This function adds two variables to existing data frame of individual tree
measurements: 1) stand basal area and 2) the number of trees per hectare
Usage
add_stand_variables(df)
Value
a data frame with added stand variables: total stand basal area and
the number of trees per hectare
Arguments
df
a data frame with individual tree measurements that include basal
area and the upscale factors. All trees should also be described with plotID
and year variables