December 2012 Number 1 - Utah Native Plant Society

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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), Utah Native Plant Society 186 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.
Calochortiana December 2012 Number 1 The underlying geology at the sites is complex. The Yegua and Jackson Formations were deposited in part of the Gulf Coast geosyncline known as the Rio Grande Embayment during Eocene cycles of sedimentation. Land subsidence and marine transgressions and regressions produced fluctuating sea levels and deposition of “complexly interbedded sands, silts, and clays” as well as marine shales (Preston 2009). Dumble (1902) provided a detailed site-specific description of this complex stratigraphy. Reporting on outcrops of “buff sandstone” near Roma (in the vicinity of our study sites), he describes sections of “greenish-yellow clays” with gypsum, oyster beds, buff clays, sandy clays, and indurated sandstone. At two of our study sites that were on bluffs along the Rio Grande, two layers of buff to yellowish sandstone were interspersed with other substrates or oyster shell deposits. Fossil oyster shell beds occurred upslope from all four study sites. Gypsum crystals occurred on the soil surface in many places. The alternating layers of sandstone with fossil oyster shell, shale and clay may explain the presence of gypsum at the sites, even though the sandy soils (Copita and Zapata) are only weakly gypsiferous. The layers of different substrates may create microsites where water is more available due to seepage from relatively permeable layers located over impermeable or less permeable layers. Several species we encountered in this study are members of genera that are documented as tolerating gypsum, particularly Tiquilia, but also Nama, Eriogonum, and Acleisanthes (Moore and Jansen 2007). We have no evidence that P. thamnophila is a gypsum endemic; however, it tolerates gypsum. All four sites were undergoing active gully erosion to the degree that some slopes could be called badlands. All four sites also appeared to have high rates of sheet erosion as well, especially in the bare areas between shrubs. While erosion probably does not directly benefit P. thamnophila, high erosion rates likely reduce the number of competing species that can live in the site, and perhaps their densities. High erosion rates may be one factor contributing to the positive association between shrubs and P. thamnophila, especially P. thamnophila seedlings within our sites (Fowler et al. 2011). By slowing the rate of erosion in their immediate vicinity, shrubs may increase seedling survival there. High erosion rates may also explain why roller-chopping part of Cuellar increased the density of P. thamnophila there (Fowler et al. 2011), as the woody debris left by this treatment may also have reduced the erosion rate. However, we cannot exclude other positive effects that shrubs may have upon P. thamnophila. These edaphic features (high erosion rates, highly calcareous soils, perhaps the presence of gypsum) could be used to search for sites where additional populations might occur. They also provide some guidance for identifying sites suitable for introduction or reintroduction. Although we do not believe that P. thamnophila requires high soil erosion rates, the presence of gypsum, or even calcareous soils, all of these probably reduce the number and density of competing species. Ecologically, P. thamnophila is apparently a stress-tolerator (sensu Grime 1977, 2001), rather than a strong competitor or a ruderal species. This may also be true of many of its associates. Conservation Applications The Recovery Plan (USFWS 2004) called for (a) identification of sites where additional populations might occur; (b) identification of sites most suitable for attempts to establish new populations; (c) identification of tracts most appropriate for mitigation purposes, should that be necessary; (d) development of management plans; and (e) development of habitat restoration objectives. The vegetation structure and composition data presented here, especially that of Table 1 and Figure 5, provide quantitative objectives for each of these tasks. Whereas visually dominant species mentioned in previous work can still be used to search for general areas of suitable native thornscrub vegetation, the species composition presented here provides a finer scale focus for identification of suitable habitat. The qualitative description of likely soil types provided above could be used to guide searches for new populations and identify sites for introduction or reintroduction. It would also be helpful to know the gypsum content of the soils. Management and restoration projects can use Table 1 and Figure 5 to help set quantitative objectives. It should be noted that this study was not designed to compare sites with and without P. thamnophila, so results do not definitively identify what it is about these four sites that made them different from similar sites in the region. Additionally, we did not attempt to characterize small, remnant, disturbed sites, which are also important to the species’ conservation. In any case, with so few P. thamnophila populations, the absence of P. thamnophila from any particular site is hard to interpret. P. thamnophila may be absent from suitable sites due to chance, poor dispersal, disease, herbivory, or other factors, which is often a problem in studying endangered species (Hanski and Ovaskainen 2002). Experienced biologists will no doubt make their own judgments from the data we provide. ACKNOWLEDGEMENTS We thank the Lower Rio Grande Wildlife Refuge staff for their assistance; landowner Jorge Gonzales and family for access to Santa Margarita Ranch; and Tom and Elena Patterson, Thomas Adams, Robyn Cobb, Loretta Pressley, Jim Manhart, and Alan Pepper for 187
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