After a difficult year for establishing new trees, a Danish tree manager has given UK colleagues a breakdown of how scheduled irrigation overseen by a tree management system not only prevents losses but also boosts growth.
Lars Schultz-Christensen, who previously worked as head gardener for the city of Copenhagen, told the recent National Tree Officers Conference in Telford that "data-driven maintenance" enables trees in the Danish capital to grow "much faster than we expected" and so deliver more ecosystem services sooner.
"Previously, when we only kept paper records, our results weren’t impressive," he said. "We used to lose 20% of street trees in the first 20 years, and not knowing which trees to irrigate was likely to have been a factor."
Now though, a geographical information system (GIS) "for us is a planning tool, ensuring systematic maintenance at the tree-specific level", he said. "It doesn’t forget, as humans do."
The city’s tree management regime now specifies 13 irrigation dates during each tree’s first growing season, including four in July, followed by seven further scheduled waterings in each of the next two growing seasons.
"By knowing in advance when you have to irrigate, it helps you plan," he explained. "In Stockholm they now water every week."
To justify the resulting expenditure, Schultz-Christensen’s team recorded the annual increase in trees’ height, then sought correlations with the frequency of irrigation. "We chose height because our politicians understand it better than diameter," he said. "We found we are getting 82cm a year in street elms."
His team also trialled "irrigating bags" around the base of trees, which Schultz-Christensen said led to extra circumference growth, though the extent of this depended on species. Circumferences also continued to increase at a comparable rate over the next three years after irrigation had ceased, they noted. "When you combine these with a GIS-based irrigation programme, you get enormous growth. Twenty years ago, we were satisfied with 30cm a year. Now we are getting 70-90cm — a huge difference.
"The focus tends to be on how little the maintenance department needs to irrigate. Irrigation is expensive — for us about 100 kroner (£12) each time. Over three years, that will cost 2,700 kroner, which is almost the same price as the tree. But in five years the tree will increase five times in value. If I could, I would irrigate also in the fourth and fifth seasons, just for the extra growth."
This growth boost is obviously dependent on species, the star performer being alder, which can reach more than 10m in six years, followed closely by Robinia and plane. "Now they don’t die," he added. It’s hard to believe but we just don’t lose young trees."
Denmark, like the UK, experienced a prolonged warm, dry summer this year. "But our irrigation concept kept them going. It has been a success," he said. This contrasts with forestry planting, where five-million newly-planted trees, more than half the total for Denmark, are believed to have perished this year, at a cost of up to 40 million kroner, while the cost to the industry from reduced growth of established trees is expected to be even greater.
The Copenhagen results have been given added significance by an ongoing planting boom in the city backed by the local council, which has set a target of planting 100,000 trees by 2025. Its own surveying found that 69% of residents back more greening of urban areas.
Potential benefits of a data-driven maintenance and irrigation system
- Reduces mistakes made due to human error
- Results in improved growth and tree health
- Saves money by reducing losses of trees
- Resulting efficiencies and improvements in tree stock can boost the argument for further investment in greening and tree planting
The London Tree Officers Association executive committee says: "Post-planting watering is vital and hugely cost-effective. The cost of sourcing, purchasing and planting a tree dwarfs watering costs by 300-400% and shows the intent of the organisation planting that it is being taken seriously and not left to chance. Reputational risk outweighs the cost of watering and within a few years you have a functioning tree delivering all the public amenity and ecosystem services you planted it for in the first place."
Viewpoint: Russell Horsey, urban forester and former senior arboricultural officer, Bristol City Council
"It's been a tough year for all newly planted trees including forestry whips due to the prolonged drought. My experience is also that watering, along with weed control, is key to getting new trees established. I am not surprised at Lars’ results. I have just never had time to measure trees I have managed."
A 50m stretch of pavement in front of a Copenhagen café marks a scalable solution to both flood attenuation and green infrastructure irrigation, according to its backers.
Designed by architecture studio Tredje Natur (Third Nature) and unveiled in September, the Klimaflisen (Climate Tile) paving has regular holes to allow water to percolate into storage below, where it can irrigate an adjacent strip of trees and herbaceous planting, while also preventing drains becoming overwhelmed during downpours.
Tredje Natur now plans to commercialise the format with Danish concrete supplier IBF. Partner at the practice Flemming Rafn Thomsen says: "Pavements are an underappreciated part of our infrastructure that hold great potential for future communities in our ever-growing cities."
Project manager for the city’s climate change adaptation centre Jan Rasmussen adds: "We look forward to following the Climate Tile, what role it can play in the city’s climate adaptation and how the citizens welcome it."