December 2012 Number 1 - Utah Native Plant Society

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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 166
Calochortiana December 2012 Number 1 were cube-root transformed, and the software used was JMP 8.0 (SAS, Cary, NC). In October 1997 we established ten new experimental plots for root trenching within the low-elevation burning study area. Each plot was paired with a nearby control plot from the original burn experiment. The number of trenched plots was eventually dropped to eight due to off-road vehicle damage and other factors. The below ground effects of tree removal were simulated by digging a narrow trench approximately 1 m deep around each plot. Trenches were located 50 cm outside the plot boundary to avoid physical disturbance within the measured area and were lined with plastic sheeting to minimize tree root regrowth (Milne 1979). Trenches were backfilled immediately after lining, and plots were re-measured in August 1998, September 1999, August 2000 and August/September 2008. P. clutei density was analyzed as described for the prescribed burning study above. In 2008, we collected two soil seed bank samples from each of the 80 prescribed burning study plots and also from the trenched plots (176 total). Samples were collected to a depth of 5 cm, approximately 15 cm away from the original plot center to the east and west, and each core was approximately 70 cm 3 in volume. We also collected 30 targeted seed bank samples 15 cm to the east and west of reproductively mature individuals that were located outside of plots. Seeds are dispersed in the fall and winter, and germination is thought to occur during late spring and early summer rains, so we collected soil seed bank samples in late August and early September, presumably after germination occurred, but before new seeds were dispersed, in an effort to capture seeds in the persistent soil seed bank. We sieved samples to remove large cinders and placed the soil samples on potting soil in gallon-sized pots. Samples were placed in the greenhouse in September 2008 and received artificial light, one application of Miracle Gro® and daily watering for five months, using the seed emergence method (Ter Heedt et al. 1996). RESULTS In 2008, thirteen and fourteen years after spring and fall burning, respectively, there was no significant difference in P. clutei density between burned and unburned plots (Figure 2). Mean density in burned plots (combined spring and fall burns) was 0.9 live plants, and mean density in control plots was 0.6 live plants. Over the course of this study, there has been a general decline in P. clutei in both burned and control plots. However, there was still a significant difference in mean density between trenched and control plots ten years Figure 2. Mean density of Penstemon clutei plants following a prescribed burn study near Sunset Crater National Monument in northern Arizona. Pre-treatment data were collected in 1994 and 1995. Bars indicate standard error. 167
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December 2012 Number 1 - Utah Native Plant Society

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