Rare Plant Surveys

Pediomelum cyphocalyx

Point of Contact

Kim Norton Taylor

Conservation Research Botanist

Rare Plant Surveys:

Pediomelum cyphocalyx (turniproot scurfpea)

Pediomelum cyphocalyx
Pediomelum cyphocalyx

Introduction

Pediomelum cyphocalyx (A.Gray) Rydb. (Fabaceae) is a perennial herb with a tuberous root up to 2 cm in diameter. It grows to a height of up to 1 m, with stems usually solitary, erect, and simple or sparingly branched above (Rydberg 1919, Diggs et al. 1999). It can be distinguished from the other species in the genus within the region by the presence of cauline leaves, dense, many-flowered spike-like inflorescence, peduncles exceeding the subtending petioles, and linear-lanceolate or narrowly oblong-lanceolate leaflets (Diggs et al. 1999).  It is most often confused with P. cuspidatum, and P. linearifoium, both of which share a similar habit and habitat to P. cyphocalyx. The longer and narrower leaflets and dense inflorescence, respectively, will serve to differentiate P. cyphocalyx from these similar taxa (Diggs et al. 1999).

Pediomelum cyphocalyx has a NatureServe Conservation Status Rank of G3G4 at the global scale and S3S4 at the Subnational scale for the state of Texas (NatureServe 2017, TXNDD 2017). The species is endemic to Texas with records occurring in 18 counties (Kartesz 2015) and two level III ecoregions: Cross Timbers, and Edwards Plateau (OmernikRegionsLevelIV). 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). This region is equivalent to the “Grand Prairie” land resource area as described by United States Department of Agriculture (2006). Diggs et al. (1999) note the species occurs on limestone outcrops, which are abundant within the region. Gray (1850) described the habitat as “Rocky prairies”. Little additional information is available about the specific habitat requirements of the species.

The Grand Prairie, a system of limestone prairies with Fort Worth at its center, forms the central band within the Cross Timbers region (Gould et al. 1960). This region in particular is under-explored botanically, with significant range expansions discovered in recent years (Taylor et al. 2012; Taylor & O’Kennon 2013, 2014). SGCN within this region face the imminent threat posed by the rapid growth of Fort Worth and surrounding areas, but without a thorough understanding of the distribution and habitat needs of these rare species, we cannot begin to comprehend the impact this urban expansion will have on them, let alone work toward their conservation.  Pediomelum cyphocalyx is one plant SGCN within the Cross Timbers region with its center of distribution primarily within the Grand Prairie region. This project seeks to fill the information needs for this species to better understand its current range and status in the Grand Prairie.

Results

A total of 15 populations of Pediomelum cyphocalyx were identified in the Fort Worth Prairie. Eleven of the 15 populations were visited in 2017 with plants observed at 8 populations in 2017. A total of 246 plants were counted. Sites with plants present had an average of 30.75 plants per site with two sites having as few as five plants and one site supporting 77 plants. The oldest record for Pediomelum cyphocalyx in the region dates back to 1902 but thirteen of the 16 sites were first observed in the last 10 years. Plants were not relocated at the three sites with the oldest records. Plants were relocated at all but five of the contemporary (seen in last 20 years) sites.

In addition to the populations in the Fort Worth Prairie, there were seven records mapped within the Lampasas Cut Plain Ecoregion. Five of the records date to between 1932 and 1985, while the remaining two were from Fort Hood Military Reservation in 2008 and 2009. Forty records were mapped within the Edwards Plateau. Records date from between 1847 and 2013, with 17 of the records prior to 1960, 18 records between 1960 and 1997, and only 5 records from within the last 20 years.

map of distribution of pediomelum cyphocalyx
Map showing distribution of Pediomelum cyphocalyx.

Habitat

The Fort Worth Prairie, Lampasas Cut Plain, and Edwards Plateau are underlain by Cretaceous limestones, clay, or mudstone formations (Stoeser et al. 2005). Topography across the regions vary with gently sloping hills and cuestas in the Fort Worth Prairie, flat-topped mesas and escarpments in the Lampasas Cut Plain, a mostly flat elevated plateau in the western Edwards Plateau to deep canyons in the eastern Edwards Plateau. The uniting characteristics across the region include shallow limestone soils overlying layers of resistant limestone. Specimen label information indicates P. cyphocalyx is found in shallow limestone soils typically on hillsides, a habitat which is abundant within the region.

Pediomelum cyphocalyx habitat
Pediomelum cyphocalyx habitat.

Field observations indicate Pediomelum cyphocalyx grows in thin to moderately deep soils over Cretaceous limestones. The plants within the Fort Worth Prairie occur primarily on the Walnut Clay formation, a Cretaceous, fossiliferous, erosion resistant limestone (Stoeser et al. 2005). Plants are typically found on limestone hillsides with a mean mapped slope of 6.2 degrees (Gesch et al. 2013), where soils are comparatively deeper than the exposed bedrock found at higher or lower topographic positions. Seven populations of plants mapped to Paluxy Sand. These areas are in close proximity to mapped Walnut Clay and appear to represent unmapped areas of this formation. Field observations support this with indications that all plants were found in limestone soils.

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, Glen Rose Limestone, or Edwards Limestone. All three formations are Cretaceous in age and resistant to erosion. The softer limestones and mudstones of the region do not appear to support the plant (e.g. Goodland Limestone).

Threats

Several threats to Pediomelum cyphocalyx plants were recorded in the field. The most prevalent threats include Transportation and Service Corridors, Tourism and Recreation Areas, Natural Systems Modifications, and Invasive Species.

Conservation Status and Implications

Pediomelum cyphocalyx currently has a global conservation status rank of G3G4 (Vulnerable/Apparently Secure) and is endemic to the state of Texas. The ambiguity of this rank is likely due to the lack of up to date information on current population numbers. Forty-eight of the 71 records (67.6%) across the range of the species were collected over 20 years ago, with 43 of these collected without geographic coordinates prior to the widespread use of GPS units. Of the 22 records from the Fort Worth Prairie, only eight sites had verified plants present in 2017. Only seven of these records were historic (>20 years old) compared to 41 historic records out of the remaining 48 records for the Lampasas Cut Plain and Edwards Plateau. The likelihood of relocating a population decreases the longer the population goes without being observed, particularly for records with vague location information. At least 12 of the Lampasas Cut Plain and Edwards Plateau records have location descriptions too vague to confidently relocate the original populations. 18 of the remaining sites with adequate location information are in close proximity (<15 miles) to a town or urban area. The combination of this potential loss of populations and small population sizes (~30 plants per site), potentially warrants a revision of the NatureServe Conservation Status Ranks at both the global and sub-national scale for P. cyphocalyx.  The data gathered during this project will be used to inform an updated NatureServe Conservation Status Rank for the species.

 

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