Diversity and classification of grasses

Introduction

Grasses belong to the Grass Family, a group of plants with the scientific name Poaceae or Graminae. Grasses are the fifth-largest family of angiosperms (flowering plants), and the second-largest family of monocots after the orchid family (Orchidaceae). The grasses include three major groups, the early diverging lineages, the BOP clade, and the PACMAD clade. (A clade is a group made up an ancestor and all of its descendants.) The groups are divided into 12 subfamilies, which are themselves divided into tribes, genera, and species. 

The subfamilies of grasses are discussed below. Familiar grasses in each subfamily are highlighted in order to illustrate the breadth of grass diversity and the high importance of grasses to humans.

Early diverging grasses

The three early diverging subfamilies of grasses include forest grasses found primarily in tropical habitats. These subfamilies are Anomochlooideae, Pharoideae, and Puelioideae. All three subfamilies are very small, with about 7 genera and fewer than 30 species among them. The most widely distributed is Pharoideae, which in found across the tropics worldwide. Anomochlooideae is found only in the Americas, whereas Puelioideae is restricted to Africa. The basally diverging grasses are all thought to be C₃ grasses. Anomochloodieae is the only subfamily of grasses that does not have spikelets.

BOP clade

The "BOP" clade is named for the three subfamilies that make up the group: Bambusoideae, Oryzoideae, and Pooideae. All members of the BOP clade are C₃ grasses. This group is sometimes also called the "BEP" clade, since an alternative name for Oryzoideae is Ehrhartoideae.

Subfamily Oryzoideae

Subfamily Oryzoideae has about 19 genera and 115 species in two tribes. This relatively small subfamily includes one very important cereal crop: rice (Oryza, especially Oryza sativa). Wild rice (Zizania), which is under much more limited cultivation and is also gathered from the wild, is in this subfamily as well. Examples of other members of this subfamily include cutgrass (Leersia) and watergrass (Luziola).

Subfamily Bambusoideae (bamboos)

Bambusoideae are the bamboos. This is a large subfamily of grasses, with more than 100 genera and 1400 species. Bamboos are distributed worldwide and are found in the neotropics (American tropics), paleotropics (African and Asian to Australian tropics), and in temperate regions of eastern North America and eastern Asia. Bamboos are most diverse and abundant in Asia, although they are also very well represented in South America. While bamboos may be thought of as "woody" grasses, these plants do not produce true wood; nevertheless, many bamboos have stems with hard tissues that give them similar characteristics to true wood.

Bamboos are divided into three tribes. More than half of bamboo genera and species are tropical woody bamboos in the tribe Bambuseae, which includes more than 70 genera and perhaps more than 900 species. The temperate woody bamboos are in tribe Arundinarieae, which includes the eastern North American canes (Arundinaria), related eastern Asian species, and some bamboos from Africa and Madagascar. The smallest tribe is Olyreae, the herbaceous bamboos (also known as olyroid bamboos), which are primarily found in the Americas.

Subfamily Pooideae (cool-season grasses)

Pooideae are the largest subfamily in the BOP clade, with more than 200 genera and 3900 species in 15 tribes; grasses from this subfamily are found all over the world, although they are mostly temperate.

Most members of subfamily Pooideae are in tribe Poeae, which includes about 120 genera and more than 2500 species, among them species of bentgrass (Agrostis), oat (Avena), reed grass (Calamagrostis), orchard grass (Dactylis), fescue (Festuca and Vulpia), timothy (Phleum), bluegrass (Poa), alkali grass (Puccinellia), and oatgrass (Trisetum).

Tribe Triticeae (27 genera, about 500 species) includes the economically important cereal crops barley (Hordeum vulgare), rye (Secale cereale), and wheat (Triticum). Other types of grasses in subfamily Pooideae include brome or cheat grass (Bromus) and needlegrass (Stipa).

PACMAD clade

The "PACMAD" clade is named for the six subfamilies that make up the group: Panicoideae, Aristidoideae, Chloridoideae, Micrairoideae, Arundinoideae, and Danthonioideae. PACMADs include C₃ and C₄ grasses.

Subfamily Aristidoideae

Subfamily Aristidoideae includes three genera and more than 350 species mostly found in the tropics. The lemma of the florets in this subfamily have three awns (pointed extensions), which gives the three awn (genus Aristida) one of its common names. These grasses generally grow in dry environments and disturbed areas. Most Aristidoideae are C₄ grasses. 

Subfamily Panicoideae

Subfamily Panicoideae is among the largest subfamilies of grasses, with nearly 250 genera and more than 3200 species in 14 tribes; many are C₄ grasses.

The largest tribe in Panicoideae is tribe Paniceae (about 80 genera and more than 1200 species). This tribe includes regionally important crops like Pearl millet (Cenchrus americanus) and foxtail millet (Setaria italica), grown in parts of Africa and Asia, as well as Proso millet (Panicum millaceum) from Eurasia. Examples of other members of this tribe are crabgrass (Digitaria), satintail (Imperata), panicgrass (Panicum), fountaingrass (Pennisetum), and signalgrass (Urochloa).

One of the most economically important tribes in subfamily Panicoideae is Androponogoneae (98 genera, more than 1200 species), which is named after the bluestem (Andropogon). This tribe includes maize or corn (Zea mays), the most widely grown cereal crop in the world, as well as sorghum (Sorghum bicolor), which ranks fifth among cereals. Sugar cane (Saccharum officinarum), one of the major sources of table sugar (sucrose), is also in this tribe. 

Examples of other grasses in subfamily Panicoideae include wild cane (Gynerium sagittatum), which has many uses (for example, this grass is used to make arrows, hats, and mats); the grain crop Kodo millet (Paspalum scrobiculatum) from India; and tiger grass (Thysanolaena latifolia), which is used an an ornamental and to make food wrappings and brooms in parts of Asia.

Subfamily Arundinoideae

Subfamily Arundinoideae includes about 14 genera and 40 species of C₃ grasses divided between two tribes. Among members of this subfamily are the reeds (Arundo and Phragmites), which include the widespread grasses giant reed (Arundo donax) and common reed (Phragmites australis).

Subfamily Micrairoideae

Subfamily Micrairoideae includes about 8 genera and more than 180 species in four tribes. Two genera of Micrairoideae (Eriachne and Pheidochloa) are C₄ grasses. Examples of members of this subfamily are Wanderrie grass (Eriachne) and bloodgrass (Isachne).

Subfamily Danthonioideae

Subfamily Danthonioideae includes C₃ grasses, with about 20 genera and more than 290 species in one tribe. Members of this family are adapted to cool environments. Examples of grasses in this subfamily are pampas grass (Cortaderia) and oatgrass (Danthonia).

Subfamily Chloridoideae

Chloridoideae are a large subfamily of mostly C₄ grasses, with nearly 125 genera and more than 1600 species in five tribes.

More than half of grasses in this subfamily are in tribe Cynodonteae (94 genera, 850 species), which includes buffalograss (Buchloe dactyloides), Bermuda grass (Cynodon), goosegrass (Eleusine), and sprangletop (Leptochloa). Finger millet (Eleusine coracana) is a regionally important crop in parts of Africa and Asia.

Tribe Eragrostideae has about 14 genera and more than 480 species. It includes teff (Eragrostis tef), an important grain crop in Ethiopia and Eritrea, and other lovegrasses (Eragrostis).