December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society ¡Viva thamnophila! Ecology of Zapata Bladderpod (Physaria thamnophila), an Endangered Plant of the Texas-Mexico Borderlands Dana M. Price, US Army Corps of Engineers, Albuquerque District Christopher F. Best, US Fish and Wildlife Service, Austin Ecological Services Field Office Norma L. Fowler, Section of Integrative Biology, University of Texas at Austin, and Alice L. Hempel, Department of Biology and Health Sciences, Texas A&M University- Kingsville Abstract. Conserving rare plants is dependent upon our ability to identify, manage and restore their habitat. We examined the plant community associates and habitat requirements of Physaria thamnophila, an endangered herbaceous perennial of the Tamaulipan shrubland of south Texas, at four sites from 2003 to 2007. At each site, vegetation height structure and species composition were sampled concurrently with intermittent censuses of P. thamnophila. We found significant and interesting differences among sites and years, as well as between our quantitative results and previous descriptions of P. thamnophila’s community and habitat. Existing plant community descriptions and mapped soil types do not provide a close match with our field observations. Finally, we discuss the application of these results to conservation of P. thamnophila and restoration of its community. An endangered plant that is a member of a fragmented and altered remnant plant community poses many challenges to conservation workers. In addition to threats to its (usually) few and small known populations, two additional challenges are faced. One, the detection of additional populations is often hampered by uncertainty in identifying its habitat; and two, uncertainty about its habitat requirements hampers management and restoration efforts. Physaria thamnophila (Zapata bladderpod; Brassicaceae) is one such plant. This endangered herbaceous perennial of south Texas grows in remnants of Tamaulipan thornscrub, likely on a specific but as yet poorly defined soil type and geologic substrate. Prior to the present study, the structure and composition of the specific plant community in which Zapata bladderpod occurs had been examined quantitatively for only one site (Sternberg 2005). The U.S. Fish and Wildlife Service (USFWS) Recovery Plan (USFWS 2004) highlights the need for better knowledge of the habitat and community in which P. thamnophila lives for several purposes, including discovering additional populations, locating sites for establishing new populations, and restoring and managing its habitat. This study addresses these goals by providing a detailed, quantitative description of the habitat and community of four sites with persistent (as defined by NatureServe 2002) P. thamnophila populations. The results will not only be useful for P. thamnophila conservation, but can improve our ability to manage and restore one of the communities that comprise the Tamaulipan thornscrub ecosystem. SPECIES Physaria thamnophila (Rollins and E.A. Shaw) O’Kane and Al-Shehbaz (formerly Lesquerella thamnophila) is a federally listed endangered species (USFWS 2004) with a global conservation ranking of G1 (critically imperiled; NatureServe 2009). It is a shortlived perennial with a rosette of silvery- or gray-green leaves covered with stellate trichomes (Figure 1) and one to several flexuous, sprawling to ascending flowering stems. The yellow flowers give rise to subglobose silicles on recurved pedicels (Rollins and Shaw 1973) (Figure 2). It usually flowers in spring (February to April), but can flower during midsummer or as late as September in response to rain (Poole et al. 2007; Sternberg 2005). Germination has not been observed, but probably occurs in response to rainfall in cool weather. Under prolonged dry conditions, all of the plant’s leaves may die, making some surviving plants very difficult to locate. 172
Calochortiana December 2012 Number 1 Figure 1. Physaria thamnophila rosette leaves. SITES All known populations of P. thamnophila (eight verifiably extant, one not accessible, and one historic population; data on file, Natural Diversity Database, Texas Parks and Wildlife Department, Austin, TX) are in Starr and Zapata Counties, Texas, between Zapata and Roma, a distance of 69 km (Figure 3). All populations occur in remnant Tamaulipan thornscrub vegetation (Poole et al. 2007). For this study, we examined all sites with at least 100 plants to which we had access. Other sites were excluded due to small populations, lack of access to private land, or discovery too late to include in the study. Three of the four sites in this study (Arroyo Ramirez, Arroyo Morteros, and Cuellar) were in the Lower Rio Grande Valley National Wildlife Refuge. The fourth study site, Santa Margarita, was on private land. All four sites were within 18 km of each other in Starr County, Texas, between Falcon Dam (26° 33’ N, 99° 09’W) and Roma (26° 24’N, 99° 01’W). The climate in this region is hot and often dry. At Falcon Dam, the nearest station in the National Oceanic and Atmospheric Administration (NOAA) data base, the months of July and August have the highest means of daily maximum temperatures (37.6 o C and 37.5 o C, respectively) and January has the lowest mean daily minimum temperature (7.7 o C) (NOAA 2009). Between 1963 and 2007, average annual precipitation at Falcon Dam was 515 mm. However, annual precipitation varied widely, as during those 45 years, there were 7 years with more than 600 mm and 9 years with less than 400 mm annual precipitation. During our study, 2003 and 2004 were wet (804 mm and 671 mm, respectively) and 2005 was dry (353 mm). However, precipitation in the area occurs largely as isolated thunderstorms, and likely varied among sites. Most known populations occur on the edges of terraces above the Rio Grande flood plain. Physaria thamnophila populations have been found on the Jackson, 173 Figure 2. Physaria thamnophila fruiting stem. Yegua and Laredo formations (Bureau of Economic Geology 1976; Poole et al. 2007; USFWS 2004), all of which are Eocene calcareous sandstones and clays. All four study sites were on Eocene sandstones: Cuellar and Arroyo Morteros on the Yegua Formation, and Arroyo Ramirez and Santa Margarita on the Jackson Group (Bureau of Economic Geology 1976). Arroyo Morteros, Arroyo Ramirez, and Santa Margarita were located along the edge of the cliff that marks the edge of the flood plain of the Rio Grande; Cuellar was ~ 3000 m from the flood plain in an area without a cliff. Known P. thamnophila populations occur on shallow, well-drained sandy loam soil. Soils at known sites are mapped as members of the Zapata, Maverick, Catarina, or Copita series as described by the Natural Resources Conservation Service (NRCS) (Poole et al. 2007; USFWS 2004). These highly calcareous soils are derived from Eocene sandstone, clay and shale. Catarina soils are derived from Frio and Yegua formation parent material; these and Maverick soils contain up to 15 percent gypsum. Copita soil is derived from weakly consolidated calcareous sandstone of the Jackson Formation and is only slightly (2 percent) gypsiferous (NRCS 2009; Thompson et al. 1972). The soils at our study sites have been described as follows: Arroyo Morteros: Copita, Zapata and Catarina; Arroyo Ramirez: Jimenez-
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December 2012 Number 1 - Utah Native Plant Society

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