Earlier in the year, Pro Climb was asked by the Auckland Council to provide a tree management solution for the notable Norfolk Island Hibiscus (Lagunaria patersonii) in Monte Cecilia Park, Auckland. This blog discusses how the team at Pro Climb approached the tree management options for this tree which included bracing and pruning. It also shares how the team worked together to determine the best outcome for the longevity and vitality of the tree.
The proposal meant we needed to address how to mitigate the defect in the trunk of the tree. Sadly, 14 years ago the trunk of the tree split from the crown to the base.
Most likely, the tree was planted around 1850 by William Hart who purchased the surrounding property in 1844. If you would like to read more about the history of the Monte Cecilia Park and the Pah Homestead, please follow this link to Wikipedia article. (https://en.wikipedia.org/wiki/Pah_Homestead)
Due to the heritage value and the focal point the tree provides within the park, the clients wished to retain the tree. Unfortunately, previous attempts (to stabilise the split by installing dynamic bracing and conducting some crown part reduction pruning) had been unsuccessful.
Creating our tree management solution
It was our job to create a tree management solution to stabilise the trunk, retain the aesthetics and preserve the tree for future generations. We were honoured to have the opportunity to provide the tree management solution, to work with this notable tree and to be trusted with this chance to sustain and support this amazing tree.
The current tree dimensions are:
- BHD: 204.5 (measured at 1.4m)
- Crown Spread: 20.5m (measured N-S and E-W)
- Height: 22m (measured with the app Arboreal from W and SE)
- Specific wood weight: 0.8722
When inspecting the tree, we found that although the split in the trunk increased to 30cm over the past years, the tree still showed a closed canopy.
This was one of the indicators that the tree has great vitality and would be able to deal with reduction pruning, and invasive bracing if necessary. A tree with less vitality would have shown a gap in the canopy, mirroring what was happening with the trunk.
The calculations showed that the tree is unlikely to fail as a whole. However, there was a risk of the tree splitting apart due to its crown weight and the defect in the trunk. The split in the trunk had to be stabilised if the risk of losing the tree was to be mitigated. A careful and considered tree management plan needed to take into account all of these realities. Our approach to pruning and bracing the tree needed to also be mindful of risks.
Bracing and Pruning the Hibiscus Tree
The tree has two large limbs and four leaders that are weighted as follows:
- Leader A – Upright leader weighted towards NNW
- Leader B – Horizontal large limb weighted towards NW
- Leader C – Upright leader weighted towards NNE
- Leader D – Horizontal large limb weighted towards E
- Leader E – Upright leader weighted towards SSE
- Leader F – Heavy leaning upright leader weighted towards SSW
By understanding the lean of each leader, we designed the bracing system to use counterbalance principles.
After calculating/estimating the total above-ground tree weight, we designed a static bracing system that would be able to carry the weight of the tree and prevent the further opening of the split.
The static bracing is designed from heavy lifting chains, shackles, hammerlocks, turnbuckles and webbing sling. It is designed to last a minimum of five years and will be inspected every 12 months.
We chose the triangle configuration for the bracing so that each leader will be supported by two bracings with a total load capacity of 34 metric ton.
We also conducted a 15% crown reduction on the trees canopy and removed all the previous dynamic bracing.
Ongoing tree management
The hibiscus tree will be pruned every 3-5 years. Through this repeated pruning, the canopy will be reduced from its current size to a size of 17m height and 15m crown spread over the next ten years. Once these dimensions are achieved, the tree will be retained at that size.
By staging the pruning over a longer time period the tree can adjust to the foliage loss and any negative effects of the pruning are mitigated.
We look forward to monitoring the development of the tree over the next years and decades to come. It will be interesting to continue to evaluate the tree management approach and to see the tree thrive.
The team at Pro Climb are grateful for the opportunity to work with this beautiful tree and delighted that everyone visiting the park can continue to enjoy it.
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