December 2012 Number 1 - Utah Native Plant Society

Utah Native Plant Society
were established by the U.S. Forest Service, but results
were inconclusive (Nagiller 1992). We initiated a study
in 1992 to test the hypothesis that restoration of historic
ecosystem conditions may enhance the sustainability of
this species (Fulé et al. 2001). This study encompassed
several components, including seed germination studies,
a field seeding trial, a prescribed burning study and a
trenching experiment. We initiated the prescribed burning
component in 1994 to test the hypothesis that prescribed
fire would increase P. clutei density by removing
litter and competing vegetation. The results suggest
that prescribed burning caused a significant decline in
density by as much as 75%. However, density also declined
in two of the three control areas (in one area also
by as much as 75%). So, while prescribed burning appears
to be responsible for the death of mature plants,
natural population declines may also occur in the absence
of disturbance.
After evaluating results from the prescribed burning
experiment, we investigated the possibility that vigorous
responses following fires were a result of mortality of
overstory trees and removal of root competition (Fulé et
al. 2001). We initiated a study in 1998 to test the hypothesis
that cutting root competition through trenching
would increase P. clutei density. In 1999, one year following
the trenching, there was a significant difference
in density between trenched (mean of 104.9 plants/plot)
and control plots (14.0 plants/plot), mostly in the form
of seedlings. By 2000, densities had declined to an average
of 30.6 vs. 1.5 plants in the trenched and control
plots, respectively, mostly due to the death of seedlings.
Two preliminary conclusions were drawn from the
trenching study: 1) trenching had a positive effect on P.
clutei reproduction, and this trend was still evident a
year later, and 2) increases in P. clutei were likely due
to reduced root competition with overstory trees. Although
our earlier germination experiments indicated
that P. clutei did not exhibit innate seed dormancy under
laboratory conditions (see Fulé et al. 2001), we were
puzzled over the dramatic field response to root trenching.
A field seeding trial of P. clutei showed very poor
rates of establishment (0.1-0.6%), with no seedlings establishing
after an April seeding, and only a minimal
number establishing following an October seeding (Fulé
et al. 2001). Determining whether P. clutei maintains a
persistent soil seed bank is crucial for conservation and
management efforts. The combined results from our
previous studies suggest that it does not form a persistent
seed bank, that there may be dissimilarities in germination
rates between plants from different habitats,
and that field emergence is extremely low and/or seedling
mortality is high. Collecting P. clutei seeds from
additional habitats could yield new information on
whether this species exhibits cyclic dormancy patterns
or dormancy that differs in contrasting habitats.
It remains largely unknown, then, how long populations
of P. clutei plants persist on the landscape following
disturbance, what mechanism this species employs
to colonize an area following disturbance, or how it is
able to disperse across the landscape. In an effort to gain
answers to some of these questions, we revisited the
study area ten years after root trenching and 13-14 years
following prescribed burning. Our objectives were to
assess the long-term effects of the prescribed burning
and trenching treatments and to evaluate the importance
of a persistent seed bank in population dynamics.
METHODS
Fulé and others (2001) described methods of our previous
prescribed burning and root trenching studies in
detail, but we will also summarize them here. The experimental
studies described in this and the 2001 paper
were established in 1992-1994 and conducted on Coconino
National Forest lands in the vicinity of O'Leary
Peak, adjacent to Sunset Crater National Monument
(Figure 1). The elevation of the study area is approximately
2100-2300 m (6890-7550 ft). Soils are cindery
and deep, well-drained Vitrandic Ustochrepts and Typic
Ustorthents (Miller et al. 1995). Weather records from
Sunset Crater National Monument, 1 km south of the
study area, include an annual precipitation average of
42.7 cm (16.8 in) (1969-2008), with most precipitation
occurring in winter and during the summer monsoon
(July-September). However, annual precipitation has
varied widely in recent decades from a low of 23.6 cm
(9.3 in) in 1989 to 66 cm (26.0 in) in 1992. The average
minimum temperature in January is -11 o C and the average
maximum temperature in July is 29 o C.
We established an experiment to study the effects of
prescribed burning on the P. clutei community in 1994.
Forty P. clutei plant-centered plots were established,
each with a 2.5 m radius circle (area = 19.6 m 2 ) centered
0.3 m northwest of an existing plant. P. clutei was tallied
in four categories: seedling, second-year plant, mature
plant, and dead. Field experience indicated that the
distinctions between the living plant categories were
approximate. Plots were randomly selected for burn or
control treatments, and burning was conducted in September
1994. Burn season effects were tested on a second
experimental site immediately north of the fall
burning site, and twenty randomly selected plots were
burned in April 1995. The fall burn plots were re-measured
in July 1995. All 80 plots, including spring and fall
burns, were re-measured in August/September 1996;
August 1997; August 1998; and August/September
2008. Changes in P. clutei density were analyzed with
repeated measures analysis of variance (Systat 8.0,
SPSS Science, Chicago). Data were square-root transformed
to meet ANOVA assumptions. In 2008, data
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