Olympic Habitat Development Study
Three flying squirrels are flattened out having smashed into large trees in a forest. Captions says WHY FLYING SQUIRRELS DON'T DO WELL IN STEM-EXCLUSION FORESTS.

Forests naturally go thru a stem-exclusion stage -- this means there are too many stems for that size and trees will begin to die. Stands in the stem exclusion stage are typically simple in structure and stems may be close together. These stands are unsuitable for flying squirrels (and many other species) because they don't provide room to maneuver (glide if you are a flying squirrel or fly if you are a bird), and offer very few perches, little cover, and possibly limited food sources. Variable-density thinning is designed to move stands out of the stem exclusion stage more quickly and accelerate the development of characteristics such as structural complexity, large-diameter trees, variation in tree size and species composition, and of course, woody debris.