Plant life form classification sensu Raunkaier (1934) is a simple but still useful way of functionally classifying plants. A more detailed way is given under 2.3. Growth form. Robert Whittaker described life form classification as follows (Whittaker 1975, with our modifications): “...instead of the mixture of characteristics by which growth forms are defined, the relation of the plant's perennating tissue to the ground surface. Perennating tissue is what enables the plant to survive harsh, unfavorable (cold or dry) seasons. For the purposes of this classification, perennating tissue comprises not only buds (including those of specialized perennating organs mentioned in 2.1. Life history and maximum plant lifespan), but also seeds. For perennial plants (as contrasted with annuals), its location relative to the ground surface is an essential feature of a plant’s adaptation to climate. The harsher the climate, the fewer plant species are likely to have perennating buds far above the ground surface, fully exposed to the cold or the drying power of the atmosphere”. Locating the perennating buds below the surface provides added protection from harsh-season conditions. For species that may be subject to disturbances, such as grazing and fire, the position of buds or bud-forming tissues allows us to understand the likelihood of their survival. The definitions given below refer to the highest perennating buds for any particular plant.
How to record?
Life form is a categorical trait assessed from field observation, descriptions or photos in the literature. Many floras give life forms as standard information on plant species. Five major life forms were initially recognised by Raunkaier; his scheme was further expanded by various authors. Here we give a simple, only slightly modified scheme (SM 1 - Fig. 1a-f).
a) Phanerophytes: plants that grow taller than 0.50 m and whose shoots do not die back periodically to that height limit
b) Chamaephytes: plants whose mature branch or shoot system remains below 0.50 m, or plants that grow taller than 0.50 m, but whose shoots die back periodically to that height limit
c) Hemicryptophytes: periodic shoot reduction to a remnant shoot system, such that the buds that produce new growth following the annual harsh season are located about at ground level
d) Geophytes: annual reduction of the shoot (and often also of the root) system to perennating organs located below the soil surface.
e) Hydrophytes: plants that grow in aquatic habitats, and whose perennating organs remain, during the harsh season, either in the water or (much more often) in the mud or soil at the bottom of that body of water. Aquatic plants that perennate from a bottom-dwelling organ such as a rhizome or tuber but whose shoots emerge, and display their leaves, above the water surface during the growing season, are termed emergent hydrophytes or helophytes (Greek helos, marsh, see f) Helophytes below).
f) Helophytes: plants that perennate under water from a bottom-dwelling organ such as a rhizome or tuber but whose shoots emerge, and display their leaves, above the water surface during the growing season.
g) Therophytes: plants whose entire shoot and root system dies after seed production, so perennate only through their seeds.
Bulb, corm and tuber-bearing plants, and those that perennate via subsurface rhizomes, are geophytes. These plus hydrophytes are collectively termed cryptophytes, because their perennating organs are hidden from aerial view. Prostrate subshrubs, perennial cushion and rosette forms (see 2.3. Growth form) and plants perennating by rhizomes that grow on the ground surface, are all hemicryptophytes. Dwarf shrubs, and most tussocks, would be classified as chamaephytes. Climbers and epiphytes would be classified, along with shrubs and trees, as phanerophytes since they normally extend, or occur, more than 0.5 m above the ground surface. Therophytes include not only annuals, but also monocarpic perennials (c.f. sec. 2.1. Life history and maximum plant lifespan) which, however, for the seasons prior to their seed reproduction, must fall into one of the other life-form categories.
References on theory, significance and large datasets: Raunkaier (1934); Cain (1950); Ellenberg and Müller-Dombois (1967); Whittaker (1975); Box (1981); Ellenberg (1988); Hickey and King (2000); Bell (2008).
Supplementary Material 1. References
Bell AD (2008) ‘Plant Form: an Illustrated Guide to Flowering Plant Morphology’. (Timber Press: Portland).
Box EO (1981) ‘Macroclimate and plant forms: an introduction to predictive modeling in phytogeography’. (Dr. W. Junk Publishers: The Hague).
Cain SA (1950) Life-forms and phytoclimate. The Botanical Review 16, 1-32.
Ellenberg H (1988) ‘Vegetation ecology of central Europe. 4th edition’. (Cambridge University Press: New York).
Ellenberg H, Müller-Dombois D (1967) A key to Raunkiaer plant life forms with revised subdivisions. Berichte des geobotanischen Institutes der ETH, Stiftung Rübel 37, 56-73.
Hickey M, King C (2000) ‘The Cambridge illustrated glossary of botanical terms’. (Cambridge University Press: Cambridge and London).
Raunkiaer C (1934) ‘The life forms of plants and statistical plant geography’. (Clarendon Press: Oxford).
Whittaker RH (1975) 'Communities and Ecosystems, 2nd edition' (Macmillan Publishing Co.: Inc., New-York)