62 Moore: Wind-Thrown Trees: Storms or Management? aſter the tree had been wind thrown, the large number of smaller blackened roots, and the sog- giness of the soil suggested that there had been problems with waterlogging over a longer period. A second condition arises when the patterns of Figure 8. Some of the many fallen elms growing along path- ways in a public garden in Melbourne, Australia, after a major storm event in 2005. Both trees proved to be positive for criteria 1–6, and both have hinged close to the trunk on the leeward side. In almost all cases, there was evidence of major interference with the tree root systems (Table 4). This interference may have been from trenching and other construction work, or from mowing prac- tices, which repeatedly damaged the roots that had come to the soil surface through secondary growth. Such wounds may provide access for pests and dis- eases. While root damage was a common factor associated with the failure of many of these speci- mens, there was also a strong correlation of fail- ure with changed soil/water conditions (Figure 7). These changes to soil/water conditions were of two major types. The first was the existence of waterlogged soils. In many instances, the soil under the trunk of the fallen tree was so wet that there was a pool of water at the base of the hole at the time of inspection. Over time, waterlogged soils have a significant impact on descending roots, which oſten die back and leave a root plate with very few, if any, descending roots if the tree has been grow- ing under such conditions for long enough. In most cases, the pungent odor of the exposed soil Eucalyptus Ulmus Acacia Cupressus Melaleuca Lophastemon Populus Ficus Total Trees positive for criterion (%) No. 18 30 15 5 4 2 4 2 80 Crit 1 7 28 2 2 0 2 4 2 47 58.8 Crit 2 14 29 11 5 3 2 4 2 70 87.5 Crit 3 16 29 10 5 3 2 4 2 71 88.8 soil water movement are altered by construction work, adding soil as fill, or re-contouring surfaces, all of which can inadvertently divert flows from the tree’s root system. The trees oſten benefitted from these subterranean flows for decades and then face a sudden imposition of a water deficit. Under these circumstances, the trees show the effects of wilting and are oſten significantly stressed. These water-deficient trees were the only speci- mens that appeared to show symptoms of canopy dieback and significant amounts of deadwood. In one case, a tree was wind thown the day aſter an inspection recommended its removal due to canopy dieback and a lack of stability of the root plate. In these situations, the canopy dieback may well be an indication of stress that has affected the health and integrity of the root system, which may increase the risk of windthrow. However, many wind-thrown trees had intact and healthy cano- pies, and trees with significant canopy dieback and deadwood remained standing, which suggests caution when using these canopy characteristics in the visual assessment of the risk of windthrow. Compaction was assessed using a narrow screw- driver with a 300 mm blade, which was pushed into the wet soil within 24 hours of the trees being wind thrown. In some cases, around the bases of fallen trees, it was not possible to penetrate the soil at all, and in others, penetration was only to a depth of 10–30 mm. In undisturbed soil, the blade could be pushed into the soil up to its handle (Table 4). Soil was described as compacted if the blade did not penetrate 30 mm into the soil. The Table 4. Assessment of eighty wind-thrown trees against the criteria listed in Table 3. Genus Crit 4 11 25 3 3 2 2 4 2 52 65.0 Crit 5 4 21 0 0 0 0 3 2 30 37.5 Crit 6 9 23 4 1 2 1 3 2 45 56.3 Crit 7 10 10 2 0 0 1 3 0 26 32.5 ©2014 International Society of Arboriculture
March 2014
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