The jury at the inquest into the death of Erena Wilson at Kew Gardens in 2012 has found there was insufficient evidence to attribute the cause of the branch failure that led to her death to summer branch drop.
Arboricultural consultant Jeremy Barrell, who produced a report which blamed summer branch drop for the Cedar of Lebanon branch fall, said he was "disappointed" but accepted the jury's decision.
The coroner declined to write a Prevention of Future Death report on summer branch drop's threat or on tree management systems, because she said Kew's systems had now changed and because summer branch drop was not proven to be to blame for the branch failure.
Barrell is now writing a paper on the phenomenon: "I support the idea that further research into summer branch drop is needed. Indeed, I would be happy to collaborate with Kew or any other suitable individual or body to share the information that I have collected with a view to clarifying what summer branch drop is, the conditions that trigger it and what precautions can be taken to reduce the risk of harm.
"Summer branch drop is not quite as rare as some sources suggest. My hope is that I can contribute to the development a management protocol that will reduce risks in a sensible, balanced and proportionate manner.
"I firmly believe that it is possible for trees and people to share the same space, but there is still some fine-tuning to do on the detail of how that is achieved in practical terms."
Wilson's parents, Chris Wilson and Elizabeth Shelley, said: "Our hope is that the circumstances of Erena's death will raise public awareness of the deadliness of the summer branch drop and lead to urgently needed funding and research into this phenomenon. If one death could be prevented and one family saved from having to go through this, then some good will have come from this tragedy."
Commenting on the phenomenon of summer branch drop after the inquest, International Society of Arboriculture (ISA) executive director Ian McDermott said it was difficult to "warn for something we don't fully understand".
He added: "London does seem to have an issue around summer branch drop because of the island heat effect which can mean it's 4 to 6 degsC warmer and is more likely to have the drying effect than in Birmingham."
He added that Kew is upgrading its Tree Risk Assessment and Management System (TRAMS) system, adapted 20 years ago from the Matheny and Clarke/ISA system to the ISA TRAQ system, which replaces 1-10 numeric risk ratings with a low/medium/high rating based on a series of likelihood assessments.
Kew said the "improved standardised and systematic process promotes the safety of people and property".
McDermott said Kew's tree management was good: "The jury considered it a robust management system."
McDermott said Dr Claus Mattheck is the world expert on summer branch drop but even Mattheck can't fully explain the summer branch drop phenomenon, identified in 1979 by Keith Rushforth and further researched by Richard Harris for a 1983 Journal of Arboriculture article.
The verdict Tree failure ruled out
Kew horticulture director Richard Barley said: "The jury had found that there was no identifiable cause of branch failure from this tree which caused this accident. We of course continue to extend our greatest sympathy to the family and friends of Miss Wilson affected by this tragedy.
"However, we can reassure everybody that Kew places the utmost importance on the safety of its tree collection for all of our visitors and employees. And as noted by the independent expert in this case there was nothing that could be foreseen, the were no action, that he felt could or should have been taken and that the trees were being managed in an entirely appropriate and responsible way.
"Summer branch drop is a very loosely-defined phenomenon. It's not well researched and there's very little data on it and, hence, I think it is unfortunate to seek to make, as in this case, something fit that loose definition."