December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society Figure 3. Mean density of Penstemon clutei plants following a root trenching experiment near Sunset Crater National Monument in northern Arizona . Pre-treatment data were collected in 1997. Bars indicate standard error (each plot was 19.6 m 2 ; n=80; p value in 2008 was
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- 169
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December 2012 Number 1 - Utah Native Plant Society

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