December 2012 Number 1 - Utah Native Plant Society

Utah Native Plant Society
Figure 4. Non-metric Multi-dimensional Scaling (NMS) of the plant species composition at the two study sites (Eagle
and Gypsum). Triangles represent quadrats and plus signs are plant species with species codes as labels. Red vectors
represent correlation coefficients with R 2 > 0.20 and are directionally aligned to plant species to which the vector
variables are positively correlated. The length of the vector represents the magnitude of the correlation.
data from the primary matrix. The vector for ‘species
richness’ is oriented towards the sample space primarily
occupied by the Eagle plots and represents a positive
correlation between the Eagle samples and plots with
higher species richness. The ‘dominance’ vector represents
a ratio of P. harringtonii and A. tridentata densities
and documents the positive correlation of the higher
penstemon ratios with the Eagle samples. ‘Site’ is a
categorical or ‘dummy’ variable necessary to code the
sites and distinctly segregates the two study sites. Ordination
analysis of soil moisture data and densities of
Harrington’s penstemon or sagebrush did not determine
any significant relationships or strong correlations. Although
simple linear regressions determined that the
Gypsum site had a weak negative relationship (R 2 =
0.15) between the penstemon and sagebrush densities.
Using linear regression to compare species richness with
penstemon or sagebrush density, the two study sites had
weak R 2 values, but the general trends were always
opposite between the sites. At the Eagle site, species
richness was positively related to penstemon density (R 2
= 0.0856) or sagebrush density (R 2 = 0.0857). Conversely,
at the Gypsum location the densities of penstemon
and sagebrush were negatively correlated to species
richness (R 2 = 0.108 and R 2 = 0.125, respectively).
DISCUSSION
The monitoring data documents that the P. harringtonii
populations at Eagle and Gypsum are stable over
the 13 years of monitoring (Figures 2 and 3), although
they fluctuated greatly and reached the lowest population
sizes during the droughts of the early 2000s. Increasing
drought severity or frequency could have negative
consequences for this species, especially if its habitat
becomes more fragmented by development or degraded
due to overuse. These results cannot be extrapolated
to all populations of Harrington’s penstemon, but
provide quantitative data concerning the population
118