Bryophytes and Lichens

As shown above (Sect. 6.1, Fig. 6.1) bryophytes and lichens may constitute a considerable floristic diversity and biomass among the epiphytes in tropical rainforests, especially in the cloud forests at higher elevations with their cooler nights (Seifriz 1924; Sipman 1989). Epiphyllous liverworts are involved in a reciprocal transfer of nitrogen with their host leaves, where the amount of nitrogen obtained by the epiphyllous liverworts from the host leaves varies between 1% and 57% of their entire demand, and vice versa host leaves obtain mostly up to 2.5% of their nitrogen demand from leachates of the epiphylls (Wanek and Pörtl 2005).

Green and Lange (1994) have provided a comparison of photosynthesis in mosses and lichens. A major difference between the two groups is the effect of water relations on photosynthesis. In mosses, the CO2-exchange surface is external, and the mosses have special water storage volumes, i.e. special cells - often dead cells - and capillary structures, which are separated from the gas exchange areas. Therefore bryophytes may constitute most important water stores in the epiphytic habitat (Freiberg 1997). Lichens have an internal CO2-exchange surface with the phycobionts embedded in a relatively compact fungal tissue, and any water storage will tend to hinder gas exchange either within the compact tissue or at the outer lichen surface. Therefore, as compared to mosses, lichens tend to have lower maximal water content on a dry weight basis and there is the risk that high thallus water content impairs CO2-uptake and assimilation. This difference between the two groups possibly explains the particular dominance of bryophytes in very wet habitats, while lichens can also be very successful in dry habitats. Bryophytes are also well adapted to the large variation of temperatures in their montane, cloud and elfin forest habitats. They show constitutive temperature resistance to both rather cool temperatures often encountered in these habitats and quite hot temperatures that may occur during light fleck events (Lösch and Mülders 2000).

Floristics, taxonomy and habitat occupation by tropical lichens has been well studied (Galloway 1991). Less work is available on their ecophysiology. However, ecophysiological studies have investigated lichens in the temperate rainforests, e.g. in New Zealand (Green and Lange 1991; Green et al. 1991; Lange et al. 1993) and also in forests of the wet tropics (Lange et al. 1994; Zotz and Winter 1994a). Lange et al. (1994) have studied the gas exchange, water relations and potential productivity of the cyanobacterial basidiolichen Dictyonema glabratum living epiphytically, saxicolously and terrestrially in a lower montane tropical rainforest in Panamá with an annual precipitation of 4,000-4,500 mm and a mean annual temperature of 21 -22 °C (Fig. 6.5). This lichen occurs both in shaded and exposed sites, and it is quite frequent in this forest. It has a rather unusual ecophyiological behaviour with respect to water saturation of its thallus and a number of additional traits, which explain its high productivity in the habitat.

Normally photosynthetic net CO2-uptake is impaired in lichen thalli including lichens in tropical forests (Lange et al. 2000, 2004) when they are oversaturated with water due to diffusion limitation from surface water or blocked air channels in the mycelium (e.g. see Sect. 11.2.2 and Fig. 11.22). However, not in all lichens water saturation inhibits CO2-diffusion. The reasons are not known. It is not related to a formation of secondary chemical lichen compounds and must be due to morphological features (Lange and Green 1997; Lange et al. 1997). D. glabratum maintains maximum rates of net-CO2 uptake when it is fully hydrated up to a water content of 1,000% of its dry weight (Fig. 6.6). This allows maximum benefit from the heavy rain storms occurring in the habitat. The lichen also possesses a mechanism for concentrating internal inorganic carbon by energy dependent transport, which occurs in many algae and also higher water plants (see Griffiths 1989; Badger et al. 1993). This mechanism allows photosynthesis at elevated intracellular CO2-levels. In corticulous crustose green algal lichens in the understorey of a lowland tropical rainforest periods of thallus suprasaturation with water are reduced by the presence

Fig. 6.5 The lichen Dictyonema glabratum (syn. Cora pavonia) growing epiphytically on a 6-m-tall bush in a lower montane tropical rainforest in Panama. (Photograph courtesy B. Budel; see Lange et al. 1994)

Fig. 6.6 Net photosynthesis of the lichen Dictyonema glabratum in relation to water content in percent of dry weight. (Lange et al. 1994)

Fig. 6.7 A daily course of net CO2 exchange and water content of the lichen Dictyonema glabratum with air temperature and light intensity (PPFD) in its natural habitat in a lower montane rainforest in Panamá; measured on 23 September 1993 by Lange et al. (1994)

Fig. 6.6 Net photosynthesis of the lichen Dictyonema glabratum in relation to water content in percent of dry weight. (Lange et al. 1994)

of water-repelling surface structures of the hyphae of the mycobiont as well as the production of a hydrophobic fungal protein, hydrophobin (Lakatos et al. 2006).

Net CO2-exchange on many days is typically bimodal with a peak in the morning, when thalli are wetted from dew and early fog, and another peak after midday when heavy showers may occur (Fig. 6.7). In between, the thallus may dry out and CO2-uptake ceases. In fact, drying out occurs for a few hours almost every day, and similar to most lichens, D. glabratum is also desiccation tolerant (Sect. 11.4.2). However, unlike many other lichens it does not reactivate photosynthetic CO2-fixation immediately after rewetting following desiccation, but it needs a recovery period of about 60 min. Thus it appears that lichens of the very moist lower montane rainforest show subtle changes in rehydration and reactivation characteristics as compared to lichens from temperate or arid habitats (Sect. 11.4.2).

Another important trait is the thermophily of D. glabratum. It has been noted above that epiphytic mosses and lichens are particularly abundant in forests at higher elevations. The better supply of water in the epiphytic habitat by dew and fog may only be one reason for this distribution. An even more critical factor may be the reduction of respiration at lower night temperatures because respiration is a critical factor in the net productivity of lichens in the tropics where respiratory losses may be substantial. Indeed, the cooler nights much reduce respiration and thus nocturnal loss of carbon, which is a considerable factor in overall productivity, may be decisive for the general preponderance of epiphytic mosses and lichens in cloud and fog forests in the tropics. In D. glabratum respiration increases only slightly with temperature up to 22 0C, but then increases sharply with higher temperatures. Net photosynthesis increases up to 22 0C and then declines in parallel with increasing respiration, so that gross photosynthesis calculated from net photosynthesis and respiration remains at a constant high level up to 40 0C. The balance of net photosynthesis is positive up to 35 0C (Fig. 6.8). In fact the maximum rates of net CO2-uptake in D. glabratum are quite high even in comparison to sun plants among vascular epiphytes (see below: Table 6.6). On a thallus area basis the highest rate observed in the field was 8 jumolm-2 s-1. Calculations have suggested that the annual relative production under the habitat conditions of D. glabratum in Panamá is 2.28, i.e. a gain of 2.28 g of carbon per 1 g of initial thallus carbon. Thus, even with leaching of carbon under the influence of regular heavy rain which is frequently observed in lichens (Bruns-Strenge and Lange 1992), D. glabratum must retain sufficient surplus to allow rapid growth.

glabratum in relation to tem-

Fig. 6.8 Net photosynthesis, dark respiration and calculated gross photosynthesis of the lichen Dictyonema

perature at a light intensity (PPFD) of 150 ^mol m-2s-1 and at high thallus water content. (Lange et al. 1994)

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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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