|MadSci Network: Environment & Ecology|
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.
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