As long as the dryads are a part of the forest, the forest is a part of the army.

Natural Affinity

All lands become 2/2 creatures until end of turn. They're still lands.

Ninth Edition

Until end of turn, all lands become 2/2 creatures that are still lands.

Eighth Edition

Mercadian Masques

All lands become 2/2 creatures until end of turn. They still count as lands.

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.