Hi Geoff, With your approach the RMSE is the estimate of standard deviation between predicted and actual biomass for each tree. The stand-level error is the standard error of the tree-level error, calculated as RMSE/sqrt(n) where n is the number of trees in the stand. Remember that RMSE applies to the population used to fit the model, the standard deviation of the predicted values (applied to the new population) is larger but it may not be possible to calculate it with the information provided with the biomass equations.
Hope this helps. Peter
