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Schoenoplectiella hallii (Hall’s bulrush)
Introduction
The 2012 Texas Conservation Action Plan (TCAP) for the Cross Timbers Region identifies the lack of information and lack of processing of existing data for Species of Greatest Conservation Need (SGCN) as two major issues for the region. Little information is available about the distribution and habitat needs of SGCN within the region and little has been done with what data does exist. This knowledge gap makes it “difficult to focus or prioritize management objectives or share information with private landowners about the importance of some sites, populations, or communities” (TCAP 2012). Without a thorough understanding of the true status of SGCN, informed conservation decisions cannot be made to protect these threatened species. This can lead to uninformed, and often expensive, conservation actions which are either overly aggressive or not aggressive enough.
SGCN within the Cross Timbers region face the imminent threat posed by the rapid growth of Dallas, 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.
One such SGCN occurring within the region is Schoenoplectiella hallii (A. Gray). It is an annual tuft-forming sedge in the Cyperaceae family found in 10 states, mostly across the Midwest and Northeast, with historical occurrences in three more states (NatureServe 2017). In the last 10 years, S. hallii sites have been found widely scattered in Illinois, Indiana, Kansas, Kentucky, Michigan, Missouri, Nebraska, Oklahoma, Texas, and Wisconsin (NatureServe 2017). Schoenoplectiella hallii was first discovered in Texas in 2003 in the Lyndon B. Johnson National Grasslands (LBJNGL), Wise County. It likely arrived in Texas from the northern populations via migratory waterfowl with local dispersal attributed to connected waterways or the movement of wildlife or cattle. There are no other occurrences of the species within the state and little information is available on the size, extent, habitat or status of the S. hallii populations in the LBJNGL.
Schoenoplectiella hallii was petitioned to be listed under the Endangered Species Act in 2010 and is currently under review. Information from this project will be used to inform the listing decision for this species.
Results
Examination of herbarium specimens and the scientific literature identified 18 ponds in the LBJNGL which contain the plant presently or historically. These ponds are dispersed across six discrete, discontinuous units within the property which can be grouped into 5 naturally breeding populaitons.
Plants were observed at 10 of the 18 ponds in 2017 but only three of the five populations. Using sampling plots and total habitat area, we estimated a total of 16,266 plants across three populations. Among ponds with plants present, a mean of 1807 plants were found in each pond. The majority of plants were found in population 1, with 16,157 plants across eight ponds.
Habitat
The Texas sites of Schoenoplectiella hallii are all margins of sandy clay ponds with widely fluctuating boundaries, often shallow, both in forested openings and in open prairie. All ponds occur over Antler’s Sand in the Western Cross Timbers level III Ecoregion. This area is characterized by sandy loam soils supporting post oak-blackjack oak (Quercus stellata-Q. marylandica) woodlands interspersed with open prairies and savannas (Cross Timbers and Prairie Ecological Region 2017). Over 1,000 ponds occur within the LBJNGL representing a large area of potentially suitable habitat for S. hallii. Examination of aerial imagery indicates the ponds were constructed sometime prior to 1995, likely as early as the mid-1950s. Ponds were dug for cattle use and are still used for this purpose.
Common associates found within the vegetation plots, growing in areas of high S. hallii density, include Eleocharis engelmannii (7 ponds), Pilularia americana (5), Rotala ramosior (5), Eleocharis atropurpurea (4), Eleocharis palustris (4), Ludwigia peploides (3), and Sagitaria platyphylla (3). The taxon with the highest cover within the plots was Pilularia americana with a mean coverage of 52.5% in the five plots where it was located. Schoenoplectiella hallii had a mean coverage of 13.18% within the vegetation plots. Eleocharis parvula had a mean cover of 20% but was only found in two ponds. Four additional taxa were found in a single pond with a coverage of approximately 37.5% including Eclipta alba, Bergia texana, an unknown grass and an unknown dicot seedling. All other taxa were found with a mean coverage of less than 20%. A total of 30 taxa were recorded from within the vegetation plots. Vegetation plots had a mean bare soil coverage of 51.8% with a range of 20 to 70 percent. Plots had a mean herbaceous layer coverage of 50.9% with a range of 20 to 80 percent. The remaining strata (rock, cryptograms, shrub, tree) each had a mean percent cover of less than 5% and never exceeded 20 percent. Areas where S. hallii is most abundant are open, sunny locations with an abundance of bare soil. Plots averaged 6.45 taxa per plot, suggesting S. hallii tends to grow with a small number of associates, all with relatively low coverage. This suggests S. hallii is a poor competitor. Taxa that did have the highest coverage within the plots are low growing, or have little horizontal spread, suggesting light competition (not below ground competition) may be the limiting factor.
Taxa typically found in high numbers and with high coverage in ponds where Schoenoplectiella hallii was not found in 2017 (but had been previously documented) include Eleocharis quadrangulata, Juncus marginatus, Diospyros virginiana, and Salix nigra (found in at least 2 ponds with >12.5% cover). With the exception of Salix nigra, none of these species was found in the vegetation plots where S. hallii was present. Other taxa encountered in more than one pond without S. hallii were Sagittaria platyphylla, Panicum virgatum, Ludwigia peploides, and Juncus nodatus. It appears the abundance of rhizomatous perennials and woody vegetation may serve to exclude S. hallii from these ponds, as is noted by McKenzie et al. (2007).
Discussion
Schoenoplectiella hallii populations in Texas only occur in ephemeral bodies of water. It is likely that fluctuating shorelines reduce competition by perennials and woody species. Unlike many northern populations which visually disappear for decades, only persisting in the long-lived seed bank (McKenzie et al. 2007) the populations of S. hallii at the LBJNGL generally appear year after year, never disappearing for long periods of time. Plants have been observed in at least a few ponds every year since their discovery in 2003 and can often be located any month during the year. Often, the only plants visible in winter months are those that are submerged when water levels rise. These plants often persist through the winter, completing their reproductive cycle when water levels retreat exposing the plant the following year (O’Kennon pers. obs.).
We attribute this ability to the fact that ponds in Texas do not typically freeze over the winter. None of the ponds with S. hallii present have been observed to freeze since the species was first discovered in Texas in 2003 (O’Kennon pers. comm). O’Kennon indicates water temperatures in winter of 2006 (the period the plants are typically submerged) never dropped below 55 degrees Fahrenheit in one pond where S. hallii was present, even when surrounding air temperature was well below freezing (O’Kennon unpublished data). Plants on the shoreline growing out of the water senesced and disappeared during this time, while those under water persisted. The ability of the plant to persist under water through otherwise unsuitable climatic conditions helps ensure the persistence of populations.
Many of the ponds which were lacking S. hallii plants in 2017 were very full of water with little available shoreline suitable for S. hallii at the time of survey. It is likely the plants at these sites were alive but submerged. Submerged plants were noted at several of these sites 21 in the past and possible submerged plants were noted in 2017, but their identification could not be confirmed. Several young plants were found at multiple other sites, suggesting seed germination was still occurring at the time of the survey. If water levels drop sufficiently in Units 21 and 27, it is likely there is still time for plants to germinate at these sites. Two of the ponds which lacked plants in 2017 were mostly dry at the time of survey. It is likely the ponds were too dry to stimulate seed germination. Three additional ponds without S. hallii plants were dominated by perennial taxa or woody vegetation, which we believe outcompetes S. hallii. High coverage of perennial taxa and encroachment by woody vegetation are known threats to the species (McKenzie et al. 2007).
Threats
According to McKenzie et al. (2007), threats facing S. hallii on a national scale include irrigation, development (residential, commercial, agricultural, and recreational), heavy grazing, herbicides, perennials, invasive and woody encroachment, polluted runoff, lack of management, predation by waterfowl and insects, and hybridization. At the Texas sites, signs of cattle, presence of perennials, and woody encroachment were all observed. At one site, hybridization with Schoenoplectiella saximontana was previously observed, but no plants of either species were found in 2017. The potential for hybridization is a continued threat as S. saximontana is documented from within three ponds in the LBJNGL.
Invasive species, irrigation, off road vehicles, herbicides and polluted runoff were not noted as threats at any of the ponds. The most frequently encountered threats include high grazing pressure as all ponds are in areas grazed by cattle; and the presence of perennials and encroachment by woody taxa. Eight ponds were noted as having encroachment by woody vegetation, though S. hallii was seen in all eight of these ponds in 2017. Some of the largest population sizes occurred in ponds with early evidence of woody encroachment. Perennial vegetation was noted as a threat in all ponds, with most ponds lacking S. hallii dominated by rhizomatous perennial vegetation, including Eleocharis quadrangulata, Juncus marginatus, Sagittaria platyphylla, and Panicum virgatum. Woody taxa commonly seen as encroaching on the ponds include Diospyros virginiana and Salix nigra. Quercus stellata and Quercus marylandica were also culprits, though they were typically not within the pond high water line. While the absence of large populations of perennial taxa does not ensure the presence of S. hallii within a pond, they do appear to eventually outcompete S. hallii if allowed to persist and encompass the whole pond.
The only threat that appears to have impacted populations is the establishment of perennial vegetation. Encroachment by woody taxa could seriously impact populations in the future if allowed to continue unchecked. Grazing pressure does not appear to be detrimental to populations, and in fact may be essential to reduce establishment of perennials. Any drastic modifications to grazing pressure could harm the populations so proper management must continue. Though evidence of cattle was seen (cow patties, footprints), there was no direct evidence of cattle or any other herbivore eating the plants. Evidence of soil disturbance was seen in several ponds, with evidence of feral hogs in six ponds, deer hoof prints in four ponds, and cattle trampling in three ponds. Only one pond had evidence of human traffic. Moderate levels of soil disturbance, while possibly detrimental to a population in the short term, is likely important to limit establishment of perennials.
Conservation Status and Implications
Schoenoplectiella hallii currently has a global conservation status rank of G2G3 (Imperiled/Vulnerable), indicating the species is at a high risk of extinction. The species has a large, widely disjunct distribution but is rare throughout its range. State conservation ranks vary ranging from S1 (Critically Imperiled) to S2S3 (Imperiled/Vulnerable) (NatureServe 2017). Three states (Georgia, Iowa, Massachusetts) have a state rank of SH (Possibly Extinct) indicating the species is known from only historical occurrences and is potentially extinct within the state. The Texas conservation status rank is S1, which seems appropriate due to the small number of populations (TXNDD 2017). All of the Texas plants occur on federally protected land.
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