2.3. Growth form

Growth form is mainly determined by the direction and extent of growth, and any branching of the main-shoot axis or axes. These affect canopy structure, including its height, and both the vertical and horizontal distribution of leaves. Growth form may be associated with ecophysiological adaptation in many ways, including maximising photosynthetic production, sheltering from severe climatic conditions, or optimising the height and positioning of the foliage to avoid or resist grazing by particular herbivores, with rosettes and prostrate growth forms being associated with high grazing pressure by mammals.

How to record?

Growth form is a hierarchical trait assessed through field observation or descriptions or figures or photographs in the literature. Because we are classifying types along a continuum, intermediate forms, between the categories recognised here, may be encountered, as well as occasional unique forms lying outside any of these categories.

    (A) Terrestrial, mechanically and nutritionally self-supporting plants

(1) Herbaceous plants have either no or at most modest secondary growth, with stem and root tissues that are rather soft compared with typical wood.
 
(a) Rosette plant. Leaves concentrated on a short, condensed section of stem or rhizome (see Category C under Section 2.5 for a definition of rhizome), at or very close to the soil surface; with an inflorescence (or single-flower peduncle) bearing either no or reduced leaves (bracts) produced from the rosette axis, above ground level. Graminoids whose principal photosynthetic leaves are attached to the base of their aerial stems (e.g. ‘bunch grasses’) fall in this category.
 
(b) Elongated, leaf-bearing rhizomatous. The permanent axis is an elongated rhizome that directly bears photosynthetic leaves that extend individually up into the light. The rhizome can be located either at or below ground level (e.g. Pteridium aquilinum (bracken fern), Viola spp., Iris spp.), or (epiphytes) on an above-ground support such as a tree branch. Aerial inflorescences (or single-flower peduncles) with either reduced leaves (bracts), or none, may grow out from the rhizome.

(c) Cushion plant (pulvinate form). Tightly packed foliage held close to soil surface, with relatively even and rounded canopy form (many alpine plants have this form).

(d) Extensive-stemmed herb develops elongated aerial stem(s) whose nodes bear photosynthetic leaves that are distributed nearly throughout the canopy of the plant, except when shed from its more basal parts during later growth, and lacking in distally developed inflorescences. Graminoids (rhizomatous or not) with leafy aerial stems fall here.

(e) Tussock. Many individual shoots of a dense colony or clone grow upward, leaving behind a tough, mostly dead supporting column topped by living shoots with active leaves (e.g. the Arctic cotton grass, Eriophorum vaginatum).

(2) Semi-woody plants. Stem without secondary growth but often toughened by sclerification (or, alternatively, with relatively feeble, soft or ‘anomalous’ secondary growth).

(a) Palmoid. Bears a rosette-like canopy of typically large, often compound leaves atop a usually thick (‘pachycaulous’), columnar, unbranched or little-branched stem (e.g. palms (Pandanus), tree ferns). Certain tropical or alpine Asteraceae such as Espeletia spp., cycads, Dracaena, arborescent Yucca spp. and some Bombacaceae can be regarded as having this growth form, although their stems undergo more extensive secondary growth (see also ‘Corner model’ within references below).

(b) Bambusoid. An excurrently branched (cf. Point A.3.d.i in the present Section) trunk lacking or having only weak secondary growth is stiffened by sclerification to support a vertically extensive, sometimes tree-sized canopy (bamboos; various tall, herbaceous dicots such as Chenopodium, Amaranthus and Helianthus).

(c) Stem succulent. A usually leafless photosynthetic stem with extensive, soft, water-storage tissue and only limited secondary growth (cacti, and cactoid plants of other families; most leaf succulents fall instead into one of the subclasses of Points A.1 or A.3 in the present Section).

(3) Woody plants develop extensive, usually tough, secondary xylem and phloem from vascular cambium, and corky outer bark from cork cambium (woody vines are covered in Point B.3 of the present Section).

(a) Prostrate subshrub. Long-lived woody stem growing horizontally at ground level (examples include many Arctic willows and ericoids).

(b) Dwarf shrub, or subshrub, with usually multiple, ascending, woody stems less than 0.5 m tall.

(c) Shrub. Woody plant between 0.5 m and ~5 m tall, with canopy typically carried by several trunks that are usually thinner and younger than typical mature tree trunks.

(d) Tree. Woody plant usually >5 m tall, with main canopy elevated on a long-lived, substantial, usually single (but upwardly branching), trunk. (i) Excurrent. Single main axis (trunk) extends up to, or almost to, the top, with shorter, ascending or horizontal branches giving a conical or (in mature trees) columnar form to the crown. (ii) Deliquescent. Trunk divides, somewhere above its base, into two to several, more or less equal branches that continue branching upward to produce a wider, more flat-topped crown.

(e) Dwarf tree. Morphology as in one of Types (i) or (ii) above, but substantially <5 m tall. Many forest understorey trees, but also in various climatically or nutritionally challenging, unshaded habitats, such as ‘pine barrens’, semi-deserts, certain tropical cloud forests, bogs and near-timberline vegetation.
 
    (B) Plants structurally or nutritionally supported by other plants or by special physical features
 
(1) Epiphyte. Plant that grows attached to the trunk or branch of a shrub or tree (or to anthropogenic supports) by aerial roots, normally without contact with the ground (e.g. many tropical orchids and Bromeliaceae).

(2) Lithophyte. Plant that grows in or on rocks (e.g. many species of ferns, species of Nepenthes, Utricularia forestii, Cymbalaria muralis).
 
(3) Climber. Plant that roots in the soil but relies, at least initially, on external support for its upward growth and leaf positioning.

(a) Herbaceous vine. Usually attaches to its support either by twining or by means of tendrils.

(b) Woody vine, including liana. Often attaches to a support by means of aerial roots.

(c) Scrambler. Grows up through a sufficiently dense canopy of other plants, without any means of attachment (e.g. Galium spp.).

(d) Strangler. May start epiphytically (but become soil-rooted) or by climbing from ground level. However, by secondary growth, it later becomes self-supporting, and may eventually envelope the initially supporting stem (e.g. certain tropical Ficus spp.).

(4) Submersed or floating hydrophyte. Herbaceous, aquatic plant that relies on surrounding water for physical support. (Emergent hydrophytes (‘helophytes’) mostly fall into one of the subgroups of Point A.1 in the present Section.)

(5) Parasite or saprophyte obtains important nutritional needs directly or indirectly from other vascular plants (parasite) or from dead organic matter in the soil (saprophyte) (see Nutrient uptake in Material S2 where other more specific forms of parasitism are covered).
 
References on theory, significance and large datasets: Cain (1950); Ellenberg and Müller-Dombois (1967); Whittaker (1975); Barkman (1988, and references therein); Rundel (1991); Richter (1992); Box (1996); Ewel and Bigelow (1996); Cramer (1997); Lüttge (1997); Medina (1999); McIntyre and Lavorel (2001).

More on methods: Barkman (1988, and references therein).