Photo by Sybille Hauessler: Northern Grasslands
Grasslands are anomalies in the context of world vegetation units. They simply should not exist in the natural world. However, they have managed to more than 'exist' - they have prospered, and extended their range to become one of the most successful of all of the world vegetation types and also to become one of the most commercially important of all species. The reason for the phenomenal success of grasslands is contained in the phrase commercially important because grasslands have been utilised by generations of farmers as the source for all our cereal crops.
Grasses belong to the plant family called Graminaceae. This family first appears in the fossil record at the comparatively late stage of the Cretaceous era about 100 million years ago. It has been hypothesised that the first grasses probably evolved at high elevation sites above the tree line because all grasses have one common attribute. They cannot tolerate shady conditions. Therefore, the first grasses faced a major problem - where could they grow? Luxuriant forest ensured that almost 90% of the land surface was unsuited to grasses as beneath the forest canopy only a tiny fraction of the incoming solar radiation penetrated the thick forest cover. The pollen of grass species is notable by its absence beneath all types of forests but as soon as tree pollen begins to decline (indicating a possible thinning of the forest cover) then grass pollen makes a sudden appearance in the pollen profile. This dramatic appearance of grass pollen is difficult to explain. Most grass pollen is small, light and wind dispersed; its sudden appearance in the pollen profile may be due to nothing more than an in-blowing of grass pollen from sites away from the forests. However, the decline in tree pollen is usually accompanied by other evidence. Often there appear layers of charcoal at the site of investigation. Again, this poses a problem. Was the charcoal a result of a natural fire or was it evidence of burning by our ancestors. Some sites give clear support of the latter occurrence; flakes of flint indicating primitive tools; the existence of pollen grains from specie we now associate as the weeds of agriculture, for example the dandelion, daisy and plantain. There is also the unexplainable fact that pollen from grass species makes an appearance at almost the same time in many different parts of the globe. It may be that there was a marked climatic drying out making conditions unsuited for forests.
No matter what explanation is relevant we can be certain that by the time the first of the settled agricultural societies were established in the ancient city of Jarmo (in modern-day Iraq) some 5000 years before the present day, farmers were cultivating several different cereal crops the preserved seeds of which are directly comparable to contemporary wheat, barley and rye. Although we have no preserved evidence, these crops must have been evolving for many thousands of years prior to 5,000 B.P. Some archaeologists have suggested that these agricultural crops, known as cultivars, may have evolved purely by chance, growing on the edges of the first clearings made by humanoids, possibly finding the midden heaps that surrounded the encampments to be highly fertile ground that encouraged the grasses to undergo spectacular evolution from the small wild form to those which produced large seed heads. The large seed heads would have proved a natural food source. But how the transition was made from a human society based on the gathering of seed heads to one in which grass seeds were deliberately planted to produce a crop is not known.
The rise in the importance of the grasses as a species that would eventually challenge the might of the forests is paralleled by the rise in human numbers. As human beings increased in numbers and colonised more and more of the surface of the Earth then the spread of Homo sapiens is accompanied by the Graminaceae. The grasses possessed many natural advantages over other plant species. For example, they are one of the few vegetation types that actually thrive on being eaten, burnt, trampled upon. They can grow at sites ranging from sea level up to the edges of high altitude glaciers. They can tolerate salinity, acidity and alkalinity. They can adapt to exposure and shelter. One reason for this vast environmental range is due to their genetic adaptability. By means of sexual reproduction they can quickly evolve new varieties to suit new growing environments. But they have one other advantage over almost all other plant species. Most plants generate new growth from a special growth cell called the meristem. The meristem is usually located a few millimeters behind the apical growing point, the leading shoot of plant stems. The apical growing point can be easily damaged by frost, exposure, or from the teeth of herbivores. Grasses differ in that not only do they have conventional apical growing points, they also have meristems located at intervals along their stems, the so-called intercalliary meristems. If the apical growing point is damaged, then the intercalliary meristems become active and take over the growth of the stem. Grasses have yet a further advantage over other plants in that in addition to sexual reproduction with its attendant advantages of producing hybrid plants, they can also reproduce asexually. When actively growing they can develop side shoots either above ground (runners) or below ground (stolons or rhizomes). These outgrowths produce 'daughter plants' with an identical genetic structure as the parent plant (in effect these are clones). If the parent plant is well suited to the habitat in which it is growing then the production of clones is an excellent way of generating large numbers of identical plantlets which will also be ideally suited to the habitat in which they grow.
There are three types of grasses: annuals which complete their life cycle in one year; biennials which germinate late in the growing season of one year and flower and seed in the next year; and perennials which continue their growth from year to year. When grasses form the dominant species in a plant community then the biome name is grassland. Most grasslands comprise complex plant associations, that is a group of similar plants growing in a uniform environment and containing one or more dominant grass species. Grasslands are usually dominated by perennial grasses over the annual and biennial types. However, some grasslands may be grasslands in name only and, in particular, agricultural 'grasslands' that have been sown to support grazing animals can be dominated by leguminous species such as clover, vetch, alfalfa and lucerne. Conversely, acid upland 'grasslands' in Scotland can be dominated by sedges, mosses and lichens in which grasses have been over-grazed by excessive numbers of sheep or deer.
Nowadays we tend to think of grasslands as typical of the agricultural landscape of north west Europe or the wetter areas of North America. These agricultural grasslands are very different in species composition from natural grasslands. The picture is made confused because many natural grasslands have been taken into agricultural land use. For example, the North American Prairie grasslands are sometimes considered to be an example of true natural grassland. Recent work has shown that even before the arrival of the European settler from the 1750s onward, the Native Amer-Indians was practising extensive management of the Mid West Prairies through the use of fire, both as a means of encouraging new growth of grasses and to assist in the hunting of buffalo. The true 'naturalness' of the Prairies will probably never be determined.
A short discussion of agricultural grasslands can be found by clicking [here]. The remainder of this lecture will consider the 'natural' grasslands of the world. The term 'natural' grassland is considered by some biogeographers to be a contradiction of terms, claiming that almost every grassland owes its origin to human management and therefore, cannot be described as ‘natural’. There are, however, some grasslands that have been in existence for many thousands of years and have taken on the apparance of ‘natural’ grasslands. The term 'old' grassland should be used in place of 'natural' grassland. This convention will be followed here.
The world distribution of old grasslands is shown in the Figure 4.You will note that there are two distinct distributions: the temperate grasslands shown in green and the and the savanna (or tropical) grasslands shown in orange.
The main temperate grasslands include:
Although these areas are located in separate continents they share one common habitat condition - a shortage of annual moisture. Towards the higher latitude locations of the temperate grasslands, annual rainfall totals are usually less than 500 mm. Nearer the equator, where evaporation rates are higher, annual rainfall may reach 750 mm. Where the higher rainfall figures prevail, the grasses may grow to over one metre in height. The tall grass prairies of the eastern Great Plains reach 1.5 metres tall. The tallest grasses grown in distinct clumps and often the clumps are separated from one another by bare ground. The turf forming grasses that are common place in agricultural grasslands tend not to occur in old grasslands. Farmers often refer to the clumps of grass as 'bunch grass'.
The clumps of grass are separated from one another because there is insufficient moisture to support a denser network of grasses. Excavating into the soil shows that beneath ground the grass roots grow horizontally until they meet the roots spreading out from the adjacent plant. Thus, below ground, all the living space is occupied by the roots. Above ground the bare patches of ground have given rise to the term 'open community' imply that space exists between plants. In reality, it is unlikely that any additional vegetation could be encouraged to grow in these 'open communities' due to the lack of moisture and the full utilisation of below ground space.
Apart from the low total rainfall figures, the temperate grassland sites also experience a distinct wet and dry seasonal distribution. Rain occurs in the summer time, when the sun moves towards the tropic, bringing with it the period of maximum temperature (July average as high as 30oC, though more usually in the high 20s). Three-quarters of the annual rain may occur in the summer season, falling as heavy convectional showers. Winters, by contrast, can be cold with average January temperatures of -10oC. Precipitation may fall as snow which may lie for up to three months.
Under this surprisingly harsh climatic regime the grasses grow rapidly in the summer months, with the seeds ripening in the short lasting autumn period before they fall to the soil surface to lie dormant until next spring. Most of the seed will be eaten by the great herbivore herds and by seed eating migrant birds. The parent grass plant may live for up to ten years before being replaced by a new generation. However, this sequence can be drastically altered by the occurrence of fire.
Fire regularly sweeps across the old grasslands, sometimes as frequently as every two or three years. Only grass species that can survive such drastic treatment will be found in these areas. The main way of surviving fire is to produce fire resistant seeds which lie protected in the top-most layer of the soil. The heat of the fire stimulates the dormant seeds to germinate in the aftermath of the fire, and to make use of the fertile ash as a medium to support rapid growth.
At the end of each growing season, apart from the production of seeds, the grasses also produce large quantities of leafy growth (the total vegetable growth is called the biomass). This organic material remains on the surface of the soil, little changed during the cold dry winters but come the spring and the return of moist, warm conditions, the dead grass is quickly converted into humus and incorporated into the soil by the teeming soil micro-organisms and worms. Soils beneath old temperate grasslands are a very distinctive dark brown or black colour. Beneath the prairies, the Prairie Soil may be 1 metre deep, dark brown in colour due to the high organic content and alkaline in reaction (pH 7.5). Beneath the steppe lands of Russia, an even more distinctive soil type has developed, the Chernozem soil. These soils are almost black in colour, up to 1.5 metres deep and calcareous (pH >7.5). Not unexpectedly, agriculturalists long ago turned their attention to these fertile soils. The native grassland species have been replaced by cereal crops, in particular by wheat. The Prairie Soils and the Chernozems have been labelled the 'bread baskets' of the world as it is these locations which yield prodigious wheat harvests and in particular, allow Canada and the US to export grain to the world.
Some of the worst problems experienced in the temperate grasslands have been associated with over-exploitation by intensive grazing, with the conversion of unsuitable areas to arable agriculture. One of the worst examples was that of the US Dust Bowl. Elsewhere, disturbance of the fragile temperate grassland ecosystem has often resulted in considerable changes in the species composition of the vegetation of these areas especially when humans have 'assisted' by introducing new species under the mistaken belief that they can 'improve' the ecosystem. For example, in Nevada and Utah, introduction of a Mediterranean grass the 'cheatgrass' has become a serious pest and has made the native grasslands more susceptible to damage from fire. The native bunch grasses were spaced sufficiently far apart to prevent fire from jumping from one plant to another whereas the cheatgrass tends to form a more continuous ground cover.
It has been suggested by some workers that temperate grasslands will become particularly prone to the impact of climate change. Increasing drought and higher temperatures will lead to an expansion of semi-arid conditions. This will be of particular concern in those areas where the native grassland has been converted to grain producing regions.
Savannas (also sometimes spelled savana and savannah) are located much nearer the equator than the temperate grasslands. In Africa, the savannas cross the equator in the vicinity of the East African highlands, uniting the northern and southern hemisphere grassland units. Typically, the savannas occupy latitudes between 5o and 20oN and S of the equator. The grass species making up the savannas are quite different from those species found in the temperate grasslands and a further difference is that trees (most commonly, members of the Acacia family) usually form a prominent feature of the vegetation community.
The climate of the savanna regions is typically that of tropical semi-humid climate, with a hot, rainy summer season and a cooler and drier 'winter'. The hottest monthly average temperature is usually greater than 35oC and the coolest monthly average temperature is always greater than 10oC. A marked temperature and rainfall gradient is shown across the latitudinal range. On the equatorial edge of the savannas, annual rainfall total may be as high as 2500mm, easily sufficient to support thick forest. On the savanna edges nearest the tropics, annual rainfall totals may be as little as 500 mm and the savanna merges into the semi-desert areas that mark the descending air associated with Hadley cell circulation.
Even the most cursory study of the savannas suggest that both the climate and the soils could support a forest vegetation. The extensive presence of trees in the present day savannas are further proof that these vegetation units are, at best, transitional areas between the tropical forest and the semi-desert scrub typical of latitudes 20 - 30oN and S. And yet other evidence suggests that the savannas are ancient vegetation types. Archaeological evidence suggests the presence of tropical grassland in East Africa 6,000 years ago while the work of the anthropologist, David Leaky, has postulated grasslands which existed 5,000 years ago.
Savannas, therefore, display conflicting evidence in support of the 'naturalness'. Some savannas are undoubtedly very ancient, but are they also 'natural' grasslands, or do they owe their origin to some other cause? As for the temperate grasslands, fire plays a major role at the present day in shaping savannas into their present species composition. But was fire also responsible for creating the savannas? For many years it was considered that tropical rain forest was too wet too burn. We have written evidence from the earliest Roman seafarers that they witnessed the burning of the African coastline from their ships. Were these fires the results of lightening strikes? Or were they the actions of the many tribes peoples that inhabited Africa from the dawn of human civilisation? Today, we find abundant evidence of charcoal layers beneath savanna vegetation suggesting that great fires were a feature in the evolution of these latitudes.
A second shaping force has undoubtedly been the impact of vast herds of grazing herbivores. These animals roamed the grasslands, following the movement of the rainfall belts which stimulated the new growth of grasses. But did these vast herds develop in response to the abundant feeding areas on the grasslands that had already been formed before the increase in herbivore numbers?
Thirdly, some workers have suggested that savannas are found on sites that are extremely ancient and that have never been glaciated or disturbed by volcanic deposition or by climate change. These workers have suggested that because of a lack of disturbance the soils have become 'senile', and have lost their fertility and therefore cannot support a luxuriant forest. If lack of disturbance is a relevant cause for the existence of savannas then exactly the same causes should apply to latitudes nearer the equatore and where tropical and equatorial forest is still found.
Fourthly, the impact of human activity must be considered. We know with some certainty that the earliest huminoids, Australopithicus, evolved on the high plains of Tanganyika and that they spent much of their time hunting on open grasslands which probably existed prior to the appearance of humanoids. We are therefore no closer to proving whether the grasslands were the pre-existing resource that allowed the development of Australopithicus and the many other humanoids and also the herbivores, or whether animals (including early humans) were responsible for causing the savannas.
Savanna soils are generally poor in their nutrient content, and this may limit the availability of some nutrients, especially nitrogen. Some of the dominant grass species growing in savannas are known to release chemicals that inhibit the action of the bacteria that help to 'fix' atmospheric nitrogen. This seemingly unhelpful tendency may be a specific adaptation to deliberately prevent the accumulation of nitrogen in the soil and thus prevents the invasion of competitive species.
Bradshaw, M. & Weaver, R. (1993) Physical Geography. pp 586 - 589 Grassland Biomes. Mosby
Cole, M. (1960) Geography Journal 126 pp 168
Cox, C. B. & Moore, P.D. (1993) Biogeography. An Ecological and evolutionary approach. Basil Blackwell, Publisher. See pages 96 - 100
Furley P.A. & W.N.Newey (1983) Geography of the Biosphere. Savanna pp.291-301 Butterworths. (R.C.)
Goudie A. (1993) The Nature of the Environment Tropical seasonal forest and savanna. pp175-176. Blackwell. (R.C.)
Hopkins, B. (1974) Forest and Savanna. Heinemann.
Miller G T (1993) Living in the Environment. section on major types of terrestrial ecosystems covers the Grasslands.
Tivy, J. (1971) Biogeography. Ch 13 Grasses and the Grassland Ecosystems. Oliver & Boyd.
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