Plant life forms and biological spectra

3.1.1 Variation in vascular plant and bryophyte life form

In addition to taxonomic classifications which endeavour to place closely related species in the same family, botanists have for centuries attempted to distinguish particular life forms, any one of which may be adopted by quite unrelated species. The simplest of these is the distinction between woody and herbaceous plants. Raunkiaer (1934) developed the most widely known scientific description of life forms, and then used it to initiate the use of biological spectra to compare different floras. The main feature of this ecologically valuable system is the position of the vegetative perennating buds or persistent stem apices during the cold winter or dry summer forming the unfavourable season of the year. The main life forms shown in Fig. 3.1 form a sequence showing successively greater protection from desiccation, indicating the position of the vegetative buds when the plant is dormant.

It was assumed that the flowering plants evolved when the climate was more uniformly hot and moist than it is now, and that the most primitive life form is represented by the phanerophytes which still dominate tropical vegetation. These large terrestrial plants can grow continually forming stems, often with naked buds, projecting high into the air. Those whose buds are protected from cold or desiccation by bud scales are considered to be more highly evolved. Tropical evergreens like the gum tree Eucalyptus orientalis lack the protective bud scales of evergreen phanerophytes of the temperate zone, such as holly Ilex aquifolium and Scots pine Pinus sylvestris. Ash and larch belong to a third group formed by deciduous phanerophytes with bud scales. These large plants are divided into four height classes: nanophanerophytes, woody plants with perennating buds between 0.25 and 2 m above the ground; microphanero-phytes, between 2 and 8 m; mesophanerophytes, between 8 and 30 m; and

Examples Therophytes
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Figure 3.1 Diagram of the chief types of life form, apart from the therophytes which survive the unfavourable season as seeds. 1, phanerophytes; 2-3 chamaephytes; 4, hemicryptophytes; 5-6, geophytes; 7, helophytes; 8-9, hydrophytes. Parts of the plant which die in the unfavourable season are unshaded; while the persistent axes and perennating buds are in black. The sequence represents an increasing protection of the surviving buds, which are most exposed in the phanerophytes. (From Raunkiaer, 1934. The Life forms of Plants and Statistical Plant Geography. Clarendon Press.)

megaphanerophytes of over 30 m. Using the two criteria of height and bud protection, Raunkiaer originally divided the majority of phanerophytes into 12 groups, but he also recognized others, such as the epiphytic forms (including many aroids and orchids) which often grow on the trees of tropical and subtropical forests.

Woodward (1989) considered that temperature is the most important factor controlling leaf type and duration in the major type of tree present in any area with adequate rainfall. Broadleaved evergreens tend to dominate from the equator to the Mediterranean region. Of these the holm oak Quercus ilex can tolerate the lowest temperatures, surviving brief periods at —15 °C, but none are present in northern forests. Broadleaved trees, such as pedunculate oak Quercus robur, that can survive minimum temperatures below —15 °C are almost all deciduous; many can survive —40 to —50 °C. Forests where minimum temperatures fall below —50 °C are dominated by needle-leaved conifers -typically pines, firs and spruces - of which a few (larches - Larix spp.) are deciduous. Even here a few broadleaved deciduous species of birch and poplar manage to survive.

Woody climbers such as ivy Hedera helix, honeysuckle Lonicera periclyme-num, Old Man's beard Clematis vitalba and the tropical lianas are specialized phanerophytes profiting from the stature of their neighbours. Mueller-Dombois and Ellenberg (1974, p. 449) give a more complex life form system using five main stem or trunk forms - normal woody trees, tuft trees, bottle trees, succulent and herbaceous stem trees - to subdivide the phanerophytes. Trees can also be classified as grading from pachycaul forms, including the tree ferns and palms, with thick, unbranched or little-branched main stems, bearing a terminal crown of large compound leaves, to 'twiggy', much-branched leptocaul species bearing smaller undivided leaves, such as the elms. Common ash Fraxinus excelsior, with its pinnate leaves and stubby twigs, has a tendency to the pachycaul habit, but no British tree has the massive frost-sensitive apical meristem of a true pachycaul. Perhaps the most well-known pachycaul is the baobab (Adansonia digitata) of the African savannas, with a trunk so massive that hollow specimens have been used as rooms and even a jail.

Chamaephytes are low-growing woody or herbaceous plants whose peren-nating buds are on aerial branches not more than 25 cm above the soil, and frequently much lower, where the wind is not so strong and the air is damper (Fig. 3.2). Perennating buds of hemicryptophytes are at the surface of the soil where they are even better protected, while those of geophytes are buried beneath the soil on rootstocks, rhizomes, corms, bulbs or tubers. Therophytes survive unfavourable periods as seeds, being abundant in deserts and open habitats. Common in the early stages of reversion of bare land to scrub, they become rarer as it progresses to mature woodland.

Life form is essentially an adjustment of the vegetative plant body and plant life history to the habitat; it is primarily determined by heredity and selection. Under some circumstances the environment directly influences life form, as when severe winter or other conditions kill the upper buds so that individual plants fall into the life form below that normal to the species, resulting in the dwarfing of trees at high altitudes or in almost constant wind (see Section 3.5). Conversely, stinging nettle Urtica dioica, normally a hemicryptophyte, may overwinter as a herbaceous chamaephyte in mild winters.

Fine divisions in life form can also be categorized using leaf size. Leptophylls, with an area of up to 25 mm2 per leaf, were the smallest of the six sizes of leaf used by Raunkiaer (1934). The upper area limits of the next four members of the series - nanophyll, microphyll, mesophyll and macrophyll - increase by a factor of nine in each instance. The largest leaves (megaphylls) all exceed a nominal value of 164 025 mm2 in area, equivalent to a leaf about 40 x 40 cm. The tropical aroid genus Monstera (climbing shrubs often maturing as epiphytes with aerial roots reaching the soil, and sold as houseplants under the name 'Swiss cheese plants') has megaphylls which have gaps or rounded holes when mature. Leaves tend to be large in the hot wet tropical rain forests, medium-sized in temperate

Bluebells Perennating Buds Location

Figure 3.2 Seedlings of two woodland chamaephytes (a) wood-sorrel Oxalis acetosella and (b) yellow archangel Lamiastrum galeobdolon. The seeds of these species appear to require a period of chilling to break dormancy and are difficult to germinate experimentally. In nature considerable numbers of seedlings often appear together in early spring. Very occasionally Lamiastrum seedlings have three cotyledons. Wood-sorrel can also exist as a rhizome geophyte or a rosette hemicryptophyte. Bluebell, an example of a bulbous geophyte, is shown in Fig. 10.9. (Drawn by John R. Packham. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 2.2. With kind permission of Springer Science and Business Media.)

Figure 3.2 Seedlings of two woodland chamaephytes (a) wood-sorrel Oxalis acetosella and (b) yellow archangel Lamiastrum galeobdolon. The seeds of these species appear to require a period of chilling to break dormancy and are difficult to germinate experimentally. In nature considerable numbers of seedlings often appear together in early spring. Very occasionally Lamiastrum seedlings have three cotyledons. Wood-sorrel can also exist as a rhizome geophyte or a rosette hemicryptophyte. Bluebell, an example of a bulbous geophyte, is shown in Fig. 10.9. (Drawn by John R. Packham. From Packham et al., 1992. Functional Ecology of Woodlands and Forests. Chapman and Hall, Fig. 2.2. With kind permission of Springer Science and Business Media.)

woodlands, and small in the cold or dry conditions of tundra and heaths. In tropical rain forests at least 80% of the species of trees present have leaves of the mesophyll class (2025-18 225 mm2): most are also unlobed sclerophylls ('hard leaves') with pronounced drip tips. Microphylls predominate in montane rain forests where the climate is cooler. This gives some justification for using a leaf size spectrum, based on percentages of the different leaf sizes present, to characterize different vegetation types. However, light intensity and soil conditions, particularly available nitrogen and phosphorus, also have an important influence on leaf size even within the same genotype.

Life form systems can also be of use with lower plants, indeed Gimingham and Birse (1957) developed a very useful life form classification for bryophytes. This involved five main types: cushions (dome-shaped) e.g. Leucobryum

MM M N Ch H G Hel Th (a) Relevé 1 (18) Hàllmarkstallskog

MM M N Ch H Hel (b) Relevé 2 (18) Hàllmarkstallskog

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Responses

  • Mauri Tikkanen
    What are the subdivisions of hemicryptophyte?
    6 years ago
  • Gherardo
    What is perennating buds?
    2 years ago

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