Tree care with AirSpades

 

I’m very excited to have finally purchased my own AirSpade 2000. I’ve been using AirSpades in Arboriculture since 2003 – 14 years – to rehabilitate stressed and/or injured trees, to decompact soil, to introduce soil ameliorations, to excavate near trees whilst minimising damage to roots, to locate and map root systems.

IMG_0366
Construction damaged Purple Beech (Fagus sylvatica f. purpurea) 11 years after rehabilitation in 2006 with AirSpade and soil improvement. Photograph taken July 2017.

An AirSpade functions by directing air at supersonic speeds (MACH 2) on to soil. The soil is displaced by the air, while more solid items such as roots, stones, cables remain in-situ.

Why decompact soil?

Every season, to remain healthy, a tree needs to grow new roots. When Soil Bulk Density (SBD) exceeds approximately 1.38 g/cm3 (depending on soil type), plant roots can no longer grow in soil.

Just a single pass of a tyred vehicle can be enough to raise SBD to over the critical 1.38 g/cm3 that means root growth can no longer occur. Such an event can be sufficient to tip an ageing, slightly stressed tree over a threshold and into a declining condition.

By using an AirSpade on compacted soil, it is possible to reduce Soil Bulk Density to a level conducive to tree root growth. At the same time it is possible to retain the vast majority of existing roots, whilst even creating a better growing medium for roots.

Soil improvement

Whilst decompacting soil with an AirSpade, it is possible, and often recommended, to improve the condition of the existing soil to facilitate tree root growth. Such improving material can be worked into the existing soil using an AirSpade. Examples of improving material include composted woodchip, composted garden clippings (with a high, at least 60%, carbon content). It is important that the introduced material has a high carbon content, and relatively low nitrogen content.

This technique of using AirSpades to decompact and improve soil was pioneered by  Bartlett Tree Experts in the U.S.A (more about Bartlett and AirSpades).

Case study

The Copper Beech in the above photograph was damaged during the construction of the Princesshay Shopping Centre in Exeter, United Kingdom, during the early 2000s. The tree’s rooting area was reduced when a new footpath was constructed to the west, and a road to the east was reconfigured. Usage of the root zone by construction traffic was also suspected.

In the summer of 2006, the tree exhibited typical symptoms of construction damage – areas of dieback in the crown, a thinning crown, and foliage size smaller than typical.

Tree vitality was assessed both visually and with Chlorophyll Fluoresence. Indications were that the tree should respond well to decompaction and soil improvement.

In the autumn of 2006, approximately 10m radius of the grass sward around the tree was removed. Soil in 50% of this 10m radius circle was decompacted to a depth of 40-50cm using an AirSpade. The 50% area consisted of three segments evenly spaced around the tree.

Following decompaction, an approximately 15cm layer of composted garden clippings (as described above) was laid over the 3 segments. This compost was then worked in with the AirSpade.

Following this the entire 10m radius rooting zone was mulched with a 5-10cm deep layer of fresh woodchip, with care taken to keep the woodchip away from the stembase.

In the summer of 2007, tree vitality was assessed again, both visually and with Chlorophyll Fluoresence.

In the autumn of 2007, the mulch was raked away, and the decompaction and soil improvement process repeated on the three segments that had been left untouched in 2006. The woodchip mulch was then relaid and replenished after this work.

After a number of years, soil management under the tree was returned to formal cut grass.

As you can see from the photograph, the tree has responded very strongly to the AirSpade treatment. I guestimate that this tree’s life has been extended by at least 20-30 years.

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