During the 19th century, much of the mountain was denuded by logging, fires, and grazing.
Forests have since reclaimed the mountain.
Several forest communities exist on Mount Greylock.
Lower slopes are inhabited by northern hardwood forest species while upper summits are dominated by boreal balsam fir and red spruce.
The ridgeline of Greylock, between Mount Fitch on the north and Saddle Ball on the south, is the only place in Massachusetts where a taiga-boreal or sub-alpine forest flourishes.
Researchers have identified 555 acres (2.25 km²) of old growth forest on the mountain.
The steep western slopes (which include The Hopper) contain northern hardwood forest biome species up to 350 years old, including a 120-foot-tall (37 m) red spruce.
Because of its extensive stands of red spruce old growth, The Hopper has been designated a National Natural Landmark.

Mount Greylock is designated as an important bird area (IBA).
 There are a number of species of birds which breed in the taiga or boreal forests at the higher altitudes of the mountain, which are not normally found breeding in Massachusetts.
 These species include the blackpoll warbler and Bicknell's thrush.
eBird has records of 132 species of birds on Mount Greylock.
 There is less known about birds visiting the mountain in winter, as the mountain is more difficult to access during this time.
During the 19th century, much of the mountain was denuded by logging, fires, and grazing. Forests have since reclaimed the mountain. Several forest communities exist on Mount Greylock. Lower slopes are inhabited by northern hardwood forest species while upper summits are dominated by boreal balsam fir and red spruce. The ridgeline of Greylock, between Mount Fitch on the north and Saddle Ball on the south, is the only place in Massachusetts where a taiga-boreal or sub-alpine forest flourishes. Researchers have identified 555 acres (2.25 km²) of old growth forest on the mountain. The steep western slopes (which include The Hopper) contain northern hardwood forest biome species up to 350 years old, including a 120-foot-tall (37 m) red spruce. Because of its extensive stands of red spruce old growth, The Hopper has been designated a National Natural Landmark.
Mount Greylock is designated as an important bird area (IBA).  There are a number of species of birds which breed in the taiga or boreal forests at the higher altitudes of the mountain, which are not normally found breeding in Massachusetts.  These species include the blackpoll warbler and Bicknell's thrush. eBird has records of 132 species of birds on Mount Greylock.  There is less known about birds visiting the mountain in winter, as the mountain is more difficult to access during this time.
Mount Greylock