Geography 140
Introduction to Physical Geography

Lecture: Biomes Dominated by Grasses

     F. Grasslands are, surprisingly <G>, dominated by grasses, which 
        can form a continuous or discontinuous cover.  There can also be found 
        trees, shrubs, and forbs, too, depending on the area.  I'll discuss 
        four subtypes.
        1. The first two are tropical grasslands.
           a. The first of these is savanna.

              
    
                i. This is dominated by medium to tall grasses (up to 3-4 m!), 
                   which form a continuous or discontinuous cover.
                   a. This grass is soft and green and humid soon after the 
                      rainy summer season starts, affording high quality 
                      forage for large herds of a few species of grazing 
                      animals (e.g., zebras, wildebeests) and for herds of 
                      domestic animals.  Great numbers of them move into 
                      savanna as summer begins. It grows to really impressive 
                      heights by the end of the wet season, and it'll be 
                      virtually continuous in good years and more 
                      discontinuous in drier years.
                   b. Comes winter, the hot dry season, this grass gives a 
                      dry, brown, coarse appearance, and it can be quite 
                      dangerous to hike in, because of the silica deposited on 
                      the blades of the grasses as they struggle to input 
                      scarcer and scarcer soil water.  Nasty stuff.  The great 
                      herds of animals migrate out of it then.  It is very 
                      prone to massive fires at this time.
               ii. The peoples who have lived with this vegetation for 
                   hundreds and thousands of years will often themselves set 
                   it on fire just before the rainy season, to turn all that 
                   dead biomass into ash fertilizer and to knock it down so 
                   sun can reach the soil.  This stimulates really rapid 
                   growth of tender grass shoots, which is great for those 
                   herds of wild animals or their own domestic herds of 
                   cattle, horses, goats, and camels.  There is some 
                   speculation that savanna and many grasslands in fact may 
                   not be strictly natural vegetation forms!  The argument is 
                   that people have been messing with fire for perhaps 100,000 
                   years and, so, changing the balance between grass and 
                   trees.  Sometimes, when firing ceases or recurs less 
                   frequently, you see tropical deciduous woodland moving back 
                   in, filling the grassy spaces, so there might be something 
                   to this.
              iii. Interspersed with the grass is a grouped cover of smaller 
                   tree-form plants, including acacias, palms, even giant 
                   cacti. In drier or overburnt portions, this second cover is 
                   in shrub form.           
               iv. There is also an understory of forbs, most evident in the 
                   early rainy season.
                v. Tropical savanna is associated with the drier portions of 
                   the tropical wet and dry climate (the wetter portions of 
                   that climate being occupied largely by tropical deciduous 
                   woodland). That would be the Aw climate in the Köppen 
                   system.
               vi. Accordingly, you can expect to find savanna in such places 
                   as southern central Brazil, northeast South America, Africa 
                   well south of the Sahara, East Africa, and much of southern 
                   Africa (the African savannas are the most extensive, and 
                   people have lived there the longest), southern and central 
                   India, interior Southeast Asia, and Australia some distance 
                   around the tropical desert there.

              

           b. Sudan, sometimes called tropical steppe.

               

                i. This is a tropical grassland, dominated by a continuous or 
                   discontinuous cover of short grasses, normally interspersed 
                   with a very discontinuous layer of small shrubs.  Both are 
                   generally from 15 cm to 1 m in height, so this is a pretty 
                   low-lying vegetation.
               ii. In good years, the grass cover is quite lush, forming a 
                   pretty continuous cover and attaining greater height; in 
                   dry years, the grass cover becomes very sparse and short, 
                   which brings the shrub component to visual dominance.
              iii. This vegetation type is associated with BSh climates in the 
                   Köppen system:  The tropical semi-arid or the tropical 
                   steppe climate.  As such, it can be seen as a nearly 
                   perfect transition from savanna to tropical desert.  In 
                   good years, it resembles the savanna; in bad, the desert.  
               iv. So, you can find it in a long, slender strip immediately 
                   south of the Sahara (often called the Sahel).  It is also 
                   found in southern Africa inland from the Namib Desert, in 
                   an area called the Kalahari Steppe (sometimes called a 
                   "desert").  It's also found in northern Australia, just 
                   north of the Australian desert but south of the 
                   savannalands there.
                v. The recurrent hazard here is drought.  The drier a climate 
                   is,  the less predictable is its precipitation.  So, if the 
                   ITCZ doesn't quite get to this vegetation one year (or 
                   several in a row), the sudan takes on a desert-like 
                   appearance.  Famine accompanies drought here.  Sometimes we 
                   see that human tendency to respond to low level, recurrent 
                   hazards by setting society up for the rarer high-magnitude 
                   event.  In the Sahel, international agencies have sunk 
                   boreholes to tap fossil water and give people a place to 
                   get water for their animals even in drought.  The problem 
                   is that cows aren't all that stupid:  They hang out around 
                   the boreholes, and their hooves crush and pulverize the 
                   soil, making it susceptible to wind erosion.  Their 
                   overgrazing kills the local grasses and shrubs.  The result 
                   is "desertification," the degradation of non-desert 
                   vegetation into something resembling the desert.  This, of 
                   course, makes the landscape even less able to support the 
                   human population, which becomes really apparent when the 
                   rains fail and marginal farmers and animal herders lose 
                   their few resources and starve by the dozens of thousands.
        2. The second two subtypes of the grassland biome are temperate ones.
           a. The first of these is tall-grass prairie.

              

                i. This consists of a variety of medium and taller grass 
                   species (1-2 m in height).  
               ii. These normally form a continuous, even a dense cover, 
                   though in dry years, you'll see a discontinuous cover 
                   dominated by the shorter of the local grass species.
              iii. There is a second layer of smaller forbs, which are very 
                   evident in the early spring before the taller grasses shade 
                   them out.  There is a lovely wildflower display at this 
                   time.
               iv. You will also see riparian or "gallery forests," too -- 
                   trees and shrubs and many forbs growing along the banks of 
                   rivers flowing through the prairie.  In late summer, these 
                   look like green snakes crossing the light green or beige 
                   grasslands.
                v. You occasionally see a tree or small grove of trees around 
                   a spring or seep, too.  Otherwise, the prairie is utterly 
                   treeless.
               vi. This prairie is associated with the moister sides of the 
                   BSk climate, Köppen's cold semi-arid climate.  You can 
                   also find it on the drier side of the humid subtropical and 
                   humid continental climates and the west coast marine 
                   climate. This is a transition from the lusher vegetation of 
                   these climates to the steppes and deserts beyond.  In fact, 
                   there is a lot of evidence that human firing may, in fact, 
                   have created this prairie or extended its coverage beyond 
                   any natural distribution.  When fires are suppressed, 
                   succession results in the replacement of prairie by 
                   temperate broadleaf deciduous and mixed forest!
              vii. American prairie extends from eastern Kansas, Arkansas, 
                   eastern Oklahoma, Missouri, eastern Nebraska, and the 
                   eastern Dakotas, into southern Saskatchewan. The South 
                   American prairie is the pampas of Argentina, while in 
                   eastern Europe, we have the Hungarian puszta.
             viii. Not surprisingly, the great natural hazard here is prairie 
                   fire.  It does renew the prairie by cleaning debris off the 
                   ground, killing a lot of disease organisms, and creating 
                   fertilizer.  So, again, mimicking nature, Native Americans 
                   and other human occupants of such grasslands have regularly 
                   torched them.
           b. Steppe or shortgrass prairie.

               

                i. This grassland is dominated by short to medium grass 
                   species (15 cm to maybe a meter in height). 
               ii. They usually form a discontinuous cover, ranging from 
                   pretty continuous in good years (which also favors the 
                   taller of the local species of grass) to discontinuous in 
                   dry years (which favors the shorter of the local grasses).
              iii. Low shrubs are usually present also, forming a very 
                   discontinuous cover, which is more apparent and visually 
                   dominant during dry years.  In this, the steppe strongly 
                   resemles the sudan.
               iv. Steppe is associated with the temperate steppe or temperate 
                   semi-arid climate (the BSk climate in the Köppen 
                   system discussed in lecture 20).
                v. It is transitional from prairie to temperate desert in 
                   character, sometimes looking more desert-like and other 
                   times looking more like lush prairie.
               vi. You can find it on the American Great Plains (eastern 
                   Colorado; western Kansas, Nebraska, and the Dakotas; 
                   Wyoming; Montana; southern Alberta; northern Texas; and 
                   western Oklahoma); the northern Basin and Range (southern 
                   Idaho and northern Nevada, parts of eastern Washington and 
                   Oregon; parts of the Great Central Valley of California); 
                   the Russian and Ukrainian steppes, extending into 
                   Transcaucasia, southernmost Siberia, and parts of N. China; 
                   parts of Turkey and Iran; central Spain; southeastern and 
                   southwestern Australia just south of the Australian desert.
                   
                   

                   

Well, that takes care of the world's biomes.  In this lecture, we reviewed 
four vegetation formations dominated by grasses.  There were two tropical 
grasslands, namely, savanna and sudan, associated with the drier tropical wet 
and dry climate and the tropical semi-arid climate, respectively.  There were 
also two temperate grasslands, prairie and steppe.  Both are associated with 
the temperate semi-arid climate, but the former also extends into the drier 
parts of the humid subtropical (Cfa), west coast marine (Cfb), and humid 
continental (Dfa and Dfb, especially).

So, in all, this week we reviewed three major groups of biomes:  those 
dominated by trees, those dominated by shrubs, and those dominated by grasses. 

Throughout the biosphere section, we've looked at the biosphere, including the 
environment of organisms. The environment (both the physical environment and 
the biotic environment of other organisms) provides the selective forces
operating to shape various kinds of life in various situations. These 
different kinds of life can be classified according to their evolutionary 
relations (genetic classifications). They can also be classed according to the 
structures and functions imposed on each by the selective forces in their 
environment, hence, the life form and functional classifications. The 
assemblages of living things can be further classed into biomes, by the plant 
life forms dominant in a particular association. Biomes and their subtypes
are sent to reflect broadly the climatic (and sometimes soil) conditions
of their environments, so that's why I related each to one of the Köppen 
climatic categories.

Document and © maintained by Dr. Rodrigue
First placed on web: 11/06/00
Last revised: 07/02/07