Arboricultural Assessment

Transcription

1 Arboricultural Assessment Site Address: Client: The Proposal: St Omer Park Lake Esplanade Queenstown Queenstown Lakes District Council Arboricultural assessment of a failed Lombardy poplar tree growing within St Omer Park, Queenstown Report Compiled by: Samuel Earp Report Reviewed by: Craig Webb Date of Report: January 2014 Page 1 of 10

2 1.0 Introduction 1.1 GreensceneNZ Limited has been engaged by Mike Weaver, Parks Manager, Queenstown Lakes District Council (QLDC) to carry out an arboricultural assessment of a Lombardy poplar (Populus nigra Italica ) which failed in recent high winds causing damage to surrounding property. The Lombardy poplar was one of 23 trees of the same species growing within St Omer Park, Queenstown. 1.2 The purpose of this report is to provide an arboricultural assessment of the subject tree to try and ascertain what caused the tree to fail in the manner that it did. The analysis of the tree failure event has been extrapolated from a post-failure visual tree assessment and a resistograph test (used for determining structural integrity of wood). 1.3 The above inspection and test was undertaken on the basis of using the failed specimen as a control tree in relation to the pending assessment of the remaining mature Lombardy poplar trees growing within St Omer Park. 1.4 The visual tree inspection and resistograph test of the failed Lombardy poplar was carried out on the 18 th and 20 th January A site plan showing the approximate growing location of the subject tree is included within Appendix A of this report. 1.6 Photos of the subject tree are contained within Appendix B of this report. 1.7 Results of the resistograph tests undertaken on the subject tree are contained in Appendix C of this report. 2.0 Background 2.1 In strong winds on the evening of 17 th January 2014, a mature Lombardy poplar growing within St Omer Park failed in a northerly direction. The tree came to rest across Lake Esplanade causing damage to surrounding property, including cars and a motel. 2.2 Upon arrival to the site of the fallen tree on the morning of 18 th January 2014, the bulk of the main canopy had already been removed to clear Lake Esplanade. A review of available photos suggests that the foliage was of good health and density. The visual assessment on site of the remaining portions of the failed tree was also undertaken. 2.3 There are numerous Lombardy poplars growing within St Omer Park. These trees have been the subject of previous attention/assessment by QLDC, primarily due to their age, size and stature, as well as their growing location in relation to a high target area. 2.4 A previous report on the Lombardy poplar trees growing within St Omer Park compiled in 2007 by Mr. Richard Wanhill (Horttraining NZ Ltd) for QLDC was reviewed by GreensceneNZ Limited to ascertain whether there were any health or structural issues pertaining to the failed tree. Whilst it wasn t clear from the report exactly which tree had failed (due to a numbering issue), the report suggests that the Lombardy poplars within the eastern aspect of the park were, generally, in good condition. It is believed that Mr. Wanhill had carried out a resistograph test on the south-western side of the subject tree Page 2 of 10

3 2.5 A further inspection of the Lombardy poplars within St Omer Park was carried out in 2010 where the data from a visual tree inspection (only) was compiled in the form of an excel spreadsheet. Again, it was unclear from the spreadsheet exactly which tree was the one that had recently failed, however, the data does not in any way strongly imply that the tree was in imminent danger of failing or suggest ill health. 3.0 Site Description 3.1 The Lombardy poplar tree that failed is located within the eastern aspect of St Omer Park which bounds Lake Esplanade to the north and Lake Wakatipu to the south. St Omer Park is a popular recreational facility which is enjoyed by tourists and local residents alike. The park also serves as a thoroughfare between the eastern end of Lake Esplanade and the Fernhill Road / Glenorchy-Queenstown Road roundabout. 3.2 The subject tree was one of several Lombardy poplars that form a discontinuous stand of trees growing within the eastern aspect of St Omer Park. Further Lombardy poplars form a contiguous row of trees within the western aspect of the park. The Lombardy poplar trees are a prominent feature of St Omer Park providing amenity value to the lake-front and Lake Esplanade. The trees are also highly visible from the western extent of the Queenstown township. 4.0 Tree Assessment Methods 4.1 Resistograph tests were used to carry out analysis of the structural integrity of the failed Lombardy poplar. 4.2 The resistograph drills a fine diameter needle into the wood and measures drill resistance as it rotates. The drill is between mm in diameter and 300mm or 500mm in length. The drill resistance concentrates at the tip which is between 2mm 3mm in diameter (Rinn et al. 1996). During a resistograph test, a chart is produced which depicts the profile of the drilled wood. From the chart, structural integrity, growth rates and areas of decay can be assessed. 4.3 A visual assessment of the failed tree was carried out in conjunction with the resistograph analysis. 5.0 Arboricultural Assessment Species Profile 5.1 The Lombardy poplar is a variety of the black poplar (Populus nigra); so-called because it originates from the Lombardy region of Northern Italy (Tebbs, 1984). The Lombardy poplar spread worldwide in the eighteenth century (Wood, 1994) and was introduced into New Zealand during the 1830 s as a shelter and amenity tree on farms and public places (National Poplar and Willow Users Group, 2007). Page 3 of 10

4 5.2 The Lombardy poplar is a rapidly growing tree with a distinctively columnar shape, often with a buttress base (Wood, 1994) and attains a height of 30 metres (100ft) (Tebbs, 1984). The Lombardy poplar is a shallow rooted tree but stabilises itself by forming a large root plate and stiff buttresses. The species generally forms buttress roots on the windward side of the tree, especially when growing on high water table sites. The tree is resistant to root delamination and, if it does fail, it is usually because of extensive butt-rot. The Lombardy poplar s restricted rooting depth anchors itself in a way that tropical trees balance themselves on wet land by means of their giant buttress roots (Mattheck and Breloer, 1994). Despite the Lombardy poplar tree s buttress root form, like other members of the genus, the wood is weak and is prone to breaking (Wood, 1994). Whole Tree Failure Analysis 5.3 Being a 30 metre high veteran tree, the Lombardy poplar that failed in St Omer Park was generally consistent with the age and form of the neighbouring trees of the same species growing within the park. 5.4 The Lombardy poplar tree failed in high winds, causing it to snap at the base of the tree. Upon post-failure inspection of the tree, parts of the base of the trunk were hollow with large areas of rotten wood (indicative of heartwood decay) present. 5.5 A closer inspection of the decayed area revealed soft spongy wood indicating the Lombardy poplar was afflicted with white rot. White rot fungi cause the lignin (wood) and hemicellulose within the wood to break down first leaving behind the cellulose fibres which provide the tree with tensile strength and flexibility (Weber and Mattheck, 2003). In this case, the degradation of lignin has resulted in a loss of strength within this aspect of the tree. 5.6 It is common for shallow rooted trees such as the Lombardy poplar to topple like a coat stand should whole tree failure occur (Mattheck and Breloer, 1994) however in this instance, the tree snapped at the base, leaving the root plate firmly in the ground. This is consistent with the observations and personal comments of a local tourist who anecdotally described the subject tree as bending sharply in the wind before it snapped at the base and came to a rest across Lake Esplanade. 5.7 The Lombardy poplar is likely to have snapped rather than uprooted due to the hollow area within the base of the tree. The abrupt transition from the hollow base to the solid main stem of the tree has resulted in a concentration of stresses around this transition zone. As the tree has not been able to optimise its form to accommodate the cavity, the increased force applied by the recent wind event has resulted in a cross-sectional collapse (Mattheck and Breloer, 1994). The remaining sections of the trunk have taken on the form known as devil s ear (depicted in Figure 3 of this report which shows where the tree broke at the hollow/solid wood transition zone (Mattheck and Breloer, 1994)). 5.8 An inspection of the trunk flares of the subject tree revealed internal decay present within the specimen. However, at the time of the post-failure inspection, of the area immediately accessible for visual inspection, there were little or no obvious outward signs to suggest the presence of significant internal decay or a compromised base. No sign of fungal fruiting bodies on the external portions of the trunk were immediately visible at the time of post-failure visual inspection. 5.9 Lombardy poplars are fast growing, short lived trees and it is not uncommon for decay to be present within the base of a trunk. Given there were no significant outwardly visible signs of ill health, in order to ascertain the presence of extensive internal decay, it would have been necessary to undertake an internal decay detecting assessment (i.e. through resistograph tests). Page 4 of 10

5 Resistograph Analysis 5.10 The results of the resistograph tests undertaken on the failed Lombardy poplar are consistent with the hypothesis that the cause of whole tree failure was due to a large hollow and areas of decay present within the base of the main stem of the tree. Please refer to the photos and resistograph results in Appendix B and C respectively that support this hypothesis Figure 6 (contained in Appendix C of this report) depicts a resistograph reading of the cross section of the buttress roots at the base of the southern aspect of the tree. The results show a consistent low amplitude within the graph indicating that decay within this area of the tree. These results are visible in Figure 4 and 5 (as shown by the discoloured wood) Resistograph tests carried out within the main stem approximately 1.4 metre from the base of the tree show a much higher amplitude within the resistograph readings. These results suggest that the main area of decay was within the base of the tree thus allowing the creation of a hollow within the trunk (refer to Figure 7 in Appendix C of this report). 6.0 Conclusions 6.1 A mature Lombardy poplar located within St Omer Park, Queenstown failed in strong winds coming to rest across Lake Esplanade and causing damage to surrounding property. 6.2 The Lombardy poplar failed due to heartwood decay caused by a white rot within the base of the trunk. This caused the tree to snap at the base rather than uproot (i.e. root plate failure). The results of the resistograph test provide further countenance of the tree s failure. 6.3 A post-failure inspection revealed little outward visible indications that the subject tree was afflicted with internal decay before it failed. References Barry Tebbs, Trees of the British Isles. Orbis Publishing Limited, London. Christina D. Wood, A Most Dangerous Tree : The Lombardy poplar in Landscape Gardening. Arnoldia, The Arnold Arboretum of Harvard University Vol 54, Number 1, Winter Claus Mattheck and Helge Breloer The Body Language of Trees, A Handbook for Failure Analysis. TSO (The Stationary Office) Publishing. K. Weber and C. Mattheck, Manual of Wood Decays in Trees. Published in the UK by the Arboricultural Association. National Poplar and Willow Users Group, Growing Poplar and Willow Trees on Farms, Guidelines for Establishing and Managing Poplar and Willow Trees on Farms. Rinn, F Resistographic visualization of tree-ring density variations. Radiocarbon 1996, pp R. Wanhill, Resistograph Tree Assessment Reports, Speargrass Flat Road, Queenstown & St Omer Park, Queenstown Arborist Report. Page 5 of 10

6 APPENDIX A TREE LOCATION PLAN Lombardy poplar (approximate location) Lake Wakatipu

7 APPENDIX B PHOTOS Figure 1: Eastern view of failed Lombardy poplar indicates how the tree snapped at the base Figure 2: Photo depicts areas of white rot decay within the base of the tree

8 Figure 3: Devil s Ear formation defines the transition zone between the hollow base and the solid main stem Figure 4: Cross section of the trunk flares on the southern side of the tree. A resistograph test was carried out in this area. The low amplitude of the graph shows where the drill was meeting minimal resistance and this corresponds with the clearly visible decay (refer to following photo [Figure 5] for a clearer picture)

9 Figure 5: Cross section of decayed trunk flare with resistograph reading superimposed to illustrate the extent of the decay (so as to be readable). The amplitude of the graph reading is low or flat-lining in the most decayed areas where the drill has met little resistance within the wood

10 APPENDIX C RESISTOGRAPH READINGS Note that resistograph readings are read from right to left. Figure 6: Resistograph reading taken from the decayed section of one of the trunk flares as shown in Figure 5. The low amplitude and flat-line sections of the graph indicate areas of decay Figure 7: Resistograph reading taken approximately 1.4 metres from the base (estimated breast height). The reading shows that, whilst there are some low resistance areas, the structural integrity of the wood is what could be considered to be normal for this species. Note that where the reading has flat-lined on the right side of the graph, this is due to the bark which the needle drills through first.