variables</code> generates a list in which
each element represents one of the considered plot designs (fixed area,
k-tree and angle-count plots). These elements are data frames containing
the estimated metrics and variables at stand-level as columns (described
below). Further columns include information about the plot, such as the
identification number (<code>id</code>) and the stratum identification
(<code>stratum</code>) both coinciding with the respective columns in
the output file of the <code>tree.dete