This is the Torah: when a man dies in a tent, all who come into the tent and all that is in the tent shall be unclean seven days.
Torah Temimah: Shabbos 28a — Any product of a tree does not convey tent-impurity except flax. From where do we know this? Rabbi Elazar said: He derives “ohel” “ohel” from the Mishkan. It is written here “a man who dies in a tent” and it is written there (Pekudei) “and he spread the tent over the Mishkan”; just as there flax is called an ohel, so here flax is called an ohel.
English: Any product of a tree does not convey tent-impurity except flax. From where do we know this? Rabbi Elazar said: He derives “ohel” “ohel” from the Mishkan… just as there flax is called an ohel, so here flax is called an ohel.
Gezeirah shavah on the word “ohel.”
For the purpose of conveying corpse impurity as a tent, only a covering of linen (flax) qualifies among plant materials; other tree products do not.