To open the Guardian Tree in Kena: Bridge of Spirits you need to collect the three owl statues around the Guardian Tree entrance and place them on the blue stones to the right of the entrance using the Rot.
The first owl statue is to the left of the Guardian Tree entrance behind a log.
Use the rot to pick up the statue and guide them over to the three blue stones where they need to set down the statue onto one of the stones.
The second statue is to the right of the Guardian Tree entrance near some big rocks and a tree stump.
The third and final owl statue is behind the Guardian Tree entrance near some cliffs overlooking the river.
Here are all the owl statue locations on the map.
After you have put all three owl statues on the blue rocks you need to stand in front of the Guardian Tree entrance on the big round stone and use the pulse ability to open the Guardian Tree.