Discussion

The present center of Nassella’s diversity is Argentina (Table 1), where 72 of its 116 currently recognized species grow. Within Argentina, the genus has its greatest diversity in the northwestern part of the country. Within Bolivia and Chile, it also appears to be most abundant in the regions adjacent to Argentina, i.e., in the central Andes. To demonstrate this pattern more convincingly would require determining the collecting locality for individual specimens from herbaria in all the relevant countries, a task that is beyond our current resources.

One species, N. tenuissima, grows in both South and North America, but its distribution is disjunct, the southern portion being restricted to Argentina and the northern portion to Mexico and southern Texas, U.S.A. One species complex, comprising N. mucronata, N. mexicana, and N. leucotricha, extends from South America into the southern U.S.A. Nassella mucronata intergrades with N. mexicana in northern South America, but it intergrades with N. leucotricha in northern Mexico.

Mexico has eight species of Nassella, three of which (N. linearifolia, N. mexicana, and N. mucronata) extend into South America and the remaining five (N. cernua, N. lepida, N. pulchra, N. leucotricha, and N. tenuissima) into the U.S.A. As noted above, N. tenuissima also grows in South America, but its North and South American ranges are disjunct. Within the U.S., Nassella grows primarily from California to southern Texas but N. cernua, N. lepida, and N. pulchra grow only in California, whereas N. leucotricha and N. tenuissima grow only in Texas and neighboring portions of adjacent states in both the U.S.A. and Mexico. The other North American species, N. viridula, has a rather different distribution, extending from the Central Plains into the southwestern portion of northern Canada.

Nassella has not been found at lower elevations in Central America, nor in the islands south and east of North America (Acevedo-Rodríguez 1996 [St. John, U.S. Virgin Islands]; Correll & Correll 1982 [Bahamian Archipelago]; Howard 1952 [Grenadine Islands]; Judziewicz 1990 [French Guiana, Suriname, Guyana]). We were unable to find any information for Nicaragua, Cuba, or Haiti.

Sixty species of Nassella are endemic to a single country, 26 of them to Argentina. One species, N. mexicana, grows in eight countries and two, N. mucronata and N. neesiana, are found in seven. These figures do not, of course, indicate which species are the most, or least wide ranging, because of the widely differing size of the countries involved.

Eleven species have been reported as growing outside their native range. Of these, N. trichotoma is generally regarded as the most serious weed threat, but N. neesiana is becoming a serious problem in Australia (Jacobs, pers. comm., 2000). The Australian populations do not appear to set seed in the exposed inflorescences, but they form numerous cleistogenes. Another species of potential concern is N. tenuissima. It is available through the horticultural trade and has escaped from cultivation in both California and New Zealand (Edgar & Connor 2000), but whether it can invade relatively undisturbed habitats is not clear.

The other species that have become established outside their native range do not appear to be particularly aggressive. Nassella manicata is well established on the tailings from old gold mines east of Sacramento, California, and in some grasslands near the coast, but is not known beyond these areas. Similarly, there are well established populations of N. megapotamica in Australia which are not spreading (Jacobs & Everett 1993; Jacobs & al. 1998).

During the course of this study we came across a number of specimens that appear to represent new species of Nassella, but to demonstrate that they do, indeed, represent new species, would require more time and resources than are available to us. We anticipate, however, that the genus will increase in size both from further examination of existing specimens and additional exploration of South America and Mexico. We hope that this synopsis of the genus will aid those engaged in such endeavors..

The great concentration of Nassella in South America suggests that it may have originated on that continent, but the fossil record indicates that this may be misleading. Thomasson (1976, 1978, 1979) described three species of Nassella from the Late Miocene-Early Pliocene of Colorado, Kansas, and Nebraska, and Elias (1942) described an additional species from Pliocene records of Kansas. Only one extant species, N. viridula, now occurs in these states. Its morphological intermediateness and high chromosome number suggest that it may be an allopolyploid derivative of an Achnatherum P. Beauv. – Nassella hybrid. Alternatively, if these two genera are sister genera, N. viridula may be an autoploid derivative of their common ancestor.