December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
Long-term Population Demographics and Plant Community Interactions
of Penstemon harringtonii, an Endemic Species
of Colorado’s Western Slope
Thomas A. Grant III
Program in Ecology, Colorado State University, Fort Collins, CO
Michelle E. DePrenger-Levin,
Denver Botanic Gardens Research and Conservation Dept, Denver, CO
and Carol Dawson
Bureau of Land Management Colorado State Office, Lakewood, CO
Abstract. Penstemon harringtonii is an endemic species of Colorado’s western slope. Known from only six counties,
Harrington’s penstemon is threatened primarily by habitat degradation and destruction in rural areas that are experiencing
relatively rapid development and recreational pressure. Annual demographic monitoring since 1996 has
not identified statistically significant changes in the overall number of rosettes, although significant inter-annual
variation occurs at the two study sites. Additional research has focused upon the interactions of P. harringtonii with
Artemisia tridentata (big sagebrush) and local plant species richness, and competition for soil moisture. Weak negative
correlations between P. harringtonii and A. tridentata have been documented at both study sites, although the
two sites have opposite trends in the correlation of P. harringtonii and species richness. Ordination techniques (nonmetric
multi-dimensional scaling, NMS) are being explored as a means to find patterns that could increase our understanding
of the rare species’ interactions with the dominant shrub (A. tridentata), local plant species diversity, and
soil moisture. NMS found a positive correlation between species richness and the higher density P. harringtonii
quadrats, although no strong relationships were identified between the rare species and soil moisture. Additional sites
will be sampled in 2009 and 2010 to test hypotheses concerning the potential drivers of P. harringtonii density and
provide guidance in the development of appropriate management and restoration methods.
Long-term monitoring of Penstemon harringtonii
Penland (Scrophulariaceae) was initiated in 1996 to determine
population trends of this rare species. The penstemon
is threatened by land development for homes
and ski areas, oil and natural gas development, overgrazing,
and off-road vehicle use (CoNPS 1997, Panjabi
and Anderson 2006). Formerly a Category 2 candidate
for listing under the Endangered Species Act (ESA), P.
harringtonii is endemic to western Colorado (USDA
NRCS 2009) and populations have been documented
from six counties within the state (Eagle, Garfield,
Grand, Pitkin, Routt, and Summit)(Panjabi and Anderson
2006, Spackman et al. 1997). Currently, P. harringtonii
is listed as a sensitive species by the U.S. Bureau
of Land Management Colorado State Office and the
USDA Forest Service Region 2. The species is ranked
G3/S3 by The Nature Conservancy Natural Heritage
ranking system (Spackman et al. 1997). Based upon the
S3 ranking, the species is considered vulnerable to extirpation
in the state and only 74 occurrences have been
documented (Panjabi and Anderson 2006).
The majority of populations occur in habitats dominated
by Artemisia tridentata (big sagebrush), a hyd-
raulic lifting species (Caldwell et al. 1998). The interaction
between A. tridentata and growth of herbaceous
plants such as P. harringtonii is unclear, although soil
moisture and precipitation are acknowledged as limiting
factors for primary production in semiarid sagebrush
steppe ecosystems (Horton and Hart 1998).
Initial goals of the project were to document population
trends at two study sites, although this has been
expanded into understanding the species’ relationship to
A. tridentata density, soil moisture, and plant community
composition with the ultimate goal of improving
our management of the species and its habitat. We hypothesized
that areas with lower densities of A. tridentata
will have higher densities of P. harringtonii and
greater species richness due to reduced competition for
water. Although destruction or degradation of P. harringtonii’s
habitat is the greatest threat, an understanding
of the species’ interactions with big sagebrush and
soil moisture may augment management of extant populations
and restoration of degraded areas. Additionally,
an understanding of the relationships between sagebrush,
soil moisture and species richness may assist our
ability to manage for diverse ecosystems.
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