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Table 10.5 Altitudinal distribution of C4- and C3-grasses in Kenya. The soil moisture index is in arbitrary units increasing approximately linearly with altitude (10 at 600 m; 100 at > 3,500 m). (Tieszen et al. 1979)

Altitude (m) Soil moisture index C4- vs C3-photosynthesis

< 2000 50 C4-grasses dominate

C3-grasses only in the shade

2,000 - 3, 000 60-70 Transition zone

> 3000 80 C3-grasses dominate forests of tropical lowlands (Sect. 3.2.4); CAM-epiphytes are abundant in wet forests of medium altitudes (Sects. 6.6.1 and 6.6.2.2); and at higher altitudes, terrestrial CAM species may also be frequent in dry sites of open habitats (Sect. 12.3.3).

Towards the wet side of the spectrum of savanna types (Fig. 9.10), i.e. in wetland ecosystems, C4-photosynthesis has also proved to be highly successful. For example in fertile flood plains of nutrient rich rivers and lakes (white waters) of the Amazon region in South America (Sect. 3.2.3) the perennial C4-grass Echinochloa polystachya may form monotypic stands over 5,000 km2 and displays extraordinarily high rates of net-CO2 uptake in photosynthesis of 30-40|imolm-2s-1 (compare Table 10.4) with fast growth and high productivity during the wet season of 108 tonnesha-1 year-1 (comparable to the productivity of sugar cane plantations) when the flood plains are submerged. During the shorter dry period CO2-uptake rates are 17 |mol m-2 s-1 and photosynthesis shows a midday depression (Piedade et al. 1992, 1994; Esteves 1998).

10.1.2 Trees 10.1.2.1 Phenology

The trees can develop large root systems reaching far into the soil in both vertical and horizontal directions. Nevertheless, due to the strong seasonality of water supply in most savannas, appropriate phenological behaviour remains important (Table 10.1). Trees need larger amounts of precipitation than grasses. The requisite rain can also fall during dormant periods, and so a small amount of water uptake into the trunk and branches must be possible even during drought periods.

One of the most striking phenological aspects is the flowering of savanna trees which is most attractive when it occurs in the leaf less period of deciduous trees (Fig. 10.4). An intriguing physiological question is how these flowers are supplied with water in the absence of leaf transpiration and substantial xylem water flow. In some species - although not in all of them - flowers can be supplied with water via the phloem. This requires a sink for phloem flow, which could be nectar secretion (Chapotin et al. 2003). In Africa and Australia a few trees are evergreen and most are drought-deciduous, whereas in South America evergreen trees prevail (Medina

Fig. 10.4 See next page for details

Fig. 10.4 Flowering savanna trees of the Llanos in Venezuela: Tabebuia chrysantha (A), Tabebuia orinocensis (B), Yacaranda filicifolia (C), Pseudobombax sp. (D), Palicourea rigida (E), Byrsonima crassifolia (F)

1993). Curatella americana (Fig. 10.5), a dominant tree of the Venezuelan Llanos, is evergreen with seasonal growth. Production of flowers and fruits, new leaves and shoots begins in the middle to the last third of the dry season, so that the plants are "ready" when the rainy season comes, as shown in the phenological diagram of Fig. 10.2C.

Among the deciduous trees the length of the leafless periods may be different:

• trees with a short leafless period afford low (highly negative) water potentials and high respiration rates, e.g. Bursera simaruba (Burseraceae), Spondias lutea (Anacardiaceae), Pereskia guamacho (Cactaceae);

• trees with a long leafless period have high (less negative) water potentials and low respiration rates, e.g. Tabebuia chrysantha (Bignoniaceae).

The precise phenological behaviour of plants in the tropical savannas (see also grasses, Sect. 10.1.1.1) is a reliable indicator of season. For example, using up the last water reserves for formation of reproductive and vegetative biomass in the last third of the dry season, as shown for the savanna tree Curatella americana (Fig. 10.5) in the phenogram of Fig. 10.2C, would be dangerous if the rainy season were not close. This raises the question which signals are sensed in phenological timing. It has been observed that several phenological phenomena, including water budget, leaf abscission and flowering are related to phytochrome equilibria (see Box 4.7) (Reich and Borchert 1984). Several suggestions have been made to explain phenological timing on a hormonal basis. It was also proposed that water stress itself is involved. However, accurate phenological timing is also observed in water storing stem succulent savanna tress (Sect. 10.1.2.2), and moreover, with variations of only 10 to 15 days year after year, its precision is much better than it would be given by the stronger inter annual variations of rain fall (Borchert and Rivera 2001; Rivera and Borchert 2001; Rivera et al. 2002). Directly at and very near to the equator endogenous regulation of annual rhythmicity must play a role (Wright

1991). However, even at very low latitudes away from the equator, photoperiod is the decisive signal.

The proposal that the decisive external signal is photoperiod has long been rejected a priori, because near the equator the differences between the longest and the shortest days are rather small (Reich and Borchert 1984). However, empirical facts demonstrate very precise photoperiod sensing of plants. In the forest tree Hildegardia barteri in Nigeria at 7° N where photoperiod changes by 53 min a similar decrease of photoperiod inhibits seedling leaf production (Njoku 1963; Wright 1996). An experiment with Hyptis suaveolens (Lamiaceae), although an annual species and not a tree, showed, that in principle plants can even sense differences in photoperiod close to 20 min (Fig. 10.6; Medina 1982). Seeds were germinated at the beginning of

Fig. 10.6A, B Experiments with the annual tropical short-day plant Hyptis suaveolens showing that plants can sense very small differences of photoperiod (daylength). Note that, by definition, short-day plants require daylengths below a certain species-specific threshold for induction of flowering. A The daylengths from May to September at the site of the experiment. B The columns give the number of days passed after sowing and the height of the plants attained after sowing by the end of September. Independent of both time passed after sowing and height attained, all plants flower at the end of September, i.e. the photoperiod of 12 h 24 min in August was still too long (above the threshold) but the only 18 min shorter photoperiod of 12 h 06 min in September was short enough to induce flowering in H. suaveolens at the tropical site. (After Medina 1982)

Fig. 10.6A, B Experiments with the annual tropical short-day plant Hyptis suaveolens showing that plants can sense very small differences of photoperiod (daylength). Note that, by definition, short-day plants require daylengths below a certain species-specific threshold for induction of flowering. A The daylengths from May to September at the site of the experiment. B The columns give the number of days passed after sowing and the height of the plants attained after sowing by the end of September. Independent of both time passed after sowing and height attained, all plants flower at the end of September, i.e. the photoperiod of 12 h 24 min in August was still too long (above the threshold) but the only 18 min shorter photoperiod of 12 h 06 min in September was short enough to induce flowering in H. suaveolens at the tropical site. (After Medina 1982)

each month at a location north of the equator starting in May and ending in September. At the end of September, all plants were flowering irrespective of the age and biomass they had attained during growth, such that the 180 cm tall, ~ 150 day old plants, germinated in May flowered simultaneously with the 12 cm high, ~30day old plants, only germinated in early September. Thus, flowering was not related to age or biomass. The photoperiod, which did not lead to flowering (plants germinated in August with no flowering in August photoperiod 12 h 24min), and that which elicited flowering in September (photoperiod ~ 12 h 06 min) differed by only 18 min. Borchert and coworkers have compiled a large amount of data from field observations made at frequent intervals over several consecutive years and from herbarium collections which now provide strong evidence for photoperiod sensing with a precision of at least 30 minutes in phenological synchronizations year after year. Moreover, phenological phase shifts between the northern and the southern hemisphere are found to be six months (Borchert 2000; Borchert and Rivera 2001; Rivera and Borchert 2001; Rivera et al. 2002). Thus, clearly many of the phenolog-ical phenomena observed in savannas can be regulated by photoperiod.

10.1.2.2 Morphological and Anatomical Traits

For the trees the water capacity of the soil does not need to be high (Table 10.1). In the Brazilian cerrados the soil is always deep and well drained. The ground-water table therefore is low, i.e. from 3 to 6 m down to 30-50 m. Hence, the trees develop deep roots, which reach water even during the dry season, as shown by high transpiration rates. Woody cerrado plants have substantially higher root-to-shoot ratios than trees in nearby forests (Hoffmann et al. 2004). In the Llanos in central Venezuela there is frequently a hard pan - "arecife" - of lateritic iron-oxide (see Sect. 10.2.4.1) above the ground water-table (Fig. 10.7). The roots of savanna trees must penetrate this layer to reach the ground water, which also varies on a seasonal basis.

Leaf xeromorphy is another structural feature frequently observed among savanna trees. It is very important in Australia and South America, but less so in Africa (Medina 1993). It has already been noted in Sect. 3.4.4.3 that the formation of small and longlived leathery leaves may be considered as a strategy which gives the best return for investment of resources when nutrient supply is low. In addition such leaves also offer ways to economise on water use by some of the following traits:

• thick and water tight cuticle, which reduces water loss via cuticular transpiration,

• dead hairs on the surface,

• prevailingly hypostomatic distribution of stomata, i.e. stomata only on the lower surface;

evergreen evergreen

groundwater table at the end of the rainy season

« 2m groundwater table before the beginning of the rainy season

Fig. 10.7 Relations between the vegetation, the hard lateritic ferrous-oxide layer ("arecife") and the seasonally shifted groundwater table in the Llanos of central Venezuela. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

(the latter three properties are generally assumed to reduce evapotranspiration by supporting the built up of unstirred layers although in detail leaf boundary-layer relations are very complex; Schuepp 1993);

• lignification of cell walls,

• formation of idioblasts and sclereids,

(these three properties help to stiffen the leaves, so that the trees are sclerophyllous and leaf-shape is maintained even when turgor pressure is low);

• production of etheric oils, which due to their hydrophobic nature may also assist in preventing water loss into the gas phase around the leaves.

Many cerrado trees were found to store water in their sapwood which may play a dominant role in the regulation of diurnal water deficits (Scholz et al. 2007). However, a particularly conspicuous peculiarity of some savanna trees is real stem succulence. With particularly thickened stems these trees really look very succulent, such as the Bombacaceae Pseudobombax in South America and Adansonia, the baobab (Fig. 10.8). The latter is a most spectacular plant. There is only one species on the African continent, A. digitata, which may be up to 9 m in diameter (Fig. 10.8D,E) and has a geographical distribution clearly correlated with the occurrence of savanna (Fig. 10.9). There are seven species of Adansonia in Madagascar and two in Australia. Woody stem succulent plants are abundant in seasonally dry tropical environments. A list of families and species is given in Table 10.6. They are all deciduous C3-plants, in contrast to fleshy stem succulent plants which mostly perform CAM (Sects. 5.2.2.2 and 8.2.3.2.1). Stable isotope ratios of Adansonia

Table 10.6 Families and species of tropical woody stem-succulent plants. (Borchert and Rivera 2001, and personal observations)

Family

Species

Occurrence

Anacardiaceae

Spondias purpurea

Costa Rica, Mexico

Apocynaceae

Plumeria rubra

Costa Rica, Mexico, Nigeria

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