Vascular Plant and Vertebrate Inventory of Saguaro ... - USGS

well (Swantek et al. 1999). The most prominent
habitat disturbance in the district is wildland fire.
Since the NPS began keeping records in 1937,
there have been 572 fires in the district and the
park has an active prescribed fire program. As
part of the program, park personnel monitor
vegetation responses at 71 plots located in the
higher elevation areas of the district (Saguaro
National Park, unpubl. data). Unfortunately,
there has been no comprehensive report detailing
the results of that program, so the effects of
prescribed fire on the abundance and distribution
of plants in those areas remains largely unknown.
Historically, there have been 35 major wildland
fires in the conifer forest near Mica Mountain
from 1770–1990 (Baisan and Swetnam 1990).
Other naturally occurring wildland fires have
burned through the district, and some have been
in the lower-elevation Sonoran Desertscrub,
which has not historically been subject to
fire (Steenbergh and Lowe 1977, Esque et al
2003). This relatively new phenomenon has
resulted from an increase in abundance of
non-native annual grasses (Schwalbe et al.
1999). Of particular concern to park managers
are the impacts of fire on saguaro populations
(Steenbergh and Lowe 1977). These concerns
are well founded; in the area of the Mother’s Day
fire of 1994, Schwalbe et al. (1999) found 22%
mortality of saguaro within four years of the fire.
This is considered to be a catastrophic event for
such a long-lived cactus species. Wildland fire
has important impacts on other resources of the
park such as soil, air quality, and animals. We
discuss the impact of fire on vertebrates in the
respective chapters.
Additional Research and Monitoring Needed
As mentioned earlier, it is likely that most of the
new species to be added to the district’s flora
will be found in the middle-elevation areas of
the district, particularly on the east slope of the
Rincon Mountains. In addition, invasive, nonnative
species will likely become established
in high-traffic areas such as the Cactus Forest
Loop Drive and Old Spanish Trail, where the
park staff have been surveying for them for four
years. Future funding for the park-based effort
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is uncertain and the SODN I&M program is
establishing protocols for periodic surveys in
these areas. Considerable effort has been focused
on determining the effects of fire on the high
elevation plant community and we encourage
the park to analyze and report the results of the
fire-effects monitoring program. Finally, there
are a number of long-term monitoring plots for
saguaros that have not been relocated. These and
other, recently located plots should be resurveyed
periodically. Finally, the district is also in need
of a current, detailed vegetation map, which will
likely be created in the next few years by the
I&M program (Andy Hubbard, pers. comm.).
Vegetation monitoring will be an
important component of the I&M program at
Saguaro National Park and other park units in
the Sonoran Desert Network (Mau-Crimmins
et al. 2005), yet field methods for vegetation
monitoring have not been established. Our use
of the modified-Whittaker and point-intercept
methods provides data that could inform that
program. If the goal of the I&M program is to
monitor species richness or species composition,
a plot-based method such as the modified-
Whittaker may be more appropriate than the
point-intercept method because more species
were observed on plots and the point-interecept
transects missed many species in the area of the
transects. However, observer bias in estimating
species coverage (a measure of dominance) is an
important limitation of the modified-Whittaker
and similar methods for monitoring that
parameter. In fact, estimation of coverage can be
so great as to obscure trend detection for all but
the most extreme changes (Kennedy and Addison
1987). Bias can be minimized by reducing the
size of the quadrat (Elzinga et al. 2001). With
regard to observer bias, the point-intercept (or the
similar line-intercept method) produce less biased
estimates of species coverage because there is
less opportunity for interpretation. Elzinga et
al. (2001) provide an excellent overview of the
major survey methods for monitoring vegetation
and they include a good discussion of observer
bias.
If the goal of the monitoring program is
to monitor changes in vegetation structure and
gross vegetation characteristics (i.e., dominant