Geoscience Reference
In-Depth Information
species may be the result of general ecosystem stress (Elton, 1958; Odum, 1985).
Reduced biological diversity in LGL coastal wetland communities is frequently associated with
disturbances such as land-cover (LC) conversion within or along wetland boundaries (Miller and
Egler, 1950; Niering and Warren, 1980). Disturbance stressors may include fragmentation from
road construction, urban development, or agriculture or alterations in wetland hydrology (Jones et
al., 2000, 2001; Lopez et al., 2002). Specific ecological relationships between landscape disturbance
and plant community composition are not well understood. Remote sensing technologies offer
unique capabilities to measure the presence, extent, and composition of plant communities over
large geographic regions. However, the accuracy of remote sensor-derived products can be difficult
to assess, owing both to species complexity and to the inaccessibility of many wetland areas. Thus,
coastal wetland field data, contemporaneous with remote sensor data collections, are essential to
improve our ability to map and assess the accuracy of remote sensor-derived wetland classifications.
The purpose of this study was to assess the utility and accuracy of using airborne hyperspectral
imagery to improve the capability of determining the location and composition of opportunistic
wetland plant communities. Here we specifically focused on the results of detecting and mapping
dense patches of the common reed (
Phragmites
australis)
.
18.2 BACKGROUND
spreads as monospecific “stands” that predominate throughout a wetland,
supplanting other plant taxa as the stand expands in area and density (Marks et al., 1994). It is a
facultative-wetland plant, which implies that it usually occurs in wetlands but occasionally can be
found in nonwetland environments (Reed, 1988). Thus,
Phragmites
typically
can grow in a variety of wetland
soil types, in a variety of hydrologic conditions (i.e., in both moist and dry substrate conditions).
Compared to most heterogeneous plant communities, stands tend to provide low-quality habitat or
forage for some animals and thus reduce the overall biological diversity of wetlands. The estab-
lishment and expansion of
Phragmites
is difficult to control because the species is persistent,
produces a large amount of biomass, propagates easily, and is very difficult to eliminate with
mechanical, chemical, or biological control techniques.
The differences in spectral characteristics between the common reed and cattail (
Phragmites
Typha
sp.) are
thought to result from differences between their biological and structural characteristics.
Phragmites
has a fibrous main stem, branching leaves, and a large seed head that varies in color from reddish-
brown to brownish-black;
are primarily composed of photosynthetic “shoots” that emerge
from the base of the plant (at the soil surface) with a relatively small, dense, cylindrical seed head
(Figure 18.1). Distinguishing between the two
Typha
in mixed stands can be difficult using automated
remote sensing techniques. This confusion can reduce the accuracy of vegetation maps produced
using standard broadband remote sensor data.
This chapter explores the implications of the biological and structural differences, in combina-
tion with differing soil and understory conditions, on observed spectral differences within
Phrag-
mites
using hyperspectral data. We applied detailed
ground-based wetland sampling to develop spectral signatures for the calibration of airborne
hyperspectral data and to assess the accuracy of semiautomated remote sensor mapping procedures.
Particular emphasis was placed on linkages between field-based data sampling and remote sensing
analyses to support semiautomated mapping. Field data provided a linkage to extrapolate between
airborne sensor data and the physical structure of
stands and between
Phragmites
and
Typha
stands, soil type, soil moisture content,
and the presence and extent of associated plant taxa. This chapter presents the wetland mapping
techniques and results from one of the 13 coastal wetland sites currently undergoing long-term
assessment by the EPA at the Pointe Mouillee wetland complex (Figure 18.2).
Phragmites
