December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society 63 percent in the alpine flora of New Zealand, and 41– 51 percent of endemics in South Africa and Namibia (Fischlin et al. 2007). In the Great Basin, range contractions or extinctions have been predicted for higher elevation mammals, butterflies, birds and plants (Murphy and Weiss 1992; Van de Ven et al. 2007; Wagner 2003). Populations of pika (Ochotona princeps) have already been reduced by 28 percent compared to the number of populations known earlier in the 20 th century (Beever et al. 2003). Grayson (2000) has posited that during the Middle Holocene (5,000–8,000 years before present), a period characterized by a decrease in summer precipitation and an increase in temperatures, the small mammal fauna of the Great Basin decreased in species richness and evenness as a result of a series of local extinctions and near-extinctions coupled with an increase in taxa well-adapted to xeric conditions. The objective of this study was to conduct an initial assessment of the vulnerability of the rarest plants of the Great Basin of Nevada to climate change based on geographical and ecological data from the literature, stored in data bases and files maintained by the Nevada Natural Heritage Program, Carson City, Nevada, and stored in files maintained by the U.S. Fish and Wildlife Service at the Nevada Fish and Wildlife Office, Reno, Nevada. STUDY AREA The study area encompassed the Great Basin within the State of Nevada. The term “Great Basin” was first used in 1844 by the explorer John Fremont in reference to the large closed hydrologic basin lying between the Sierra Nevada of California and Nevada to the west and the Wasatch Front of Utah to the east with slight extensions into Oregon and Idaho (Tingley and Pizarro 2000). This analysis focuses on the floristic Great Basin within Nevada (Holmgren 1972a), an area of roughly 54,741 km 2 that includes all of Nevada north of the Mojave Desert with the exception of the Carson Range along the eastern side of Lake Tahoe (Figure 1). In general, precipitation increases and temperature decreases with elevation in the Great Basin, although physiographic factors can exacerbate temporal and spatial variation in climatic patterns. The complex terrain, with its large differences in altitude and the consequent distortion of air currents creates high variability in local precipitation and short periods of intense rainfall followed by very long periods without precipitation (Hidy and Kleiforth 1990). Rapid heat loss at night results in cool air descending to valley floors where pooling in closed basins creates diurnal temperature inversions (Beatley 1975). Along the southern boundary of the study area, air and soil temperature regimes on two valley floor sites, separated by only 90 m horizontally and 1.5 m vertically, were found to influence the distribution of dominant shrub species. The composition of herb- Figure 1. Location of the study area in the Great Basin of Nevada, an area of about 54,741 km 2 ; the larger dark outline is the boundary of the Great Basin Restoration Initiative as delineated by the U.S. Bureau of Land Management based primarily on floristic similarity. aceous perennials and winter annuals on the same two sites was similar, although temperature influenced the initiation of vegetative growth in herbaceous perennials and the germination success of winter annuals (Beatley 1969, 1975). Predicting local climates and the climatic responses of highly localized endemic plant populations, therefore, is at best a challenging approximation. METHODS I used ecologic and geographic data stored by the Nevada Natural Heritage Program and the U.S. Fish and Wildlife Service, including various status assessment reports prepared for many of the rare plant taxa, to determine the reported minimum and maximum elevations of all known populations of the rarest plants within the study area. All taxa ranked as G1, G1G2, or T1 were included in this study. G1 ranked species are considered critically imperiled based on a very high risk of extinction due to extreme rarity (often five or fewer populations), very steep declines, or other factors; T1 ranks are applied to infraspecific taxa that meet the same criteria as for the G1 ranks (Natureserve 2009). I assessed a total of 167 reported locations of 33 taxa with ranks of G1, G1G2, or T1 ranks (Table 1). I used the reported minimum and maximum elevation of all known locations for each of the 33 taxa as a surrogate for the combined effects of temperature and 92
Calochortiana December 2012 Number 1 precipitation, the principal climatic factors affecting plant distribution in the Great Basin (Billings 1949; Comstock and Ehleringer 1992; Fautin 1946). Van de Ven and others (2007) used actual temperature and precipitation data to construct bioclimatic models of potential climatic change in the White Mountains of California and Nevada at the western edge of the Great Basin, but such data are relatively sparse for the interior Great Basin. Climate stations are typically clustered near towns and skewed toward lower elevations; there are few climate stations above 2500 m. Moreover, due to the complex topography of the Western United States and the coarse resolution of most climate models, even the best climate models display biases at regional scales (Bonfils et al. 2008). All 33 taxa are highly localized endemics with geographic ranges that are considerably smaller than could be predicted by the best regional climate models. While approaches that incorporate additional factors such as slope, aspect, and geologic substrate have been used with some success to develop predictive models of potential habitat for a few rare plants in the Great Basin of Utah (Aitken and others 2007), field surveys conducted by experienced botanists for many of these 33 taxa have consistently found that only a small portion of their predicted range contains suitable habitat. Therefore, the objective of this study was to compare the climatic niches of rare plants, rather than to develop predictive distribution models. RESULTS The rarest plants of the Great Basin of Nevada (i.e., those with a G1, G1G2, or T1 rank) can be placed into three broad elevation bands. Those bands reflect occurrence: 1) below the lower limits of tree distribution on or near valley floors; 2) within a narrow montane zone dominated by pinyon-juniper woodlands and associated shrublands or subalpine; or 3) near or above timberline (Figure 2, Table 1). The lowest elevation group is comprised of 14 taxa that have a median population altitude below 2000 m. All but one of these taxa are endemic to Nevada (Figure 2). Eleven taxa, Eriogonum tiehmii, E. argophyllum, E. ovalifolium var. williamsiae, E. diatomaceum, Castilleja salsuginosa, Johanneshowellia crateriorum, Boechera falcifructa, Mentzelia argillicola, M. tiehmii, Frasera gypsicola, and Potentilla basaltica have a reported elevational amplitude of less than 244 m, with nine of these distributed across less than 129 m of elevation (Figure 2). Two additional species, Sclerocactus blainei and Mimulus ovatus, are reported from an elevational amplitude of less than 50 m. The lone anomaly to the general pattern of restricted elevational range among the lowest elevation group was Penstemon floribundus, with populations spanning 1,009 m. The middle elevation group is comprised of nine taxa, all but one of which are thought to occur only in Nevada (Figure 2). The median altitudes of all known populations of these taxa fell between 2,000 m and 2,746 m, although three taxa have some populations near or above 3,000 m. Six of these taxa, Tonestus graniticus, Lewisia maguirei, Collomia renacta, Eriogonum microthecum var. arceuthinum, E. douglasii var. elkoense, and Trifolium andinum var. podocephalum had elevational amplitudes of less than 280 m. This relatively narrow elevation span is comparable to those typical of the valley taxa, and an argument could be made to make only two groupings based on a division at 2,500 m (Figure 2). Such a division would, however, mask an ecological distinction based on a notable difference between valley and montane endemics in their edaphic specialization, discussed in more detail in the following section. The remaining three taxa, Penstemon tiehmii, P. pudicus, and P. moriahensis had reported elevational amplitudes of 640 m, 782 m, and 1,128 m, respectively. The highest elevation group is comprised of ten taxa, seven of which are considered endemic to Nevada. Only three of the ten taxa, Draba serpentina, Boechera ophira, and Ipomopsis congesta var. nevadensis had reported elevational amplitudes of less than 200 m. Two taxa, Primula capillaris and Penstemon rhizomatosus, had reported elevational amplitudes of 381 m and 366 m, respectively. The remaining five taxa, Viola lithion, Eriogonum holmgrenii, Potentilla cottami, Polemonium chartaceum, and Cymopterus goodrichii had reported elevational amplitudes between 747 m and 1,158 m. Only three, Polemonium chartaceum, Eriogonum holmgrenii, and Draba serpentina, were restricted to sites above treeline (Figure 2). DISCUSSION Overview Many of the rarest plant taxa in the Great Basin of Nevada are found below the lower limits of tree distribution. These low elevation taxa (median elevation below 2,000 m) had the narrowest bioclimatic envelope, as estimated from their reported elevational amplitudes, with 11 of 14 (79 percent) spanning less than 244 m. If other elevation bands are considered, 17 of the 20 taxa (85 percent) with an elevational span of less than 280 m have a median elevation below 2,377 m (Figure 2). Among the ten highest elevation taxa, only three (30 percent) have a reported elevational amplitude of less than 200 m (one of these is likely more widely distributed as discussed below), and four of them (40 percent) are known to occur over more than 700 m of elevation. These results oppose the prediction that alpine species are at particular risk due to isolation and lack of an 93
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