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| Commemorative Moves by Andrew Cowan Planting commemorative trees is a long term commitment. Trees can take decades to establish and they may live for several hundred years.  However, the pressures of increasing human populations with an exponential demand for housing means that suitable development space is a premium in residential areas. Combine this with the need to maintain social infrastructure and services such as schools and space gets even tighter. So when a infants school near Guildford made the decision to move out of their old buildings to merge with the junior school across the road the Arboricultural department at Merrist Wood College were asked to assist with a rescue mission. The two trees concerned were of particular value to the school community because one was planted as a memorial to a student who had died in a car accident and the other to commemorate the millennium. These trees where therefore of historic significance, even though most of their historic interest was yet to be realised. The school realised it’s responsibilities and new that these trees needed to be moved to a position where they were protected from development in a location where the pupils who planted them could return in years to come and remember their youth. So a new planting site was found in the grounds of the junior school across the road. When the Merrist Wood arb team were asked to help they realised that it was important to get this right and ensure these trees were given the best possible chance of survival, because as commemorative trees planted to remember a particular point in time and in one case as a memorial to a student’s life cut short, they were both irreplaceable. It was time to explore the advantages of supersonic air-stream technology and borrow an AirKnife from ArborEcology (www.arborecology.co.uk/airknife) The AirKnife is used to blow the soil off the root system of an established young oak tree. To provide the air pressure needed to operate the AirKnife a ‘four tool’ road air compressor was hired that could maintain a continuous air flow at 125 psi. The AirKnife itself is a simple tool that enables to operator to control and direct the pressurised air-stream produced by the road compressor. As such it is a relatively simple design including a handle where the compressed air line is connected, which incorporates a trigger mechanism to allow the operator to release the air down the hollow shaft to the nozzle at the end. Releasing the pressurised air in this way means that the road compressor has to work particularly hard to keep up the pressure, and the tool is designed to run at least 90 psi up to 350 psi. In the UK road compressors are categorised according to the number of tools they can power at any one time. Most hire shops will be able to supply a single or twin tool compressor on a cash account, but the larger four tool units may only be available to account customers and are not so common. A single tool compressor is too small and cannot maintain the required air pressure to operate an AirKnife effectively. A minimum of a twin tool compressor running a constant 125 psi will be needed that can sustain the continuous release of air through the AirKnife at a pressure of between 90-100 psi. The four tool compressor hired by the Merrist Wood tree team was capable of sustaining a pressure of 125 psi without the pressure drop experienced by smaller machines, when the air is released through the AirKnife. The Merrist Wood arb department was keen to make the most of this opportunity, which provided the students with the chance to try out a new tool at the cutting edge of arboricultural work, and also offered the ability to look closely at the root structure of some establishing trees. Developing an understanding of how roots grow in the soil, to provide the tree with the raw materials for photosynthesis and the supporting anchorage to keep it upright, is fundamental to establishing a working knowledge of modern arboriculture. Long lateral roots were exposed running close to the soil surface as were dropper roots exploring deeper soil levels along their length. Using the AirKnife to remove the soil from around the two trees offered the opportunity to lift the trees with almost all their roots intact, without the damage that would have traditionally been incurred by using spades or mechanical excavation techniques. Exposing the root system in this way enabled the students to see how it’s structure had grown and spread out as it established in the years following planting. The Oak tree appeared to have a rooting pattern typical of a bare root plant and six years had produced a considerable amount of root growth. In contrast the Cherry was a container plant, given away by the compost around the centre of the roots and, again, had probably been in the ground for around six or seven years with a well-established root system. Both trees were planted in a field location with grass growing up to the trunk base, and had grown an extensive spread of roots out into the sandy soil.  The ground around the oak tree was prepared by stripping off the turf before the AirKnife was used to expose the roots. The soil was almost all sand making excavation easy and within an hour most of the main central section of the root system was exposed. Several roots ran well outside the main root-ball and these were chased out by making shallow trenches all the way to their tips. These long laterals were growing out at around 10-15 cm below ground level and sent sinkers down every so often (see picture). The AirKnife makes exposing fine roots easy, while it is also possible to retain some of the important mycorrhizal association between roots and fungi to be retained intact. Using the AirKnife to break up the soil structure holding the tree into the ground and blow the soil off the roots it is possible to expose and save even the finest fibrous portions of the root system. With extra care it is also practical to uncover and retain sections where the roots have formed mycorrhizal associations with fungi. This was the case with the oak tree, where a small area of root system was exposed with it’s associated mycorrhiza. However, it was perhaps more interesting to note that such a small proportion of the roots had formed this association. It was thought that this could be to do with the rather sterile grassland situation where the soil contained few if any of the fungi that would normally associate with tree roots and very little in the way of organic matter. Gently lifting the tree with the tractor front loader enabled the root system to be excavated down to a depth of over half a metre. Once most of the roots had been exposed some vertical pull was applied to the stem via a front loader and it was possible to hold the weight of the tree and get the AirKnife right under the main root-ball loosening the soil down to over half a metre. This allowed almost all the roots to come free of the soil, it was estimated that as little as 5-10% was lost, with the students only had to cut a few more tenacious ones. A 360 was press-ganged into digging the rather large hole required and also removed turf 2 metres square. Any longer laterals had deep slits made in the turf and inserted. As so much root was recovered, once the backfill had been firmed the double stakes were hardly needed. The planting areas were then covered in a good layer of composed woodchip mulch out to a radius of 2m, to improve the trees growing environment and reduce the loss of water from the soil surface. The cherry went much the same although its roots were thicker and more widely spread but the same techniques were effective. However, the students did not remove the turf on this one and that turned out to be a mistake on the sandy soil as the grass roots made it much more difficult to scoop the soil out. So many roots were saved when the trees were moved that some mega size planting pits were required, so it was a good thing there was a 360 excavator nearby that could be commandeered to dig the hole. Merrist Wood arboricultural lecturer Ian Rogers commented “As a transplant tool the AirKnife more than proved its worth! No preparation to move quite well established trees with the root system almost completely intact is a major innovation and we shall be purchasing one shortly.” |
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