Morphometric analysis of the Trillium pusillum Michaux complex (trilliaceae) of the Southeastern United States

Castanea, Jun 2002 by Timmerman-Erskine, Margaret, Dute, Roland R, Boyd, Robert S

ABSTRACT

The Trillium pusillum Michaux complex consists of five or six morphogeographical taxa distributed within the southeastern United States. A quantitative morphological study was undertaken to determine appropriate species and varietal ranks of these taxa, which have a confused taxonomic history. Numerical analyses based on 14 floral and vegetative characters measured from 29 populations of T pusillum revealed significant differences among varieties. These exist, however, as arrays of characters rather than as single characters. All characters were highly variable within and among populations, suggesting that quantitative measurements alone are unreliable as distinguishing characters. We concluded that all of the previously recognized infraspecific taxa lack clear morphological separation. We therefore recommend recognition of a single variable species without varietal designations.

INTRODUCTION

The genus Trillium L. is divided into two subgenera: subgenus Trillium L., which includes pedicellate-flowered species; and subgenus Phyllantherum Raf, which includes sessile-flowered species. The Trillium pusillum complex within subgenus Trillium is unique in having both flower types (Freeman 1994). The T pusillum complex consists of five or six taxa growing in widely scattered populations on the coastal plain from Maryland to South Carolina, and in isolated inland populations in Alabama, Tennessee, Kentucky, West Virginia, Mississippi, Louisiana, Texas, Arkansas, and Missouri (Timmerman-Erskine 1999).

Varieties or species in this complex have usually been split from variety pusillum using a number of criteria. In some cases, subtle morphological differences have been used to separate taxa. For example, Case and Case (1997) differentiated variety texanum from variety pusillum by its more narrow leaves with more rounded tips, and its narrow, almost linear (but variable) petal shape. Pedicel length has been an important differentiating character. For example, varieties texanum and ozarkanum have the longest pedicels whereas those of variety virginianum are short (

Habitat also has been used to justify varietal status in some cases. Most populations of T pusillum occur in low swampy woods or floodplains except for populations of variety ozarkanum which occur in coarsely cherty, well-drained rocky soils in beech-oak and mixed deciduous forests (Case and Case 1997, Freeman 1994). Likewise, a few small rare populations of variety virginianum reported from rocky, shaley soil of mountain crests along the Virginia-- West Virginia state line were separated as variety monticulum (Bodkin and Reveal 1982). Subsequent studies of these latter populations failed to show any morphological evidence (Cabe 1995) or isozyme evidence (Cabe and Werth 1995) to support taxonomic separation from variety virginianum growing in more typical T pusillum habitat.

Finally, perceived distribution disjunctions have been used to justify taxonomic separation of some elements. This was the case for variety monticulum mentioned above, whose mountain populations were located far east of the coastal plain populations of variety virginianum. Similar apparent disjunctions were important justifications for separation of T texanum (Buckley 1861) and T ozarkanum (Palmer and Steyermark 1935) from T. pusillum. However, Freeman (1994) suggested that these disjunctions are not as wide as had been thought but stem from inadequate floristic exploration of southeastern states.

Taxonomic opinion regarding members of the T pusillum complex has varied greatly. Table 1 provides a brief summary of the history of taxa in this group; more detailed taxonomic histories are provided by Freeman (1994) and Timmerman-Erskine (1999). Briefly, T pusillum was described by Michaux (1803), and several morphologically similar species or varieties have been described in the last 200 years (Table 1). For example, T ozarkanum Palmer & Steyermark and T texanum Buckley have both been considered to be varieties of T pusillum (Roe 1978, Reveal and Broome 1981). Garrett (1982) recognized two varieties in the eastern coastal states, varieties pusillum and virginianum, and a new inland taxon (variety alabamicum), but the latter was not formally published. Cabe (1995) suggested that varieties monticulum and virginianum be treated as one variety. Even the range of T pusillum, originally described from South Carolina, has been the subject of much controversy (Freeman 1994).

Cabe and Werth (1995) suggested several reasons for these taxonomic disagreements. First, populations are often disjunct and morphologically variable. Second, field observations are limited and often inaccurate because flowers are inconspicuous, plants bloom early in the season, and taxa of this complex are relatively rare. Finally, some useful distinguishing characteristics change as flowers age (Peattie 1927) or are lost or altered when flowers are pressed and dried (Duncan and Calhoun 1962).

Two Trillium species (T persistens and T reliquum) are rare enough to be listed as Federally Endangered (United States Fish and Wildlife Service 1978, 1988). As a whole, members of the T pusillum complex are rare; thus, some may be appropriate for listing. Current information on the appropriate nomenclature and taxonomic status of members of this complex is needed to determine appropriate conservation actions. This study examines quantitative morphological differences between varieties and populations of the T pusillum complex to assist in determining appropriate species and varietal ranks.