Grasses: Biology and Ecology

1 – Game Ranging / Field Guiding Course

Module # 9 – Component # 3

Introduction

Grasses are the most important plant family on Earth. This statement is based on the following facts:

There are more individual grass plants on Earth than any other type of terrestrial vegetation.

They have the highest biomass of all plants, i.e. their combined weight is greater than that of any other group of land based plants.

They cover a large surface area of the Earth. Over 30% of the Earth’s land area is covered with natural grasslands and savannah areas dominated by grasses.

They inhabit the widest diversity of habitats. In other words, they are the most adaptable modern plants. They occur from the tropics through to the Arctic and Antarctic, in swamps, on mountain tops and on the sea shores. The most widely distributed flowering plant on earth is a grass (Phragmites australis).

They are also crucial to man’s survival - our most important food crops are grasses. These include maize, wheat, rice, oats, barley, rye and millets. Civilisation began with the cultivation of grass species whose seeds had previously been gathered from wild plants. Livestock subsists in grazing areas which are not suitable for cultivation. This is also of considerable importance to the survival of man.

There are some 9700 species of grasses world-wide of which 967 species or approximately 10% occur in Southern Africa.

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The Morphology of Grass Plants

Grasses are either tufted, floating, creeping, climbing or erect. These different growth forms, however, have a modular design and are made up of repeated similar units.

Each of these modular units or phytomers consists of an internode, a node, leaf, bud and adventitious roots.

Grass roots are generally fibrous and seldom penetrate more than 1 metre below the surface. Many species develop rhizosheaths which contain nitrogen fixing bacteria. These sheaths act as protective coverings for the roots.

There are three basic types of stems in grasses. These are aerial culms, underground rhizomes and stolons which lie at the soil surface. The culms are the most visible part of the plant. They bear the leaves and inflorescences. Grass Leaves consist of three parts; the leaf sheath which covers the culm, the blade which extends from the sheath, and the ligule which is found at the junction between the two.

Typically, leaf blades are long and narrow to obtain the benefits of this structure as previously mentioned. In times of water stress, many species can curl their leaves into a tight roll to conserve water by minimising transpiration.

The flowers are borne on the inflorescences of grasses. These play an important part in the identification of grass species.

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Pollination

All grasses found in South Africa are pollinated by the wind. The pollen remains viable for only a short period of time and can therefore not fertilise other grasses over a long distance.

Pollination and flowering for every species takes place only at specific times of the day. This ensures that pollination takes place with more reliability, and that closely related species do not cross-pollinate one another.

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Seed Dispersal

Most plants only relocate twice in their lifetime - once at pollination and again during seed dispersal. Grasses are no exception.

The seeds of some species are wind borne, and float away on parachutes of long fibres. Others produce an oil which acts as an attractant to ants who collect them and carry them into their underground nests where the seeds can germinate. Other species use animals as a means of dispersal. The seeds twist their way into the fur (or clothing) of the carrier and are usually removed by grooming.

Aquatic grasses float their seeds downstream on rafts of corky material. Some seeds are self-propelled and react to changes in humidity by burrowing their way into the soil. Some species have sticky seeds which cling to the fur of animals and are thus dispersed.

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The Ecology of Grasses

There is little evidence regarding grass evolution to be found in the fossil record, but there is speculation that their rise to dominance is related to the change in dentition of grazing animals. When grazers adapted to the utilisation of grass-like plants millions of years ago, the evolution of both grasses and grazers followed similar parallel paths. Grasses can thrive while being heavily utilised. They must be utilised if they are not to become moribund and eventually die. The lack of unpalatable secondary chemicals is common among many other plant families and makes the grasses even more highly sought after by grazing animals.

The flower structures of grasses are very reduced, due to their wind pollination strategies. Co-evolution with pollinating animals such as bats, bees and wasps has been very important to other plant families where floral parts are dramatically modified to attract pollinators through chemical compounds, shape, colour and scent. These strategies have become redundant in the grasses because of their reliance on wind pollination. Since there is a reduced chance of pollen finding its way to the female flowers, grasses have a much higher proportion of male flowers to female flowers. A further adaptation that increases the chance of fertilisation is the structure of the inflorescences which are designed to channel the wind and carry pollen to the female flowers.

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Why are Grasses So Successful?

The great success of grass plants may be directly attributed to the following factors.

They Have Herbaceous Stems and Leaves Most species of grasses are herbaceous. Given good rain and sunshine, they flourish, completing their lifecycle within a short period. In poor conditions, such as droughts and during winter, they die back and conserve their resources until better conditions arise. Annual species survive these periods as seeds, whilst perennial species survive as rootstocks and rhizomes.

This factor has given rise to the tendency in Southern Africa where for every perennial species found in the wet eastern part of the country, there is an annual equivalent to be found in the arid western parts.

Their Growth Points are at the Base of Leaves and Internodes In most plants, the growing points are located at the tips of stems and branches. In grasses, however, there are two growing points. The first is at the tip of the shoot, and the second and most important is at the base of each internode and each leaf.

This may be practically demonstrated by chewing the stem of a grass. The lowest portion of the stem is always the juiciest. This is because the area of newest cell growth is located here. When grass leaves get grazed, they do not regenerate from the tips as do other plants. This means that even with heavy grazing and burning, their growth points are seldom removed from the plant thereby allowing for maximum vegetative growth throughout the season.

They Have a Modular Construction The leaves of grasses are generally long, vertically arranged and very thin. The plant has an open canopy design allowing for maximum penetration of sunlight. Light thus reaches a large proportion of the leaf surface and allows to produce a high biomass per unit area occupied.

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Photosynthetic Pathways: Consequences for Palatability

Photosynthesis is a complex process, the details of which will be omitted here (But details may be found in Component # 1 of this Module). There are however a few principles that are important to understand since these determine the palatability of grasses.

Two basic forms of photosynthesis occur in grasses: C3 and C4 photosynthesis.

Simply put, the cells of the C3 grasses photosynthesise and produce starch. The cells of the C4 species either photosynthesise, or produce starch. The result of this is that the C3 grasses tend to lack rigidity because of their loose arrangement of starch cells, whilst C4 grasses are woody and can stand erect due to their specialised arrangement of starch cells.

This has obvious implications for the palatability of the grasses.

C3 species are usually highly palatable since they are softer, easier to digest and to graze on. The woody C4 species are usually only palatable in the early growth phase of the season, where after they become rigid and unpalatable.

African grazing animals have, however, adapted digestive systems and grazing strategies quite capable of utilising C4 grasses.

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Ecological Status of Grasses

As mentioned earlier, grasses have evolved in such a way as to require utilisation to flourish. The way in which a species reacts to utilisation determines its ecological status.

There are four different terms which may be applied to grasses in respect of their ecological status.

Decreaser Species Increaser Species - Type 1 Increaser Species - Type 2 Variable Status

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Decreaser Species

Some of the more highly palatable species are not very hardy and react negatively to over- utilisation by way of grazing. These species therefore become less frequent or decrease in number in the veld thus. These are the Decreasers.

Increaser Species - Type 1

Some species prefer not to be utilised by grazers (they may rely more on fire to stimulate them) and therefore flourish in veld that is under-utilised. These are the Increaser I species. These are generally palatable species in the early period of the growing season becoming unpalatable as they grow and gain rigidity.

Increaser Species - Type 2

These are very hardy species which react vigorously (grow well) to over- utilisation by grazing animals and increase their abundance in the veld as a result. These are known as Increaser II species. These are unpalatable species.

Variable Status

Owing to their wide distribution and adaptation to differing climates, aspects, elevations and soil types. These species have differing levels of hardiness and palatability and cannot be grouped into a single category. These species are therefore said to have varying ecological status.

Amongst the Increasers, there are sub-categories which refer to the reaction of the species to light, moderate and heavy over/under-utilisation. For the purposes of this course, these subdivisions are not important.

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Veld Types

In Southern Africa, veld types can be broadly classified from a grazing point of view as sweet veld, sour veld and mixed veld.

Sweet veld occurs in the low-lying, frost-free parts of the country. The most important types of grazing plant maintain their palatability and nutritional value throughout their entire life cycle, which can be utilized throughout the year and keeps the animals in a good condition and reproductive state.

Sour veld is a type of veld in which the most important types of grazing plant lose their palatability and nutritional value at maturity. The translocation of nutrients to the roots takes place towards the end of the growing season, and can thus maintain animals in a good condition and reproductive state for 6 – 8 months. For at least 3 months of the year nutritional deficiencies occur, often protein and phosphate which are therefore important components of licks. Sour veld areas normally have a higher rainfall than sweet veld, and the subsequent leaching of the soil has the effect of a lower pH than in sweet veld regions. This leads to a low calcium and phosphate quality in the soil and generally a deficiency in calcium, phosphate and protein in the grass species.

Mixed veld usually occurs in a transitional zone between sweet and sour veld and is suitable for grazing for periods of 6 – 10 months a year.

Veld types must not be confused with Biomes, of which there are 7 in South Africa. Each Biome is further divided up into Vegetation Types of which there are 67.

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Most common grasses found in South Africa.

Below is a handy guide to some of the most common grasses found in South Africa.

Scientific Name: Common name:

Andropogon chinensis Hairy Blue Grass

Anthephora pubescens Wool Grass

Aristida adscensionis Annual Three-Awn

Aristida congesta subsp. barbicollis Spreading Three-awn

Aristida congesta subsp. congesta Tassel Three-awn

Aristida junciformis Gongoni Three-awn

Bewsia biflora False Love Grass

Bothriochloa bladhii Purple Plume Grass

Bothriochloa insculpta Pinhole Grass

Bothriochloa radicans Stinking Grass

Brachiaria brizantha Common Signal Grass

Brachiaria eruciformis Sweet Signal Grass

Brachiaria serrata Velvet Signal Grass

Cenchrus cilliarus Foxtail Buffalo Grass

Chloris pycnothrix Spiderweb Grass

Chloris virgata Feather-top Chloris

Chrysopogon serrulatus Golden Beard Grass

Cymbopogon excavatus Broad-leaved Turpentine Grass

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Cymbopogon plurinodis Narrow-leaved Turpentine Grass

Cynodon dactylon Couch Grass

Digitaria argyrograpta

Digitaria eriantha Common Finger Grass

Diheteropogon amplectens Broad-leaved Bluestem

Elionurus muticus Wire Lemongrass

Enneapogon cenchroides Nine-awned Grass

Enneapogon scoparius Bottlebrush Grass

Eragrostis biflora Shade Eragrostis

Eragrostis capensis Heart-seed Love Grass

Eragrostis chloromelas (Narrow) Curly Leaf

Eragrostis gummiflua Gum Grass

Eragrostis heteromera Bronze Love Grass

Eragrostis inamoena Tite Grass

Eragrostis lehmanniana Lehmann's Love Grass

Eragrostis pseudosclerantha Footpath Love Grass

Eragrostis racemosa Narrow Heart Love Grass

Eragrostis rigidior (Broad) Curly Leaf

Eragrostis superba Saw-tooth Love Grass

Eustachys paspaloides Fan Grass/Brown Rhodes Grass

Fingerhuthia africana Thimble Grass

Heteropogon contortus Spear Grass

Hyparrhenia filipendula Fine Thatching Grass

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Hyparrhenia hirta Common Thatching Grass

Imperata cylindrica Cottonwool Grass

Loudetia simplex Common Russet Grass

Melinis repens subsp. repens Natal Red Top

Panicum coloratum Small Buffalo Grass

Panicum maximum Guinea Grass

Panicum repens Couch Panicum

Paspalum dilatatum Dallis Grass

Paspalum distichum Water Couch

Pennisetum macrourum Riverbed Grass

Pennisetum thunbergii Thunberg's Pennisetum

Perotis patens Cat's Tail

Phragmites australis Common Reed

Pogonarthria squarrosa Herringbone Grass

Setaria lindenbergiana

Setaria megaphylla Broad-leaved Bristle Grass

Setaria sphacelata var. sericea Golden Bristle Grass

Setaria sphacelata var. sphacelata Common Bristle Grass

Setaria sphacelata var. torta Creeping Bristle Grass

Setaria verticillata Bur Bristle Grass

Sporobolus africanus Ratstail Dropseed

Sporobolus pyramidalis Catstail Dropseed

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Themeda triandra Red Grass

Trachypogon spicatus Giant Spear Grass

Tragus berteronianus Common Carrot-seed Grass

Tricholaena monachne Blue-seed Grass

Trichoneura grandiglumis Small Rolling Grass

Urochloa mosambicensis Bushveld Signal Grass

Urochloa oligotricha Perennial Signal Grass

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Self-Study:

What follows below is a handy self-study exercise. Choose 20 key Grass species that occur in your area of operation and complete their details in the tables that follow.

Key Species List:

1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)

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Scientific name: 1. 2. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

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Scientific name: 3. 4. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

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Scientific name: 5. 6. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

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Scientific name: 7. 8. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

Scientific name: 9. 10. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

Scientific name: 11. 12. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

Scientific name: 13. 14. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

Scientific name: 15. 16. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

Scientific name: 17. 18. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing:

Scientific name: 19. 20. Common name:

English Common name:

Afrikaans Distribution:

Habitat:

Perreniality:

Leaf blade:

Ligule:

Grazing value:

Plant succession:

Grazing status:

Type of inflorescence:

Description of

inflorescence: colour, size etc. Interesting facts:

Basic drawing: