This can be determined by using a variation of the Drake Equation.
T = T* fp ne fl fi fc fL
T* represents the number of tress on earth
fp is the fraction of tress that are accessible to men
ne is the number of tress per square mile capable of holding a cache
fl is the fraction of trees in ne where that can be reached.
fi is the fraction of fl where decent size holes form
fc is the fraction of fi that are found by man
fL is fraction of the trees life during which the cachers see them.
T, the number of practical trees capable of a cache in the world.
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