What is the pollination of tree ferns?

The tree ferns have the same 'pollination' process as do all other ferns. BUT it should not be called pollination, for no pollen is involved - fertilization or reproduction are better terms.

Ferns develop spores in clusters, called sori, often on the underside of the frond (probably to protect them from moisture?.) But some species develop the spores on the edges of the frond. Each sori has many spores.

These tiny dust-like spores float off and those that land by chance in a suitable area, develop into a small "proto plant" called a prothallus.

This is a tiny plant, with a single heart-shaped leaf, and it generates both some antheridium, and some sperm. The sperm of a prothallus are remarkable, for they are motile - they can locomote across a moist surface to meet up with the antheridium, and then these two join and fertilize.

From this union, the new fern plant arises. Some of the details vary among various species of ferns, and the above description is a general one.