1 – Game Ranging / Field Guiding Course Module # 9 – Component # 3 Grasses: Biology and Ecology 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus. It may not be copied, distributed or reproduced in any format whatsoever without the express written permission of WildlifeCampus 2 – Game Ranging / Field Guiding Course 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus. It may not be copied, distributed or reproduced in any format whatsoever without the express written permission of WildlifeCampus 3 – Game Ranging / Field Guiding Course 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus. It may not be copied, distributed or reproduced in any format whatsoever without the express written permission of WildlifeCampus 4 – Game Ranging / Field Guiding Course 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus. It may not be copied, distributed or reproduced in any format whatsoever without the express written permission of WildlifeCampus 5 – Game Ranging / Field Guiding Course 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus. It may not be copied, distributed or reproduced in any format whatsoever without the express written permission of WildlifeCampus 6 – Game Ranging / Field Guiding Course 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus. It may not be copied, distributed or reproduced in any format whatsoever without the express written permission of WildlifeCampus 7 – Game Ranging / Field Guiding Course 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. Botany © Copyright This course material is the copyrighted intellectual property of WildlifeCampus.