Your fungi are understory / soil saprophytes. Above them it is a thick and tangled mass of vegetation in the polar summer. No wind penetrates. If a spore is going to have a chance to find a new area suitable for growth (and not be outcompeted by its parent) it must move some distance from its parent.
Plants have this problem all the time: how to get the seed somewhere new. They have all sorts of solutions. I am not entirely clear why this is less problematic for fungi (spores are so small?) or maybe I just do not know about fungal evolutionary adaptions for this.
In any case: your fungus. If spores are released close to the ground they will land close to the parent. The higher they are released the better the chance for a breeze.
So the spore body grows, up past the plant life, pushing it aside. It grows fast, in a day or two, because it is mostly nonorganic. This spore body is largely water filled erectile tissue and maybe even gas-filled spaces - it needs to get big fast and long term sturdiness (like a plant with a place in the sun) is of no concern. The parent fungus has considerable water at its disposal and it pumps this into the spore body. This is unlike the sturdy prototaxites which did apparently persist for growth seasons, putting down growth rings like trees.
Once the tip of the fruiting body is above the mass of plants it explosively pumps its spores into the air from the tip, and collapses.
*re dead matter during daylight hours - fungi use the night time too. Rot never sleeps.