A west London business park has proved the ideal open-air laboratory to test a range of treatments to address the poor soil health that often restricts urban trees' growth and therefore their ability to provide ecosystem services.
"90% of tree disorders have their cause underground," Dr Glynn Percival, head of the Bartlett Tree Research & Diagnostic Laboratory at the University of Reading, told the recent Arboricultural Association conference whose overall theme was soil health.
He said of Stockley Park, a business park near Heathrow Airport: "It has thousands of trees but they were showing evidence of drought, water excess, compaction and salt pollution." This had led to poor canopy coverage, limited stem extension growth, sporadic dieback, stem lesions, yellowing leaves and prevalence of pests and disease — problems that the site’s management firm MJ Mapp enlisted Percival to address.
He first sent soil away to be analysed for salinity and pH, the latter being potentially "an easy fix". "The main issue though was soil compaction," he said.
This too can be measured and the bulk density of the park’s soils was found to correlate strongly with tree failure. Those planted in soils with a density below 1.34g/cc were 100% successful, ranging up to those in soils above 1.65g/cc, where failure was "total", he said, while others "were surviving but not thriving".
"A brick has a bulk density of 1.9-2.4g/cc, and roots don’t grow into bricks," he explained. "Yet at Stockley Park we were getting rock-solid soils of 1.8-2.0."
He added: "Soil decompaction in nature is provided by earthworms. Farmers know they are an indicator of soil health and they can improve soil porosity up to fourfold." As they burrow and eat, they also excrete castings that are high in usable nitrogen, and when they die they too become "part of the organic mix", he pointed out.
While worms specifically for decompaction are available commercially, "you can’t just chuck them on". Instead, a trail at the site used "vertical mulching", using two distinct species, Dendorobaena veneta and Lumbricus terrestris — "one that burrows vertically and one that burrows horizontally".
In the trial, soils in areas of high bulk density of 1.8g/cc, around 20 trees, a mix of maple, lime and horse chestnut, were treated by removing soil cores to a depth of 30cm around the base and the resulting holes were filled with a mix of biochar, John Innes soil and slow-release organic fertiliser, while in some cases worms were also applied, in some a surface cover of clover and in some cases both.
"I’m a fan of biochar and we knew what nutrients were deficient so what we needed to add," said Percival. In the event, "all treatments were pretty much as good as each other" in reducing soil bulk density, to around 1.4 within a year, with the reduction most apparent in the first six months. But he added: "It took two growing seasons for the effect of the clover to become manifest." Soils on control plots showed no change over the period.
Soil respiration, meanwhile, provides an indication of its biological activity, he said. "Soils breathe, and you can measure it. Where you have worms, you have higher soil respiration, so you can tell if they’re doing the job. Oxygen and CO2 are the important ones, and the kit to measure them isn’t crazily expensive. It’s something to think about."
On the trial soils, all four treatments were again found to have a clear benefit, each showing a roughly threefold increase in the soil’s output of CO2.
However, further tests away from the directly treated areas showed the value of the worms because they caused soil decompaction and increased soil fertility, respiration and root growth across a wider area.
Percival added that mulches, used on some of the trial plots, also have a role in maintaining soil moisture and temperature. "Soil biota like a steady temperature, and mulch helps that."
Return on investment
Percival posed the question of whether investment in tree soil health yields a return beyond simply making trees look better. "This is where the businessmen get excited and why they’ll give me more money next year," he said. "An increase in crown volume means an increase in pollution absorption, though calculating this depends on the tree architecture." But by using digital imagery "you can actually quantify it using drones".
Meanwhile, with a diagnostic system such as Arborcheck, "you can quantify the operational efficiency of a tree’s leaves", he added. "Extra growth in that tree may be giving you 4mg more pollution absorption per day — and you can also measure heavy metal accumulation and assign a value to that."
Trees’ CO2 sequestration, or rather the resulting oxygen output, can also be measured using an infrared gas analyser, said Percival, and again a monetary value assigned to the increase resulting from treatment. In all, he concluded: "A hundred pounds invested [in tree vitality] can be shown to yield a £900 increase in benefits."