A GRASS MENAGERIE: I - The Environment of the Grasslands

The world’s grasslands flourish in the middle latitudes of our planet, quite far north and south of the equator; hence, this biome is subjected to pronounced seasonal changes in environmental factors. Grasslands also tend to thrive in the drier interior of continents some distance from the oceans.

Grasslands generally receive moderate, often erratic rainfall, usually in late spring to early summer; the short wet season is followed by prolonged dry periods during which very little precipitation falls. Temperatures in grasslands can fluctuate annually from hot daytimes to very cold winters with significant bouts of frost (at least). Moreover, most grasslands occur on deep soils (often sandy or friable) that do not retain soil moisture for long.

The grasslands survive pronounced seasonal changes in a combination of environmental factors: sufficient warmth and sufficiently long day-lengths for growth, moist upper layers of soil, and frost-free nights in the summer months, with cold and frosty nights, dry soils, and short day-lengths in winter. This combination of factors gives rise to an obvious growing season for the vegetation in late spring and summer, followed by a lengthy dormant season.

In southern Africa, the grasslands are located chiefly in South Africa (on the Highveld (an elevated central plateau) and the inland areas of KwaZulu-Natal and the Eastern Cape) with much smaller, localised regions found particularly in the eastern highlands of Zimbabwe. In general, the topography is flat or undulating, but it includes the escarpment areas of the Highveld and the highlands of Zimbabwe too. The altitude of southern Africa’s grasslands varies from near sea level to close to 3000 metres in elevation.

A copious number of grass species dominates the grasslands extensively. Between the grasses also grow many non-woody flowering plants and many geophytes (bulb-producing plants); regularly woody shrublets can also survive the environmental and climatic conditions experienced in grasslands. Since the height of the vegetation is very low, plants in grasslands experience high light intensities (at least during the summer months); no true shade-plants survive here.

Taller shrubs and trees are generally absent from grasslands. There are several reasons for this absence. The deep and often well-drained soils of grasslands frequently become too dry at deeper levels for tree roots to be able to absorb sufficient moisture. In general, the leaves and smaller twigs of trees and shrubs are also not frost resistant; therefore, taller woody vegetation can not survive the lengthy cold, dry winter seasons. During the winters, fire too becomes a significant ecological factor in grasslands, removing dense old growth from the biome, making nutrients available again for the rapid growth of new grasses during the following growing season. Fire also prevents the encroachment of shrubs and trees into grassland areas.

In contrast to most woody vegetation, grasses can survive the challenging conditions of the dry seasons during which grasses become dormant. In winter, the grasses usually die back; that is, the above-ground culms dry out completely while the below-ground root systems survive the harshest of parched cycles. Alternatively, many grasses are annual plants rather than perennials – the adult lives for one season only, during which time the plant germinates from seed, grows rapidly, flowers and sets seed itself, only to die when the dry season commences. As long as the upper soil layer does not become too hot during a fire, the seeds of the previous summer can survive there and the grasslands can persist even through regular cycles of wildfires. The smaller non-woody herbs, flowering plants and the shrubs of the grasslands follow similar strategies.

Grasses form a special subset of the vegetable world in many ways. Their roots do not penetrate deep into the soil; rather, the many thin, fibrous roots spread out just under the soil surface, knitting together the top layer of soil. The decomposition of the dead leaves, culms and roots of grasses releases nutrients into the topmost, enmeshed and very fertile soil layer. The web of fibrous roots also captures much soil water before it percolates into the deeper strata following good rains. Sacrificing a long life for rapid growth, grasses with their strap-like leaves and thin culms have done away with thick trunks and large leaves. The strategy employed is one of grow, reproduce and set seed as rapidly as possible. In grasslands, plants that do not produce seeds are very rare.

Although they are flowering plants, grasses have done away with large, showy flowers. The grasslands, in general, are often too dry for much of the year to support a permanent, large insect population. There exist vast numbers of insects in grasslands, but their appearance in a local area may be unpredictable. Therefore, attracting a pollinator at the correct time – when the flowers have grown and matured, and are ready for pollination and fertilization – is a challenge for any flowering plant in a grassland environment. Instead, grasses have reverted to wind pollination and the flowers have become minute, but bright, splashes of colour in the inflorescences of grasses. Since most grasses grow to a more or less uniform height and the grass stalks are thin, wind can move over and through grasslands relatively unimpeded, except at ground level. Thus wind pollination is an effective strategy for plant reproduction in this environment.

In contrast to all other plants, the grasses possess a unique feature. In plants that are not grasses, cell division for growth of the organism takes place only in two regions of the plant: at the tips of the roots and twigs (where elongation of all plants takes place), and in woody plants just below the outer covering inside of the bark of larger shrubs and trees (so that an increase in the diameter of roots, trunks and branches can take place). In grasses, however, patches of undifferentiated tissues (the intercalary meristems) occur and cell division can take place at any regular point along the plant. As a consequence, grasses do not stop growing when their leaves and culms are grazed (that is, when the tips of the plants are removed); rather, growth of new plant material will take place from the intercalary meristems. In fact, grasses tend to thrive better if they are grazed to a certain extent. The saliva of the antelopes that graze the grasslands of Africa and elsewhere also stimulates grass growth, an interesting example of co-evolution between a predator and its prey in these two groups that evolved on our planet at the same time.

When people look at grasslands, they may conclude that the biodiversity of this biome is quite low. Grasslands look uniform – there seem to be only grasses present, and these tend to grow to the same length (more or less) in any particular area. However, in South Africa, this biome harbours a tremendously high biodiversity (second only to the fynbos biome of the southern Cape). The grasslands are often home to rare plants, particularly so in the higher-lying areas along the escarpment. These scarce species are often endangered; they comprise mainly endemic geophytes (bulb-producing plants) and dicotyledonous herbaceous plants. However, very few species of grasses are rare or endangered, almost certainly as a result of wind pollination and the production of small seeds.

All my life I have been a resident of the Highveld of South Africa. My earliest recollections of wilderness are intertwined intimately with the grasslands. As a very young photographer, I often lamented the fact that my family had not lived in a bushveld or coastal area – environments that seemed so much more photogenic in themselves and that appeared to be jam-packed with opportunities of capturing images of wildlife. However, the more I explored, the closer I looked at the grasslands surrounding my home, the more grew my appreciation of the spectacular expanses of the grasslands. Nowadays, I relish the challenge of portraying the grasslands in all their splendour; I cherish every opportunity of arresting, in an image, the fabulous grass menagerie.