Arb: What can the UK learn from the urban trees of Budapest?

IT’S funny how travel can stimulate questions about trees, even when the travel has little to do with them. A long weekend break in Budapest, Hungary, taken to meet with friends and discuss ongoing projects elsewhere, was a case in question. The itinerary comprised visits to some of the tourist attractions in the city and general relaxation. These objectives were met with a boat trip along the Danube, a visit to the parliament building, an excursion to the Basilica of St Stephen and a ride on the really impressive funicular railway to the heights of Buda, overlooking the Danube, squeezed in between a few beers to counter the stifling 37°C temperatures.

But, as with most tree enthusiasts, it is hard not to look at trees when visiting somewhere new. While it was only a short snapshot of a relatively small part of the city, several questions emerged.

The first was the use of Styphnolobium japonicum as an urban tree. The impression created was staggering and impossible to miss. The species was everywhere and in full flower. Streets, parks and many open spaces were enhanced by the full canopies of creamy yellow flowers mirrored by fallen petals retaining their full colour while laying on the ground. Those of you who regularly read this column will know I have often sung the virtues of Styphnolobium japonicum and have repeatedly asked the question as to why it is not more commonly used in the UK. 

Koelreuteria (Image: eA/KS)

For those not familiar with the Japanese pagoda tree, it might be useful to remind folks that the tree is a native to China, although it is widely planted in Japan. It was introduced to the UK in 1753 and won the award of garden merit in 2002. It has bright green compound leaves which have very small leaflets, giving it a light, airy appearance and, once mature, produces pale yellow pea-like flowers that hang across the whole canopy in racemes. The bark is interesting, being dark green in colour, exhibiting small white flecks all along the stem.

So why is it not more widely used in the UK? Some would argue that branches die back irregularly and there is a risk of failure, others point out that it doesn’t respond well when conditions are wet. Others will say that when young there is dieback in the crown which requires careful pruning as it develops towards maturity, while some would argue nurseries in the UK don’t know how to grow it properly, allowing the crown to develop too quickly. There is some truth in all of these comments, but are they overstated? If the species is so problematic, why is it that in Budapest it appears to be the dominant one, growing everywhere in the city to such dramatic effect? I have also seen the species growing successfully – although not in quite the numbers apparent in Budapest – in Berlin, Paris, Zagreb, Prague, Belgrade and many other significant European cities. So why not here? What is the prejudice preventing its wider use, given the demise of ash and the call for wider diversity within tree populations?

Having overcome the dominance of Styphnolobium it then became difficult to ignore the yellow seed pods covering the canopy of Koelreuteria paniculata, a species also apparent in large numbers across the city. Again it is worth saying a few words about Koelreuteria. It is, in my view, underused in the UK. It is known as the pride of India, but was introduced from Japan in the 1760s. The species is exceedingly tolerant of drought conditions, forming a 7–12-metre tall dome-headed tree with small, yellow flowers developing in the spring. However, it is the bright yellow lantern-shaped fruits which have the greatest impact, covering the entire canopy. These fruits gradually turn through red into a deep brown and are often held onto the tree well into the autumn and winter. 

It would, I think, be fair to say these two species give their parts of the city a yellow haze which permeated everything, creating a warm and attractive backdrop to all the other activities undertaken. This is, of course, a biased view from someone who is always looking at trees, but I would suggest that even for those not so interested the effect would be almost impossible to miss.

There were other species present in the population, of course. Celtis orientalis was present in numbers and Platanus hispanica, as would be expected, provided several fine specimens across the parts of the city I visited.

However, other species were not so apparent, so while extolling the virtues of both Styphnolobium and Koelreuteria and their dominance of the city’s tree population it was hard not to think about the implications of that dominance. Without further research it is impossible, with any degree of accuracy, to estimate the percentage of the population represented by just two species. But this does raise questions about the resilience of that population. Visually, it appeared Styphnolobium could represent as much as 30 per cent of the population and, along with the other three species mentioned, well over 60–70 per cent of the total population. It could also mean that I have become remarkably short-sighted over the years or was so overwhelmed by Styphnolobium and Koelreuteria that I stopped looking.

So the question has to be asked: how resilient is the tree population of Budapest, despite the staggering appearance of that population? It is now widely accepted that diversity within a tree population is the most effective way of ensuring the population is resilient to the challenges of imported pests and diseases and the challenges associated with climate change. Observation of the tree population of Budapest would suggest it is highly reliant on a very few species and therefore vulnerable.

Styphnolobium. (Image: eA/KS)

So, back to the questions and thoughts which, after a brief visit, stayed with me on returning to the UK. I now find it even more difficult to understand why both Styphnolobium and Koelreuteria are not more widely used in our urban spaces. Surely evidence from mainland Europe is enough to suggest that both species can be used more widely and in greater numbers?

It was also interesting to walk a city where so few species dominated and a lack of diversity was so apparent and ask the question how many cities across the UK are equally vulnerable, reliant on relatively few species coupled with a reluctance to gamble and widen that diversity by planting different species such as Styphnolobium and, to a lesser extent, Koelreuteria?