The limits of Groot’s power remain unclear, but the regenerative abilities he’s exhibited thus far call into question a Guardians of the Galaxy scene.
Groot’s reaction to his arms being removed in the first Guardians of the Galaxy doesn’t make any sense. Groot has become one of the most beloved characters in the MCU, an impressive feat considering he is simply a tree whose catchphrase—and indeed all of his other dialogue—is his own name. However, the full extent of Groot’s power still remains somewhat unclear, and his regenerative abilities are rather inconsistent throughout the franchise.
In the first Guardians of the Galaxy, Groot loses both of his arms during his first encounter with Gamora. Before being sent to The Kyln, the Nova Corps takes mugshots of Groot and the Guardians of the Galaxy, during which Groot’s arms are still noticeably damaged. But within the next few scenes, his arms have returned to their normal size, leaving barely a scratch behind as evidence of his injury.
Groot’s ability to regenerate his arms specifically is well-established throughout the franchise. Even teenage Groot grows out his limbs to provide Thor the handle for his new ax, Stormbreaker, in Avengers: Infinity War. So it is not his ability to regenerate that is confusing, but rather his reaction to having lost his arms in the first place. The dismemberment is played up as a significant injury, but with how quickly it heals, it was barely an inconvenience. This represents a clear inconsistency across the series and MCU at large.
Given that Groot regenerates so quickly, however, it seems strange that it has taken him so long to grow back to his adult form. Groot went from a smart, dancing baby in the Guardians of the Galaxy credits to a quiet, almost angsty teenager in Avengers: Infinity War, but half a decade has passed since and Groot has seemingly remained much the same size. His growth is certainly accelerated compared to that of a human, but not so significantly as one might have imagined given the speed at which he can regrow his limbs.
Director James Gunn has made clear on several occasions that this new Groot is a separate character from the Groot that sacrificed himself in the Battle of Xandar, so perhaps Groot II will always be smaller by comparison. But some fans have speculated that Groot II may even grow to surpass his predecessor, with Groot actor and Drax inspiration Vin Diesel suggesting there will yet be an adult Groot back in 2017. Regardless, the hike in power level that is likely to take place in Guardians of the Galaxy Vol. 3 is sure to further demonstrate just what exactly Groot is capable of.