Paper-bark tree (Melaleuca cajuputi) is a common exotic species introduced to Hong Kong for early afforestation (AFCD, 2006). It can be commonly found in Country Park areas. The concern tree was located in a recreational site. It was a mature tree of about 14m in height and 815mm DBH. The canopy width at the widest was 8m, with normal foliage density, foliage colour and leave size.
The overall impression of the tree was a health one. However the tree had potential structure problem. As the tree was located in a popular recreation site, soil compaction around the tree was found. The structural roots were likely to be adversely affected. The major concern was on the trunk. It had significant codominant trunks and included bark, with one trunk was slightly larger than the other one. Both of them had significant weight which could potential cause serious injury to pedestrians passing by. The tree stood at the edge of a group of tree, meaning it was subjected to wind nearby. Included bark could be a major structural weakness if internal crack was within (Slater, 2015). It was a popular tree failure cause especially after severe storm. However further assessment involving usage of resistograph or tomograph could not be done without permission. Details of the internal crack remained unknown.
Risk Abatement
Trees with heavy bark inclusion were recommended to be pruned when they were small tree (Shigo 1989). However the concern tree was already a mature one with significant size of bark inclusion. Complete removal of either one trunk would lead to a large wound which might further lead to invasion of fungi. Consider if one of the trunks breakdown at the inclusion point, heavy impact was expected.It was recommended to have a 10-15% crown reduction pruning to reduce the weight of branches, as well as to reduce the wind sail area of the canopy. Also a dynamic cabling system as suggested by Cobra (PBS, 2016) could be installed at two-thirds of the distance between the bark inclusion and the crown to restraint the movements of the trunk (Smiley and Lilly, 2007).
After Typhoon Mangkhut
This tree was re-inspected after the attack of super typhoon Mangkhut. The smaller trunk was torn apart from the main trunk,indicating the weakness caused by the included bark. It is likely the authority will remove the fallen part. Since a significant wound will be left behind after pruning, further observation is required on assessing if decay would be intrude the hardwood inside the main trunk.
Another Paper-bark tree (Melaleuca cajuputi) nearby
Another Paper-bark tree located near the one as mentioned was also studied. The aim was to show the location of the bark inclusion, as well as the sizes of the codominant trunks would cause great difference on the fate of trees.
This concerned tree was located in a popular recreational site around 20m next to the Paper-bark tree as mentioned. It also stood at the edge of a group of tree and was likely to experience similar wind force. It was a mature tree of about 15m in height and 600mm DBH. The canopy width at the widest was 8m. The foliage density was a bit lower than normal, while the foliage colour and leave size were normal.
The overall impression of this tree was health,though it was likely suffered from soil compaction and had structural defects. Consider the planted area was bounded by a small wall on one side, the growth of the root system was restricted inside drip line. Like the Paper-bark tree as mentioned, it had significant codominant trunks of very similar size. Both trunks carried significant weight. The bark inclusion was large and was close to the root.
This tree was re-inspected together after the attack of typhoon Mangkhut. There was not much difference at my first glance.However the codominant trunks were further split apart, probably caused by strong wind force. The bark inclusion was exposed to the air, causing potential fungal invasion. The major problem fell on the bottom part of the trunks. Though there was no soil upheaval found on the root plate, new cracks were found on both trunks nearby.
The tree was diagnosed as having a serious structural defect. It posed immediately danger to pedestrians and properties nearby. Neither crown reduction nor dynamic cabling system was able to mitigate the immediate danger as the trunks were too large in size. Furthermore weight added on the crown by further rain precipitation would aggravate the situation. Therefore there was no other way but recommended to remove the whole tree.
References
Agriculture, Fisheries and Conservation Department(AFCD), 2018. Enriching Our Countryside with Native Flora. Accessed on 27/11/2018 via:
https://www.afcd.gov.hk/english/conservation/con_flo/con_flo_enr/con_flo_enr.html
PBS Baumsicherungsprodukte GmbH, 2016. Cobra TreeBracing. Accessed on 20/11/2018 via:
http://www.cobranet.de/en/pdf/cobra_us_brochure.pdf .
Shigo, A.L., 1989. A New Tree Biology; Durham, New Hampshire, USA, Shigo and Trees, Associates.
Slater, D., 2015. The Anatomy and Biomechanical Properties of Bifurcations in Hazel, PhD Thesis to the University of Manchester, UK.
Smiley, E.T. & Lilly, S., 2007. Best Management Practices – Tree Support Systems: Cabling, Bracing, Guying and Propping (revised); Champaign, Illinois, USA, ISA Publication.
Portfolio of Tree Risk Cases 