Giant Rosette Plants

The most typical life form of alpine tropical regions are the giant-rosette plants of the genera Lobelia (Lobeliaceae) and Senecio (Asteraceae) in Africa, Espele-tia (Asteraceae) in South America and Argyroxiphium (Asteraceae) in Hawai'i (Fig. 12.5). They may reach heights of several meters and have developed a number of morphological and anatomical features adapting them to the diurnal type of climate in their habitat.

The tallest plant of Senecio johnstonii ssp. cottonii actually measured by Beck et al. (1983) was 9.60 m, but an even taller one was also encountered at another location. The plants bear giant rosettes of living leaves at the end of their branches. Dead leaves usually cover the entire shoot in a dense layer. The dead leaves are kept for decades and perhaps even centuries, since the larger giant-rosette plants may be of considerable age. Estimations for Senecio keniodendron suggest that about 50

Acaulescent Plants
Fig. 12.4 The five major life forms of Afro-alpine vegetation (Hedberg 1964b)

new leaves are formed annually, the stem growth rates are approximately 2.5-3cm per year giving an age of 35 years per meter of unbranched stem (Beck et al. 1980). A 10 m tall giant-rosette plant then might have an age of 350 years.

These dead leaves provide heat insulation of the stem. However, the morphology of the living leaf rosette, together with some physiological reactions, enables most important acclimatory strategies (Fig. 12.6).

The conical leaf bud in the center of the rosette is protected by nyctinastic movements of the adult leaves. Thus, a "night-bud" is formed in these plants, which of course grow during the whole year and have no dormant periods, as plants in the temperate zone have in the form of winter buds. In Espeletia schultzii the new leaves, which had just developed from the central bud, wilted and died when the formation of the night-bud (by nocturnal closure of the rosette) was experimentally prevented (Smith 1974).

In African giant-rosette plants the leaf water may freeze apoplastically during the night (see Sect. 12.4.1). The closing mechanism of the inward movement of the adult leaves in these plants is based on water loss from the cell interior and the associated decrease of pressure in the whole tissue as turgor of the individual cells declines due to cellular loss of water. The opening of the night-bud is due to instantaneous resorption of the water and restoration of turgor after melting of the apoplastic ice in the morning.

The dense packing of the developing leaves in the central bud gives this organ a massive structure with a considerable inherent heat-storage capacity. In addition,

Fig. 12.5A-D Tropical alpine giant-rosette plants. Espeletia timotensis (A) and E. schultzii (B), Aguila Pass, Venezuela, 3,600-4,000 m a.s.l.; Lobelia keniensis (C) and Senecio kenioden-dron (D), Teleki valley, Mt. Kenya, at 4,100 m and 4,300 m a.s.l. respectively. (C, D courtesy E. Beck)
Fig. 12.6A-E Giant rosettes of Espeletia schultzii (A), E. moritziana (B) and E. timotensis (C), Aguila Pass, Venezuela, 3,600 - 4,000 m a.s.l., and of Lobelia deckenii in the day position (D) and as the night-bud (E), Mt. Kilimanjaro, Tanzania, 3,850 m a.s.l. (D, E courtesy E. Beck)

excreted fluid and mucilage may also contribute to the heat-storage capacity. The bases of the adult leaves form tanks or cisternae where mucilageous fluid is collected during the day. In Lobelia keniensis, for example, this may amount to several liters per rosette. During the nyctinastic leaf movements this fluid, which warms up during the day, is pressed upwards to cover the meristematic part of the night-bud in the evening (Beck et al. 1982).

Moreover, the leaves are very pubescent and have large intercellular air spaces which add to the insulating effect (Fig. 12.7). The consequences of leaf pubescence are not straightforward. There are complex interactions between several factors

Fig. 12.7 Leaf anatomy of giant-rosette plants, with hairs and large intercellular spaces and mucilage inside and on the surface of the upper epidermis. Cross-section of an adult leaf of Lobelia keniensis in the upper cisterna region. (Beck et al. 1982)

implying non-linear behaviour of the system. The role of pubescence has been examined in more detail in Espeletia timotensis in relation to temperature, wind speed and high solar radiation in the páramo habitat (Meinzer and Goldstein 1985; Schuepp 1993). Pubescence is more effective at high wind speeds. Increased boundary layer thickness due to a coat of hairs hinders exchange between leaf surface and ambient wind, and its primary effect in cool air would be an increase in surface temperature. This may be about 7 0C in the case studied, which is associated with a small increase in transpiration 17%) due to the effects of leaf temperature on leaf/air water vapour pressure difference. In numerical simulations it was shown however, that in contrast an increase of surface temperature of about 7 0 C would result in a doubling of the transpiration rate of non-pubescent leaves. Increased transpiration will have a feed-back on leaf temperature because of the effect of transpirational cooling, which adds to the complexity of the system.

Fig. 12.8 Exponential heat decay curves and time constants, te, for inflorescences of Puya hamata (controls and denuded, respectively), P clava-herculis and P aequatorialis. te is the time when the heat decay curve crosses the line of 1 (T1 — T2) + T2 and is independent of the magnitude of the T\ — T2 temperature difference. Thus, te gives the time it takes for a 63% decrease in the total temperature difference between the plant organ (Ti) and the atmosphere (T2). (After Jones 1992; Miller 1994)

Fig. 12.8 Exponential heat decay curves and time constants, te, for inflorescences of Puya hamata (controls and denuded, respectively), P clava-herculis and P aequatorialis. te is the time when the heat decay curve crosses the line of 1 (T1 — T2) + T2 and is independent of the magnitude of the T\ — T2 temperature difference. Thus, te gives the time it takes for a 63% decrease in the total temperature difference between the plant organ (Ti) and the atmosphere (T2). (After Jones 1992; Miller 1994)

In any event, pubescence will slow down the establishment of thermal equilibrium of plant organs with air temperature. For the pubescent inflorescences of Puya (Bromeliaceae) in the equatorial páramo zone of the Ecuadorian Andes this has been quantified using time constants (te) derived from the exponential heat decay curves (Fig. 12.8). Clearly, in the non-pubescent species P. aequatorialis, which grows on rocky outcrops between 1900 and 2,100 m a.s.l., te is much lower (15 s) than in the pubescent species P. hamata (135 s) and P. clava-herculis (126 s), and in P. hamata it drops to 30 s when inflorescences are denuded (Jones 1992; Miller 1994).

All of the features discussed above in this section are mechanisms to delay cooling and provide freezing avoidance in the buds. Indeed, measurements in Kenya showed that only the temperature of adult leaves closely follows air temperature and is for many hours every night below the freezing point. However, nocturnal bud temperatures in Lobelia and Senecio are significantly higher and may even remain positive. In Senecio brassica for example, at air temperatures around —8 0C, bud temperature remains +1 0C (Fig. 12.9). We shall see below (Sect. 12.4.2) that for Andean giant-rosette plants the mechanisms of freezing avoidance based on insulation, heat storage and delay of cooling, together with some supercooling effects, may suffice for the frost-resistance of the adult leaves. However, in Afro-alpine giant-rosette plants they are insufficient and freezing tolerance is needed (Sect. 12.4.1).

Fig. 12.9A, B Comparisons between leaf temperature and night-bud temperature as related to air temperature in Lobelia telekii (A) and Senecio brassica (B) on Mt. Kenya during the dry season in March 1979. (Beck et al. 1982)

12.3.2 Other Life Forms: Tussocks, Cushions, Acaulescent Rosettes, Sclerophylls

In general, it appears that smaller plants are less threatened by frost than taller ones, as suggested by Fig. 12.10, which relates average plant height of cushions and small rosettes, shrubs and perennial herbs, giant rosettes and small trees to temperature causing injury.

In tussock and cushion plants the regenerating buds are insulated by adult leaves and dead material. Extensive studies on alpine plants in Europe have shown that the internal microclimate, temperature and air humidity, within such plant bod-

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