Presence-absence sampling for estimating plant density using survey data with variable plot size

1. Presence-absence sampling is an important method for monitoring state
and change of both individual plant species and communities. With this
method only the presence or absence of the target species is recorded on
plots and thus the method is straightforward to apply and less prone to
surveyor judgment compared to other vegetation monitoring methods.
However, in the basic setting all plots must be equally large or otherwise
it is unclear how data should be analyzed. In this study we propose and
evaluate five different methods for estimating plant density based on
presence-absence registrations from surveys with variable plot sizes. 2.
Using artificial plant population data as well as empirical data from the
Swedish National Forest Inventory we evaluated the performance of the
proposed methods. The main analysis was conducted through sampling
simulation in the artificial populations, whereby bias and variance of
density estimators for the different methods were quantified and compared.
3. Both for state and change estimation of plant density, we found that
the best method to handle variable plot size was to perform generalized
least squares regression, using plot size as an independent variable.
Methods where plots smaller than a certain threshold were excluded or
their registrations recalculated were, however, almost as good. Using all
registrations as if they were obtained from plots with the nominal plot
size resulted in substantial bias. 4. Our findings are important for plant
population studies in a wide range of environmental monitoring programmes.
In these programmes plots are typically randomly laid out and may be
located across boundaries between different land use or land cover
classes, resulting in subplots of variable size. Such splitting of plots
is common when large plots are used, e.g. with the 100 m2 plots used in
the Swedish National Forest Inventory. Our methods overcome problems to
estimate plant density from presence-absence data observed in plots that
vary in size.