Why can some Lycopodium gametophytes take five or more years to reach sexual maturity?

In many species of Lycopodium, particularly those found in the north temperate zone, the gametophyte is subterranean and grows much more slowly than surface-dwelling varieties. The primary reason these plants require five or more years to reach sexual maturity is their reliance on an associated fungus for continued growth and nutrition.1

Unlike surface-growing gametophytes that are photosynthetic and can mature in as little as eight months to a year, these underground, non-photosynthetic plants must depend on this fungal association to develop their characteristic carrot, rod, or disk-shaped forms.1