Forest structural dynamics is governed by tree growth and mortality; While the mean growth rate has been integrated in mathematical derivation of forest size structure, variance in growth is always ignored. Here by solving Kolmogorov forward equation, I show the significant difference in forest size structure predictions with and without considering of the variance, which just solved the puzzle of mismatch between observation and theoretical prediction in previous studies. An asymptotically power-law relationship between tree size and growth rate variance was found and statistically interpreted, which is the key link in connecting all the evidences in forest size structure formation.