Environmental Engineering Reference
In-Depth Information
5.2.3 When?
Timing is extremely important in vegetation sampling. Just as there are multiple
scales of spatial investigation and variability, temporal variability has multiple
scales. The most common consideration in vegetation sampling is season of the
year. Within the northern hemisphere, most wetland vegetation is sampled in mid to
late summer in order to facilitate sedge, grass, and aster identification, and to
capture vegetation at the peak of its growth. However, many early-season sedge,
mint, and violet species may be difficult to identify in late summer, and several
orchid species seem to disappear entirely after blooming. Within forested wetlands,
spring ephemerals may be missed altogether. A good strategy is to visit the wetland
site periodically early in the year in order to identify early-blooming and fruiting
species and then apply this knowledge to a more comprehensive quantitative
sampling later in the growing season when early-bloomers are in a non-flowering
state. If a comprehensive species list for the wetland is desired, then returning to
sample at multiple times during the year is necessary.
On a larger scale, interannual variability can also be important. When reporting
results of vegetation study, include information about whether the climate was
typical or unusual that year. Some plant species flourish or become more apparent
during times of high or low water levels (Warwick and Brock
2003
). Late spring
freezes can temporarily eliminate a host of spring herbaceous species. If the
sampling year is atypical, an additional year will probably be necessary in order
to fully describe the wetland vegetation. Wetlands like prairie potholes or beaver
(
Castor canadensis
) meadows experience larger-scale cycles of disturbance that
can extend for decades or longer. In order to capture the full variability of these
systems, it may be necessary to sample wetlands at different stages in the cycle. It is
important to recognize and document the site history and temporal context of your
sampling when reporting and applying results.
Questions to ask yourself before choosing when to sample:
1. Is it important to get a general description of the vegetation, or a comprehensive
species list?
2. Does the system have a characteristic frequent disturbance regime that will make
the results of your sampling only narrowly applicable?
5.3 Basic Vegetation Sampling Techniques
Entire topics have been written about how to properly sample vegetation. What
follows is an introduction to some basic techniques. For further information, the
reader is encouraged to consult Greig-Smith (
1983
), Bonham (
1989
), Kent and
Coker (
1995
), Elzinga et al. (
1998
), Krebs (
1998
), and Mueller-Dombois and
Ellenberg (
2003
).
