How canopy data sets can shore up tree cover

Managers urged to use data to set achievable targets to harness benefits of trees.

Urban areas: strategic assessment by Urban Forest Research Group shows wide disparity in level of tree canopies - image: HW
Urban areas: strategic assessment by Urban Forest Research Group shows wide disparity in level of tree canopies - image: HW

Urban tree managers need to know what their tree canopy cover consists of and then set achievable and measurable targets to increase it to harness environmental, social and economic benefits, according to the leader of a new study in UK urban tree cover.

While several towns and cities have conducted studies of their own tree populations, Forest Research's Urban Forest Research Group has carried out a strategic assessment of 265 English urban areas using the US-hosted i-Tree Canopy software package, showing for the first time the wide disparity between those with the most and least cover.

The head of the group Dr Kieron Doick, along with partners including consultant Kenton Rogers of Treeconomics, have collated this information at the urbantreecover.org website and unveiled it for the first time at the Trees, People & the Built Environment 3 conference earlier this month.

The team further compared the i-Tree approach with a ward-by-ward approach of 26 densely populated areas of England based on the National Tree Map data provided by Bluesky International. "If you take larger boundaries you are more likely to include farmland and woodland," says Doick. "But as long as you use the same boundaries, you get the same results."

Bringing these results together, Doick and colleagues found that of the 40 towns and cities with less than 10% tree canopy cover, 30 were coastal, as were four of the bottom five (see below). "It's more difficult to get the right tree species selection and get the cover in seaside areas," says Doick. "If you take tree cover to include shrubs, that's one way to increase that figure."

Based on this work, Doick recommends that towns and cities should set a tree cover target of at least 20%, or 15% for coastal towns, with "achievable target deadlines" of 10-20 years.

"There is some good work being done by some local authorities, but many more don't have baseline or target figures," he adds. "That was one of the reasons for this study. We are trying to help people to have that conversation more quickly and easily. Another is that there is anecdotal evidence of tree cover decreasing in some places, but without the figures we can't say. With this we will know what damage has been done in, say, five years' time."

Tree cover baseline

With a tree cover baseline, managers not only know where canopy is low or deficient but also its composition by species, age and size, as well as potentially condition and vulnerability to pests and diseases, he notes. "A survey can focus discussions and help make a tree population more resilient, something that requires a strategic approach."

Yet it is not a resource-intensive exercise, he insists. "You can get a figure for a city in an hour, or if you want to be more strategic by working at ward level, you can spend an hour on each ward. Rather than iTree, a GIS-based desk exercise is quicker but you would have to buy in the data. But it's not a huge amount of work either way."

An earlier set of data for Wales was provided by Natural Resources Wales' 2014 study Tree Cover in Wales' Towns & Cities. Described at the time as the world's first country-wide urban canopy cover survey, this desk-based analysis of aerial photographs for 220 urban areas put Wales' overall urban tree cover at 16.8% - "a mid-range figure" compared with other towns and cities around the world, which "suggests scope for improvement", according to the report.

TREE COVER HIGHEST AND LOWEST

TOP FIVE

Area/Tree cover

Farnham (Surrey) 45.0%
Trimsaran (Carmarthenshire) 34.3%
Tunbridge Wells (Kent) 33.7%
Woking (Surrey) 33.2%
Treharris (Glamorgan) 32.0%

BOTTOM FIVE

Area/Tree cover

Fleetwood (Lancashire) 3.3%
Sheerness (Kent) 4.3%
Blackpool (Lancashire) 4.4%/6.0%*
Boston (Lincolnshire) 5.4%
Newbiggin-by-the-Sea (Northumberland) 5.5%
* Depending on methodology


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