One of the best preserved tropical cloud forests is the forest of Rancho Grande on the northern coastal range of Venezuela (Fig. 3.6C). It is determined by the trade winds coming from a north-eastern direction (Fig. 3.8). Clouds build up over the Caribbean islands and dissolve as the wind moves south, where it then hits the coastal range of mountains on the South American continent. The hot wind cools down as it climbs upwards and on top clouds are formed almost continuously (Figs. 3.6C and 3.8). We find an altitudinal gradation of vegetation from cactus-forest, thorn scrub and dry forest to evergreen cloud forest. Clouds break up as the wind drives down the southern slopes and also move further south with the trade winds. In places, the constant exposure to the wind may lead to formation of elfin forest with dwarf trees similar to those on the top of Santa Ana Mountain on the Paraguana Peninsula of Venezuela, which are here dominated by Clusia multiflora and the palm Geonema paraguanensis about 1 m tall (Fig. 3.6D). In addition the reduction of growth and formation of dwarf forms may also be caused by reduced availability of nutrients, especially phosphorus, due to the lower mineralization rates at lower temperatures when altitude increases (Kitayama and Aiba 2002).
3.2.3 Floodplain Forests
Wetland ecosystems are found to be related to hyperseasonal and marsh savannas (Sect. 9.2, Fig. 9.10), but in addition we mention them here because we also find fresh water flooded sites in moist forests all over the world's tropics (Esteves 1998) where all stages from open water to high forest can often be traced. We may distinguish between swamp forest which is permanently waterlogged and marsh forest which is flooded only seasonally (Richards 1996; Fig. 3.9). In Brazil the Central Amazon floodplains cover an area of more than 300,000 km2 (Junk 1997) where we may distinguish nutrient poor bottomland floodplains along black water rivers (varzea) and nutrient rich periodically inundated riverine wetlands fed by white water rivers (igapo) (Esteves 1998). In Central Amazon floodplains in the pioneer species Salix maritima even the canopy can remain under water for months, while the canopy of the shade tolerant late successional species Tabernaemontana juruana always remains out of the water (de Simone et al. 2002).
Flooding leads to hypoxic and even anaerobic conditions (Richards 1996; Lobo and Joly 1998), which require morphological and anatomical as well as physiological adaptations. Phytohormones such as ethylene and auxin are involved in their regulation. At the morphological/anatomical level adventitious roots with lenticels above the water (Sect. 18.104.22.168) and root aerenchymas facilitate aeration of root tissues because plants subject to flooding have to rely on supply of oxygen from parts of the plants which are not flooded (Pimenta et al. 1998). There is adaptive hypertrophy of lenticels under the control of ethylene. In Sesbania commersoniana the height x length of the main root lenticels was 0.31 x 1.56 (mm x mm) in non-flooded controls and 0.44 x 2.06 (mm x mm) when flooded (Pimenta et al. 1998). The submerged roots may produce suberized and lignified barriers in the exodermis to reduce diffusion of oxygen from the root tissue to the ambient soil (de Simone et al. 2003a) although some loss of oxygen from the roots will also help to build up
a partially oxidant atmosphere at the root periphery important for acquisition of nutrients and control of harmful reduced compounds at the root periphery (Lobo and Joly 1998). White water fed flood plains are rich and black water fed flood plains are poor in nutrients. Symbiotic dinitrogen fixation is frequent but is discussed in Sect. 10.2.3.2 in the context of savanna floodplains. The reducing root environment during long periods of flooding also may lead to formation of several harmful reduced compounds and especially dangerously high levels of reduced iron, Fe2+, and manganese, Mn2+, and stress due to uptake and storage of these metals in the plants (Lobo and Joly 1998; de Simone et al. 2003b). At the metabolic level glycolysis and fermentation with the production of ethanol or lactic acid serve anaerobic metabolic energy turnover (Lobo and Joly 1998; Pimenta et al. 1998; de Simone et al. 2002).
Trees and herbs may respond differently to flooding. There are basically two different response patterns among trees, deciduous tree species respond to the flooded period by complete defoliation and evergreen tree species are maintaining their foliage. Thus, Fernandez et al. (1999) found two different physiological reactions, (i) a decreasing rate of photosynthesis, stomatal closure and reduced leaf conductance for water vapour with flooding, and (ii) flood tolerance, where both photosynthesis and leaf conductance were independent of flooding. The C4-photosynthesis grass Echinochloa polystachya on submerged Central Amazon floodplains showed the very high rates of photosynthesis of 30-40 |molm-2s-1 typical for C4-photosynthesis (Sect. 10.1.1.2) but much lower rates of 17 |molm-2s-1 during shorter dry periods (Piedade et al. 1994).
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