Environmental Engineering Reference
In-Depth Information
This type of sampling is also used to detect rare species (Goff et al.
1982
).
A disadvantage to this technique is that it can only be used effectively by skilled
field botanists who know the likely habitats of different species and can identify
them quickly. In addition, the practitioner must be careful to specifically check for
the small plants that might be more easily detected using plot sampling.
5.3.3 Locating Samples Within a Wetland
As discussed above, locating sample points within a wetland system is not a simple
matter due to the patterned heterogeneity of much wetland vegetation. There are
two basic approaches widely used in wetland vegetation assessment today: (1) rep-
resentative, more subjective sample placement or (2) systematic sample placement
based upon a pre-defined objective scheme. These pre-defined schemes have been
described in Chap.
1
as random, restricted-random, regular, or haphazard.
Representative sample placement is quick and efficient, but less defensible in
scientific or legal settings than systematic sampling. Nonetheless, it is common
practice for monitoring wetland mitigation sites and for rapid wetland delineations.
Using a combination of on-the-ground and aerial reconnaissance, different plant
communities are roughly delineated (frequently on a map or aerial photograph), and
a sample is described from one to several representative locations within each
community (Fig.
5.5a
). Representative sampling is easier to practice when there
are relatively distinct and homogenous communities. It is most commonly used
when a rapid, general assessment is needed and or there is a high level of trust in the
judgment of the practitioner.
Pre-defined sampling schemes frequently use a baseline plus transects, which
define a grid system for sample placement within the wetland (Fig.
5.5b
). The
baseline is established parallel to the dominant hydrologic gradient of the wetland,
and transects extend perpendicular to it and the gradient (Fig.
5.5b
). Sample sites
are then located at specific locations on the transects. Transects can be regularly or
randomly arranged on the baseline, and sample sites can be located regularly or
randomly on the transects. This method is generally perceived as more objective
and accepted by the scientific and legal community in North America. However, it
may be overkill in situations where only preliminary descriptions of vegetation are
needed. In addition, the method can be difficult to implement in very large wetland
complexes with complex or non-obvious hydrologic gradients or in wetlands with
large areas of deep water in the middle. Consider the extreme example of placing
one 1 m
2
quadrat every 200 m on a 2,000 m long transect. Clearly alternative
strategies must be devised. For large complexes, subdividing the region into smaller
representative subsections may be more practical. For wetlands ringed with vege-
tation around large central deep water, a better strategy might be establishing a
baseline around the perimeter of the wetland and running transects in toward the
center (Fig.
5.5c
). However, this method risks over-sampling the wetland's center
with more plots than the periphery.
