Tentster, you are more likely to see pines colonizing the semi-shaded understory in poplar and paper birch forests... as Dan described, the deep shade of sugar maple forests like those in the Big Crow old growth pine reserve won't allow pine seedlings to survive. IIRC, there are some poplar/birch forests west of Whitney north of Galaeiry and west of Brent in the Cauchon area (the nature reserve there was created to protect a poplar/birch forest over formerly burnt-over lands and these might be colonized with young pines if there are any seed trees in the area). And many more scattered throughout the east side of the park.

Hemlocks are the most shade-tolerant of all the dominant forest trees and they will colonize in the heavy shade of sugar maples, but the process is very slow due to the low light levels and the time needed to transform from a sugar maple forest to hemlock might take hundreds of years. This also requires freedom from disturbance such as fire, wind, insects, or logging, so the chances of any given area escaping disturbance to allow hemlocks to colonize over a time frame of hundreds of years is less. But pines and poplars will move in after disturbance and both being fast-growing species, will grow up quickly to form forest cover.

The recent burn near Achray should show some poplar, birch and pine seedlings colonizing in several years and Bob and Diana McElroy may be documenting this (see their recent post on a TR on the Petawawa).