December 2012 Number 1 - Utah Native Plant Society

landeri, cited as a community dominant in the Recovery
Plan (USFWS 2004), appeared in the vegetation data in
only one of our four study sites. This species is a dominant
at Santa Margarita immediately upslope of the P.
thamnophila population that is on a different soil type.
Prosopis glandulosa never occurred within our study
plots. The inclusion of Prosopis in the Recovery Plan
was based on a site that we did not study, an abandoned
trailer park which has subsequently been overtaken by
invasive buffelgrass (Pennisetum ciliare), leaving only a
small remnant on highway right-of-way (J.M. Poole,
Texas Parks and Wildlife Department, personal communication
20 July 2009). Other species mentioned in the
Recovery Plan that we encountered infrequently include
Celtis ehrenbergiana, Yucca treculeana, Ziziphus obtusifolia,
and Guaiacum angustifolium, which are common
visual dominants in the region that were not closely
associated with P. thamnophila at our sites. These species
may be better suited to sites with deeper soils or
more favorable soil chemistry.
Two common invasive grasses of the area, buffelgrass
(Pennisetum ciliare (L.) Link) and Kleberg bluestem
(Dichanthium annulatum (Forssk.) Stapf), are also
conspicuous by their absences or low abundances in the
four study sites. It seems likely that P. thamnophila, like
other native grasses and forbs of south Texas (Sands et
al. 2009), is out-competed by these invasive grasses.
The plots in which P. ciliare was encountered should be
monitored to determine its effect on P. thamnophila.
Associated species’ negative or positive correlation
with P. thamnophila may have a number of explanations.
Some of the negative correlations may reflect very
localized competition. For example, a plot in which
Thymophylla pentachaeta or Nama hispidum were very
abundant (especially in 2007) may have been colonized
by these species in response to winter rain, and they in
turn excluded P. thamnophila. Some negative correlations
may reflect microsites not suitable for P. thamnophila,
such as hardpan, where Tiquilia canescens was
relatively common. Synthlipsis greggii seems to use different
microsites and topographic positions than P.
thamnophila; however, we did not quantify this observation.
Acacia rigidula’s positive correlation with P. thamnophila
may indicate facilitation, probably because as a
legume it may create a soil patch with relatively high
nitrogen content. Alternatively, its presence may indicate
that the plot is not bare hardpan, but rather is favorable
to vegetation in general. The other shrub that was
positively correlated with P. thamnophila was the legume
Mimosa texana. This shrub, although not rare, has
a restricted range and may be more indicative of P.
thamnophila habitat. A “characteristic species of the
arid, sandy-soil Falcon Woodlands, which cover a small
upland part of Starr and Zapata Counties” (Ideker 1999),
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M. texana was present at all study sites in 5 to14 percent
of subplots.
The positive correlation of P. thamnophila with perennial
herbaceous species such as Melampodium
cinereum and Evolvulus alsinoides is probably related to
its similar microhabitat requirements and response to
precipitation and shrubs. Other species with significant
correlations were present in low frequency and are inconclusive.
Edaphic Requirements of P. thamnophila
Although P. thamnophila populations are mapped on
several soil series, our observations in the field indicate
very similar soils and geologic substrates at all sites. All
four populations of P. thamnophila occur on a sandstone
substrate (Figure 6), on yellowish, highly erodible,
highly calcareous soils. All other Texas populations to
which we and other observers have had access have
similar yellowish sandy soils and occur on sandstone.
Wu and Smeins (1999) report Copita and Zapata sandy
loam soils as the substrate for P. thamnophila, and clarified
that sites mapped as Catarina soils (saline, gypsiferous
clay) are actually on sandy inclusions. Their analyses
of soil from four P. thamnophila sites found very
high calcium, high sulfur, and very low nitrogen levels.
We believe that use of NRCS digital soil maps at a
level of detail beyond which they were intended has led
to confusion about the soils on which P. thamnophila
occurs. P. thamnophila has never been found on
Jimenez-Quemado soils (contra Poole 1989 and
USFWS 2004). The parent material of these soils is
gravelly alluvium, deposited by ancient, high-velocity
streams on the high terraces over the Rio Grande
(Thompson et al. 1972). The Jimenez-Quemado soil
polygons contain inclusions of “unnamed, minor components”
and rock outcrops. These outcrops, rather than
Jimenez or Quemado soils, are likely habitat for P.
thamnophila.
Figure 6: Sandstone substrate with Physaria thamnophila
plant.