Environmental Engineering Reference
In-Depth Information
a
b
Fig. 5.4 Plotless sampling methods for trees. (a) The point-quarter method of dividing the plot
space into four quadrants and measuring the distance (d
x
) and diameter at breast height of the
nearest tree. (b) The Bitterlich method in which a central figure counts trees that are “in” and “out”
using a prism. If the prism trunk image overlaps actual trunk, the tree is counted. If there is no
overlap, the tree is not counted (Published with kind permission of
M. Kuchta 2014. All Rights
©
Reserved)
Basal area (BA) per hectare for each species can then be calculated by:
BA
=
ha of species A
¼
ð
Mean BA of species A
Þ
ð
Mean density of species A
Þ
The Bitterlich method is used to very rapidly calculate the total basal area of
trees in a forest, and so has the most application in forestry. Today, hand held glass
wedges, called “prisms”, are used to carry out Bitterlich sampling. By standing in a
central location, the worker uses the prism, which is calibrated to a specific basal
area factor (BAF, usually 0.929-1.858 m
2
/acre) to determine whether surrounding
trees are included in the sample or not. In general, a tree that is closer or larger is
more likely to be included than a distant, small tree (Fig.
5.4b
). The number of each
species included in the sample is then multiplied by the BAF to obtain the number
of m
2
/acre for each species. Although the inability to obtain density information
from this method is a definite drawback, it may be useful in wetlands applications
where the primary interest is a general description of the overstory trees with a more
specific focus on understory or shrub layers.
Another plotless method frequently used to assess the plant species present
within a wetland or community is the timed meander search. The practitioner
simply walks around the wetland or community, recording all species that they
observe. The time limit means that one can be consistent in order to compare
different wetlands to each other. However, the timing should be scaled according
to wetland size. An advantage to this technique is that it can capture a larger number
of species than plot-based sampling alone because the investigator is free to explore
a larger diversity of potentially species-rich microsites wherever they occur.
For this reason, it typically leads to larger species counts than plot-based sampling.
