Estimating optimal plot size without field data
If no field data is available, the function
estimation.plot.size can be applied to
determine the optimal plot design size. This function uses the data
frame containing the list of detected trees (introduced in
tree.tls) and estimates stand-level
density (\(N\), trees/ha) and basal
area (\(G\), m\(^2\)/ha) for many differently-sized plots
for the three plot designs (circular fixed area, k-tree and angle-count)
by increasing continuously their size.
Thus, circular fixed area plots with increasing radius (increment of
0.1 m) with the maximum radius defined by
radius.max in
plot.parameters (by default set to 25, if
radius is larger than furthest tree, the horizontal distance to this
furthest tree is considered as maximum radius) will be simulated and for
each plot, the variables (N and G) are estimated. Similarly, k-tree
plots with tree numbers (k) ranging from 1 to
k.max (specified in
plot.parameters, default value set to 50 or number of trees
in the plot) and angle-count plots with increasing basal area factor
(BAF, increments of 0.1 m\(^2\)/ha) to
the maximum value specified by BAF.max in
plot.parameters (set to 4 by default) are simulated and the
respective stand-level variables are calculated. Optionally, the minimum
diameter at breast height (dbh.min, in cm)
to include the trees in the estimations can be defined. By default the
minimum dbh is set to 4 cm.
The function generates size-estimation charts i.e., plots showing the
estimated stand-level density (N) and basal area (G) on the y axes
respective to the different plot sizes (x axes). The estimations will be
performed for all existing plots. By default the output plots will
contain one line for each plot. When
average is set to TRUE, the
average of all estimations and the standard deviation will be drawn
instead of multiple lines for each sample plot. One chart for each plot
design is drawn by default. If
all.plot.designs is set to
TRUE, the line charts of all three plot design will be
drawn in one plot.
estimation.plot.size(tree.tls = tree.tls,
plot.parameters = data.frame(radius.max = 25, k.max = 50, BAF.max = 4),
dbh.min = 4,
average = TRUE, all.plot.designs = FALSE)
The black line represents the average over all plots (in the example shown here 16 plots) of the estimated density on the left and the basal area on the right. The dotted line indicates the number of plots. This figure helps to find suitable plot design for the calculation of stand-level metrics and variables. The optimal plot design and size should be chosen within a range where the estimated values for N and G reach a stable level. A too small plot size leads to high estimation errors and the true value can not be estimated. For example, the basal area estimated for fixed area plots with radius smaller than 5 m is higher than the true value. On the other hand, too large plots come along with systematic errors due to occlusion of trees. Therefore, for example the basal area of fixed area plots with radius bigger than 20 m is estimated lower than the true value. In order to avoid both types of errors, the figure helps to find a plot size range with stable values.