Dienstag, 18. Dezember 2012

Different models of plant-classification

If you go out and look at the plants around you, you may notice, that they can vary greatly in their form and shape. Some of them are large trees, while other species are small, inconspicuous herbs. In Botany, the kind of growing is a very important, morphological attribute of plants and many models were developed to characterize them. Two of the most popular models are the classification by the growing form of the trunk (“Wuchsform” in German) and the “Raunkiaer Life-Form System”, which was developed by the Danish Botanist C. C. Raunkiaer (1860 -1938).

a Riparian Forests with many perennials and small trees

It's important to say, that a growing form or life form isn't specific to a plant family or genus. A species from a genus can grow as a tree, while another from the same genus can grow as shrub.

1) The characterization of plants by the growing form

As I mentioned before plants can grow in very different shapes. Some of them are small and filigree, while others are large and robust. The morphology is also the starting point of this system. Here, the form of the stem, the maximum height and the degree of lignification are crucial for the category, in which a plant is classified. The classic system distinguish between the following categories.

  • Tree: This is the first growing form, which includes the largest plants on earth. In Botany, a tree is defined as a plant whose main stem axis grows straight and upright. It must also be lignified. The average height is between 1,50 to 40 metres. Within this category, there are also some sub-category; based by the different morphology of the stem.
  • Tropical trees: These are the large trees of the tropical rainforests. They have long stems and sweeping canopy.
  • Broad leafed trees: This common trees are characterized by their wide leaves. Many trees like acer, birch, beech and oak belong to this category. This trees can be seasonal-green or evergreen.

    Acer pseudoplatanus - a seasonal-green tree
  • Conifers: conifers have slim, needle-like leaves (often simply referred as needles). Typical conifers are e. g. pine, fir, yew or cypress. In the most cases, conifers are evergreen but there are also some seasonal-green trees (like larch).

    Cryptomeria japonica - a large conifer
  • Mangrove: Mangroves are trees, which grows in the tropical tidal range. They often have special roots, which serve as stilts. In addition, the cavities within their stems are often filled with air (Aerenchym).
  • Bottle Tree: This trees have a very thick and broad stem, whose tissue serves as a kind of cistern. This trees can be found in the dry regions of Africa like the Savanna. An example for this category is the Baobab.
  • Tuft tree: Tuft trees are a very special form of a tree. They can be found in the mountainous regions of the tropics. Here, the climate is very extreme and we have dry, hot days and cold nights with snowfall. Basically,. a tuft tree is a rosette with a woody stem. The leaves of the canopy are arranged in a rosette, which protects the bud from cold

  • Shrub: The term “Shrub” refers to a plant with multiple, woody stems. Unlike a tree, a shrub hasn't a straight trunk, but several strains of the same order, branching from the same base. However, the boundary between these two definitions is fluent and partly dependent on environmental influences. For example, a beech can grow as a shrub with multiple stems, if it grows in an unfavorable environment (like mountains). 
     
    As for the trees, there are also different subtypes of the shrub. The most important ones are
  • Evergreen shrubs As the name suggest, these shrubs are evergreen and never lost all of its leaves at the same time. Common species from this type are Prunus lauraceus, Skimmia japonica or Rhododendron catawbiense.

    Skimmia japonica - a evergreen shrub
  • Seasonal-green Shrubs: These shrubs lost most of their leaves during a season or a dry period. An example for such plants are the shrub-like species from the large genus Hibiscus, which are very popular as ornamental plants in gardens.
  • Marquis: This are sub-tropical shrubs, which are very common in the Mediterranean regions. They have leathery leaves with a thick cuticula. This is an adaption to drought; caused by the hot summers in this region.

    H. canariense is an example for a marquis
     
  • Stem succulent: This type includes all of the large cacti, which you may known from classic western movies. They are as large as a small tree, but have multiple stems with a thick skin, which protects them dehydration.
Other plants, which are wooden but not trees are grass tree like bamboo and liana. However, these aren't real shrubs but an own, unique type of growing form, which should not be considered any further now.
  • Perennial plants: perennial plants are evergreen, non-wooden plants with a maximum height of 1,60 meters. Although not wooden, their stems are very sturdy and often thick. In the most cases, the stems die after a growing season but return the next year, what is also the major difference between annual or grasses (s. below). Perennial plants haven't to be large. For example, Taraxacum officinale L. (Dandelion) is also a perennial plant. There are also some subtypes within the perennial plants. The most important one are
  • evergreen perennial plants: green over the whole year. Typical species are Phlox panniculata L., Canna indica Ker-Grawl. or the Banana (Musa spec.)
  • Seasonal-green perennial plants: These plants are green only during a certain time of the year. Many common perennial plants belong to this category, e. g. Fallopia japonica (Houtt.) Ronse Decr., or Heracleum mategazzianum Somm. & Lev.

     Fallopia japonica- a seasonal-green perennial plant
  • Succulent perennial plants: Many succulent plants (small cacti) are perennial plants, which have adapted to extreme dryness. Examples are species from the Genus Agave or Bromelia.
     
  • Creeping perennial plants: These perennials creep other the floor. Typical plants are the species from the Genera Saxifraga, Geranium or the Common Ivy Hedera helix L. Creeping plants are very popular in gardens, because they are often evergreen and can be use as turf. 

 Potentilla reptans - a small, creeping plant
  • Grasses, annual and biannual plants: These plants have also no wooden stem. Unlike the perennial plants, they wilt after one (Grasses, annual plants) or two (biannual plants) growing seasons. In the most cases, such plants creates a lot of seeds to ensure their reproduction. These seeds are very resistant and can endure in the soil for other years. This is very common in desert plants, which must withstand for years sometimes.

    Verbascum speciosum - a biannual plant
It's important to say, that these growing types and sub-categories aren't the only one within the vegetable kingdom but only the (in my opinion) most important ones.

2) The characterization of plants by their Life-Form

The model of life-forms was developed by the Danish Botanist Cristen Christian Raunkiaer in 1905 and is one of the most common and popular models to describe the different growing types of plants. One reason for this is, that the model is very simple (much more simple than the system of growing forms).

a slope with many Therophytes, Chamaephytes and Hemikryptophytes

Raunkiaer took as a reference the location of the growing buds on a plant. The growing buds are important for the re-sprouting of a plant after an unfavorable period (like Winter or drying time).Over time, plants have therefore developed various ways to protect their buds, what resulted in a different morphology. These ways are also called life-forms. The classic life-forms after Raunkiaer are

  • Phanerophyte: Phanerophytes are plants, whose growing buds are located high over the ground level at the end of their shoot axis. Here, the buds are exposed to the elements, so they are often covered by sheds or other structures. The most trees and shrubs are Phanerophytes. Tropical trees are an exception because their buds aren't exposed to the elements, so they're also not protected (because it's simply unnecessary). This life-form is also divided in Macro-Phanerophytes (5 meters and higher) and Nano-Phanerophytes. (0,5 to 5 meters).
Planatuns x hispanicus - a Phanerophyte
  • Chamaephytes: Chamaephytes are small shrubs or perennials, whose buds are located a few meters about the soil. They are very common in regions with a harsh, cold winter (like the Tundra), because the snowfall acts as a insulating snow cover, which protects the buds. 

     Vaccinium vitaes-ideaea is a Chamaephyt
  • Hemikryptophytes: The growing buds of the Hemikryptophytes are located on the ground and are often protected by the leaves of a rosette. Many small, evergreen perennials are Hemikryptophytes. The low position of the buds has two advantages. Like the Chamaephytes, the Hemikryptophytes benefits from the insulating effect of the snow cover. In addition, the buds are better protected from wind. For this reason, Hemikryptophytes are very common in the arctic and sub-arctic regions, where the cold wind is a permanent threat for plants
Taraxacum officinale - a Hemikryptophyte
  • Geopyhtes: These plants survive the unfavorable period underground. The majority dies, while the growing bud survives in the soil as a bulb, a tuber or a rhizome. Geophytes are very common in Europe and belong to the first flowering plants of the year, because they sprout in late winter or spring in order to avoid the competition with the large Phanerophytes.
Allium schoenoprasum is a Geophyte
  • Helophytes: Helopyhtes (swamp plants) are similar to the Geophyte. However, their growing buds don't survive within the soil but in the mud. The Helophytes aren't part of the classic model of Raunkiaer and were added later.
  • Hydrophytes: The Hydrophytes are aquatic plants and the growing buds are located beneath water surface. The effect is the same as with the Geophytes and Helophytes. The Hydrophyte were also added later. Together with the Helophytes and Geophytes, the Hydrophytes are described as Kryptophytes, what means “hidden plants” because the buds are hidden beneath surface.
  • Therophytes: These are annual or biannual plants, which “survive” the unfavorable period as seeds while the mother plant dies completely. Therophytes are very common in the desert regions, where the tough seeds often wait years for rain.

    Impatiens glandulifera - an annual plant and a
     Therophyte

The model of Raunkiaer not only helps in the classification of plants, but also for the characterization of vegetation zones. All major vegetation zones (Tropical subtropical, desert, temperate broad and arctic) have their own characteristic composition of life forms, what is an adaption to the climatic conditions of the different zones. Such a composition is called “Life-Form” spectrum. Here are some examples for different spectra after Raunkiaer.

Tropical
P
CH
H
K
T
61
6
12
5
16

Sub-Tropical
P
CH
H
K
T
12
6
29
11
42

Desert
P
CH
H
K
T
12
21
20
5
42

Temperate broad
P
CH
H
K
T
10
5
50
15
20

Arctic
P
CH
H
K
T
1
22
60
15
2

All figures in percent;
P = Phanerophyte, CH = Chamaephyte, H = Hemicryptophyte, K = Cryptophyte, T = Therophyte.

As you can see, the spectra are a good way to characterize the different vegetation zones, because it's very logical. For example, the Arctic (and Antarctic) regions are well-known for a harsh, cold wind. So, here are not so many Phanerophytes because their buds are exposed to the wind.

a broad temperated forest

The Hemikryptophytes on the other side are well protected against the wind and so, Hemikryptophytes are characteristic for the flora of the arctic regions. If you go online and look for some pics about Spitzbergen, Tasmania or Iceland, you will see, that there are many Hemikryptophytes.

Thus, I arrived at the end of the article. I hope to give you an overview of the growing forms and life forms of plants and these have occurred not by chance. Of course, the article includes not all forms (there are also parasites and epiphytes). For reasons of simplicity, however, the basic shapes are sufficient.

Please note: The picture of Vaccinium vitis-ideae were taken by Christopher Schwerdt. I have his permission to use these pictures in my blog. Please also visit his homepage www.schwerdtfisch.net


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