I was thinking of using Geometric Brownian Motion to forecast future prices of timber (say one variable, the stumpage price of sawtimber).
I tested the time series with Augmented Dickey-Fuller test and found the data series as non-stationary which means the series follows a random walk. Then, I went on to use it for price forecast.
However, my professor comes and says we cannot use GBM to forecast future prices that has long horizon.
In my case, I wanted to use quarterly price into 10 years or 15 years into the future. I know GBM is a good model for stock prices for short periods, but is 10 or 15 years too far considering the confidence limits and probable volatility?