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3.3.4.3 Stem Characteristics

Conspicuous life forms based on stem characteristics are the fleshy and woody stem succulents (Sects. 8.2.3.2.1 and 10.1.2.2, respectively) and the xylohemicryp-tophytes with their xylopodia or lignotubers (Sect. 10.1.2.2).

3.3.4.4 Whole Plant Modifications

Very typical special life forms of vascular plants in tropical forests are epiphytes, hemi-epiphytes and lianas to which a special chapter (Chap. 6) is devoted.

Another example are the myrmecophytes (ant plants). The symbiosis between plants and ants plays a particularly important role in the tropical environment. It is frequently found in epiphytes (see Sect. 6.6.3) but also in terrestrial plants, where the genera Tococa (Melastomataceae), Cecropia (Moraceae) and woody Legumi-nosae are the best known ant plants. In the Sira-mountains of Peruvian Amazonia, Morawetz and Wallnofer (1992) counted that 4.4% of all species were genuine myrmecophytes. The ant plants prefer disturbed sites, which may be naturally due to land-slides; 86% of all genuine myrmecophytes (38 species) were found in such sites, while only 14% (6 species) regularly occurred in the primary forest.

The plants provide hollows, so-called domatia, where the ants find protected spaces for nests, e.g. in the inflated leaf bases of Tococa (Fig. 3.23A), the hollow stems of Cecropia (Fig. 3.23B) and thorns and leaf petioles of many Leguminosae (Caesalpiniaceae, Mimosaceae). In addition the ants may receive nutrition in the form of nectar from extra floral nectaries or various food bodies (Webber et al. 2006) and special nutritive appendices containing fat and oils, protein and carbohydrates, e.g. the elaiosomes (appendices of seeds) or the Muller-bodies of Cecropia which are made up of glycogen and not the usual plant-storage carbohydrate starch.

It is thought that in return the ants provide to the plants protection from phytophagous animals especially insects (Davidson and Epstein 1989; Duarte Rocha and Godoy Bergallo 1992). It has also been observed that ants keep their host plants free of epiphytes. A most astonishing story has been reported by Morawetz et al. (1992). The Myrmecochista ants of Tococa occidentals systematically and rapidly kill all angiosperms coming closer than 4 m to their host plants. T occidentals is

Fig. 3.23A, B Ant-nest plants. A Tococa sp. with ant nests in the inflated leaf bases (arrows). B Cecropia sp. with ant nests in the hollow stem

a light demanding species and its growth and proliferation is stimulated by the ants' clearing of the surrounding competitors. Thus, after an initial T. occidentalis-plant has been colonized by the ants, pure T. occidentalis stands with a diameter of several

Fig. 3.24 Stand of Tococa occidentals (To) with a surrounding safety corridor (s) and adjacent vegetation. (After Fig. 2 in Morawetz et al. 1992, with kind permission of the author and Chapman and Hall)

Fig. 3.24 Stand of Tococa occidentals (To) with a surrounding safety corridor (s) and adjacent vegetation. (After Fig. 2 in Morawetz et al. 1992, with kind permission of the author and Chapman and Hall)

meters (10 - 30 m) may then develop, around which the ants even maintain a "safety corridor" (Fig. 3.24). Using their mandibles the ants cut the veins of the leaves of the competing plants. In the case of palmate leaves they attack the point at the base, where all veins join; pinnate venation is destroyed by cutting the first and second order veins at the base; the veins of moncotyledons (e.g. palms) are cut one by one along the entire leaves. After cutting the veins the ants inject a poisonous excretion from their abdomen. Apical meristems are also attacked. In this way the plants die rather rapidly. Although the ants can effectively kill 10 - 50 m tall trees, they do not usually attack such emergent trees at some distance of their host plants which may then form a closed canopy 10 - 25 m above the stand of T. occidentals. As the light demanding T. occidentalis-plants die away back in the shade, the stand deteriorates and the ants emigrate to start a new cycle elsewhere.

3.4 Vertical Structure

Stratification, meaning the vertical structure of tropical forests, is directly linked to local action of specific environmental factors, such as light, temperature, humidity, CO2 and minerals, the vertical distribution of which can be described. The vertical structure of tropical forests is determined by several more or less distinct and typical canopy layers. In simplified terms one may distinguish three major layers:

• a layer of emerging giant trees up to 60 - 80 m tall,

• an intermediate main canopy layer up to 24 - 26 m,

(Whitmore 1990), as shown in the schematic transect of Fig. 3.25. More realistic transects of actual forests often show a larger complexity, and there is also much diversity of vertical structures among forest types.

Fig. 3.25 Schematic representation of the strata structure of a tropical forest

The abundant plant life in these various strata determines vertical gradients of many important environmental factors such as:

• light intensity and spectral composition,

• CO2-concentration,

• mineral nutrients.

3.4.1 Irradiance

Due to absorption by the foliage intensity of irradiance may decrease exponentially from the main canopy layer down to the forest floor, which often obtains only a few per cent of the intensity received by the upper canopy or by a large forest clearing (Fig. 3.26). One consequence of such light gradients in tropical forests is that the maximum heights individual trees may reach are negatively correlated with shade tolerance and that there is a positive correlation of maximum height with light saturated rates of photosynthesis which are usually higher in sun plants (Sect. 4.1.1) (Davies et al. 1998).

Light-absorption by the canopy of forests may be treated according to Lambert-Beer's law, which is well known from photometry. The ratio of the light intensity I

intensity

Fig. 3.26 Light penetration through the canopy of a tropical forest. (After Jacobs 1988)

intensity

Fig. 3.26 Light penetration through the canopy of a tropical forest. (After Jacobs 1988)

of a beam passing through a sample solution of a thickness d and the light intensity of the incident light beam Io is given by

Io where s is the molar extinction coefficient and c the concentration of the sample. In analogy, the situation for canopies may be written as

Here Io is the light intensity outside the canopy and Ii the intensity at level l, LAIo-e is the leaf area index between the top of the canopy, o, and level i, and k is a constant. The leaf area index is dimensionless and is given by a projection of all foliage onto a certain level l or onto the ground; it thus is the ratio of

[total foliage area above a unit area at layer l ] : [unit area] , and a typical value for tropical rainforests is 8.

Light-absorption by the canopy not only reduces light intensity but also changes the light quality or spectral composition. In the red region of the spectrum absorption by photosynthetic pigments changes the red:far red ratio (R:FR) so that phy-tochrome regulated processes, which respond to this ratio are affected (Sect. 4.2.2). In a low-land rainforest in Costa Rica R:FR was found to be 1.23 in a large clearing but only 0.42 on the forest floor (Chazdon and Fetcher 1984) and in rain forests in French Guiana R:FR values of 0.10 to 0.15 were recorded (Bongers et al. 2001).

The degree to which the canopy is closed above the forest floor or leaves openings for light penetration can be determined by quantitative image analysis of photographs taken around noon with wide-angle or fish-eye lenses pointing upwards from the ground (Fig. 3.27). In addition to the diffuse light filtering through the canopy foliage, the forest floor may also obtain light in the form of light flecks. Light flecks occur when movements of leaves in the wind or the changing angle of the sun allow direct light penetration for intermittent periods of time. These light flecks may provide up to 80% of the total irradiation received by the forest floor, and their intensity ranges from 10 to 70% of full sunlight. They are important for photosynthetic productivity (see Sect. 4.2.1). Figure 3.28 shows that for short periods lower strata in tropical forests may obtain quite high irradiance, which may at times exceed that received by strata higher up. Thus, the gradual decline of irradiance from the top of the canopy to the ground shown in Fig. 3.26 does not always correspond to the actual situation.

Fig. 3.27 Vertical fish-eye camera view from the floor to the canopy of a rain forest in Panamá. (Photograph Bettina Engelbrecht)

Fig. 3.27 Vertical fish-eye camera view from the floor to the canopy of a rain forest in Panamá. (Photograph Bettina Engelbrecht)

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