2 parts trunk. This was not needed for visibility as the branches were far too small to impede clearance at this stage and at least those on the side-walk side could have been preserved. This height is some 7 meters up. The required height for streets according to the APVG is 4.5 meters.
Here are some useful drawings indicating desired crown ratios on urban trees. This illustration advises for a 70% LCR (From Gilman’s Illustrated Guide to Structural Pruning).
This honey tree (Japanese pagoda tree) was a gift to Haren from the Gemeente Groningen after the merging in 2019. The tree was immediately pruned, and then pruned again, and now so little of the crown is left, it might only survive another few years. This epitomizes the new approach towards tree care: plant it at the wrong time of the year, immediately over-prune it, forget about it, and let others watch it die within a few years.
That is me overjoyed to see the results of careful tree care for one of these ancient ancestors of ours. They support us and they deserve highly specialized care and respect.
This oak tree has been protected and now as it ages, it is supported by cables. This was taken in a city in Oregon in the USA where many such trees have been allowed to develop a good branch architecture but also pruned for clearance. Now it is more than 200 years old (and there are dozens in this area). With proper care and pruning, trees can grow old in cities, near cars and pavement, buildings, and people and even in poorer quality soils (cross the border to Germany and see the immediate difference in tree care there even on lesser quality soil). But we must have the long-term perspective, specialized knowledge, resources, and will to preserve them.

In conclusion:

We’ve asked questions about why you have done this and no answers have been forthcoming beyond “our tree workers are ETT-certified and therefore qualified to prune our trees” or “We’ve heard your critique and we don’t agree.” This is not an acceptable answer in a municipality with one of the lowest UTC (urban tree cover) ratios in the country (12%). With many trees declining and removed every year simply from bad care (not to mention all of the perfectly healthy trees removed for no legitimate reason at all), we can no longer simply ignore what is right in front of us with so many trees failing not exclusively from disease or drought or pollution, but just as often in response to such reckless pruning.

If you are a concerned citizen reading this open letter to the municipality, please submit a formal complaint against these procedures if you see them in your neighborhood. Use parts of this letter or write your own.

To submit a claim: do so on the gemeente’s website here and copy this letter.

You can use these photos and also attach this flyer on sub-standard pruning techniques:

More sources to learn about the dangers of lion’s-tailing and topping trees

Videos about lion’s-tailing and its dangers:

Lions Tailing by aborist Blair Glenn:

LION TAILING AND WHY IS IT SO DANGEROUS by certified aborist James Batton:

Pruning standards and best management practices by the TreeHusker:
–       Section on lions tailing begins at 18:00-19:00
Tree Biology and Pruning by USU Extension Forestry Webinar by Dr. Randy Miller:
–       Excellent discussion of ‘mass damping’ potential of tree branches and leaves (making them wind resistance if not lion’s tailed, 5:00-7:00)
–       Excellent explanation of topping (28:43),
–       Excellent discussion of pruning and health of tree (42:30)
–       Excellent explanation and discussion of lions tailing (55:00)

Discussion on Repairing topped trees (55:57-57:20)

Why Lion’s Tailing is Wrong by aborist Blair Glenn:–s&t=283s

Articles, flyers and books about pruning (with sections regarding topping and lion’s-tailing)

“Bomen toppen is slecht en gevaarlijk.”

“Bomen toppen.”

“Is bomen toppen hetzelfde als snoeien?” De Boomverzorging.

“De 10 grootste misverstanden over het verzorgen van bomen.” Vartago Boomverzorgers: voor gezonde en veilige bomen.

“Toppen van bomen: Waarom het bomen toppen moet stoppen.” PKS Boomverzorging.

“Waarom is toppen of inkorten van bomen zo gevaarlijk?” De Boom Dokter.

“Pruning Maturing Shade Trees.” CMG Garden Notes 615, Colorado State University.

“Topping and Lion’s Tailing are Forbidden.” National Arborist Association.

Dujesiefken, D. & Stobbe, Horst. 2002. “The Hamburg Tree Pruning System – A framework for pruning of individual trees.” Urban Forestry & Urban Greening 1: 75-82.

Ellis, Deborah. 2013. “Pruning Urban Trees: Overpruning.”

Figart, Larry. 2009. “Lions tail pruning won’t protect your trees: Pruning method leaves branches less resilient in high winds.” University of Florida.
Hanboek Bomen. 2014. Norm Instituut.

Gilman, Edward. 2012. An Illustrated Guide to Pruning, third edition. Delmare.

James, Kenneth, Nicholas Haritos and Peter K. Ades. 2006. “Mechanical stability of trees under dynamic loads.” American Journal of Botany.

James, Ken et al. 2014. “Branches and Damping on Trees in Winds.” 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), Byron Bay, Australia, S.T. Smith (Ed.).

Peltola, H., S. Kellomaki, et al. (2000). “Mechanical stability of Scots pine, Norway spruce and birch: an analysis of tree-pulling experiments in Finland.” Forest Ecology and Management 135: 143-153.

Roloff, Andreas, ed. 2016. Urban Tree Management: For the Sustainable Development of Green Cities. John Wiley & Sons.

Rottmann, M., 1986: Wind- und Sturmschä den im Wald. J. D. Sauerlä nder’s, Frankfurt a. M. (in German).

Shigo, Alex. 1991. Modern Arboriculture: Touch Trees. Shigo and Tree Associates.

Spatz HC, Theckes B. “Oscillation damping in trees.” Plant Science. 2013 Jun 207: 66-71.

More photos of substandard pruning in Groningen (and Haren)