Raunkiær's plant life forms1

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The diagram below2 was originally published in 1907 by the Danish plant ecologist Christen C. Raunkiær. It summarizes his insights into the way in which the location of the shoot apical meristem (-s) that is (are) responsible for a plant's renewal growth reflects the prevailing conditions under which the plant grows and to which it is adapted. In this diagram the plant parts that persist from year to year are shown in solid black, while those parts that typically fall off (leaves, flowers, fruits) or die back (leafy shoots, inflorescences) are shown in outline.

 

Raunkiaer life forms Wikipedia image

The life form classes enumerated in the diagram at the left that have been recognized by Raunkiær and others comprise

  1. Phanerophytes - Shoot apical meristems are borne more than 25 cm above the soil surface. Examples of phaneropytes are thus all trees and a great many shrubs, including primarily tropical, arboreal monocots such as palms. Mechanically dependant species (see below) that remain rooted in the soil, such as climbers, lianas, or vines, could be considered phanerophytes. Arguably, however, mechanically dependant species that are not rooted at ground level, warrant a class of their own, i.e. should be classified as epiphytes (see below).
  2. Erect Chamaephyte - Shoot apical meristems are borne less than 25 cm above the soil, and produce erect shoots and (or) inflorescences.
  3. Prostrate Chamaephyte - Shoot apical meristems are borne less than 25 cm above the soil surface, and produce prostrate shoots and (or) inflorescences.
  4. Hemicryptophytes - Shoot apical meristems are borne at or near soil level, as in biennial and perennial rosette plants, like Daucus, Taraxacum, or Verbascum. As indicated in the diagram, the above-ground portions of these plants die back, after flowering and (or) during adverse climatic conditions
  5. Cryptophytes - Geophyte with rhizome - Shoot apical meristems are borne below the soil level, at the tip of a more or less elongate underground stem, or rhizome. These apical meristems may grow monopodially and produce lateral inflorescences that form the above-ground, photosynthetic portion of the plant (see note at left). Alternatively, the onset of flowering may cause the rhizome to become erect and finish off in a photosynthetic inflorescence. In this case a lateral branch continues the (sympodial) growth of the rhizome.
  6. Cryptophytes - Geophyte with bulb - Shoot apical meristems are borne below the soil level, at the center or tip of a compact underground stem (corm or bulb, respectively, depending on whether the stem or the leaf bases become enlarged for storage of nutrients and water). Examples include Crocus and Allium, respectively.
  7. Cryptophytes - Helophyte - Much as in geophytes, shoot apical meristems are borne below the soil level, but under water. These apical meristems give rise both to indeterminate growth of the underwater shoot system and to an emergent, determinate, aerial inflorescence.
  8. Attached Hydrophyte - The shoot system is entirely underwater. Leaves are underwater, or float on the surface. Only the inflorescence may emerge above the water surface.
  9. Free-floating Hydrophyte - The entire plant may exist within the water column, or on the surface of the water, with no connection to the bottom of the water body. Only the inflorescence may emerge above the water surface.
  10. (not shown) Epiphytes (incl. Aerophytes) - These have also been termed "mechanically dependent species," and as such differ from phanerophytes in depending on other plants or on a more or less erect substrate, for their support. The entire plant may exist above ground, in some cases obtaining moisture directly from the atmosphere (aerophytes, e.g. Tillandsia)
  11. (not shown) Therophytes - Shoot apical meristems persist during unfavorable climatic conditions only within seeds. Annuals and desert ephemerals are examples of therophytes.

Inevitably there are exceptions to this classification, such as the genus Ruscus (Asparagaceae or Ruscaceae; formerly included in the Liliaceae). Species of Ruscus are rhizomatous geophytes, in which the above-ground inflorescence is persistent over several years. Flowers and fruits are borne on flattened inflorescence branches of determinate growth that resemble leaves (phylloclades; see illustration HERE). While the individual flowers and fruits fall off, the apical meristem that produces them remains active over the several years over which an individual, determinate inflorescence persists. In this way one could argue that Ruscus is both a geophyte (year-to-year persistence of the rhizome and its underground growing point) and chamaephyte (year-to-year persistence of the phylloclade-borne flower-producing inflorescence growing points).

 

 

Note how, in the descriptions of the life form classes at the right, the above-ground portions of hemicryptophytes and cryptophytes can be referred to as inflorescences, since typically the reproductive structures of these plants are only produced by these above-ground axes. Descriptions of the aerial stems and leaves of these plants are properly ones of the inflorescence axes and the leaves that are modified by the flowering process (bracts, etc.). It is important for writers of popular accounts, like field guides, to remind their readers of the underground stems with which these plants are associated.

Only in the case of therophytes is it critical to refer to the location of the shoot apical meristems that enable these plants to persist during unfavorable environmental conditions. In these plants, once the seed germinates shoot apical meristems may be found anywhere (below ground, at the soil surface, or above ground), but will not persist after condirions become too harsh for continued growth.

1. Image from Wikimedia Commons, accessed on 6-Sep-2010 at http://en.wikipedia.org/wiki/File:Raunkiaer1907-life_forms-small.jpg.

2. Anonymous 2010. Raunkiær plant life-form - Wikipedia, the free encyclopedia. Accessed on 6-Sep-2010 at http://en.wikipedia.org/wiki/Raunkiær_plant_life-form. In contrast with the Wikipedia article, note the focus on morphology in the descriptions of the life form spectra here. Raunkiær's life form classes can be recognized morphologically regardless of climatic conditions, in the wet tropics as well as in highly seasonal environments. For this reason, an emphasis on the position of resting (e.g. overwintering) buds can be misleading, other than in the case of therophytes, as described above. The significance of a morphological emphasis in characterizing life form classes is discussed below.

 

Plant life form spectra

Floristic Life Form spectrum

The locations of shoot apical meristems within a vegetation unit, and hence representation of the life form classes among the species that are present in that vegetation unit, may characterize the vegetation in a manner that reflects prevailing environmental conditions. In all except some typically tropical phanerophytes and chamaephytes, shoot apical meristems are protected by leaves that are modified as scale leaves or bud scales. These scale leaves and bud scales can provide protection from harsh environmental conditions. Locating the shoot apical meristems required for perennation of the plant at or below the soil level can provide additional protection from environmental insult. Shoot apical meristems located below water level may enjoy similar benefits.

For these reasons different environmental conditions may select against some life form classes more than others. Geophytes may be favored over phanerophytes in situations where fire, grazing, or freezing temperatures tend to kill or remove above-ground biomass. Conversely, the year-round availability of moisture may allow phanerophytes (and epiphytes) to dominate the vegetation.

Life form spectra are frequency histograms for the representation of the life form classes in a given vegetation unit. They may be calculated in terms of the number of species, or in terms of some measure of species abundance. The two life form spectra at the left are from a paper3 that compares three alternative formulations. The two seen here are recommended: the floristic life form spectrum and the vegetation life form spectrum (note the contrast between them, observed for the same Cerrado site). The floristic life form spectrum is based on counting the numbers of species in each life form class. The vegetation life form spectrum is based on counting the numbers of individuals of belonging to each life form class.

 

Vegetation Life Form spectrum 3. Batalha, M. A. & Martins, F. R. (2004). Floristic, frequency, and vegetation life-form spectra of a cerrado site. Brazilian Journal of Biology 64 (2). Online version accessed 6-Sep-2010 at http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1519-69842004000200004
 
 


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