Tree Specification – Do we need to be more diverse?

27th March 2020

As our ‘climax vegetation’, trees are one of the most precious natural resources in the UK.  They provide cooling in our towns and cities, help to clean the air we breathe, create habitats for wildlife and make the landscapes we live in so much more beautiful.

Whilst, in England, the proportion of woodland is low compared to other European countries (10% compared with 38% average for the EU) our towns and cities do well.  For example, did you know that London can actually be classified as a forest; boasting over 8.5 million trees and over 10% continuous canopy cover(FAO, 2015)!

Trees are also some of the planets longest living organisms.  Bowthorpe Oak in Bourne (Lincolnshire) is believed to be one of England’s oldest oak trees with an estimated age of more than 1,000 years and the Llangernyw Yew, quietly growing in a grave yard in North Wales is estimated to be at least 4,000 years old , this means it took root in the Bronze Age!

Unbeknownst to many, trees are hugely valuable to our economy, in 2015 it was calculated that London’s 8 million trees contribute a staggering £275.8M of benefits: £2.8M in storm water alleviation, £146.9 M in carbon capturing and  £126.1M in pollution removal and would cost an eye watering £6.1 billion to replace.

The importance of trees in the urban environment is unquestionable.

However, trees are facing ever increasing threats with climate change, pests and disease all posing a risk.  As more and more people travel and trade across the globe, and the demand for exotic plant species grows, so does the risk of importing unwanted disease and pests, which impact our native flora.

Ash Dieback is an example of an imported disease that is threatening to wreak utter devastation on one of the UK’s most common broad-leaf trees.  Another example is that of Sudden Oak Death;  Sudden Oak Death has caused extensive damage to trees in parts of the USA and has occurred in parts of mainland Europe and Britain. It is thought that non-native Rhododendron is a host to the fungus organism which has been implicated in sudden oak death.

Worryingly, in England, just five species of broad-leaf tree make up 59% of our broad-leaved woodland and just six species of conifer make up 81% of our conifer woodland and this lack of tree diversity increases further when entering towns and cities.

A disease that targets one of our most common trees could therefore wipe out large areas of woodland.  Equally the five most common trees found in UK urban areas are Plane tree (Platanus x hispanica), Sycamore (Acer pseudoplatanus), English oak (Quercus robur), Silver birch (Betula pendula) and Horse chestnut tree (Aesculus hippocastanum).  Loosing one or two of these species could completely change an urban landscape: “I watched the majestic elms of my youth dying. They can now only be seen in places like Brighton, where they have been carefully protected.” Dr Fred Rumsey, botanist, Natural History Museum.

Tree populations need genetic variation for survival, good growth and viability in the long term. Variation enhances resistance to pests and diseases, and the effects of global warming. Such a long-term vision and approach will help to strengthen our natural resources ensuring that trees planted today will become the healthy forests and streetscapes of tomorrow.

Mike Glover of Barcham Trees very eloquently describes the impact of using large numbers of the same tree “I recently took a trip out to Washington and saw there were ranks upon ranks of Elm (Ulmus) planted outside the White House. They looked great and so pleasing to the eye but being all the same means that the genus could be open to a pest or disease problem, in that if one gets infected they will all quickly suffer the same fate…If this disease could express itself, it must have been ecstatic when it arrived seeing that it could then predate on a huge number of potential host with no break crop to slow its progress from tree to tree.”

So what can we do as landscape architects to help protect the trees of our towns and cities?

As Landscape Architects we can help protect trees in our urban environments by diversifying the tree populations we specify.  By devising carefully balanced mixes of trees and shrubs, we can help slow/prevent diseases in an area.

When specifying trees, we must consider both species diversity and genetic variation (within and among populations) as genetically diverse populations are more likely to evolve to resist new pest and diseases. Putting it simply, large areas of monoculture should be avoided and a wider range of tree species should be encouraged.

Of course specifying a wider range of trees does not only improve resilience to pests and diseases.  The five most common trees found in UK urban areas all happen to be deciduous, this means our streets look very different as the seasons change.  The canvas of a Landscape Architect is the landscapes we design.  We are uniquely privileged in that our landscapes grow and change, bursting into an array of bright greens one month only to dazzle us with a pallet of oranges, burnt umbers and golds another.  Careful planning in relation to trees can result in a landscape that provides a vibrant array of changing colours throughout the year, providing the people who live amongst them with natures very own visual calendar.

As a profession, we must strive to guide our clients and work with planning authorities to put time in to specifying plants, rather than it being an afterthought. We need to challenge decisions which put aesthetics above nurturing healthy ecological balance. We must focus on management and select compatible trees species, companion plants, taking notes from mother nature…She knows best!

By Kate Kershaw, Practice Manager and Antony Geddes, Associate.