Roiling Canopy

Roiling Canopy's triggered ability has an "intervening 'if' clause." That means (1) the ability won't trigger at all unless, at the time a Forest enters under your control, you control five or more Forests other than that new one, and (2) the ability will do nothing if you control fewer than five Forests other than that new one by the time it resolves.

If a Forest you control leaves the battlefield between the time Roiling Canopy's second ability triggers and the time it resolves, be aware of whether that was the Forest that caused Roiling Canopy's ability to trigger or not. If it was, Roiling Canopy's count isn't affected; if it wasn't, Roiling Canopy's count goes down by one.

If multiple Forests enter under your control at the same time, Roiling Canopy's second ability could trigger that many times. Each ability takes into consideration the other Forests that entered at the same time as the one that caused it to trigger.