Environmental Engineering Reference
In-Depth Information
combination of the line-intercept and plot-based methods in which field personnel
traverse a long, linear plot, counting all individuals within the belt-like plot.
This method is most effective for low density plant populations, shrubs, or trees.
Point-based sampling is another variation on the plot method, conducted with a
plot of one dimension. A long rod is vertically placed in the ground, and each plant
that intersects or touches the rod is recorded. This method is primarily good for
assessing understory species presence, but could be combined with a canopy tube to
assess tree presence at the point. The point-based method is relatively more
objective in that few observer decisions must be made, but it also supplies less
information per sample. Since most time in the field is spent moving between
locations, it is relatively inefficient. In addition, the technique offers limited
statistical flexibility because the wetland site as a whole is quantitatively
described (by all of the points), but each point sample is not (no abundance
information is collected at each sample). For these reasons, point-based sampling
is rarely used to provide a general description of the vegetation in a wetland,
although it is used by the Washington State Department of Transportation for
wetland monitoring due to its more objective nature (WSDOT Environmental
Services
2008
).
Plotless methods are location-restricted, but do not have any distinct boundary
within which to assess vegetation. They are most commonly used to assess tree
populations or forest communities, or relatively rare plant populations. There are a
wide variety of plotless methods (Bonham
1989
). The point-quarter (PQM) and the
Bitterlich methods are commonly used.
The PQM is a distance-based method that can be easily combined with nested
plot sampling because it has a central point that smaller quadrats or plots can be
centered upon. It also has the advantage of being usable by a single investigator;
however, it is most appropriate in forested settings and not in wetlands with bands
of herbaceous vegetation. These center points can be positioned randomly or
regularly throughout the wetland area, just like other plots. Once the central point
is determined, the surrounding region is divided into four 90-degree sectors (typi-
cally aligned with compass directions, but not always). The nearest tree or plant of
interest within each sector is selected, measured, and the distance back to the center
point is recorded (Fig.
5.4a
). If trees are of interest, diameter-at-breast-height
(DBH, 1.4 m from the ground) is typically measured. The PQM data can be used
to calculate the density of different species, the mean basal area, and the frequency
of different species in a forest (Barbour et al.
1998
). In order to calculate density, a
mean point-to-plant distance is calculated for all trees, with:
¼
2
000 m
2
Total density
=
ha 10
;
10
;
000
=
ð
mean distance in m
Þ
And the density (no./ha) for each species can be calculated by:
Relative density of species A
¼
ð
total no
:
of species A
Þ=
ð
total no
:
of all trees
Þ
total density
