December 2012 Number 1 - Utah Native Plant Society

Calochortiana December 2012 Number 1
RESULTS
Differences in Vegetation Among Sites
A total of 150 vascular plant species occurred in at
least one vegetation plot in a least one site (Table 1).
Sixty-six of these species occurred in only one of the
four sites (pooling Cuellar treatments). While the vegetation
of the four sites was similar in many ways, the
relative abundances of species differed among sites
(Table 1). In Arroyo Morteros, Thymophylla pentachaeta,
Acacia rigidula, and Chaetopappa bellioides/
Aphanostephus skirrhobasis var. kidderi were most
abundant; in Arroyo Ramirez, Aristida purpurea,
Turnera diffusa, Acacia rigidula, and Polygala lindheimeri;
in Santa Margarita, Aristida purpurea and
Eriogonum greggii; and in Cuellar, Acacia rigidula,
Leucophyllum frutescens, Polygala lindheimeri, and
Melampodium cinereum. Sites also differed in the abundances
of less common species (Table 1). A MANOVA
(multivariate analysis of variance) comparing the abundances
of all species (except P. thamnophila and ‘bare
ground’, for a total of 146 ‘species’ after the pooling
described in the Methods) among the five site x treatment
combinations (Cuellar treatments not pooled) was
highly significant (Cuellar 2007 vegetation data used;
Hotelling-Lawley Trace, F = 9.44, df = 548, 114.71,
P < 0.0001).
The four sites also differed in vegetation structure
(Figure 5). The lack of vegetation above 1m in the
cleared portion of Cuellar in 2002 was due to its treatment,
while the uncleared portion of Cuellar had the
densest canopies. The regrowth in the Cuellar cleared
portion is apparent in a comparison of the 2002 and
2007 graphs (Figure 5). Among uncleared sites, Arroyo
Ramirez had the most open canopies, and Arroyo
Morteros the tallest.
Relationships Between P. thamnophila Presence and
Neighboring Species
Twenty-nine species, including P. thamnophila, were
present in 25 percent or more of the subplots. These species,
except for P. thamnophila, were used as the dependent
variables in a MANOVA. In this MANOVA,
site and P. thamnophila presence/absence in the 2007
census were the independent variables. Cuellar 2007
vegetation data were used in this analysis and the two
Cuellar treatments were considered to be 2 different
‘sites’, for a total of 5 site-treatment combinations. Both
of the independent variables were highly significant
(site-treatment combination: Hotelling-Lawley Trace F
= 20.67; df = 112,476; P < 0.0001; presence/absence:
Hotelling-Lawley Trace F = 1.93; df = 28,143; P =
0.0066).
Because the MANOVA found significant effects of
P. thamnophila presence we followed it with ANOVAs
(univariate analyses of variance). Six of 28 ANOVAs on
the same 28 species had P-values less than 0.05 associated
with the effect of P. thamnophila presence/absence.
In each of these ANOVAs, presence/absence, df = 1,
was the second factor in a hierarchical sums of squares
table that had site, df = 3, as the first factor. Thus these
P-values reflect the amount of variation that presence/
absence accounted for above-and-beyond the variation
accounted for by site-year combination. Plots that had
P. thamnophila had on average more Diospyros texana
(P = 0.0424), Acacia rigidula (P = 0.0043), unidentified
seedling grasses (P = 0.0068), Mimosa texana (P =
0.0012), and Chamaesaracha sordida + Physalis cinerascens
(P = 0.0293), and less Tiquilia canescens (P =
0.0293), than plots without P. thamnophila. These positive
and negative associations with P. thamnophila
should be regarded as tentative, due to multiple testing
issues, deviations from the multivariate normal assumption,
and correlations among abundances of the different
species. Only Mimosa texana meets the Bonferroni criterion
for significance (for 28 tests, P < 0.00183 to give
an overall P < 0.05).
Relationships Between P. thamnophila Density and
Neighboring Species
A stepwise regression was used (SAS PROC REG)
with the average 2006/2007 density of P. thamnophila
as the dependent variable. Site was included as a class
variable (df = 3). All 148 ‘species’ except bare ground
and P. thamnophila were available to the regression procedure.
A criterion of P < 0.05 was used to retain variables
in the model. Eighteen taxa met this criterion.
Thirteen of them were negatively associated with P.
thamnophila: Acleisanthes obtusa, Acourtia runcinata,
Citharexylum brachyanthum, Thymophylla pentachaeta,
Echinocactus texensis, Echinocereus reichenbachii ssp.
fitchii, Galphimia angustifolia, Nama hispidum, Oenothera
laciniata, Portulaca sp., Sideroxylon celastrinum,
Synthlipsis greggii, and Tridens muticus. Five were
positively associated with P. thamnophila: Acacia rigidula,
Cyperus sp., Evolvulus alsinoides, Melampodium
cinereum, and Passiflora tenuiloba.
DISCUSSION
Community Composition: General
Each of the study sites had a rich plant community
that contained shrubs, forbs, graminoids, and cacti
(Table 1). The shrubs Acacia rigidula and Leucophyllum
frutescens dominated all four sites. While all four
study sites were similar enough to be described as having
the same plant community, there were some interesting
differences among them. These include Turnera diffusa,
found in 59 percent of samples at Arroyo Ramirez
but absent from other sites; Eriogonum greggii, itself a
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