Modelling stem diameter class distribution with Weibull distributions


The two-parameter Weibull probability distribution is commonly used to model the distribution of stem diameters in forests. The scale and shape parameters can be used to compare different forest stands to see how their distributions vary. I created a function in R which calculates these parameters per plot, basically a wrapper around MASS::fitdistr(). It’s worth noting that FPC refers to adjustments by sampling effort that may occur in plots with a nested sampling design:

#' Fit a two-parameter Weibull distribution of diameter size classes
#' @param x dataframe of stem measurements
#' @param plot_id column name string of plot IDs
#' @param diam column name string of stem diameters
#' @param fpc column name string of stem FPC values
#' @param return_fit logical, optionally return a list of model objects instead
#'     of a dataframe of parameters
#' @return dataframe with the shape and scale parameters, and the standard 
#'     errors of each parameter for each plot-level Weibull distribution
#' @details The two-parameter Weibull distribution is a probability density 
#'     function of the form: \eqn{f(D) = c/d(D/d)^{c-1}  exp(–(D/d)^{c})}, 
#'     where $c$ is the shape parameter, $d$ is the scale parameter, and $D$ 
#'     is the stem diameter. To deal with variation in sampling effort, this 
#'     function duplicates rows based on their FPC, rounding the number of rows 
#'     up or down. E.g. a stem with FPC=0.08 would be duplicated 13 times, 
#'     because $1/0.08=12.5$. The Weibull distribution is fitted here using 
#'     Maximum Likelihood. $c$ is closely related to the mean stem diameter 
#'     in most cases. $d<1$ indicates a concave exponential which becomes more 
#'     extreme as $d -> 0$, while $d>1$ indicates a hump-shaped 
#'     relationship which becomes increasingly left-skewed as 
#'     $d -> inf$.
#'     Note that the Weibull distribution is not capable of representing 
#'     bimodal or more complex diameter distributions, which can occur in 
#'     disturbed woodlands. 
#'     Any NA diameters will be excluded from the calculation
#' @examples
#' @importFrom MASS fitdistr
#' @export
diamWeibullGen <- function(x, plot_id = "plot_id", diam = "diam", fpc = "fpc", 
  return_fit = FALSE) {
  x_split <- split(x, x[[plot_id]])

  out <- lapply(x_split, function(y) {
    y_dup <- y[rep(row.names(y), round(1 / y[[fpc]])),]

    y_fil <- y_dup[![[diam]]),]

    fit <- tryCatch({
      suppressWarnings(MASS::fitdistr(y_fil[[diam]], "weibull"))
    error = function(cond) {

    if (return_fit) {
      ret <- fit
    } else { 
      if (!is.list(fit)) {
        ret <- data.frame(
          weibull_shape = NA,
          weibull_scale = NA,
          weibull_shape_se = NA,
          weibull_scale_se = NA,
          plot_id = unique(y[[plot_id]])
      } else {
        ret <- data.frame(
          weibull_shape = fit[[1]][1],
          weibull_scale = fit[[1]][2],
          weibull_shape_se = fit[[2]][1],
          weibull_scale_se = fit[[2]][2],
          plot_id = unique(y[[plot_id]])

  if (!return_fit) {
    out <-, out)
    names(out)[names(out) == "plot_id"] <- plot_id


I created another function which fits a Weibull distribution and extrapolates it to estimate the number of stems within arbitrary size classes:

#' Estimate lower size-class abundances in plots with higher minimum diameter thresholds
#' @param x dataframe of stem measurements
#' @param plot_data dataframe of plot data
#' @param binwidth 
#' @param min_diam_thresh column name string of minimum diameter threshold in \code{plot_data}
#' @param diam column name string of stem diameter in \code{x}
#' @param min_limit lower diameter limit down to which to estimate stem abundance
#' @param size_classes vector of size classes 
#' @return dataframe of stem abundances per size class
#' @examples
#' @export
diamWeibullEst <- function(x, plot_data, bins, 
  stem_plot_id = "plot_id", plot_plot_id = stem_plot_id, 
  min_diam_thresh = "min_diam_thresh", diam = "diam", fpc = "fpc") {

  # Get plot IDs in stems and plots
  plots_all <- unique(c(x[[stem_plot_id]], plot_data[[plot_plot_id]]))

  # Which plot IDs are shared?
  good_plots <- plots_all[plots_all %in% x[[stem_plot_id]] & plots_all %in% plot_data[[plot_plot_id]]]

  # Plots not in stems 
  plots_not_in_stems <- plots_all[!plots_all %in% x[[stem_plot_id]]]

  # Plots not in plots
  plots_not_in_plots <- plots_all[!plots_all %in% plot_data[[plot_plot_id]]]

  # Plots with no min diam thresh
  plot_no_thresh <- plot_data[[[min_diam_thresh]]), plot_plot_id]

  # Warning for plots with no min diam thresh
    "Some plots have NA diameter thresholds:", "warning", 15)

  # Warning for any plots which have been dropped 
    "Some plots not present in plot data, will be NA:", "warning", 15)

    "Some plots have no stems, will be 0:", "warning", 15)

  # For each plot 
  out <-, lapply(plots_all, function(y) {

      # Get stems and plots
      x_fil <- x[x[[stem_plot_id]] == y & ![[diam]]),]

      plots_fil <- plot_data[plot_data[[plot_plot_id]] == y,]

      # Find min diam thresh
      if (nrow(plots_fil) > 0) {
        min_diam <- min(plots_fil[[min_diam_thresh]], na.rm = TRUE)

        # Filter to plot diameter threshold
        x_fil <- x_fil[x_fil[[diam]] >= min_diam & ![[diam]]),]
        # Calculate weibull 
        if (nrow(x_fil) > 0) {
          weib <- diamWeibullGen(x_fil, diam = diam, plot_id = stem_plot_id, 
            fpc = "fpc", return_fit = FALSE)

          # Get replicated rows by FPC
          x_dup <- x_fil[rep(row.names(x_fil), round(1 / x_fil[[fpc]])),]

          # Extract Weibull parameters
          weib_shape <- weib$weibull_shape
          weib_scale <- weib$weibull_scale

          # Find number of stems used to generate the distribution
          nstems_obs <- length(x_dup[[diam]])

          # Find predicted proportion of stems this represents
          prop_obs <- unname(exp(-(min_diam / weib_scale)^weib_shape))

          # Estimate total number of stems
          nstems_total <- nstems_obs / prop_obs

          # For each bin, calculate estimated proportion of stems
          n_est <- unlist(lapply(bins, function(z) {
            # Estimate proportion of stems in category of interest
            prop_cat <- unname(exp(-(z[[1]] / weib_scale)^weib_shape) - 
              exp(-(z[[2]] / weib_scale)^weib_shape))

            prop_cat * nstems_total 

        } else {
          n_est <- 0
      } else {
        n_est <- NA_real_

      # Create pretty bin chars
      diam_class <- unlist(lapply(bins, function(z) {
        paste(z[[1]], z[[2]], sep = "-")

      # Create dataframe of output
      ret <- data.frame(y, diam_class, n_est)
      names(ret)[1] <- plot_plot_id 

      # Return

  # Return

The function models stem numbers reasonably consistently among plots, but tends to under-estimate the lower diameter size classes:

Estimated diameter size classes vs. observed

In the plot each set of points connected by a line is a plot, and each point is a diameter size class, with the proportion of stems observed within that size class on the x axis, and the proportion of stems estimated by diamWeibullEst() on the y axis. As you can see, large diameter size classes are slightly over-estimated in their proportional abundance, and the smallest size class (5-10 cm) is consistently under-estimated. This is probably because the Weibull distribution will dip down towards zero unless the shape parameter (k) is less than 1.