Environmental Engineering Reference
In-Depth Information
Table 5.2 Common cover class systems
Cover class
Braun-Blanquet (
1965
) (midpoint)
Daubenmire (
1959
) (midpoint)
+
<
1 % (0.5 %)
1
1-5 % (3 %)
0-5 % (2.5 %)
2
5-25 % (15 %)
5-25 % (15 %)
3
25-50 % (37.5 %)
25-50 % (37.5 %)
4
50-75 % (62.5 %)
50-75 % (62.5 %)
5
>
75 % (87.5 %)
75-95 % (85 %)
6
95-100 % (97.5 %)
especially important in population studies, and can be more easily collected for
shrubs and trees than other measures of abundance. Difficult decisions must be
made when defining an individual, however. Many plants are connected to each
other by vegetative reproductive structures (e.g., blanket-like clones of grasses,
grass tussocks, branching tree trunks, or multiple shrub stems connected under-
ground). What is an individual in these circumstances? Investigators typically
choose a definition that will capture the influence of the individual on the system
of investigation. Trees are often considered separate individuals if they are
separate at breast height. Shrubs are frequently measured using stem counts
with disregard for underground structures. Grasses can be counted using number
of shoots, although it is highly tedious. Boundary decisions (is the plant inside or
outside the plot?) can also be difficult. Elzinga et al. (
1998
) provides a good
overview of boundary decisions. Comparisons of density between different
species are most informative when species are of similar size.
Cover: Percent-cover is a measure of abundance that describes the horizontal
area that a plant species or individual occupies. All plot-based techniques can be
used to measure cover. It is frequently faster to use than density for very dense
populations, and it is more effective than density for mat-like, low-growing
vegetation. Some also argue that it is a better measure of the species' influence
on the community than density, because it takes the size of the individuals into
consideration. A disadvantage of cover measurements is that they change dra-
matically over the course of a season, so consistency in time-of-year is important.
There are numerous methods of assessing percent-cover, and the boundary
decision problem still applies here. One of the most common methods is visual
percent-cover. The observer stands over the plot and assigns cover to each species
present. Naturally, this process is highly subjective and can differ dramatically
between different plots and different observers (Kercher et al.
2003
). In order to
minimize observer bias, cover classes are typically used (Table
5.2
). Mid-points of
cover classes are typically used when analyzing the data, although this introduces
substantial uncertainty into the data (Podani
2006
). Care must be taken when
analyzing ordinal data derived from cover classes (Podani
2006
).
In order to avoid the observer bias inherent in the visual-percent cover system,
some practitioners use a pin-frame system. This is a metal or plastic grid frame with
attached vertical pins (10-100 pins) that fits over the top of a plot. Just as in the
