Systematics of the Corkwoods (Leitneria; Simaroubaceae)

Plants of the genus Leitneria (corkwood) are shrubs that thrive in shade or sun and grow well in either saturated or drained soils.  Although large leaves and upright, thicket-forming stems distinguish this taxon in the landscape, Leitneria is rarely available in nurseries but might be marketed as a stress-resistant specialty plant to those particularly interested in species indigenous to North America.  The genus is restricted to five widely disjunct populations in southeastern Missouri, eastern Arkansas, southeastern Texas, southern Georgia, and northwestern Florida.  While morphological variation among the populations has been acknowledged for decades, ours is the first formal taxonomic assessment of Leitneria at the infrageneric level.  Leitneria historically has been considered a monotypic genus with Leitneria floridana its only species.  Our goals were to assess genetic variation among populations, to determine if infrageneric divergence exists, and if so, to provide a formal treatment of infrageneric taxonomy.  Based on leaf morphology, ISSR polymorphisms, ITS sequence character, and geographical isolation, we describe one new species, Leitneria pilosa, and one new subspecies, L. pilosa subsp. ozarkana.  Our infrageneric assessment revealed that L. pilosa, which occurs as western disjunct populations in Missouri, Arkansas, and Texas, has undergone species-level divergence from plants of L. floridana indigenous to Florida and Georgia.  Leitneria pilosa forms dense trichomes on both abaxial and adaxial leaf surfaces, while trichomes are sparse to absent on the leaves of L. floridana.  Leaves of L. pilosa are smaller, about half the length, and have a lower length : width ratio than leaves of L. floridana.  Within L. pilosa, subsp. ozarkana indigenous to Arkansas and Missouri differs from subsp. pilosa indigenous to Texas by its consistently larger, elliptic to slightly oblanceolate or obovate leaves with longer petioles.  Leaves of L. pilosa subsp. pilosa are narrower and lanceolate.  These new taxonomic designations are supported strongly by phenetic and cladistic analyses of ISSR and ITS markers and by geographic disjunction of the taxa.  Foliar differences documented among plants in their native habitats were confirmed on plants grown in a common environment and may influence subsequent selection of genotypes for use in horticulture.