Tree researcher explains value of Air-Pots, mycorrhizae and mulching

A senior Italian academic has given UK tree professionals some practical insights from research into urban tree establishment.

Professor Francesco Ferrini. Image: Barcham Trees
Professor Francesco Ferrini. Image: Barcham Trees

Professor Francesco Ferrini of the University of Florence, Italy said his team's research had showed long-term benefits from using UK-made Air-Pots, which have a perforated, "knobbly" profile, over conventional smooth-sided pots at the nursery stage.

Not only did standard pots eventually produce deformed roots in the small-leaved lime (Tilia cordata) and smooth-leaved or English elm (Ulmus minor), but tree transplanted to a field showed improved performance two years later, and even after four years, root circling was still much higher in the trees inititally grown in standard containers.

Ferrini's team has also attempted to produce site- and species-specific mycorrhizae for inoculating trees' roots in the nursery and at planting, by harvesting fungal populations from healthy urban trees and propagating them.

Such "mycorrhization" in the nursery did not enhance the growth of container-grown maples, limes and oak, but did yield physiological benefits such as the maintenance of less negative leaf water potential, a higher apparent carboxylation rate, higher RuBP regeneration and higher quantum yield of PSII under water shortage - suggesting that inoculated plants were better able to cope with water stress.

When hackberry (nettle tree, Celtis australis) and ash (Fraxinus excelsior) were planted in streets and car parks, mycorrhizae yielded better trunk growth, shoot growth and chlorophyll content in hackberry, but only improved shoot growth in ash, though both showed higher carbon assimilation.

In the more favourable environment of an urban park, inoculated common or English oak (Quercus robur) showed better shoot growth and chlorophyll content than controls, though Ferrini reminded delegates that measurements need to be taken for at least three years to have real value.

He concluded that inoculation with native mycorrhizae generally improved plant growth and physiology, but the time of response depends on the inoculated species, and the effect of mycorrhizae on host growth is dependent on environmental conditions.

On mulching, a study looking at the effect of soil management techniques on growth of common lime (Tilia × europaea) and red horse chestnut (Aesculus × carnea), found that the height, trunk diameter and shoot growth of the mulched horse chestnut all improved significantly, but such improvements were only manifested later in the lime.

The soil temperature around mulched trees was also found to be lower than around unmulched trees - an important factor in Italian summers, but less so in British ones, he said, while soil respiration and biological activity were higher on mulched trees.

Ferrini added that he considered the optimum mulch depth around trees to be 8-15cm, with the material a blend of nutrient-poor large particles and nutrient-rich fine particles.

The seminar was the latest of a series for arboriculturists held at Barcham Trees in Cambridgeshire.

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