Ao

Temperature °C

This behaviour of a tropical rainforest lichen is quite remarkable in comparison to the slow growth otherwise observed among lichens. The observations of Lange et al. (1994) put the biomass production of lichens and its ecological importance in tropical fog and cloud forests in a new perspective. Other lichens may have similar capacities for high productivity (Zotz et al. 1998; Lange et al. 2000, 2004). Lichens with cyanobacteria as phycobionts are particularly frequent in the moist tropics (Budel et al. 2000). In the cyanobacterial lichens dinitrogen fixation is an additional advantage. If we average maximum rates of net photosynthesis given by Lange et al. (2000, 2004), a superior performance of cyanobaterial lichens compared to green alga lichens is observed with rates in pmolm-2 s-1 of 5.3 ± 1.9 (11) and 3.0 ± 1.1 (3), respectively (SD with number of measurements including different species). In the balance of overall productivity, however, it must be noted that maximum rates are only possible for short periods each day (Fig. 6.7) and respiratory losses are significant (Fig. 6.8; Lange et al. 2000, 2004). In the deep shade of the understorey of tropical rain forests lichens may also make effective use of light-flecks (Lakatos et al. 2006; Sect. 4.2.1).

6.3 Lianas, Climbers, Vines and Hemi-Epiphytes

Lianas, climbers and vines are rooted in the soil and use other plants, especially trees, as support for growth away from the ground (Holbrook and Putz 1996a). It is mostly assumed that the particular advantage of this habit is to allow these plants to escape from deep shade and to reach the upper canopy of forests. This implies that their seeds would germinate in the shade and seedlings initially would grow upwards and develop in the shade. In the tropics, lush growth of lianas and climbers, however, is mostly found adjacent to clearings and in sites disturbed by man, and it appears that these forms need high irradiance for establishment and development. Lianas in fact can slow down successions in the reinvasion of gaps (Sect. 3.3.3), in that they suppress climax species, and thus, support the growth of pioneer species (Schnitzer et al. 2000). Growth of saplings of trees is not only inhibited by lianas via competition for light but also below ground via competition in the root medium (Schnitzer et al. 2005).

The plants climb using tendrils formed from modified leaves or parts of leaves, shoots or adventitious roots. Shoots wind around branches or form coils, which are then modified by the secondary thickening, so that the wood develops in the form of bands (Fig. 6.9), or it is fragmented to individual strands forming rope- or cablelike structures which are resistant to torsion.

Some species in the aroid genera Philodendron and Monstera and the Cyclan-thaceae Asplundia begin their life with rooting in the soil and climbing up a phoro-phyte, but later their old roots degenerate. By growing at the tip and continuously degenerating the base of their shoots, they literally crawl up their hosts. Hence, they begin as lianas and later become epiphytes. They have been termed secondary hemi-epiphytes. However, this term is not all that convincing; strictly they are sec-

Fig. 6.9 Spirally twisted flat and band-like shoots of lianas

ondarily epiphytes. Moreover, in some cases aerial adventitious roots can be formed again, which may hang down from these plants like curtains (Fig. 6.3C) and establish contact with the ground for a second time. Thus, secondary hemi-epiphytes would become primary hemi-epiphytes, a term used for plants which start their life epiphytically but subsequently establish soil-contact.

The latter include the stranglers, a true group of "murderers". Among them is a genus with an extreme plasticity, namely Clusia (Clusiaceae, Order Theales) (see also below: Sect. 6.6.2.3). Clusia-seedlings may get established terrestrially and grow directly as independent trees. However, these plants, like other stranglers, may begin their seedling stage as humus epiphytes, using accumulations of humus in knotholes or between branches of phorophytes for establishment (Fig. 6.10A). Alternatively, they germinate in epiphyte gardens together with several other epiphytic species (Fig. 6.10B-D), where tanks of bromeliads or nest and basket-forming ferns provide the required substrate. Then, adventitious roots develop, some of which grow positively gravitropically to the ground whilst others are attached to the host tree (Fig. 6.11). First they may only compete with their host for light. Subsequently, after rooting in the ground, they also compete for nutrients in the soil. Eventually they strangle and kill their host. Their adventitious roots surrounding the trunk of the host tree hinder the secondary thickening and clamp the phloem in the bark with the sieve tubes, the tender pathways for long distance transport of assimilates. Prevented from adequate partitioning of supplies, the phorophyte dies. It seldom falls

Fig. 6.10A-D Epiphytic seedlings of Clusia rosea in humus accumulation of tree forks (A) in an epiphytic garden (B), and inside tanks of the bromeliad Aechmea lingulata (C, D). In D the tank of A. lingulata has been cut open showing the accumulated humus and the root system of a Clusia seedling
Fig. 6.11 Adventitious root-system of the strangler Clusia rosea

down. The roots of its ungrateful visitor often form a veritable net with anastomoses via parenchyma bridges, and inside this hollow cylinder of adventitious roots the stem of the former host rots away. Thus, the originally epiphytic strangler becomes an independent tree with a pseudostem of adventitious roots. Such behaviour is not only observed by Clusia, of course, but equally by other genera with stranglers, e.g. Ficus species (Fig. 6.12). Other species of Ficus, i.e. F. pertusa and F. trigonata live in palm trees. They generate negatively gravitropic roots which suspend them in the crowns of the palms, where they find humus between the leaf bases (Putz and Holbrook 1986). A most successful form is represented by Ficus bengalensis. The original aerial roots of a single plant, after gaining ground-contact, may form an entire forest of pseudostems. Walter and Breckle (1984) described an individual, which was only 26 m tall but had an average crown diameter of 170 m, a crown circumference of 530 m and a crown area of 22,000 m2. (See also Ficus microcarpa in Fig. 6.12C.)

By clasping other plants lianas are saving investment in thick stems which would provide independent support for their heavy plant biomass. Only the first shoots produced after germination are self supporting and their Young's modulus is high indicating higher stiffness or lower elasticity, then the shoots start to climb which is associated with anatomical changes and a decreasing Young's modulus (Rowe and Speck 1996; Speck 1997). The reduction of supporting tissues in lianas is also true for their leaves which are found to have a lower leaf mass per unit leaf area and this is also beneficial for optimising nitrogen use in relation to photosynthesis of

Fig. 6.12A-C Network of anastomosing strangler roots of Ficus sp. (A) and interior of the hollow cylinder marking the stem of the original host (B) in a cloud forest above Lake Coté, Costa Rica. "Forest" of pseudostems of one individual tree of Ficus microcarpa (Foster Botanical Garden, Honolulu, Hawaii) (C)

Fig. 6.12A-C Network of anastomosing strangler roots of Ficus sp. (A) and interior of the hollow cylinder marking the stem of the original host (B) in a cloud forest above Lake Coté, Costa Rica. "Forest" of pseudostems of one individual tree of Ficus microcarpa (Foster Botanical Garden, Honolulu, Hawaii) (C)

fast growing lianas (Kazda and Salzer 2000). Using the support by their host plants lianas can afford to have very wide xylem vessels reducing friction for the transpiration stream and facilitating transport over long distances (Ewers et al. 1990). Vessels of up to 0.7 mm in diameter have been observed. The xylem sap readily flows out of these wide vessels when they are cut open. Wandering around one may serve oneself from such lianas for a cool drink. Vareschi (1980) reports on a 1 m long piece from which 205 ml of water were collected within 3 min.

Thus, lianas and hemi-epiphytes evidently develop a particular hydraulic architecture to support a huge leaf biomass via relatively thin shoots. The theoretical expectation is that compared to free standing trees (Zotz et al. 1997):

• lianas and hemi-epiphytes show significantly higher specific stem conductivity, Ks,

• lianas and hemi-epiphytes show less wood cross-section per unit leaf area, i.e. lower Huber-values, Hv,

• lianas and hemi-epiphytes tend to have less conductive stems per unit leaf area, Kl.

A quantitative comparison between free standing trees of three species and hemi-epiphytes of four species of Ficus, respectively, and the hemi-epiphyte Clusia uvi-tana has been presented by Patiño et al. (1995), where Ficus species are C3-plants and C. uvitana is a C3/CAM-intermediate species (Table 6.2). The ranges of Ks values for the hemi-epiphytic and the terrestrial Ficus species overlap, but as expected much higher maximum values are reached in the hemi-epiphytes. However, for the hemi-epiphytic C. uvitana the Ks value is much lower. As also expected Kl values are lower in hemi-epiphytic Ficus species than in free standing trees, but the values for C. uvitana are still much lower. The Huber-values, Hv, are similar in both the C3 and C3/CAM-hemi-epiphytes and as expected lower than in the free standing trees. The differences in Ks and Kl values between the hemi-epiphytes with the different modes of photosynthesis show that in addition to hemi-epiphytism hydraulic architecture is also related to the performance of CAM and that due to the water saving mechanism of CAM (Sect. 5.2.2.2, Box 5.1) C. uvitana can afford lower specific stem conductivity, Ks, and lower leaf specific conductivity of stems, Kl.

The wide vessels of lianas are potentially prone to cavitation and embolism which would lead to loss of conductivity. However, Andrade et al. (2005) observed that maximum sap flow densities in co-occurring lianas and free standing trees were comparable at a similar stem diameter. In the tropical vine-like bamboo Rhipido-

Table 6.2 Hydraulic architecture parameters of free standing trees and hemi-epiphytes of the genus Ficus and the hemi-epiphyte Clusia uvitana (rounded up values from Patino et al. 1995)

Ficus C. uvitana

Free standing Hemi-Epiphytes Hemi-Epiphyte trees

Specific stem conductivity Ks (kg s-1 m-1 MPa-1) Cross section per unit leaf area Huber-value, Hv x 104 Conductive stem per unit leaf area Kl (kg s-1 m-1 MPa-1)_

cladum racemiflorum (Cochard et al. 1994) the rather wide xylem vessels were found to be surprisingly resistant to cavitation. Experimentally, xylem water potentials (see Box 6.1 for explanation of terminology) of -45 bar were required to induce 50% loss of hydraulic conductivity, but at the end of the 1993 dry season potentials of only - 37.5 bar were reached with a loss of conductivity of 10% due to cavitation and embolism. In the wet season high root pressures possibly repair cavitation. When transpiration was low in the rainy season, i.e. at night and during rain events positive hydrostatic potentials up to 1.2 bar were built up. The hemi-epiphyteMonstera acuminata built up pressures of 2.25 bar in the aerial roots reaching the soil, which are among the highest root pressures known (Lopez-Portillo et al. 2000).

In contrast to the vine R. racemiflorum, C. uvitana is found experimentally to be very vulnerable to cavitation loosing 50% of hydraulic conductivity at a stem water potential of only -13 bar (as compared to -45 bar in R. racemiflorum). It is possible that CAM is important in helping to prevent additional damage by insuring low transpiration rates and avoiding dehydration. Indeed, in comparison with two other epiphytes with different adaptive strategies, C. uvitana performed equally well in terms of long-term carbon gain. The comparison (Zotz and Winter 1994b) included:

• the evergreen C3/CAM intermediate C. uvitana,

• the drought-deciduous C3 -orchid Catasetum viridiflavum,

• the evergreen C3-fern Polypodium crassifolium.

In C. uvitana, during the four months dry season, mean daily carbon gain was reduced by ca. 40% following the shift from C3-photosynthesis to CAM, with strongly decreased daytime CO2-uptake. C. viridiflavum grew new leaves in the second half of the dry season with greatly reduced carbon gain. In the wet season rates of CO2-uptake by these leaves doubled. Growth occurred until the end of the wet season, when leaves were shed again. In P. crassifolium the daily carbon balance was negative during the dry season, and the epiphyte showed characteristics of chronic photoinhibition. Nevertheless, in all of the three species there were similar rates of annual carbon gain (1,000 g CO2 m-2 year-1) and long-term nitrogen-use efficiency (i.e. annual carbon gain/mean leaf N content was around 1.1g CO2 kgN-1 year-1). The long-term water use efficiency (WUE) of net CO2 uptake in C. uvitana was more than twice that in the other two species.

Box 6.1 (Continued)

ty is best explained using an osmotic system of two chambers separated by a semipermeable membrane each having a vertical tube. The solvent particles (• = H2O molecules) can pass the semipermeable membrane, whereas the solute particles (o = solute molecules) cannot permeate.

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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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