Since also entire plant societies are to be modeled later in this book, In this chapter we are also concerned with the description of vegetation. The vegetation geography is the field that is occupied with the evolution of plant communities. Areal science is a part of vegetation geography that describes the spatial distribution of plants. Generally, we speak here of spatial patterns that are summarized in a spatial map.
The development of vegetation is determined by two factors: firstly by ecologic factors that describe the existing conditions of the habitat at the time of observation; secondly by floral evolution, stemming from the development of the earth’s history. Regardless of these factors, different plants can be found in areas with similar climatic and geological conditions. Usually the reason is that it was not possible for plants from one area to migrate to other areas due to large distances or adverse conditions.
The observation that in different continents different plant families and vegetation types are dominant, is not new. Alexander von Humboldt tried to systematize the different factors that influence the development of flora and fauna, though he did not create a world-wide taxonomy. This was done in the 19th century. In 1922, the Swedish botanist G. Turesson (1892-1970) furnished evidence of the existence of stable varieties occurring in nature by showing that species consist of genetically different populations (also called ecotypes) subject to selection due to the environmental conditions in their respective habitats.
In taxonomy, two different approaches are employed: firstly, the floral taxonomy, which divides the earth into floral regions and floral kingdoms, according to plant families unique to the endemic species, i. e., the local habitats; secondly, vegetation taxonomy, which proposes physiognomic-defined units, often in combination with ecological characteristics.
Schroeder  differentiates between six major floristic kingdoms with a total of 38 floristic regions; Richter  applies a division of 40 regions, according to combinations of regional differentiations introduced by Mattick  with 43 regions; and 35 regions as suggested by Takhtajan . So far the division into six floristic zones seems to have been predominantly accepted: the Holarc- tic and Holantarctic floristic kingdoms span from the outer edges of the polar
Chapter 2 regions to the tropics, where the neotropical floristic kingdom encompasses Plants the South American region, and the Palaeotropical floristic kingdom encompasses Africa and the Pacific region. Australia presents its own floristic kingdom, while the Cape-region in South-Africa is a spectacular exception with its own unique concept of 8,500 flowering plants and 6,500 endemics . vegetation division ^ As we already mentioned, the science of vegetation division, in contrast to floristic science, is oriented toward physiognomic aspects, and thus concerns itself with the appearance of a landscape. Here the predominant terms, such as forest or semi desert are old, and only during recent decades they were accurately defined using the term formation. This will be discussed in more detail in Sect. 2.9.
The substantial ecological factors for the scientific vegetation classification of a landscape are the climate factors temperature and water. Accordingly, in 1986 Oskar Drude divided the earth into six vegetations zones. Today, however, usually seven zones are referred to.
In the equatorial area, we find the tropical vegetation zone, which is essentially characterized by the absence of frosts. Climatically it forms the most favorable zone with an abundance of vegetation. The zones of temperate deciduous forests are located between the tropical and the Arctic and Antarctic zones. However, these areas are not uniform, and in the Northern Hemisphere the zone is divided into three zones. The meridian zone forms the southernmost range and borders on the tropics and on expanded desert areas like the Sahara. Here temperate, evergreen deciduous forests prevail. The nemoral vegetation zone, situated in the north, is characterized by deciduous, broad-leaf forests. Further to the north, evergreen coniferous forest is dominant: the boreal zone. Towards the poles, the temperature sinks to a point that does not allow plants to maintain a metabolism, which is essential for the development of tree structures. These zones are called the Arctic and Antarctic tundras.
The Southern hemisphere is less differentiated. Here the temperate evergreen laurel forest stretches up to the Antarctic forest border, and unites into only one zone, the Australian vegetation zone. Certainly there are different opinions; other authors use a larger or smaller number of vegetation zones. areal (definition) ^ On this abstract level, both in the floristic and in the vegetation scientific classification, regions are usually only typified by relatively rough descriptions of domestic plants. Little is stated about the quantitative distribution of the plants in both cases. Also regional characteristics do not play a role on this level, which can be seen from the fact that in both classifications Central Europe is treated as a uniform zone. A regional differentiation happens only if an area is subdivided more finely into so-called areals.
According to Richter  one speaks of an areal when a larger number of plant groups have a similar basic structure. Related groups are identified as floristic or geoelements. Schroeder differentiates areals as follows:
Areas that are defined by geographical conditions. Related species, e. g.,
families or groups, are termed geoelements. For example, the red beech and the sycamore maple, together with other related species, represent the Central European areal type, which is one of the parts of the Holarctic floristic region.
■ Biographical units
Units, which are also termed floristic areal types, denote the natural vegetation zones.
■ Areas with ecological characteristics
Often identification of a type is determined based on ecological principles. Meusel  uses a mix of ecological and biogeographical criteria. He uses vegetation zones and merges them with the criterion oceanicity, an ecological characteristic, which is determined by the humidity and the temperature amplitudes of the climate.
By the nature of the description methods, the definition of areal types includes regional specifics, such as the Alpine areals. However, little quantitative information with regards to plant population are at hand at this time, and would also not be of any significance for the purpose of this book. If needed, a division of areal types is possible by comparing general quantitative values of selected plants in given test regions.
Occasionally, we will later speak about ecosystems. While in the classification used by Richter the term does not play a separate role, and instead areals are considered to be spatial patterns on a larger or smaller scale, in  it is proposed that the term has to be viewed as an independent habitat for plants and animals. An ecosystem can extend from a few square meters up to many square kilometers. Schroeder  speaks in this connection simply of vegetation, which is conclusive as the term ecosystems includes all kinds of organisms, but here we are only concerned with vegetation itself.
Schroeder refers to the term “vegetation” based on integrated effects of environmental factors on classified cohabitation of plant families in the respective habitat, which at the same time indicates characterization of vegetation in a local framework. In the following, we often speak of vegetation in a local environment; we will use the term ecosystem as a synonym for our purpose.
In computer graphics, such ecological systems have their own place in that on this level plants still must be modeled and visualized as individual objects, in order to receive a realistic general impression. If larger areas are to be shown, work without geometrical abstraction is no longer possible. For example, entire plant communities must here be rendered as precomputed images in the context of an aggregate scene, because their detailed geometrical description is too complex. For the computer-graphical visualization of such an area the dominant structures must be converted into mathematically precise plant distributions. Additionally, the conditions at the borders, where special interactions between the plants of the respective areas take place, must be considered.