Tropical Forests V Mangroves

7.1 Phytogeography

Mangroves are a characteristic and important type of tropical and subtropical forests, with a unique capacity to tolerate large short-term changes of salinity. The name comes from the Spanish "mangle" for Rhizophora, a mangrove genus, and the English "grove". Mangroves may also be considered as "tide-forests", since their ecology is determined primarily by the tides at the three typical sites where they occur (Fig. 7.1):

• coastal mangroves,

• estuarine mangroves and

• coral mangroves, i.e. mangroves along the coastlines, in river estuaries and around coral reefs and coral islands. However, salinity in mangroves is not only influenced by the tides, but also by the climate. At high tide salinity in the rooting medium of mangroves, of course, will be determined by sea water. At low tide, however, it will be higher or lower depending on the climatic conditions, i.e.

• humid climate with rainfall frequently diluting and leaching salt,

• arid climate with salt normally concentrated, so that in any case mangrove sites are characterized by conditions of very variable salinity, which may even change rhythmically. Climate impacts on mangrove trees are also reflected in an annual cyclicity. Although as tropical trees mangrove trees lack distinct growth rings, high resolution profiles of stable carbon isotope ratios (518O, 8 13 C) in the wood of the stems reveal a seasonal cyclicity related to physiological processes under the environmental driving forces of salinity and water

Fig. 7.1A-F ►► Coastal mangroves of the Morocoy National Park near Tucacas at the Caribbean coast of Venezuela (A, B) and in Queensland, Australia (C). Estuary mangroves in Trinidad (D) and Costa Rica (E Las Baulas, F Rio Partita)

Fig. 7.1 Continued

potential of soils and other factors such as relative air humidity (Verheyden et al. 2004, 2005).

Mangroves delimit most tropical coast-lines and also extend into the subtropics (Fig. 7.2), such that 60-75% of all tropical coast-lines are occupied by mangroves (Popp 1991). The area covered by mangroves is 140,000 km2, which is about 0.1% of the total land surface of the earth. The total global biomass of mangroves is estimated to be 8.7 gigatons dry weight (Twilley et al. 1992). There are about 50 - 75

Table 7.1 Na+ and Cl levels in different mangrove species collected from all over the world. (Popp et al. 1984)

Table 7.1 Na+ and Cl levels in different mangrove species collected from all over the world. (Popp et al. 1984)

Fig. 7.2 Global distribution of mangroves with numbers indicating the approximate number of tree and shrub species in the mangrove vegetation. (After Vareschi 1980, with kind permission of R Ulmer)
Fig. 7.3 Co-occurring mangrove species in the Morocoy Park of Venezuela (see Fig. 7.1A,B): Avicennia germinans, Laguncularia racemosa, Rhizophora mangle

different species of mangroves in 20-26 genera in 16-20 families (Ellison 2002), some of which are depicted in Fig. 7.3 and listed in Table 7.1. Floristic diversity is poor in the Americas (1-5 tree species) and also in Africa (four species in West Africa, eight species in East Africa and Madagascar) but quite respectable in Asia (about 25 species in India and 30 species in SE-Asia), although in general terms mangroves are floristically much poorer than other tropical forests.

7.2 Site Characteristics and Contrasts in Salinity

Mangroves are characterized by their trees. Trees in mangrove forests may become quite tall although often mangroves have a scrub-like physiognomy (e.g. compare Fig. 7.1B and F). The woody mangrove species are frequently distributed in a banded zonation pattern oriented in parallel to the shore line. This pattern is correlated with the frequency and duration of tidal immersion modulating the degrees of salinity stress, and it is also influenced by dispersal of propagules, competition among mangrove species and herbivory (Ball 2002). Taller trees are formed in the

Fig. 7.4 The mangrove fern Acrostichum aureum (Costa Rica)

fringe forest near the water's edge and dwarf forms further inland at higher elevation in the intertidal zone (Cheeseman and Lovelock 2004; Lovelock et al. 2006a; see also Sect. 7.6). Lin and Sternberg (1992a,b, 1993) have compared scrub and tree life-forms of the mangrove species Rhizophora mangle with trees in the fringe forest at lower levels (24 cm above sea level) and the scrub formation at higher levels (60 cm a.s.l.). The scrub form is associated with high salinities occurring at the higher levels during the dry season. In the rainy season, the scrub mangroves can also take up fresh water from rain, and R. mangle is therefore a facultative halo-phyte. However, frequent stress is caused by the changes in salinity following shifts between flooding by ocean water and fresh water introduced by rain. Such variations can lead to a significant decrease in photosynthesis and plant growth in the scrub mangroves, in contrast to constant salinity which maintains the salt load in the substratum.

On the silty substrate of mangroves the undergrowth of vascular plants is usually poor (Ball 1996) although the vigorous growth of large terrestrial ferns of the genus Acrostichum often is a striking feature (Fig. 7.4). These "mangrove ferns" are shade-tolerant plants, which, however, have their maximum development and productivity under full exposure. Acrostichum aureum is quite salt tolerant, although perhaps somewhat less than the mangrove trees. However, the gametophytes are only resistant to mild salinity stress and can survive the full salinity of sea water only for short periods, so that establishment is a problem and the fern remains restricted to the landward side of mangrove swamps (Medina et al. 1990; Li and Ong 1998; Sun etal. 1999).

7.3 Morphological Characteristics of the Mangrove Tree Life Form

At the littoral habitats the wood of the trunks of mangrove trees needs to resist particularly strong winds as well as the pressure of tides. The major stress factors related to morphological characteristics of the mangrove tree life form, however, are salinity and the additional stress of low O2-partial pressure resulting from the inundation of the silty hypoxic substratum in which they root. This causes particular demands on morphology of roots for aeration and hydraulic architecture for lifting water to the shoots against the low water potential of the saline substratum.

7.3.1 Hypoxia in Inundated Swampy Soils, Root Morphology and Aeration

The root systems of mangrove trees are most remarkable. In comparison to other tropical forest communities root biomass is greater in mangroves (Ball 1996). Highly conspicuous is the diverse range of strangely shaped bizarre root systems above the soil surface. They must have evolved to provide anchorage as well as aeration in the silty muddy soils. With respect to the latter function they have been named pneumatophores. Diffusion of gases is highly limited in the inundated soil. Therefore, only contact of the root system with the atmosphere or with the sea water, depending on tidal level allows gaseous exchange. Figures 7.5 and 7.6 show a variety of some of the most frequently observed aerial root systems with stilt roots, planks and buttresses and finger- or knee-like protrusions above ground.

The root aeration provided by these pneumatophores is reinforced by a physiological mechanism. The exposed parts of these roots usually have lenticels, which

Fig. 7.5A-C Root types of mangroves. A Stilt-, B Buttress-type roots. C Knee- or finger-like pneumatophores. (Vareschi 1980, with kind permission of R. Ulmer)

Fig. 7.6A-D Stilt roots of Rhizophora mangle (A-C) and finger-like pneumatophores of Avicennia germinans (D)

are openings in the bark where gas but not water can penetrate. The influx of water is prevented by surface tension in the intercellular spaces of the lenticels. A pneu-matophore aerenchyma may occupy as much as 70% of total root volume (Curran 1985). During high tide, root respiration reduces the O2-concentration in the intercellular spaces of the root aerenchyma to a hypoxia of as little as 4 — 8% (Kitaya et al. 2002). Photosynthetically active cells at the surface of the pneumatophores can feed some O2 into the aerenchyma for root respiration (Aiga et al. 1995; Kitaya et al. 2002), but pneumatophore photosynthesis is decreased with flooding due to reduced irradiance (Kitaya et al. 2002). Thus, the low O2-concentration in the submerged pneumatophores of only a quarter to less than half of the atmospheric concentration causes considerable O2 gradients along the roots and gas-pressure deficits in the aerenchyma to minus 1.7 kPa (Chiu and Chou 1993; Skelton and Allaway 1996; Youssef and Saenger 1996; Losch and Busch 1999), because the O2 consumed in respiration can not be reabsorbed readily from the sea water, while the CO2 liberated can be dissolved as bicarbonate and released. At low tide, when the roots establish contact with the air again, the pressure deficit effectively leads to air being sucked into the root air spaces via the lenticels. Hypoxia (see also Sect. 3.2.3) is an additional stress to salinity in mangrove trees and, in view of the energy costs of salinity tolerance (e.g. salt exclusion, K+/Na+-selectivity, see Sects. 7.4 and 7.6), such mechanisms for the control of hypoxia at the root level are quite important in addition to ventilation from the photosynthesizing shoot via the aerenchyma.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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