Environmental Engineering Reference
In-Depth Information
plots could be random or representative, depending upon the number of plots and
the study purpose. They should be located so as to minimize potential for human or
animal disturbance.
Three different types of permanent plots are common: circular (marked by
central point), square (marked by four corners), and transect (marked by two
endpoints). Researchers should not expect enough accuracy from a compass to be
able to mark only one endpoint of a transect. In addition, although global position-
ing system (GPS) technology has advanced to a stage where most practitioners can
get sub-meter accuracy and occasionally sub-foot, this is not high enough accuracy
to avoid leaving monuments in the field. GPS can help to narrow down plot
location, but then permanent marking structures should be installed to identify
exact plot location. Of course, leaving structures in the field brings risks that they
will be disturbed by humans, animals, or acts of nature (flood, landslide, fire).
Non-visible, ground-level markers minimize the chance that they will be disturbed,
but also minimize chances of relocation in the future. The ideal marker is rugged
and hard to remove (such as rebar), visible or detectable by metal-detector, and
designed to minimize harm to or detection by passers-by. Elzinga et al. (
1998
)
provides a detailed discussion of monumentation considerations. Within wetlands,
it can be extremely difficult to use a metal detector in areas of thick litter and
vegetation, because the vegetation dampens the signal.
A picture is worth a thousand words. Photopoints not only communicate change
over time in a visible way, but they can also help locate permanent plots in the field.
A picture is typically located at some type of permanent marker and associated with
a compass direction. In order to associate a particular photopoint with a location,
record photographs containing location information can be taken in between
photographs of the plant community or population. It is also good practice to locate
these points using GPS.
5.3.7 Assessment Techniques for Specific Attributes
Substantial research and invention has been invested into how to best assess
different vegetation attributes. Provided here is a non-exhaustive list of some
commonly-used techniques and considerations for their use.
Definitions: Meaningful definitions are important to establish and consistently
use throughout the study. In vegetation studies, understory is typically defined
by its height (i.e.,
breast height (1.4 m)), the shrub layer defined by its number
of stems, height and diameter (i.e.,
<
>
1.4 m tall, with DBH
<
4 cm), while a tree is
4 cm DBH). In some cases, definitions of
functional groups will need to be created. When assessing plant traits, clear
definitions of leaves, stems, and flowers are important.
Density: The number of individuals per area can be estimated using the plot-
based counts or plotless techniques described above. Information on density is
defined by its diameter (i.e.,
>
