March...A blog by Andy Hirons, How do we select trees for the urban environment?

For the last eight months, I have been involved in a Green Infrastructure project (sponsored by NERC and supported by TDAG) aimed at improving the way we select trees for urban environments. As part of this project, we conducted a survey that sought to uncover the way we currently select trees. Indeed, some of you may have actually taken part in the survey – if so, many thanks. For this blog, I thought that I would give you a brief synopsis of the survey’s findings.

The first thing to say is that over 300 people took part in the survey and although there was a slight bias towards London and the southeast, all parts of the country were well represented.

Reassuringly, just over 70% of respondents ‘always’ or ‘mostly’ used site investigations to influence their species choices. Most of these investigations involved an evaluation of the soil and assessing the proximity to above ground infrastructure. However, 25% of respondents only use site investigations to influence their species choice ‘sometimes’. Six people even conceded that they never used site investigations to influence species choice. For the most part then, those specifying trees do consider the planting site – good news!

There are, of course, a number of different places that you can get guidance on tree selection from.

 

 

These are essentially categorised as: dendrological literature – publications such as Gerd Krüssmann’s Manual of cultivated broadleaved trees and shrubs or William Bean’s Trees and shrubs hardy in the British Isles; online selection tools – for example, the RHS online selection tool or the Forestry Commission’s The Right Trees for a Changing Climate Database; tree nursery catalogues; or, recommendations from a tree nursery. To some extent, all of the potential sources of guidance are used. However, in a boon to tree nurseries 60% of people ‘always’ or ‘mostly’ use tree nursery catalogues to make their tree selection decisions. Tree nursery catalogues dominated tree selection decisions by landscape architects, local authority tree officers and arboricultural consultants. Perhaps this is unsurprising given that the catalogues provided by the nurseries are often well illustrated and readily available.

Nevertheless, this finding does have important implications. Most people only make their tree selection decisions based on the catalogue of their favoured nursery. This is clearly good for the nurseries, but is it good for the urban forest? As a community, it would surely be better if the consumers had a greater role in determining what was planted in the urban landscape. If we are not careful, we will end up only specifying those trees that we are told we can buy, with few options to drive diversity within the urban forest: something of strategic importance.

Some respondents also offered their own insight into the risks involved with relying on tree nursery catalogues:

 

  "Many of the nursery catalogues offer conflicting advice”

"Never depend on nursery catalogues alone”

“Hard technical information is very difficult to come by with nursery information often being very superficial and based on aesthetic features.”

 

In defence of the nurseries, they need to have confidence that novel species introductions will ultimately have a market. If they only sell a narrow range of species, can we blame them for only growing a narrow range of species? I think not. It is, therefore, critical that those procuring trees work with nurseries and encourage them to grow new species so that have good market potential.

Other key findings from the survey were that mature tree size and drought tolerance were seen as the most important criteria to be included in selection guidance. Of course, many other criteria were also cited as being important. Interestingly though, the aesthetic attributes of trees were rarely a priority. Perhaps the importance of site suitability and potential longevity are beginning to trump the visual characteristics in the minds of those selecting trees.

Personally, I think this is refreshing: there are few dead trees that look better than a healthy tree!

The subject of tree selection remains a stimulating area for research, at least for me. As this project continues, I will be putting together some guidance on species selection that reflects the needs of the urban forest community. I’ll make sure that I keep you informed on progress and opportunities to attend species selection seminars, later in the year.

In the mean time, updates will be available via the TDAG website. Please feel free to contact me if you would like further information about the project. ahirons@myerscough.ac.uk