2.5. Clonality, bud banks and below-ground storage organs

Clonality is the ability of a plant species to reproduce or regenerate itself vegetatively, thereby producing new ‘ramets’ (above-ground units) and expanding horizontally. Clonality can give plants competitive vigour and the ability to exploit patches rich in key resources (e.g. nutrients, water, light). Clonal behaviour may be an effective means of short-distance migration under circumstances of poor seed dispersal or seedling recruitment. Clonality also gives a plant the ability to form a bud bank, which can be a very important determinant of recovery and persistence after environmental disturbances. The bud bank consists of all viable axillary and adventitious buds that are present on a plant and are at its disposal for branching, replacement of shoots, regrowth after severe seasons (winter, dry season, fire season), or for vegetative regeneration after injury (adventitious buds that arise after the injury, which are an important means of regeneration in some plants, apparently lie outside the ‘bud bank’ concept). Both the characteristics of the bud bank and the type of clonal growth exhibited by plants determine their ability to recover from disturbances (see Material S3 for a protocol for Characterisation of the bud bank, based on Klimeš and Klimešová 2005). Clonal organs, especially below-ground ones, also serve as storage and perennating organs; a sharp distinction between these functions is often impossible.

How to collect and classify?

For above-ground clonal structures, observe a minimum of five plants that are far enough apart to be unlikely to be interconnected, and that are well developed. For below-ground structures, dig up a minimum of five healthy-looking plants (Appendix 1). In some cases (large and heavy root systems), partial excavation may give sufficient evidence for classification. It is best to assess clonality and bud banks near the end of the growing season. Remove the soil and dead plant parts before counting buds or classifying the organs. The species is considered clonal if at least one plant clearly has one of the clonal organs listed below (see References below in the present Section for discussion).

Categories are then

    (A) clonal organs absent;
   
    (B) clonal organs present above ground, including the following:

(1) stolons – specialised, often hyper-elongated horizontal stems whose axillary bud growth and nodal rooting yields ultimately independent plants (e.g. strawberry (Fragaria vesca), saxifrage (Saxifraga flagellaris));

(2) bulbils – deciduous, rooting bulblets produced from axillary or what would otherwise be flower buds, or by adventitious bud growth on leaves (e.g. Cardamine pratensis, Bryophyllum); analogous vegetative propagules of bryophytes are termed gemmae; and

(3) simple fragmentation of the vegetative plant body (mostly aquatic plants, and bryophytes); and
   
    (C) clonal organs present below ground, including the following:

(1) rhizomes – more or less horizontal, below-ground stems, usually bearing non-photosynthetic scale leaves (e.g. many grasses and sedges), and sometimes instead bearing photosynthetic leaves that emerge above ground (e.g. Iris, Viola, bracken fern (Pteridium)); aerial, vegetative and/or reproductive shoots grow up from axillary (or sometimes terminal) buds on the rhizome; most rhizomes can branch, after which decline and decay of the portion proximal to the branch point yields independent, clonally generated individuals;

(2) tubers and turions – conspicuously thickened, below-ground stems or rhizomes, functioning as carbohydrate storage organs and bearing axillary buds, that can propagate the plant (e.g. potato Solanum tuberosum, Jerusalem artichoke (Helianthus tuberosus)); similar organs formed on aquatic plants are termed turions;

(3) bulbs – relatively short, below-ground stems that bear concentrically nested, fleshy scale-leaves that act as storage organs, the whole globose structure serving to perennate the plant and, through growth of axillary buds within the bulb into daughter bulbs or ‘offsets’, to multiply it vegetatively (e.g. tulip (Tulipa), onion (Allium));

(4) corms – vertically oriented, globosely thickened underground stems that serve as storage organs and bear either scale or foliage leaves; axillary or terminal buds on the corm function for perennation and to a limited extent for clonal reproduction (e.g. Dahlia);

(5) tuberous roots – thickened roots that serve primarily for storage but can form adventitious buds that permit clonal propagation (e.g. sweet potato (Ipomoea batatas));

(6) suckers – shoots developed from adventitious buds produced on ordinary, non-storage roots (e.g. aspen (Populus tremuloides), wild plum (Prunus spp.)); the sucker shoots can become independent plants once the root connection between them and the parent is severed or dies;

(7) lignotuber – a massive, woody expansion just below the ground surface, produced by secondary growth of the ‘root crown’ in many shrubs in fire-prone vegetation; after a fire that kills the shrub’s aerial canopy, adventitious buds on the lignotuber grow out to regenerate the shrub’s canopy (see Section 6.6), normally not resulting in clonal multiplication; and

(8) layering – ordinary vegetative shoots that lie on or bend down to the ground, there produce adventitious roots and continue apical growth, becoming independent plants when their connection with the parent is severed (e.g. blackberry and raspberry (Rubus), certain spp. of spruce (Picea) and hemlock (Tsuga)).

If a plant species has clonal growth (Categories B or C above in the present Section), classify it according to one or more of the following categories:

(1) regenerative clonal growth, occurring after injury and normally not multiplying the number of individuals, as with resprouting from a lignotuber;

(2) additive (also termed multiplicative) clonal growth, which can be either the plant’s normal mode of multiplication or can be induced by environmental conditions such as high nutrient availability, and serves to promote the spread of the plant;

(3) necessary clonal growth is indicated when clonality is required for the year-to-year survival of the plant, as with many plants that perennate from rhizomes, bulbs, tubers or tuberous roots and have no, or weak, seed reproduction.

Clonal growth may fulfil more than one of these functions, in which case it may not be possible to distinguish between them. In some cases, the functional nature of clonal growth may be simply

(4) unknown or not evident, in which case it may be recorded as such.
 
References on theory, significance and large datasets: De Kroon and Van Groenendael (1997); Klimeš et al. (1997); Van Groenendael et al. (1997); Klimeš and Klimešová (2005); Knevel et al. (2003); Klimešová and Klimeš (2007).

More on methods: Böhm (1979); Klimeš et al. (1997);Van Groenendael et al. (1997); Weiher et al. (1998); Klimeš and Klimešová (2005).