Physaria engelmannii (Engelman’s bladderpod)
The Dallas-Fort Worth metropolitan area spans two ecoregions, the Cross Timbers and the Northern Blackland Prairie (Gould et al. 1960). The Blackland Prairie begins on the eastern side of the metroplex in Hunt, Kaufman, and Ellis counties, includes most to all of Collin and Dallas counties, and the eastern edges of Denton, Tarrant, and Johnson counties (Gould et al. 1960). This historic tallgrass prairie region is named for the deep black soils that characterize the area (Blackland Prairie Ecological Region). These highly fertile soils were prized for food and forage crops and most of the area has been plowed. Today, much of the Blackland Prairie has been developed and further expansion, particularly to the north of Dallas (Plano, Frisco, Allen, McKinney), threaten what little native habitat remains.
On the western side of the metroplex, the Fort Worth Prairie spans portions of Denton, Wise, Parker, Tarrant, Johnson, and Hood counties (OmernikRegionsLevelIV). This system of tallgrass prairies with Fort Worth at its center, forms the central band within the Cross Timbers region, with the East and West Cross Timbers on either side. Rocky, thin, limestone soils characterize the region and extend to the south into the Lampasas Cut Plain and the Edwards Plateau (Cross Timbers and Prairie Ecological Region). Areas with deeper soils were cultivated, with shallow soil areas used for livestock grazing operations. Areas that have not been developed remain primarily as open ranchland. The Fort Worth Prairie region is under-explored botanically, with significant range expansions discovered in recent years (Taylor et al. 2012; Taylor & O’Kennon 2013, 2014). Rare species within this region face the imminent threat posed by the rapid growth of Fort Worth, particularly on the North and West sides.
Without a thorough understanding of the distribution and habitat needs of rare species in the Fort Worth Prairie and Blackland Prairie, we cannot begin to comprehend the impact urban expansion will have on them, let alone work toward their conservation. One SGCN threatened by urban expansion in the region is Physaria engelmannii (Engelmann’s bladderpod). Physaria engelmannii (Brassicaceae) is a densely pubescent perennial herb with few to several unbranched stems from the base. Bright yellow flowers up to 2.5 cm in diameter form in dense, sub-umbellate racemes. Fruiting pedicels are relatively short, ascending, with fruits globose to ellipsoid and 5 to 8 mm long (O’Kane 2010). It can be distinguished from the only other perennial taxon within the region, P. fendleri, by its dense inflorescence, fruits with a stipe 0.5 to 1 mm long (vs. no stipe), and branches of stellate hairs free or fused only basally (vs. fused from base to half their length) (Diggs et al. 1999).
Physaria engelmannii has a NatureServe Conservation Status Rank of G4 at the global scale and S3 at the Subnational scale for the state of Texas (NatureServe 2017, TXNDD 2017). The species is endemic to Texas and Oklahoma with records occurring in 21 Texas counties, eight Oklahoma counties (Kartesz 2015), and three ecoregions: Cross Timbers, Blackland Prairie, and Edwards Plateau (Gould 1960). Within the Cross Timbers, the species is limited to limestone soils in the Lampasas Cut Plain (or Limestone Cut Plain) and the Fort Worth Prairie (including limestone outcrops on the eastern edge of the West Cross Timbers). O’Kane (2010) notes the species occurs on “limestone prairies, rocky ridges, pebbly shores, thin caliche soils, [and] limestone outcrops.” This project seeks to fill the information needs for this species to better understand the current range and status in the regions surrounding the Dallas-Fort Worth metropolitan area.
The region of interest, the area surrounding the Dallas-Fort Worth metropolitan area includes sites in the Fort Worth Prairie and the northern half of the Blackland Prairie. Twenty-seven populations of Physaria engelmannii fell within this area. Sixteen of the 27 populations were not previously mapped in the Texas Natural Diversity Database.
Twenty-three of the 27 populations were surveyed to search for the plant in 2016 to 2017. Plants were observed at 10 of the 23 sites visited. A total of 26,647 plants were estimated across the combined locations. Sites with plants present had population sizes ranging from 12 to 23,558 plants with a mean of 2664 plants per site. Only one site had numbers greater than 1500 plants. Removal of this outlier gives a mean of 343 plants per collection site. The oldest record for P. engelmannii in the region dates to 1939 but 13 of the 32 sites were first observed in the last 20 years. Plants were not relocated at any sites first observed prior to 1979, though five of the 18 sites were not visited. Only four contemporary sites were not relocated.
Several additional sites were identified outside of the Dallas/Fort Worth Metroplex. Nine records were mapped within the Lampasas Cut Plain. Eight of the records date to between 1935 and 1960, while the remaining site was documented in 2013. Sixteen records were mapped from the Balcones Canyonlands on the Edwards Plateau. Records date from between 1900 and 2005, with seven of the records prior to 1960, six records between 1979 and 1991, and only 2 records from within the last 20 years. Nineteen records mapped within the Blackland Prairies, dating between 1931 and 2010. Only two new records from this region were found within the last almost 40 years. Thirteen additional records dating between 1850 and 1983 occur in the southern portion of the range but location information was not sufficient to confidently map these occurrences to an ecoregion or they occurred in other regions of the Edwards Plateau. Across the entire range of the species, only 17 records are from within the last 20 years, 12 of which are from the Fort Worth Prairie.
A total of 26,426 of the 26,647 (99%) documented plants located in 2017 occur within or on roadsides adjacent to the Lyndon B. Johnson National Grasslands (LBJNGL), Wise County, Texas. The LBJNGL occurs in the West Cross Timbers level IV ecoregion with soils derived primarily from Antlers Sand. Several outcroppings of Cretaceous limestone mapped to Goodland Limestone or Walnut Clay occur at the highest topographic positions across the landscape in a handful of units. Several additional outcroppings too small to be mapped also occur. These outcroppings represent a western extension of the Fort Worth Prairie. Physaria engelmannii is found on almost all limestone outcroppings occurring within the LBJNGL.
The Fort Worth Prairie, Lampasas Cut Plain, Edwards Plateau, and Blackland Prairie are underlain primarily by Cretaceous limestones, clay, mudstone, or chalk formations (Stoeser et al. 2005). Topography across the regions vary from nearly level to gently sloping in the Blackland Prairie, gently sloping hills and cuestas in the Fort Worth Prairie, flat-topped mesas and escarpments in the Lampasas Cut Plain, to deep canyons in the Balcones Canyonlands. Soils in the Blackland Prairie are typically deep and highly fertile, while soils in the remaining regions are often shallow and rocky. Specimen data from sites within the western range (Fort Worth Prairie south to the Edwards Plateau) indicates Physaria engelmannii plants are found on a “limestone outcrop in prairie”, in “shallow clay soil”, on “rocky limestone slopes”, and on “limestone hillslope, partially eroding just below ridgetop”. Blackland Prairie sites are apparently similar with plants growing in a “limestone outcrop”, on a “rocky prairie”, a “caliche outcrop”, or in “shallow soil with outcrops of Austin Chalk”.
All sites visited with verified plants were found in rocky limestone soils mostly in full sun with little competing vegetation. Rocky, open ground was abundant surrounding plants. Plants were found on mostly level, open ground at a few sites though most plants were found on hillsides or eroding ledges. Plants were not found in the shallowest soils where an abundance of exposed bedrock occurs, unless a crack in the bedrock was present. Soils must be sufficiently deep to support the perennial root system, though soil of as little as a few inches in depth appears sufficient. Soils much deeper than this also tend to support larger bunch grasses that may shade out P. engelmannii. Hillsides and eroding ledges appear to offer the ideal balance of soil depth and lack of competing vegetation.
The plants within the Fort Worth Prairie occur primarily on the Walnut Clay formation, a Cretaceous, fossiliferous, erosion resistant limestone (Stoeser et al. 2005). Populations in the Lampasas Cut Plain and Edwards Plateau also appear to prefer Walnut Clay and similar formations with most records mapped to Walnut Clay or Glen Rose Limestone. Both formations are Cretaceous in age and resistant to erosion. The softer limestones and mudstones of the region do not appear to support the plant. Records in the Blackland Prairie primarily map to Austin Chalk, another Cretaceous limestone formation.
Several threats to Physaria engelmannii plants were recorded in the field. The most significant threat is Transportation and Service Corridors (plants along roadsides), with several additional threats with a low impact: Tourism and Recreation Areas, Agriculture and Aquaculture, Energy and Mining, Human Intrusions and Disturbance, Natural Systems Modifications, Invasive Species, Geologic Events, and Climate Change and Severe Weather.
Transportation and Service Corridors
All of the 10 sites with verified plants in 2017 had plants occurring directly adjacent to a road or in a road right of way. Three sites have plants occurring only within the roadside right of way. However, most of the sites are along rural roadways with little traffic and are unlikely to undergo lane additions in the near future. It is unlikely that most sights would experience road work significant enough to drastically reduce population sizes.
Conservation Status and Implications
Physaria engelmannii currently has a global conservation status rank of G4 (Apparently Secure). With a total of 70 records across the Texas range for the species, the plant would appear to be secure. Of these 70 sites, only 17 (24.3%) are from within the last 20 years. While the historic range of the species was apparently widespread, the lack of new records for the species brings its current distribution into question. Less than half of the sites visited in 2017 had plants present. All plants located occurred in the Fort Worth Prairie. While all populations surveyed are in close proximity to the Dallas-Fort Worth metropolitan area, rural populations in the southern portion of the range may still be extant. No sites were relocated within the Blackland Prairie. Two contemporary records from this region that were not visited represent the last known populations for the species within the Blackland Prairie Ecoregion. This represents a drastic reduction in range for the species. A majority of plants seen in 2017 occur within the Lyndon B. Johnson National Grasslands. A lack of sites outside of this protected area is concerning. The decline in populations within the Blackland Prairie may warrant a revision of the NatureServe Conservation Status Ranks at both the global and sub-national scale for P. engelmannii.
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