"There is insufficient evidence to say with confidence that an offence of felling without a felling licence has been committed by SCC and Amey," it stated.
"However, the FC has identified a number of areas regarding the Streets Ahead programme where SCC has fallen far short of good practice."
- Poor record keeping; "the information shared by SCC in relation to the felling of each tree could and should have been more substantial" the report said;
- Poor engagement, both with the public and with the FC investigation itself;
- Felling as a first resort; "SCC consistently chose to meet their duties under the Highways Act by felling trees, rather than by maintaining them in situ."
- Poor strategic management of street tree population; "Rather than diversifying and creating a resilient urban forest, SCC and the contractor have perpetuated the same issues for later generations that they have been aiming to resolve for this one."
"SCC, and other local authorities, must take note of these lessons learnt for future operations," the report added.
The Commission said that while it had received allegations that trees were being illegally felled under the programme in autumn 2016, it had been content to accept the council's claim that the work was in accordance with its statutory duty under the Highways Act and the Equality Act.
However in early 2018, terms of the contract between SCC and Amey were made public, which included a commitment to fell 200 trees a year.
"This commitment left open the possibility that those trees were not felled in response to a statutory duty, but as a result of a contractual agreement and SCC policy decision," the FC report says. "As such, there was a credible possibility that a felling licence may have been required."
The Commission drew its conclusion from having reviewed historic photographs and Google StreetView images for "a significant sample" of tree felling records.
Sheffield Tree Action Groups representative Paul Selby said: "I’m disappointed that the Forestry Commission felt unable to proceed with a prosecution, although I understand that bringing such a case would be challenging given the burden of proof required, and the fact that Sheffield Council appear to have deleted all the evidence required.
"However I fully agree with their criticisms of Sheffield Council and Amey."