Re: DO clearings occur naturally in forests

First, this answer will depend in part on how you define a clearing.  Do 
you mean the absence of trees?  And how big an area do you mean?
Two other major reasons for clearings are (1) EDAPHIC (soil based).  In 
many areas, there may be small zones of naturally shallow, extremely 
wet or extremely sandy or naturally toxic soils (for example those that 
develop from serpentine).  Any of these soil factors could prevent trees 
from becoming established. Barren areas can be found on mountainsides for 
similar reasons.
(2) Competition.  In some cases a natural disaster (fire, windstorm, 
disease) can cause a large opening in a forest. If grassses and shrubby 
plants become established, they can delay succession for a long time.  I 
know of areas where raspberry thickets, laurel thickets and grapevines have 
prevented tree invasion for at least 30 years.  I do not know how long they 
will be successful at preventing trees, but they do create a dense shade.