A tree located at a park, used by members of the public every day.
A Lime that I had surveyed only a few weeks prior with no major concerns…other than works had been carried out by a water company who had left plastic gravel grids, designed to protect the root system, in the RPA. I recommended their removal as I was concerned about root compaction. Before this was completed vandals had set them alight.
Whilst on site assessing the condition of the tree I was able to speak to the owners of the adjacent properties who informed me of what had happened, the fire had reached approximately 15ft and burnt for 20 minutes before it had been put out. Residents wanted the tree to remain in place. I asked if any of the Lime tree has failed in past (there had been no signs of tear outs when I had surveyed the tree stock previously) and they confirmed that they had never had a problem with trees.
The fencing system that the water company used had remained in place so the area was effectively fenced off from the general public. With agreement from the Parish council it was arranged that we would monitor the condition of the tree over a period time to see how it responded to its wounds.
Diagnosis:
I knew the tree to be in a good condition before the fire. The damage was to the bark on the South West side of the tree and only the lower part of the crown. My concern was if the sapwood had been impacted, effecting the hydraulic abilities of the tree.There was no research on fire damaged urban trees that I could find, but existing research was vital in understanding the trees reaction please see APPENDIX
Additional Research:
Research by Ordonez et al (2004,112) found that the differences in survival rate of trees in a forest fire was significant, 55.2% of small trees survived compared with 84.7% of large trees. They classed a large tree as anything with DBH >20cm. They also concluded that crown damage on both parts of the crown resulted in a higher chance of death. This was explained as the thickness of the bark was the main mechanism to protect the cambium against fire and crown height in higher trees than in smaller ones. Considering that photosynthetic efficiency is greater in upper parts of the crown (Helms,1970) a fire at the base of the tree will burn less productive parts.
Injury to trees from fire can of course result in mortality that is not immediately present, due to environmental stresses the tree will face in the years afterwards (Hood&Bentz,2007) Research by Ganio & Progar support theory that percentage of crown volume scorched and the cambium kill rate are good predictors of post fire morality for tree species but note that errors can occur and it is possible to predict a dead tree will live and falsely predicting live tree will die (57,2017)
It is well known that many tree failures occur at the root plate and stem failure during strong winds, so it’s important to consider if the fire damage will impact the strength of the tree. A study by Cannon et al (2015) could not definitively conclude that fire damage increases tree vulnerability to wind damage due to the sample size but a revealing trend was witnessed. Fire damaged trees are more likely to uproot than snap, which is line with healthy trees, as the critical turning moment for failure was found to be 17% lower than the critical turning point for snapping.(196,2015)
Prognosis:
Theory suggests it’s a difficult call to make, on one hand the large is a mature specimen with a DBH of 203cm, the lower laterals are at 2.5m. The lower laterals on the north side have now died back, but it appears that the tree has developed buds that can be seen throughout the canopy. The very top of the canopy looks to be in a good order, but of course this cannot be confirmed without a climbing inspection. I would suggest that hydraulic activity and transportation is functional at this stage and the outer bark is still in place in many areas that I would have expected it to fail.
I would not wish to stress the tree any further and therefore would not recommend a reduction at this stage. I would recommend the removal of the lowest lateral as it is clearly dead and hanging over a footpath when it is reopened. I suggest the tree is monitored every three months to see progression and how its defences respond to inevitable secondary attack from fungi and bacteria.
Clearly it is a very difficult injury to predict or advise upon, hence why so little research has been carried out on this topic. More certainly is required. A risk averse society will make the call to remove, rather than provide time to recover. In this case a lack of funds, the trees location and my intervention have allowed a different approach.
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