That's a very good question. A couple of wild ideas: - They state there is a "significant ammonium concentration" in hair ice. Ammonium is a frequent byproduct

That's a very good question. A couple of wild ideas: - They state there is a "significant ammonium concentration" in hair ice. Ammonium is a frequent byproduct of metabolism, that becomes potentially harmful if concentrations become too high. Maybe hair ice allows the fungus to get rid of ammonium. (I'm extrapolating this from my knowledge of animal cell metabolism, where getting rid of ammonium is a recurring theme. I might be wrong on how well it applies to fungi.) - Hair ice also seems like it has the perfect shape to prevent the formation of solid sheet ice on top of the deadwood. No sheet ice -> better ventilation. - I also like sibling's theory of using it a means of dispersal, but the paper makes no mention of hair ice containing any fungus/spores at all.