December 2012 Number 1 - Utah Native Plant Society

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
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