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Remote Sensing and GIS Techniques for
Assessing Prescribed Fire in the Madrean
Archipelago
Larry K. Clark, Research Assistant, Department of Geography and
Regional Development, University of Arizona, Tucson, AZ
W
hile fire is generally considered an essential component in the natural
development of most ecosystems (Hardy and Arno, 1996), its occurrence
on the landscape of the American Southwest has been greatly reduced during the
past century. Land management policy decisions (Leopold, 1963; Schullery, 1989;
Arno and Brown, 1991) and changes in plant communities due to overgrazing,
logging, and nonnative plant invasion (Pyne, 1982; Bahre, 1991; McClaran and
VanDevender, 1995) have been influential deterrents. In an effort to reverse the
trend of fire suppression while improving techniques for assessing ecosystem health,
the USDA Forest Service and the University of Arizona have embarked on research initiatives related to fire and natural resources management in the borderlands region of the Southwest.
The Maverick prescribed fire study is applications- based research to determine the viability and accuracy of using easily accessible terrain and remotelysensed datasets in a geographic information systems (GIS) environment to map
and analyze the 1997 Maverick prescribed fire in the Southwestern Borderlands
Ecosystem Research Area in Arizona and New Mexico. Study objectives were to:
a) compute topographic data layers (elevation, slope, and aspect); b) produce a
map of prefire fuel types within the fire perimeter; c) create a prefire canopy density map within the fire perimeter; d) register the prefire fuels and canopy cover
maps to the topographic data layers; e) map the burn severity; and, f) analyze the
relationships among fuels, topography, and burn severity. A variety of image processing tools and techniques were used in the study to produce intermediate and
final display maps, for use by natural resource and other land managers as visual
management aids.
Remote Sensing and GIS
Recording information about fire in wilderness areas is often difficult due to
accessibility, weather conditions, safety concerns, and areal extent. Increasingly,
remote sensing techniques are being used to effectively gather data on fire activity
and ecosystem response. Remote sensing is reconnaissance at a distance- gathering information about an object without touching it.
All objects reflect or emit electromagnetic radiation over a range of wavelengths producing a distinct spectral signature. For remote sensing, the spectral
wavelengths are measured in micrometers (mm) and range from ultraviolet (UV)
(0.01 to 0.4 mm) to visible (0.4 to 0.7 mm) to infrared (IR) (0.7 to 1,000 mm) to
microwaves(> 1,000 mm). In this study, the radiation measurement occurs primarily in 2 regions of the spectrum: the visible range where the component bands
produce blue (0.4 to 0.5 mm), green (0.5 to 0.6mm), and red (0.6 to 0.7 mm)
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Remote Sensing and GIS Techniques for Assessing Prescribed Fire in the Madrean Archipelago
light; and the near IRrange (0.7 to 1.5 mm), which is invisible to the human eye.
Because vegetation shows the most reflectance in the near IR band, with the next
highest reflectance in the visible green band, the near IR band and the upper
range of the visible spectrum were the primary bands used in this study.
As wilderness vegetation is altered through natural succession, changes in spectral signatures also occur over space and through time. Disturbance, such as a
major fire, an1plifies otherwise subtle changes within an ecosystem, and correspondingly, the spectral response may be more extreme. Previous studies have
shown that using data from the Landsat Thematic Mapper (TM) satellite is effective to map change from fire disturbance at a scale sufficient to discern ecosystemlevel variation (Collins and Woodcock, 1996; White et al., 1996; Pattersonand
Yool, 1998).
TM imagery is one of several components necessary to accomplish a comprehensive change analysis. Disturbances, such as forest fires, operate within a framework of multi temporal and multispatial scales, whose impact is often unclear (Averill
et al., 1994; Rogers, 1996). Consequently, ecosystem response to wildland fires is
an integrated response that produces interrelated data for examination and evaluation.
Because a multitude of variables should be considered when assessing vegetation response to wildland fires, a relational database using a variety of unique
datasets is a logical approach to the study ofwildland fires (Sample, 1994). Such
an assemblage of variable data in a layered information structure, which can be
simultaneously analyzed and visualized, is a GIS. A GIS provides the analytical
framework to map and assess the spatial distribution of topographic features, vegetative fuels, and patterns of fire behavior at the ecosystem level.
The Maverick Fire and Its Geographic Context
Mechanically ignited by the Forest Service on June 24 and 25, 1997, the
Maverick fire smoldered into July. Of the total acreage within the primary burn
containment perimeter (approximately 17,000 acres), the Forest Service has estimated between 6,000 and 8,000 acres burned, based on aerial inspection. The
total area of the study site (primary burn containment perimeter and secondary
spill-over containment perimeter) was more than 40,000 acres of public and private land; an unprecedented areal extent for a management-ignited prescribed fire
in the Southwest. The purpose of the burn was to reduce invasive trees and shrubs,
increase herbaceous plant materials, return the natural forces of fire to the ecosystem (Encinas, 1997), and create a mosaic ofburned and unburned areas (Pickett,
1985).
To comprehend the complexity of the burn and the analytical component of
this investigation, it is important to understand the location and nature of the
study area. The site is in the Peloncillo Mountains of the Madrean Archipelago
(Brown, 1994). Bounded by the Chihuahuan Desert to the east and the Sonoran
Desert to the west, the site straddles the Arizona/New Mexico border just north
of Mexico (figure 1).
The Peloncillos, a subtle north south arc crossing the southern Arizona/New
Mexico border, are one of about 40 sky island complexes that comprise the Madrean
Archipelago. Stretching along a roughly north south axis from the Mogollon Rim
to the Sierra Madre Occidental, the Madrean Archipelago is a complex of alternating mountains (islands) and valleys (seas), that are either barriers or bridges for
colonization by various species, depending on dispersion patterns, transport cap a-
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Remote Sensing and GIS Techniques for Assessing Prescribed Fire in the Madrean Archipelago
Clark
Figure 1. Biogeographic provinces of the southwestern U.S.
and northwestern Mexico. From
Biotic Communities, Southwestern United States and Northwestern Mexico, David E. Brown,
Editor, 7994. Reproduced with
permission from the University
of Utah Press.
bilities, and adaptability (Warshall, 199 5). The biodiversity found throughout
this complex is unique and extensive (McLaughlin, 1995 ).
Unlike the timber rich regions of previous fire research, the Maverick site is
abundant in plant communities ranging from semidesert mesquite grasslands
at lower elevations to dense Madrean evergreen woodlands of oak, juniper, and
pinyon at upper reaches with riparian corridors of sycamores and other deciduous tree species interspersed throughout. This stratification is characteristic of
sky island complexes. The survival of these plant communities is partly due to
the region's cycle of mild winters and warm, wet summers interrupted by a dry
spring hiatus. The dramatic geophysical nature of the site, typical ofvolcanically induced geologic disturbance (Mcintyre, 1988 ), is characterized by elevations from 1289 m to 1964 m, with predominantly west and south aspects,
and slopes from flat to nearly vertical.
Data and Methods
A wide range of digital and nondigital data were used for this project including United States Geological Survey (USGS) digital elevation models
(DEMs) provided by the Forest Service; TM imagery from June 17, 1995, and
June 24, 1997 (prefire), and July 3, 1997 (post fire), obtained through USGS;
USGS black and white digital orthophoto quarterquads (digitized aerial photographs corrected for earth curvature distortion); true color low and mid
altitude aerial photography provided by the Forest Service; and true color ground
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Remote Sensing and GIS Techniques for Assessing Prescribed Fire in the Madrean Archipelago
photography. The software that was used for processing and analysis included
UNIX-based ERDAS Imagine, IDRISI for Windows, ARC/INFO, andArcView.
Classification of land cover change as a result of the Maverick fire required
preparation of preanalysis theme maps. First, the topographic layers were created
by deriving basic elevation, slope, and aspect maps from the DEM. Then, they
were reclassified to impose a priority ranking that was weighted relative to fire
behavior; slope had the most influence and elevation had the least. An overlay of
these 3 maps identified areas of similar topographic pose and depicted the range
of topographic variability on the site. Another map layer, derived from aerial and
ground photography and other related research efforts in the study area (Sundt,
1997; Muldavin et al., 1998), delineated vegetation composition, distribution,
and density. An analysis of prefire vegetation fuels relative to topographic pose is
also important.
Mter preprocessing for radiometric and atmospheric correction and geometric rectification, the TM data will be examined and processed to identifY the most
successful means of mapping the fire. Processing techniques for this step include
the Kauth-Thomas transform (greenness, brightness, wetness indicators), principal components analysis, and computation of various vegetation indices. Once the
transformed prefire and post fire maps are created, change detection will be accomplished through subtraction of post fire from prefire maps. For each transformation technique performed, the resulting change map, along with aerial and
ground photography and field observations, will be used to develop a classification of burn severity and to estimate total burned acreage. When all data layers are
assembled, an in-depth assessment of the relationship among the characteristics of
burn severity and fuels, canopy cover, and topographic pose will be performed.
Conclusion
The use of remote sensing techniques in a GIS environment provides an opportunity to study ecosystem disturbance on a broad-scale, while assisting in natural
resource management. Analyzing and mapping the spatial distribution of fire behavior in relation to topographic variability and natural fuel composition and distribution is important to understanding wildland fire behavior, and returning it to
an ecosystem. This is particularly true in a region where fire has been suppressed
for a century or more. Although it is bordered by desert, the Madrean Archipelago still relies on fire to help regulate and maintain diversity.
Employing easily accessible terrain and remotely-sensed data to develop new
tools and techniques for visualization of ecosystem development is essential as
technology rapidly evolves, natural resource management responsibilities expand,
and increased oversight efficiency is demanded. Building a comprehensive program documenting the Southwestern Borderlands Ecosystem Research Area will
have a profound impact on understanding ecosystem processes and successfully
managing the natural resources in the borderlands area and other environmentally important regions.
This project was funded by the Southwestern Borderlands Ecosystem Research
Program, USDA Forest Service, in cooperation with The University of Arizona.
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Clark
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