Nassella: Species diversity and distribution
Barkworth and Torres
Longer version of a paper to be published in Taxon later in 2001

Introduction

During the past decade several changes have been proposed for generic delimitation in the Stipeae. They have met with varying degrees of acceptance. One difficulty that we and other members of the Stipeae working group[1] seek to address is that most of the changes have been based on rather limited knowledge of the tribe’s diversity. This is not surprising, for the tribe is large (450-500 species[2]), widely distributed, and not clearly delimited (Jacobs & al. 2000). In this paper we present a review of one of its genera, Nassella E. Desv.

Barkworth (1990) expanded Nassella from a genus of approximately 9 species (Parodi 1947) to one of 79 species. Most of the additional species had previously been included in Stipa s.l. Thomasson (1976, 1978, 1979) suggested such an expansion earlier, but did not present the necessary nomenclatural changes. The characteristics that unite the expanded genus, all of which appear to represent derived states within the Stipeae, are the strongly convolute lemma, the reduced, usually glabrous, short palea, the lemma epidermal pattern (Thomasson 1976, 1978, 1979; Barkworth 1990), and the ITS sequence data (Jacobs et al. 2000). All species of Nassella have a crown, but so do species of Piptochaetium J. Presl and Hesperostipa (Elias) Barkworth. Only in Nassella, however, does the crown vary from being evident only on dissection to highly conspicuous and ornamented. No vegetative characteristics are known to distinguish Nassella from other stipoid grasses (Arriaga & Barkworth, 2000).

The expansion of Nassella has not been universally accepted (e.g., Renvoize 1998; Torres 1993; Zanín & Longhi-Wagner 1990), but the morphological and anatomical data supporting its recognition (Barkworth 1990) are almost as strong as those supporting Parodi’s (1944) now universally accepted expansion of Piptochaetium. After examining a wide range of stipoid species, Torres (1997) became convinced that, contrary to her previous decision (Torres 1993), the expansion of Nassella is appropriate. It was also accepted by Jorgensen & León-Yánez (1999), Peñailillo (1998), and Rojas (1997).

Recent work has revealed several species of Nassella in addition to those listed by Barkworth (1990). Torres (1997) described 11 new species and presented 11 new combinations for the species occurring in northeastern Argentina. Additional species and combinations have been published by Rojas (1997), Laegaard (1998), Peñailillo (1998), and Jorgensen & León-Yánez (1999). We provide combinations for three more species, a list of all species currently included in the genus, a summary of their geographic distribution, and a synopsis of the morphological characteristics commonly used to distinguish the species.