What is the ruling on "eco-burials" or "biodegradable funerals" that involve placing the deceased in a biodegradable pod and burying it underground with a tree planted above the pod, considering this as "reviving" or "giving life back" to the spirit of the deceased?

The burial method mentioned in the question, involving placing trees, is impermissible because it is an innovation (bid'ah) and an addition to the religion. Allah Almighty has legislated burial in the earth, and the prophets and messengers acted accordingly. Allah Almighty said: (From it We created you, and into it We will return you, and from it We will extract you another time.) [Taha/55]. Al-Baghawi said in his commentary on Allah Almighty's saying: (Have We not made the earth a container for the living and dead?) [Al-Mursalat/25-26], that the earth contains the living upon its surface and the dead within its belly.

Ibn Abbas narrated that the Prophet (peace be upon him) said: (The lahd (niche) is for us, and the shaqq (trench) is for others.) Both are within the earth.

It is not permissible to plant trees on graves, neither cacti nor any other type, nor to sow them with barley or wheat, because the Messenger of Allah (peace be upon him) did not do so, nor did his Rightly Guided Caliphs. What the Prophet (peace be upon him) did, by planting a palm branch on two graves, was specific to him and to the two graves whose torment Allah had revealed to him. The Prophet (peace be upon him) prohibited introducing anything into the religion that is not part of it, and he forbade plastering graves, building upon them, sitting on them, and writing on them.

As for the belief that planting a tree or placing it on a grave brings new life or "revival" or "restoration of life," this is among the delusions of the people of Jahiliyyah (pre-Islamic ignorance) and the beliefs of the atheist transmigrationists. A person's life, once it has departed, never returns to him except when Allah raises those in the graves, and his soul is with its Creator; his soul does not go to a tree or an animal.