Sorry @ZarulSaurus - I misspoke:
The box plots in the upper figure represent the distribution
of the selected alpha diversity metric for each group of
samples at each even sampling depth. The lower and upper
whiskers of the box plot are the 9th and 91st percentiles of
the distribution (respectively), while the lower and upper
extents of the box are the 25th and 75th percentiles of the
distribution (respectively). The horizontal bar through the
middle of the box is the median of the dstribution
(i.e., the 50th percentile). Outlier points of these
distributions are not shown.