According to Trigger Night Vol.6, due to how closely magic is tied to trees (particularly, Yggdrasil, being the very source of magic itself), it grants witches longevity. In the case of more powerful witches, due to magic's constantly flow through them, they would transform into a tree upon death, which explain why Mrs. Holbrooke's grave is shown with a large tree grown on it, and why Beatrix's spirit appeared before Diana near a giant tree where the Ascension ritual was to be held for her to become head of Cavendish family. I managed to confirm it thanks to following references which I found after painstaking research:
https://www.reddit.com/r/LittleWitchAcademia/comments/6smxg9/lwa_info_from_trigger_night/
https://little-witch-academia.fandom.com/wiki/File:Boards_4chan_Witch_and_Tree_Connection_LWA.jpg
That being said, there's something I still uncertain about the intriguing info, which is whether witches physically (and literally) becoming the tree itself upon dying or simply manifesting a tree that grow over her grave following her burial with said tree's appearance reflects on amount of power she had in life and roots that shield her remains. I thought the latter idea more plausible that I included in Magic page in the wiki.
How about you guys?