AN ASSESSMENT OF THE EFFICACY OF USING GPS FOR BIOLOGICAL
MONITORING OF THE EFFECTS OF DEER HERBIVORY ON FOREST VEGETATION
Emily Yates and Dr. Thomas Rosburg, Drake University
and
Kyle Swanson, Iowa Natural Heritage Foundation
ABSTRACT
A method of relocating individual plants or populations is required in ecological
studies involving monitoring plant populations. Global Positioning Systems offer an attractive
method for plant relocation, but only if they can achieve the accuracy and resolution necessary
to map on the fine scale of individual plants. The accuracy of GPS was investigated in this
study considering three possible sources of error: 1) normal differential processing vs. high
accuracy carrier phase processing, 2) location of base data for differential corrections, 3)
presence or absence of a forest canopy.
Measurements of data were made in conjunction with baseline inventories of the
community composition of forest plots either inside or outside of deer exclosures in Polk
County. Locations of individual plants in a vegetation plot were made with GPS and with an
X/Y grid. Data was compared to determine the amount of deviation between the two locations.
GPS Analysis
A method of locating and relocating individual plants is required in ecological studies that involve
monitoring plant populations over time to assess the effects of particular factors. Grids established from
permanently marked corners offer a conventional technique to map the spatial occurrence of plants, but this
method is extremely time consuming and must rely on some type of permanent marker. Global Positioning
Systems (GPS) offer an attractive alternative, but only if they can achieve the resolution and accuracy
necessary to map at the very fine scale of individual plants. In this study the utility of GPS to provide fine
scale mapping for biological monitoring of vegetation will be studied. Even more specifically, three
factors that affect the accuracy of GPS will be investigated. 1) The mode in which the GPS receivers will
be used. Conventional differential correction was compared with the high accuracy carrier phase. 2) The
proximity of the base station used for differential correction. Two base stations were utilized, one within
50 miles and a second base station within 300 miles. 3) The influence of a forest canopy. A study done by
Gerlach (1989) suggests that a canopy makes GPS less accurate because the canopy vegetation interferes
with the satellites ability to position accurately by triangulation. Research by Hansen (1996) confirms this
and also states that with a canopy, readings can take longer to obtain. Data were collected at one of the
forested sites for the deer browse study and compared to data collected on an open field on the Drake
campus.
Data sets collected with both types of GPS units were processed with the appropriate
Pathfinder software with differential correction applied. Base data were obtained from two locations
- the Iowa Department of Transportation base station in Ames, Iowa (approximately 40 miles away)
and the Trimble base station in Onalaska, Wisconsin (about 200 miles away). Post-processed shape
files were imported into GIS to assess variation in points located with both GPS and the X/Y grid.
Location of the plot’s four corners were grouped and used to establish the location of the
plot in GPS space. GIS was then used to superimpose an X/Y coordinate system over the plot. For
each of the 7 plants (or flags) within the plots, the amount of deviation between the grid location and
the GPS location was measured. Both direction and distance of the deviation were recorded. The
mean amount of deviation distance will be calculated and a one-tailed T-test will be used to
determine if the mean deviation is less than various levels of accuracy (e.g., 5, 10, 25, 50 cm).
RESULTS
GPS Accuracy
METHODS
Study Site
A deer exclosure was established in the fall of 1997 at Brown’s Woods, a 200-hectare park
located in southwestern Des Moines. The location of the study plots was on an area of level upland
topography where the vegetation is dominated by black oak. The deer exclosure was approximately 27 x
57 m in order to accommodate a 20 x 50 m vegetation plot and allow a buffer strip between the plot and
the fence of the exclosure. Exclosures were constructed of wire fencing approximately 2.5 meters tall. The
bottom of the exclosure fence was approximately 20 cm above the ground to allow movement of small
mammals.
GPS Measurements
The 20 x 50 m vegetation plot utilized to record plant community composition was used with a
grid system to assess the accuracy of GPS locations (Fig 3). Grids were established at Brown’s Woods to
collect location data under a forest canopy and on central campus of Drake to ascertain GPS performance in
an open habitat with no canopy. Use of the campus lawn also facilitated more accurate establishment of the
grid system than did the natural vegetation and deadfall in Brown’s Woods. Corners of the grid system
were marked with flags, and four lanes were established within the plots to provide baselines for obtaining
X and Y coordinates of the specific plants. Various species of plants occurring sporadically throughout the
plot were chosen for flagging because they were easily recognizable and identifiable. These included
Mayapple, Jack in the Pulpit, White Snakeroot, and Forest Sedge.
At Brown’s Woods 7 plants were selected and flagged within the 20 x 50 m plot. At the campus
location 7 flags were placed in a random pattern within the plot. In each case, the location of the plant (or
flag) within the plot was determined by measuring its X and Y coordinates in the grid system. Coordinate
locations were recorded to the nearest cm. GPS data were collected at each plant (or flag) within the plot,
as well as at all four corners of the plot. Prior to collection of GPS data, Trimble pathfinder software was
used to determine the optimum time of day to collect data based on satellite geometry. Two Trimble
Geoexplorer II’s were used to record GPS data (Fig 2). One was used in coarse acquisition (basic)
differential mode. It was positioned on a monopod approximately 1.3 m high and data points were
collected for 5 minutes at five-second intervals. Depending on the surrounding interference, the number of
points collected varied from a minimum of 7 to a maximum of 55. The set up of this unit was designed to
represent a less labor-intensive approach with a lower time requirement. The second unit was set up in
high accuracy, carrier phase mode. An external antennae 3.7 m high was used, and points were collected
for 10 minutes at five-second intervals. Each location was measured with 120 data points. This unit was
designed to represent a more labor-intensive approach with a higher time requirement.
Figure 1: Polk County including Des Moines and Brown’s Woods.
INTRODUCTION
Global Positioning Systems (GPS) has experienced an explosion of applications in a variety
of industries in recent years. Precision farming in agriculture, industry, and transportation are many
typical GPS applications. Natural resource managers have used differential GPS for large scale mapping
and land surveys, but with recent advances GPS now can be used for very fine scale positioning and
mapping. The accuracy of GPS may be sufficient for biological monitoring of small plant populations or
even individual plants.
This study will assess the efficacy of GPS for biological monitoring. It will be employed in a
baseline inventory of a long-term study designed to assess the impact of deer herbivory on forest
vegetation. Deer are nearly 50 times more populous than a century ago (Mooney 1997), and this
overpopulation has caused browsing to have a detrimental effect on plants in some areas. Recently in
Polk County, Iowa, an urban deer task force was formed to evaluate various cultural and natural
problems associated with deer overpopulation. A recent deer survey completed by the task force
indicated that some areas of Polk County have deer populations in excess of 170 individuals per square
mile. Wildlife managers consider a sustainable deer population density to be in the range of 8 to 15 deer
per square mile.
Deer overpopulation causing high levels of herbivory suggest two effects on the forest
community. One is the direct influence of forest plant species due to selective herbivory. Overgrazing
by deer has reduced vertical growth, diameter, and distance to the next species in species of forest plants
(Shelton 1995). Many species sensitive to overgrazing could decrease or even disappear, while a few
species that are tolerant may increase and become unnaturally dominant. There is evidence that in some
cases these sensitive species are plants that are high-quality forest species; therefore their loss represents
a serious impact on the integrity of forest communities. A second more indirect influence is the
potential negative effect that overgrazing may have on other forest animals, particularly songbirds, via
changes in the structure of the forest vegetation and the availability of nesting sites. In short, the
concern about negative effects of intense deer herbivory on forest biodiversity has elevated the deer
overpopulation issue to prominence.
A long-term study was initiated in 1998 in Polk County to evaluate the effects of deer
herbivory on forest vegetation. It will require the capability of long-term monitoring to allow sufficient
time for vegetation to respond to the absence of deer herbivory inside the deer exclosures. In other plant
studies using long-term monitoring it has been shown that fewer sample plots were required to obtain
reliable results, whereas shorter term studies required up to 45 times as many plots to achieve the same
significance (Lesica and Steele 1997).
GPS and X/Y grid data were collected at Brown’s Woods during the last week of
September. Data from the Drake campus plot were collected during the third week of October in
order to assess the effects of a forest canopy. Data have been compiled as shape files and have
been imported into ArcView. Measurement of the distance from X/Y locations to each of the
two GPS locations is underway. Base data from Onalaska, Wisconsin and Ames, Iowa have
been acquired and will be used to measure the effect of proximity of base data for differential
correction.
DISCUSSION
Applying this study to NASA and society as a whole, we can see that if the accuracy of
GPS is high enough to locate specific plant species in ecosystems with or without a canopy, then
we can be confident in GPS’s applicability to do many other things such as locating specific areas
of crops needing irrigation or increased or decreased amounts of fertilizer. Looking at a broader
scale, GPS technology could possibly be applied to locating and mapping points in outer space as
a tool for space exploration. Although applications like these would require many technological
developments, the possibilities can be made apparent through studies that test the reliability and
accuracy of GPS.
ACKNOWLEDGEMENTS
This study was funded by NASA through the Iowa Space Grant Consortium and by the
Polk County Conservation Board. For the portion of the study depicted in this presentation Dr.
Thomas Rosburg was essential in providing guidance with the data collection, analysis and
presentation. Assistance in collecting and processing GPS data was also provided by Kyle Swanson
from the Iowa Natural Heritage Foundation.
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Figure 3 : Method of locating plants (or flags) with X/Y grid system.
Figure 2: The GPS Unit.
Rosburg, T.R. 1995b. Pre-burn inventory of forest community composition and structure on
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Figure 4: Using GPS high accuracy mode within the
campus plot.
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