Flora of North America species comparison

Cynodon dactylon is a variable species, but taxonomists disagree on just how variable. Caro and Sanchez (1969) limited C. dactylon to plants with conduplicate leaves, placing those with convolute leaves in a number of other species, such as C. affinis Caro & Sanchez and C. aristiglumis Caro & Sanchez; de Wet and Harlan (1970) do not mention this character in their study of Cynodon, Caro and Sanchez also employed several other characters in the key separating C. dactylon from the species with convolute immature leaves, but the overlap between the two sides of the lead is substantial. Pending further study, the broader interpretation, in which C. dactylon includes plants with both convolute and conduplicate leaves, has been adopted.

Several varieties of C. dactylon have been described, in addition to which numerous cultivars have been developed, some as turf grasses for lawns or putting greens, others as pasture or forage grasses. Their useful range is limited because C. dactylon is not cold hardy, going dormant and turning brown when nighttime temperatures fall below freezing or average daytime temperatures are below 10°C.

The most commonly encountered variety, both in the Flora region and in other parts of the world, is C. dactylon var. dactylon, largely because it thrives in severely disturbed, exposed sites; it does not invade natural grass-lands or forests. Determining how many other varieties are established in the Flora region is almost impossible, because there has been no global study of variation in the species. The presence of numerous cultivars complicates an already difficult problem. The two varieties keyed out below are the only two that grow in the Flora region according to de Wet and Harlan (1970), but these authors do not appear to have considered the taxa recognized by Caro and Sanchez (1969). For most purposes, it is probably neither necessary nor feasible to identify the variety of C. dactylon encountered.

Cynodon dactylon is considered a weed in many countries and it is true that, once established, it is difficult to eradicate. It does, however, have some redeeming values. It is rich in vitamin C, and its leaves are sometimes used for an herbal tea. It is claimed to have various medicinal properties, but these have not been verified. It is considered a good pasture grass, in addition to which it is sometimes grown as an ornamental and for erosion control on exposed soils.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

The subfamily Chloridoideae is most abundant in dry, tropical and subtropical regions. In the Flora region, it reaches its greatest diversity in the southwestern United States (Barkworth and Capels 2000). Almost all its members, and all those in the Flora region, have C4 photosynthesis. Most employ the NAD-ME or PCK pathways, but Pappophorum utilizes the NADP-ME pathway.

The subfamily has been recognized, with essentially the same limits as here, for some time, although reservations have been expressed concerning its monophyly (Campbell 1985; Jacobs 1987; Kellogg and Campbell 1987). More recent studies, both morphological (Van den Borre and Watson 1997, 2000) and molecular (Soreng and Davis 1998; Hilu et al. 1999; Hsaio et al. 1999; Grass Phylogeny Working Group 2001; Hilu and Alice 2001) support its recognition as a monophyletic unit. There is less agreement concerning the subfamily's closest relative, some studies pointing to the Arundinoideae (Grass Phylogeny Working Group 2001) and some to the Danthonioideae (Barker et al. 1995; Hilu and Esen 1993; Hilu and Alice 2001).

There is considerable disagreement concerning the tribal treatment within the Chloridoideae, the number of tribes recognized varying from two (Prat 1936) to eight (Gould and Shaw 1983). Hilu and Wright (1982, p. 28) concluded, on the basis of their morphological study, that "... the boundaries between most of the tribes in this subfamily are not pronounced." They noted that Savile (1979) reached the same conclusion from considering the host specificity of various pathogenic fungi.

More recent work supports Hilu and Wright's conclusion. Van den Borre and Watson (1997, 2001) recognized eight informal groups within the subfamily. Five of the groups were large, the smallest including around 133 species and the largest around 380. The other three groups, which correspond to the Orcuttieae, Pappophoreae, and subtribe Triodiinae, include 9, 42, and 54 species, respectively. The difference in size is of no concern; the fact that all three of the small groups are embedded within one of the five large groups, the Pappophoreae and Triodiinae in a group than includes Eragrostis subg. Eragrostis and the Orcuttieae in the group that includes Muhlenbergia, is disturbing. Van den Borre and Watson noted that part of the problem was that that Eragrostis, and probably some of the other large genera, are not monophyletic.

Hilu and Alice (2001) recognized four clades within the Chloridoideae. Like Van den Borre and Watson, they found the Orcuttieae and Triodiinae to be monophyletic, although their placement within the subfamily was not clear. Unlike Van den Borre and Watson, Hilu and Alice found Pappophorum, and hence the Pappophoreae, to be polyphyletic.

The treatment presented here is conservative in recognizing the Orcuttieae and Pappophoreae as distinct tribes. It departs from most other treatments in merging all other North American taxa into a single tribe, the Cynodonteae. Consensus on how the Cynodonteae sensu lato should be broken up is unlikely to be reached until the generic limits of its members have been more thoroughly examined.

(Discussion copyrighted by Flora of North America; reprinted with permission.)