December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
point to a number of studies showing that root trenching
can lead to an increase in the availability of mineral nitrogen.
Research by Selmants and Hart (2008) indicates
that there are large carbon and nitrogen pools and fluxes
under the canopies of one-seed juniper (Juniperus
monosperma) in soils around Sunset Crater. Soils under
juniper and pinyon pine canopies are generally higher in
both carbon and nitrogen than are intercanopy sites, but
these levels vary according to the age of the soils, which
are of volcanic origin in this region.
Abella and Covington (2006) obtained samples from
a number of soil types across the Coconino National
Forest, including black and red cinder soils in the vicinity
of Sunset Crater, and determined that black cinder
soils contain the driest surface soils among those tested.
These soils are very sandy (>90% concentration at 0-15
cm depth), and contained the fewest plant species per
plot. Red cinder soils are also quite sandy (averaging
63% concentration at 0-15 cm depth). Soil samples
taken from red cinders contained no calcium carbonate,
but these soils had higher organic carbon and total nitrogen
than the black cinder soils, and they also had higher
soil moisture. P. clutei populations in pinyon-juniper
woodlands are typically found on these older, red cinder
soils.
The soils on which P. clutei grows, then, are arguably
some of the harshest in northern Arizona and are
susceptible to extreme environmental fluctuations.
Seedling mortality is high, but once plants reach maturity,
they have adapted to the harsh, arid environment by
means of a large taproot or thick lateral roots (D.W.
Huffman, personal observations, 2008) and thick leaves.
The species also must have developed adaptations to be
able to rapidly colonize following disturbance, perhaps
through longevity, rapid dispersal, high germinability,
or a persistent soil seed bank. Soil seed banks buffer
populations against environmental variation, and seed
dormancy is a mechanism of escape from unfavorable
conditions in time (compared to dispersal, which is an
escape in space) (Doak et al. 2002). Short-lived perennials
in areas of high environmental variation, which includes
most rare plants in the Southwest, often rely on
the soil seed bank for recruitment (Doak et al. 2002). P.
lemhiensis (Lemhi penstemon) and P. palmeri have
been documented to buffer populations against environmental
fluctuations by maintaining a persistent soil seed
bank (Heidel and Shelly 2001, Meyer and Kitchen
1992). Conversely, long-lived perennials are often more
reliant on growth and survival of established plants than
on recruitment from seed or soil seed banks (Lesica
1995). If a species does not exhibit innate dormancy, it
is unlikely that it forms a soil seed bank. Because P.
clutei plants have been observed to appear in large numbers
following a disturbance such as the Hochderffer
Fire (P.Z. Fulé, personal observation, 1997 and 1998),
conventional thinking is that this species forms a persistent
soil seed bank (Phillips et al. 1992), but it may also
maintain genetic diversity through existing reproductively
mature plants scattered across the landscape, or
exhibit rapid dispersal rates following disturbance.
While it does seem from our study that P. clutei may
form a minor persistent seed bank, the degree of its importance
in recovery following disturbance is unknown,
and larger sample sizes from additional habitats are necessary
in order to make inferences about its significance
for recruitment following disturbance.
Meyer et al. (1995) found a diversity of germination
timing mechanisms in 38 Intermountain West Penstemon
species. Seeds of many of these species diverge
into two fractions. One fraction does not exhibit dormancy
and will germinate readily under optimal conditions
in the first year. The other fraction may respond to
chilling cues and become nongerminable, allowing for
between-year carryover in the soil seed bank. Meyer and
her co-authors (1995) found this strategy to be especially
common in populations of penstemons from middle
elevation areas that have unpredictable winters.
Meyer and Kitchen (1992) discovered that P. palmeri
seeds undergo cyclic dormancy changes in the field.
Moist chilling induces secondary dormancy in about
half of the seeds, while moisture combined with summer
temperatures removes secondary dormancy. These
mechanisms allow for a persistent soil seed bank and for
seeds that can persist from year to year without burial.
The result is that some seeds germinate in the spring,
while those seeds that are rendered dormant by chilling
are carried over in the soil seed bank. Another fruitful
area of research for this species could include seed augmentation
studies to determine if a paucity of viable
seeds may be limiting establishment. Abella (2008) conducted
such a study with P. virgatus (upright blue
beardtongue) in a ponderosa pine forest not far from our
study site and found that under particular experimental
conditions, the site environment (e.g., tree overstory)
apparently was more limiting to recruitment than either
leaf litter thickness or seed availability.
Our results also indicate that prescribed burning
alone does not seem to be a useful management tool for
this species, as it appears to kill reproductively mature
individuals leading to potential decreases in available
seeds for future recruitment. An experiment involving
use of prescribed fire as a management tool for P. lemhiensis
returned variable results (Heidel and Shelly
2001). Fire appeared to cause mortality of adult plants
ranging from 25-75%. However, the burning caused
an increased recruitment rate of 4600-6400%. As we
pointed out in our previous paper (Fulé et al. 2001),
patchy tree mortality does appear to benefit P. clutei.
Tree mortality from the 2002-2003 bark beetle outbreak
among pinyon pines appears to be correlated with dra-
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