Tree risk case 1

Case 1

The first case to be introduced is a mature Ficus microcarpa.  It is evergreen.  It is a pavement tree in tree pit form and is located outside No. 137-139 Fuk Wah Building, Luk Hueng Lane, Tai Po, Hong Kong.  The tree pit size is about 1.6 m x 2 m.  It is close to auxiliary residential buildings.

The tree had a single trunk.  Its diameter at breast height (DBH) measured at 1.3 m (AFCD, 2006) was 630 mm and the age class was mature.  It was 18 m tall with around 14 m crown spread.  The crown spread was measured in four directions as follows: 4.5 m in northeast (NE); 8.5 m in southeast (SE); 7.5m in southwest (SW); 8.5 m in northwest (NW). (Figure 2)

The tree had multiple stems.  They were divided at about 3 m above ground level, namely Stem 1, Stem 2 and Stem 3.  Stem 1 was growing towards NW with diameter about 300 mm.  Stem 2 was growing towards SE with diameter about 150 mm.  Stem 3 was growing towards SE and upwards with diameter about 100 mm.  (Figure 3)

There were epicormics found on tree crown but less than 10%.  They mainly found on Stem 1.  The tree vigor was good in general.  Furthermore, it leaned towards NW, with leaning 45° from vertical when measured at ground level and leaning 34° from vertical when measured at 1.3 m above ground.  It showed a sign of self-correction under phototropism according to its division and growing angle of the stems and branches.  Cement on the pavement was damaged behind lean but no soil cracks found.  It was a sign of root development on tension side.  (Figure 4)

Two major mechanical wounds, namely Wound A and Wound B, were found on the trunk (Figure 5).  Wound A was about 350 mm (L) (Length) x 245 mm ( W ) (Width) and at NW.  Wound B was about 100 mm (L) x 50 mm ( W ) and at SW.  The collar of Wound A was not formed completely.  According to mallet test, hollow sound was heard in the middle of Wound A.  Furthermore, there were a sum of lignified aerial roots formed under Wound A but were damaged.  For Wound B, the collar was formed and no further sign of decay found.Apart from the leaning mentioned in General Condition, there were no signs and symptoms of decay found on the trunk and there were few lignified aerial roots found damaged or encroached the trunk and formed a new part of the trunk.  Besides, there was some illegal parking (wheelchair) under the Tree.

The tension side of the Tree is at SE.  According to the aggressive root growth of the subject species, the surface roots were greatly restricted by the tree pit and surrounding cement.  Cement damage was found at the SE seriously but no root plate movement was observed. (Figure 6)

Recommendation

Short term remedial action

1. To remove the branches with fungal fruiting bodies such that the likelihood of failure of branches will be decreased from probable to possible.
2. Crown thinning and cleaning are recommended to removal the dieback twigs.

Long term remedial measures

1. To minimize the potential of damage to the surrounding buildings during inclement weather, crown reduction around 20-25 % after the breeding season of birds is recommended.  As the tree is near Tai Po Egretry, one of the Site of Special Scientific Interest (SSSI) Anon. (2016), AFCD should be consulted before commencement of tree works.
2. To enlarge the tree pit in phases is recommended for better growth space of tree root.  Extension in tension side at SE first is preferable with 0.5m extension to avoid sudden removal of weight on tree root.  Then SW, NW and NE.  Other works departments like Highways Department and Transports Department will be consulted for this enlargement of tree pit. 
3. To transplant the underneath young tree to another location for a better growth space for both the young tree and the subject tree.
4. To relocate the underground utilities near the tree.
5. To remove rental bicycle by fencing off the subject tree pit with horticultural fencing.
6. Level 2 and 3 assessments (application of sonic tomographic device on the major wound found on trunk) are recommended yearly.