Stemming originally (after the ruling of Rylands v Fletcher  relating to a landowner’s duty of care) from Giles v Walker  and Noble v Harrison , the latter having the judge conclude that “I see no ground for holding that the owner is to become an insurer of nature, or that default is to be imputed to him until it appears, or would appear upon proper inspection, that nature can no longer be relied upon…“, and slightly more recent court cases, beginning with Brown v Harrison  and Lambourn v London Brick Co Ltd , both of which concluded in line with Noble v Harrison , provide background guidance for modern-day court cases. Such modern-day cases also take the lead from the cases of Goldman v Hargrave  and Leakey v National Trust , of which the latter drew influence from the former.
Before visiting more contemporary cases however, further exploration is warranted into older cases. As noted in the case of Rylands v Fletcher , a landowner must accept that he or she has a duty of care to those using neighbouring parcels of land for their own purposes, by ensuring trees upon their land (that may affect adjacent properties) are as safe as is reasonably possible. This sentiment is echoed in Donoghue v Stevenson , where the judge remarked that a tree owner has a duty of care to “avoid acts or omissions” that can be reasonably foreseen to, if allowed to persist, cause harm to a neighbour. Of course, only where a defect is outwardly evident to a reasonable and prudent landowner will a successful case be made, as was recognised in the case of Kent v Marquis of Bristol . In this instance, it was ruled that if the elm tree in question had been inspected, then it would have been outwardly recognisable that the cavity present upon the structure was a hazard, and therefore should have been dealt with prior to a subsequent incident occurring.
Adding to this, where a tree becomes defective as a result of vandalism or at the hands of a trespassing individual, as soon as the landowner becomes aware of the issue (or should be reasonably expected to already be aware of the issue) then the tree must be made safe. This was the case in Sedleigh-Denfield v O’Callaghan , where the local authority trespassed onto a parcel of land and undertook works to ensure a ditch drained properly (by installing a culvert and grate), unbeknown to the landowner. However, such works proved to be ineffective because of the improper installation of a grate, which led to the culvert frequently being blocked. Because of this, the land frequently flooded, and eventually onto a neighbouring property where extensive damage was caused. The defendant on whose land the flooding emanated from claimed that he had no knowledge of the work being undertaken by the local authority, though the judge ruled that because the defendant had continued the allow the nuisance (which should have readily been identifiable) that he was to be liable for the damages. This has implications for the arboricultural industry, as it suggests that as soon as one is aware of a hazard relating to a dangerous tree, it must be appropriately actioned.
Similarly, where a tree has had pruning operations undertaken unto its structure, which have had the impact of making the tree less structurally safe (such as topping causing an abundance of sprouts, though even if pruning the tree in accordance with BS 3998), there is added impetus to follow-up with further inspections to ensure the tree is not posing an unreasonable danger to the target zone beneath. Such a stance was held in the case of Chapman v Barking and Dagenham LBC , and prior to this the case of Caminer v Northern & London Investment Trust ruled that whilst the lopping of the elm tree in that instance would be appropriate to remedy the situation that manifested when the tree failed and caused injury to the claimant, it would make the tree more dangerous in the long term and must therefore be inspected more frequently. In addition to this ruling, the case also held magnitude because the judge ruled that, as the tree was outwardly not defective, there was no reasonable justification in expecting the landowner to have instigated further investigation into the tree’s state. However, if the tree had warranted further investigation to such a layperson, they would have been expected to employ an expert to assess the tree more vigorously.
Reverting attention back towards more recent cases, Leakey v National Trust  was concerned primarily over a landslip and who was liable, if anyone, for the damage caused. The court did however utilise the case to expand upon the issue of natural phenomena, which ultimately included relevant conclusions for trees. The court concluded that there is a general duty with regards to ensuring hazards, naturally-borne or man-made, do not stem from the land of the owner and affect neighbouring land owners or guests on the site. The duty is “to do what which is reasonable in all circumstances, and no more than what, if anything, is reasonable, to prevent or minimise the known risk of damage or injury to one’s neighbour or his property.” However, somewhat critically, it did determine that naturally-borne hazards that could have been be prevented can only having the ruling applied to them where “the defendant has, or ought to have had, knowledge of the existence of the defect and the danger thereby created.” As a result, one can expect that only hazards relating to trees that are relative to the expected knowledge of the owner of the land, or those employed to look after such land, to be actionable. Such a standard was in fact elucidated to in Khan & Khan v London Borough of Harrow & Helen Sheila Kane , where the court ruled that the issue of reasonable foreseeability is not a subjective test, but is an objective test as to what ought to have been known to a reasonable person in the position of the defendant: “In this case, the relevant person is a reasonably prudent landowner.” However, a defendant’s subjective knowledge can impose a higher standard, the court found. Therefore, local authorities and organisations who are responsible for trees may indeed be held to such a higher standard. Such a precedent has since been mirrored by many other more recent cases, including:
- Corker v Wilson , where as the defendant had carried out informal observations of the oak tree, which had seen one of its limbs fail and strike a passing motorist, injuring him and damaging the vehicle, on an ongoing basis, and all the evidence was that the tree was in good health. The judge ruled that there was nothing about the tree which should have alerted the defendant, or led him to obtain a more detailed inspection by an arboriculturalist.
- Atkins v Scott , where the judge ruled that it was “neither probable nor reasonably possible for a competent inspector to have observed the crack in the branch that failed“, after the branch of an oak tree failed and struck and injured a passing motorist during windy weather.
- Selwyn-Smith v Gompels , where an ordinary landowner’s tree fell onto a neighbouring garage. The claim was rejected as the judge deemed that the defendant had no requirement to engage with an arboricultural expert “unless and until reasonable inspection by the standards of that [the defendant’s] knowledge discloses or should disclose that the tree might be unsafe”.
- Ulsterbus Ltd v Sufferin , in a case where a double decker bus carrying school children collided with a branch within the crown of a tree and caused both damage to the bus and injury to some passengers, after it veered slightly to the left to allow for a large van to pass heading the other way, saw the court rule that “the defendant in this case was not aware of the risk” and “should not have become aware with reasonable care of any danger posed by the branch with which the bus collided.” The court drew influence from British Road Services v Slater , where the court ruled that the case should fail based on the fact that the claimant had pulled into his nearside to enable an oncoming vehicle to pass, thereby leaving the carriageway in part, in addition to the fact that both the claimant and the defendant had not, until this point, considered the tree a hazard.
- Micklewright v Surrey County Council , where it was found that extensive internal decay was a major factor in the failure of a large branch of a mature oak tree. The judge found that nobody had seen any external signs of decay and ruled that, even if the local authority had had in place a proper system of inspection, the extent of the decay, and the danger it posed, would not have been revealed.
- Bowen (A Child) & Ors v The National Trust , where the court ruled that a “risk assessment in any context is by its very nature liable to be proved wrong by events, especially when as here the process of judging the integrity of a tree is an art not a science, as all agree. [The court] accept[s] these inspectors used all the care to be expected of reasonably competent persons doing their job, and the defendant had given them adequate training and instruction in how to approach their task. To require more would serve the desirable end of compensating these claimants for their grievous loss and injuries. But it would also be requiring the defendant to do more than was reasonable to see that the children enjoying the use of this wood were reasonably safe to do so. I regretfully conclude that I cannot find that the defendant was negligent or in breach of its duty in respect of this tragedy.“
- Stagecoach South Western Trains Ltd v Hind & Anor , where it was determined that “there was nothing that should have alerted her [the defendant], or put her on notice, that the tree was anything other than healthy, or required a closer inspection by an arboriculturalist.” This case related to a 150-year old ash tree, whose stem had fallen onto the railway line from the garden of the property, which then resulted in a collision with an oncoming train. The court in this case drew influence from both Corker v Wilson  and Selwyn-Smith v Gompels , concluding that a system of informal observations by the landowner was adequate, and that an inspection by an expert arboriculturalist was only necessary if there was something revealed by the informal inspection which suggested that a more detailed inspection was required. The arboriculturalist was also found to not be liable for any wrongdoing, having worked on the tree more than one time prior to the incident. Again, there were no outward signs to suggest to him that there was evident risk associated with the ash’s presence.
However, in specific instances, it has been deemed that the land owners have not done enough to prevent hazards from materialising. Such cases include:
- Quinn v Scott , where it was decided upon that because the decay of the tree was so visibly apparent, it should have been felled in response to the clear hazard presented. The judge stated: “The duty of the Trust is to take such care as a reasonable landowner – and that means a prudent landowner – would take to prevent unnecessary danger to users of the highway adjoining the Trust’s land. There is not to be imputed in the ordinary landowner the knowledge possessed by the skilled expert in forestry… But, in my opinion, there may be circumstances in which it is incumbent on a landowner to call in somebody skilled in forestry to advise him, and I have no doubt but that a landowner on whose land this belt of trees stood, adjoining a busy highway, was under a duty to provide himself with skilled advice about the safety of the trees”.
- Chapman v Barking & Dagenham LBC , where it was concluded that “the defendant council did not at any relevant time appreciate the distinction between making lists of trees and routine tree maintenance, and systematic expert inspection as often as would reasonably be required”, thereby finding them liable of negligence and not remedying a hazard that could have readily been identified.
- Poll v Bartholomew . This High Court case covered the standard of the duty of care and decided that, in this set of circumstances, a drive-by check was not a sufficient level of inspection and the claimant succeeded. The claimant had collided with a fallen tree that had extensive decay, though this was not picked up on by inspectors given the poor extent of inspection. This case is particularly important as it suggests different levels of inspection and competence are to be applied, depending upon the knowledge extent of the owner.
However, the case of Stovin v Wise  raises an interesting point, in that “The mere existence of statutory power to remedy a defect cannot of itself create a duty of care to do so. A highway authority need not have a duty of care to highway users because of its duty to maintain the highway“. For example, under Section 154 of The Highways Act 1981, a local authority is not necessarily to be held liable if they do not exercise the power under Section 154 to remove a threat to the highway and its users.