Article 2 - Qercus ilex, Vansittart Park, Mount Gambier

Prior to the City of Mount Gambier's annual "Carols by Candlelight" concert in Vansittart Park, a large limb was shed from a mature Quercus Ilex causing what appeared to be a significant amount of basal damage to the stem.

On initial visual inspection of this tree it would be safe to assume that the tree is in a state of structural instability. The wound caused by the loss of a large structural limb, along with the facts that the majority of the remaining structural limbs emanate from the point of previous branch failure, and the evidence of decay on two of these structural limbs would undoubtedly result in the conclusion that the tree was in a poor structural state and that removal would be inevitable.

Due to concerns for the safety of the tree and the fact that it was directly adjacent to the area of the proposed concert Mount Gambier City Council contacted ENSPEC to carry out a Picus Sonic Tomograph test to determine whether the failure was due solely to included bark or whether an element of decay had contributed to the failure.

Q. ilex Vansittart Park, Mt. Gambier

The tree tested was situated in Vansittart Park located in the centre of the City of Mount Gambier. Due to the location of the tree and its highly visible nature, its loss would have contributed significantly to the deterioration of the visual amenity of the local area.

The loss of the tree would have further impacted on a further Q. ilex situated directly adjacent to this tree. This is due to the canopies of both trees having formed a large single canopy; therefore if the damaged tree needed to be removed it would have left the remaining tree with an open, one-sided canopy that would have been exposed to the full extent of the prevailing wind conditions rendering it susceptible to future failure.
A visual inspection of the tree identified a large wound at approximately 1.8 metres above ground level (the point of branch attachment) that extended down the stem to nearly ground level, caused by the limb tearing out.

Large wound on Q. ilex Mt. Gambier

This was the main point of concern for the city council and accurate information was required on the amount of internal decay, if any, and whether the tree had other internal faults that could result in further failures if the tree was retained.

Areas of decay on large structural limbs on Q. ilex Mt. Gambier

The visual inspection also revealed the occurrence of what appeared to be large pockets of decay on two of the major structural limbs situated on the southern side of the tree.

The main concern with these was the fact that they were large heavy limbs that overhang the main road and any failure could result in significant damage

Therefore four individual Picus tests were undertaken: -
· The main stem at point approximately 1.8 metres above ground level (the point of previous attachment),
· The main stem at ground level and,
· One for each of the structural limbs that was identified with areas for concern.

Main stem at 1.8 m above ground level
Q. ilex Mt. Gambier.

The test at 1.8 metres was performed to determine the extent of any further internal problems and to assess whether any other limbs attached at this point were showing problems associated with included bark.

The test on the stem at 100mm above ground level was undertaken to determine the extent of any basal decay within the tree, whilst the two tests on the structural limbs were to assess the extent and spread of the decay identified.

The test undertaken at 1.8 metres identified that the failure of the limb was not as a result of any internal decay. The results also showed that the other limbs emanating from attachment points at this location were not exhibiting any internal problems associated with included bark.

The test clearly shows that all the internal parts of the tree, at this spatial position, consisted of 96% solid timber with only 4% altered or modified wood. When determining the structural stability of the tree and the potential for future limb shedding it can be concluded that further failure due to weakly attached limbs is negligible and no evidence identified to substantiate such a diagnosis.

The test undertaken at the base of the tree identified the entire circumference to be of 100% solid timber, with no evidence of internal decay present. This clearly showed that there were no other internal problems with the tree system that could be associated with, or contribute towards the failure of the large structural limb.

This test therefore allowed sound judgements to be made on the long-term retention of this large mature tree and conclusions drawn that the remaining tree section was structurally sound.

Main stem at 100mm above ground level Q. ilex Mt. Gambier

The two tests undertaken on the main structural limbs revealed only minimal decay beneath the area identified as point of entry during the VTA. The results clearly showed that the tree had effectively compartmentalised the decay and formed a barrier zone just beneath the surface of the point of entry. Therefore, conclusions were drawn from the results that the limbs are structurally sound with 100 % solid wood and could safely be retained.

Lower structural limb of Q. ilex Mt. Gambier - Upper structural limb of Q. ilex Mt. Gambier

The use of new technology allows detailed internal inspections of the tree system (at points determined via visual inspection) to provide conclusive evidence on the internal makeup of the tree thus allowing accurate recommendations to be made on tree retention and risk management procedures.

The Picus tests carried out on this tree have resulted in the recommendation that the tree should be retained, whilst some minor work was carried out to the thin the ends of the long heavy limbs, and future tests be carried out to determine the implications the damage caused by the tree shedding the limb has had on the tree as a whole.

Due to the size and location of the wound it will be inevitable that decay will have access to the basal area of the tree stem. However, due to the ability of the tree to effectively compartmentalise decay (as demonstrated on the two structural limbs) it can be assumed that the main stem will also resist the spread of any decay pathogens that access the wound.

The use of the Picus Sonic Tomograph can be used to effectively monitor trees over a set period of time, allowing retention of trees that under normal circumstances would have been removed.