Akira Carroll was struck on the head by the tree while on a firewood gathering trip with family at Mt Lloyd in the state's south on August 22, 2015.
He died in hospital the following day.
Joshua George Hector Clark, 40, the then-partner of Akira's mother Sierra Lynd, was sentenced in the Supreme Court of Tasmania in Hobart on Thursday.
Clark told Ms Lynd, Akira and his two siblings to stay in a ute while he took a chainsaw into the bush to cut down a green tree holding up a dead tree that he intended to use for firewood.
Clark, an experienced tree cutter, failed to follow his usual safe practice of parking 100 metres away from the tree or cutting a wedge into it.
The ute was only 22m from the tree, which was more than 30m tall.
Justice Stephen Estcourt said Clark lost control of the falling tree, describing the incident as an "aberration".
He noted Clark was remorseful immediately afterwards and during his trial and now suffers from post-traumatic stress disorder.
However, Justice Estcourt said Clark was culpably negligent and his actions were "so far short of what a reasonable person would have done".
He said if Akira had survived, Clark would have faced a lesser punishment.
"But he did cause death and he must accept responsibility for doing so," Justice Estcourt said.
Justice Estcourt said Akira's death had left his family devastated.
In a statement read to the court last week, Akira's father Brett Carroll described his son as a "sweet and gentle soul" who had a phenomenal bond with his brother and sister.
"(She) hears screams in her head and has to excuse herself from class ... because the screams are so loud," Mr Carroll said.
"He was her best friend and her biggest fan. She still can't fathom that she'll never get to see him again."
Clark will be eligible for parole after serving half of his sentence.
Justice Estcourt noted significant delays in the case, including in Tasmania Police's initial file preparations and during a period when police said Clark couldn't be found after he moved to Victoria.
Justice Estcourt said Clark, who was charged and extradited from Victoria in September 2018, had come from a background of trauma which was part of the reason he became involved in bushland wood cutting.
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