Environmental Engineering Reference
In-Depth Information
Microtopography: Microtopographic features in wetlands (i.e. hummocks,
pools, stumps, or tussocks) can exert strong control of the local plant community
(Peach and Zedler
2006
). Depending upon the study purpose and scope,
microtopography can be measured quantitatively (more intense, smaller scope)
or qualitatively (less intense, larger scope). Quantitative measurements of high
and low points within plots (associated with topographic breaks) can be
measured with high-accuracy GPS units associated with local base stations
(Werner and Zedler
2002
) or using meter sticks to determine tussock height
(Peach and Zedler
2006
) or maximum height difference within the plot. For
studies that are broader in scope, a plot can be assigned qualitative
microtopographic scores corresponding to all types within a plot (e.g. stump,
high hummock, hummock, low hummock, hollow, flat, or pool). For data
analysis purposes, these can be transformed into ordinal scores, and plot
microtopographic richness, mean score, or maximum difference can be calcu-
lated (Little et al.
2010
).
Canopy cover: Canopy cover is an important environmental variable to mea-
sure in forested wetlands, because many wetland understory species respond to
shade. There are three common ways to measure canopy cover, increasing in
accuracy: (1) canopy tube, (2) spherical densiometer, and (3) digital camera
with fish eye lens and image-processing software. Canopy tubes are simply
vertical tubes with a cross hairs and some type of leveling mechanism. These
can be sophisticated tubes with mirrors, or home-made toilet-paper tubes with
a dangling level inside. Visual percent-cover of canopy within the tube is
recorded from the middle of the plot. Alternatively, the crosshairs can be
used to determine presence/absence of canopy at a set of points per plot
(Ganey and Block
1994
). A spherical densiometer is a small, handheld
gridded mirror with a leveling bubble. It is held above the plot, and the
observer views how many grid cells are occupied with canopy cover by
visualizing a series of dots within the cells (Lemmon
1956
). The most sophis-
ticated and accurate measurements of canopy cover use a fish eye (hemispher-
ical) lens with digital image-processing software to calculate canopy cover and
light transmission (Englund et al.
2000
). However, these cameras are very
expensive, and data can only be collected a certain times of day under specific
weather conditions - limiting their utility.
Spatial data: A detailed discussion of spatial data collection and autocorrelation
is beyond the scope of this chapter, but practitioners should consider whether
important spatial relations may exist within or between wetland systems of
study. Landscape ecology approaches may be needed to assess relations between
wetland sites (consult Turner et al. (
2001
) for ideas). Within sites, numerous
workers have found interesting relations between hydrological features and plant
communities using measures as simple as distance of plot from a feature (Grace
and Guntenspergen
1999
). Since hydrological and dispersal gradients often vary
with distance in wetlands, it can be a helpful, and easily-measured surrogate for
other variables.
