genus Trillium (Liliaceae) in Texas, The

Castanea, Sep 2002 by Singhurst, Jason R, Nixon, E S, Caldwell, William F, Holmes, Walter C

ABSTRACT

Five species of Trillium are recognized as occurring in Texas. One species, Trillium pusillum var. texanum has the major part of its distribution in the state, while another species, T. gracile, is endemic to Louisiana and Texas. Three species, T. ludovicianum, which is reported as new to the state, T. recurvatum, and T. viridescens are considered to be peripheral species. The study, based upon field work and study of herbarium specimens, includes a key to species, partial synonymy, descriptions, specimen citations, and a distribution map of the species.

INTRODUCTION

Trillium (Liliaceae) is a woodland genus of about 45 species distributed in temperate North America and temperate east Asia (Freeman 1975). Freeman attributes eight species to Asia, seven species to western North America, and about 35 species to eastern North America. The genus is about equally divided into two subgenera, those with pedicellate flowers (Trillium L.) and those with sessile flowers (Phyllantherum Raf.). Texas has four species of sessile flowered Trillium, three of which may be considered "peripheral species;" i.e., the major portion of their distribution occurring outside of Texas. One species, T. gracile, is endemic to east Texas and adjacent Louisiana. The only pedicellate flowered species in the state, T. pusillum var. texanum, is part of the T. pusillum complex, which occurs in a series of widely disjunct populations throughout much of the southeastern United States (Case and Case 1997). This variety is largely confined to Texas, but is known from extreme northwest Louisiana. The genus is generally limited to the East Texas Forest Region (the "pineywoods" of Gould 1962), which Correll and Johnston (1970) consider to be part of the vast southeastern United States pine-hardwood forest. Trillium gracile is also known to occur in the Post Oak Savannah and the Gulf Prairies and Marshes vegetation areas (as delineated by Gould 1962), both conterminous with the pinewoods, while T. viridescens also occurs in the Post Oak Savannah. However, both species attain their greatest abundance in the pineywoods.

As a group, members of the genus Trillium are commonly known as trillium, wakerobin, toadshade, squawroot, or carrion flower and they have recently gained interest as garden plants. Trillium pusillum var. texanum was a former candidate for listing as state threatened but has not gained protective status because of its abundance. Trillium recurvatum and T viridescens attain the southwestern limits of their overall distribution in the state while T ludovicianum reaches the western extent of its distribution in southeast Texas. The uncommoness of the latter three species in the state is reason enough to warrant monitoring their known populations and, at the same time, searching for additional occurrences.

With one exception, knowledge of Trillium in the state has been the result of botanical studies conducted since 1962. For example, Gates (1917), in his systematic treatment of the genus in North America, does not attribute any species to Texas. Overlooked was Buckley's (1861) proposal of the binomial T. texanum, now considered a variety of T. pusillum (Reveal and Broome 1981, Freeman 1994). In a checklist of the vascular flora of the state, Gould (1962), also missing the Buckley proposal, listed only T. ludovicianum for the state. Nixon et al. (1970), however, considered Gould's report to be based on a misdetermination of T. gracile, a species not proposed until 1969, as T. ludovicianum. Trillium viridescens was initially reported in Texas by Freeman (1969a), but it did not become generally known as occurring in the state until the publication of his 1970 treatment of the genus, which appeared after the Nixon et al. (1970) manuscript reporting T. recurvatum as new to the state. To date, the most comprehensive treatment of the genus for the state was by Freeman (1970) in which four species are included. Freeman (1975) included three species that occur in Texas in his important work on the sessile-flowered Trillium. More recently, the four species known from Texas are treated in Case and Case (1997), but T. ludovicianum was not known to occur in the state at that time.

This particular treatment considers all species of the genus known to occur naturally within the state. Descriptions, flowering dates, and the key are based upon Texas materials. The taxonomy generally follows the concepts of Freeman (1975, 1994) and to a lesser extent, that of Case and Case (1997). Specifically the treatment is based upon field studies conducted in east Texas, with a limited amount of study in nearby states, and examination of herbarium specimens from the following herbaria of the state: ASTC, BAYLU, BRIT, LL, SMU, TAMU, TAES, and TEX. The lists of specimens examined are limited to one record per county (usually the oldest numbered collection) except for Trillium ludovicianum in which case all studied specimens are cited.

The purpose of this paper on Trillium is to report a species of the genus new to Texas, provide a means for identification, update the known distribution of each of the species by county, and provide other information of interest.