In individuals, populations and communitiesLife-history; habit; the Raunkiaer classification; meristem determinacy; perenniality and annuality; monocarpy and polycarpy; semelparity and iteroparity; biogeographical vegetation classification; evergreens; r- and K-selection; the CSR model of plant functional types At the whole plant and population level, life-history and habit are critical features of fitness, adaptability and survival. Raunkiaer (1934) introduced a classification of plant life-forms based on the survival of apical meristems in their active or dormant forms. The various life-forms are characterised by the extent to which shoot axes persist (phanerophytes, chamaephytes), retrench (hemicryptophytes, cryptophytes) or die outright (therophytes) at the end of the growing season. Annuals and many biennials are therophytes. For a plant to be perennial the apical meristem of at least one of its shoot axes must remain indeterminate beyond the first growth season. Determinacy is usually associated with development of the floral meristem. Although there are many exceptions, where axes bearing inflorescences that are usually terminal can resume vegetative growth (Battey and Lyndon 1990, Thomas et al. 2000), the shoot apical meristem is normally indeterminate for as long as it remains vegetative. Conversely, some pathological conditions can cause determinate reproductive apices to produce leaves instead of floral organs, but these meristems remain determinate (for example Jones and Stoddart 1971). Meristem determinacy is an important, but not the only, factor in determining a plant's position in the Raunkiaer classification. The formation of resting structures and the progressive programmed senescence and death of organs are critical also.  Raunkiaer life-forms 1. Macrophanerophyte. 2. Climbing macrophanerophyte. 3. Nanophanerophyte 4. Shrubby chamaephyte. 5. Creeping chamaephyte. 6. Caespitose (tufted) hemicryptophyte. 7. Rosette hemicryptophyte. 8. Upright hemicryptophyte. 9. Creeping hemicryptophyte. 10. Rhizomatous geophyte (cryptophyte). 11. Bulbous geophyte. 12. Therophyte, 13. Hydrohemicryptophyte. 14. Hydrogeophyte. 15. Floating hydrophyte. Image source: http://follavoine.chez-alice.fr/f2_an_glos_typesbio.htm Active chamaephytes and certain hemicryptophytes, for example creeping species like clover, are horizontal perennials (Thomas 1994). They forage for resources in their environment (Stephens and Krebs 1987, Grime and Hodgson 1987, Van Kleunen and Fischer 2001) by apical proliferation, elongation growth and subsequent tissue senescence, death and decay (Gallagher et al. 1997, Turner and Pollock 1998). The plant survives for as long as young proliferating tissues can keep ahead of the wave of senescence and tissue death behind them. The shoots of phanerophytes (shrubs and trees) also move out into the environment but do so in the vertical plane. Older tissues become senescent and die, but do not undergo post-mortem decay, persisting instead in the form of wood. The wave of cell death moving along the shoots of woody plants is seen clearly in species such as Rhododendron. Here the current year's growth is green and the previous year's growth suberises and lignifies in a zone that moves up, killing axillary buds and triggering senescence and abscission of leaves as it passes. Root systems forage through the soil and pass through the apical proliferation-growth-senescence-death sequence rather like inverted shoots of vertical perennials (Spaeth and Cortes 1995, Eissenstat and Yanai 1997). We may conclude that Raunkiaer's life-forms, as they relate to degree of annuality or perenniality, are a direct expression of the extent to which proliferation at apical meristems outpaces a pursuing wave of (programmed) tissue senescence and death. This view of life history is reminiscent of Hagemann's (1999) concept of the "blastozone" and "necrozone" as archetypal features of morphogenesis in the earliest land plants. Annual plants, which grow, reproduce and die in a single season, seem to obey the "live fast, die young" rule (Kaufmann 1996). Biennials, which generally devote the first year to vegetative growth and the second year to reproduction and death, have life cycles that are qualitatively no different from those of annuals. There are also species such as Agave which may survive for many years in the vegetative condition but then produce flowers and fruits and die. In all these cases of monocarpy (semelparity) there is clearly a relationship between reproduction and whole-organism death. Amongst polycarpic (iteroparous) perennial species, where flowering and whole-plant senescence are not obligately linked, the range of lifespans is striking, ranging from less than 10 years in some herbaceous species to more than 2000 years in woody conifers. Asexual reproduction propagates clones which often remain attached to the parent plant and can proliferate to establish community-sized "individuals" of extraordinary longevity, maybe in excess of 10000 years (there is evidence that the endangered proteacean species Lomatia tasmanica is more than 43000 years old – Lynch et al. 1998). In this respect clonal plants resemble the huge and ancient underground hyphal networks of certain fungi (Smith et al. 1992). Senescence in temperate regions defines a whole season of the year. Autumnal changes in the appearance of the foliage of trees and other perennials signify a relocation of resources from disposable vegetative parts to storage tissues, similar in principle to the salvage of nutrients that occurs during monocarpic senescence in crop and like short-lived species (Chapin and van Cleve 1989, Valjakka et al. 1999, Matile 2000). Some biogeographical classifications of vegetation types are based on canopy structure and dynamics (to which senescence makes a decisive contribution) – for example the 6-biome model of Nemani and Running (1996). A distinctive feature of temperate deciduous senescence is the part it plays in morphological remodelling, leading to the reconfiguration of the plant to survive winter. A similar strategy is characteristic of species adapted to habitats in which summer growth is water-limited. The defoliation consequence of senescence is the visual signature of mediterranean-type vegetation in summer and temperate forests in winter. Evergreens do not display synchronised senescence. The evergreen trait may arise in one of two ways. Evolution may have introduced mutations that stifle expression of part or all of the senescence syndrome. Comparable genetic variants occur in normally yellowing species where they may exhibit some of the characteristics of xeromorphic or scleromorphic evergreens – conservation and reduced internal cycling of nitrogen, for example (Hauck et al. 1997), or tolerance of drought and other stresses (Borrell et al. 2000). On the other hand many evergreens clearly have the capacity to initiate and execute a recognisable senescence program but because it is not synchronised or seasonal, the plant gives a general impression of unvaryingly stable pigmentation. A survey of RCC reductase, a key enzyme of chlorophyll catabolism, identified the activity to be present across a wide taxonomic range, including several evergreen cryptogams (Hörtensteiner et al. 2000). In many evergreens the photosynthetic apparatus undergoes periods of partial dismantling and rebuilding throughout its lifespan. This is seen not only on a seasonal scale in the conifers of high latitudes (Ottander et al., 1995) but also over the course of the daily cycle in certain succulents and cacti (Wilkins, 1992). This pattern may be related to the phenomenon of regreening, in which expression of catabolism is repressed and genes for plastid assembly reactivated. Differences in plant life-form and adaptability make critical contributions to community behaviour, habitat and biodiversity. In order to rationalise, model and analyse their roles within ecosystems, organisms have been classified into two or more functional types. One influential two-component model is the r- and K-selection theory of McArthur and Wilson (see Pianka 1970). Grime (1974) introduced a three-component functional classification of plants (the CSR model) and has explored its ecological and evolutionary implications (Grime 2001). The model considers plant behaviour in terms of productivity and habitat disturbance – essentially as rates of (biomass) construction versus (habitat) destruction. Competitors (the “C” of CSR) are characterised by high productivity in relatively undisturbed habitats, Stress tolerators are relatively unproductive under the same circumstances and Ruderals are highly productive under intense disturbance. In general, the R strategy is associated with highly dynamic canopies (intrinsically short leaf lifespans) and (possibly) root systems. The roots and canopies in the S strategy show low turnover and are often extremely long-lived. In terms of senescence of above- and below-ground parts, the C strategy comes somewhere in between R and S. Conventionally the three contrasting strategies are represented as a triangle with C, S and R at the vertices and species are located in CSR space as points within the triangle. This has been done for Raunkiaer forms, which as we have seen in turn relate to patterns of whole-plant senescence in time and space (Grime 1977). An example of a three-component model of vegetation strategies that relates both to CSR and Raunkiaer is Westoby's (1998) LHS scheme. References
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