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6.6.2.1 Acquisition of Water Shaping Life Forms of Epiphytes and Hemi-epiphytes

The availability of water is the most pronounced problem for epiphytes and hemi-epiphytes which have no root-soil contact (Zotz and Hietz 2001). Rada and Jaimez (1992) compared terrestrial and epiphytic plants of the facultatively epiphytic Ara-ceae Anthurium bredmeyeri growing close to each other in a tropical Andean cloud forest. The epiphytic plants were affected to a greater degree by the decrease in water availability during the dry season. They showed a larger decrease in leaf conductance and lower leaf water potentials during the dry season than the terrestrially growing plants as well as a reduction in stomatal densities in new leaf growth. Clearly, the water factor can have a large influence on life form of epiphytes.

Most of the lower plant epiphytes, i.e. aerial algae, lichens, bryophytes and even some ferns, are poikilohydrous and desiccation tolerant (Table 6.3: III 1; see also Sect. 11.4.2). They are truly resistant to drought stress, because they can dry out without suffering damage, overcoming drought periods in a non-hydrated state and becoming viable again when water can be absorbed from precipitation. Of the lichens only those, which have green algae as the photoautothrophic symbionts, are able to acquire their water and reactivate photosynthesis from the water vapour in the gas phase (Lange et al. 1986, 1988). This also holds for pleurococcoid aerial green algae (Bertsch 1966). However, lichens having cyanobacteria as symbionts require water in liquid form to reactivate photosynthesis.

Tillandsoid trichomes have no effect on leaf boundary layers and any associated reduction in transpirational water loss (Benz and Martin 2006). Some atmospheric bromeliads may take up water from the gas phase of the atmosphere by equilibration of the hygroscopic cell walls of the dead scale cells in the trichomes which densely cover their surface. Thus, one may observe a peak of water-vapour uptake when the relative air humidity (RH) increases at the beginning of the night (Fig. 6.20). However, this is matched again by a loss of water vapour at the beginning of the day when RH decreases and therefore the bromeliad leaf cells do not have a net gain of water from this mechanism (Schmitt et al. 1989).

In consequence, angiosperm epiphytes have developed a range of other adaptations which often are equally related to the nutrient "stress" factor (Sect. 6.6.3), e.g. formation of tanks or humus collecting baskets, in which they effectively create their own soil with a limited water storage capacity. Water demanding animals like small frogs may even live in tanks of bromeliads (Fig. 6.21), which in some species

Fig. 6.20A-C Night-day cycle of water-vapour exchange by plants of Tillandsia recurvata L. A Water-vapour exchange of normal living plants shows a peak of net uptake (negative values of JHlO) as the dew-point temperature (T) decreases and relative air humidity (RH) increases at the onset of the dark period, and a peak of net release (positive values of JH2o) with the opposite changes of T and RH at the beginning of the light period. B These peaks are also observed with plants killed in boiling water. They are restricted to passive hygroscopic equilibration of dead structures. C Subtraction of JH2O by the dead plants from that of the living plants shows true transpirational water-vapour loss, which is much higher throughout most of the dark period in this CAM bromeliad than during the light period. (Schmitt et al. 1989, from Luttge 1989)

Fig. 6.20A-C Night-day cycle of water-vapour exchange by plants of Tillandsia recurvata L. A Water-vapour exchange of normal living plants shows a peak of net uptake (negative values of JHlO) as the dew-point temperature (T) decreases and relative air humidity (RH) increases at the onset of the dark period, and a peak of net release (positive values of JH2o) with the opposite changes of T and RH at the beginning of the light period. B These peaks are also observed with plants killed in boiling water. They are restricted to passive hygroscopic equilibration of dead structures. C Subtraction of JH2O by the dead plants from that of the living plants shows true transpirational water-vapour loss, which is much higher throughout most of the dark period in this CAM bromeliad than during the light period. (Schmitt et al. 1989, from Luttge 1989)

Fig. 6.21 Small frog in a tank of a flowering plant of Bromelia humilis

can impound 5-10 l of water. Water storage tissues in leaves and stems may also be prominent, so that leaf and stem succulence occurs in most bromeliads (Horres and Zizka 1995), orchids and the epiphytic cacti (Fig. 6.22). In this relation it has been underlined that independent of age plant size of epiphytes matters a lot because availability and especially storage capacity of water is highly size dependent and this has effects on many other functions including allocation and partitioning of nutrients and area based photosynthetic capacity. Identical environmental conditions impose different degrees of stress on co-occurring smaller and larger plants (Zotz and Andrade 1998; Schmidt and Zotz 2001; Schmidt et al. 2001; Zotz et al. 2001, 2004; Zotz and Hietz 2001).

6.6.2.2 CAM and Water Relations Parameters

The pre-eminent role of water in limiting the life of epiphytes has resulted in the frequent occurrence of CAM, the mode of photosynthesis which conserves water (see Sect. 5.2.2.2). At Barro Colorado Island, Panamá, 25% of the epiphyte flora are CAM plants (Zotz and Ziegler 1997). Some authorities have counted about 13,500 species of epiphytes with CAM. This corresponds to 57% of all epiphyte species, while only 10% of all vascular plants are CAM species. The advantage of CAM for epiphytic life is:

Fig. 6.22A-C Stem succulent epiphytic cacti (A Epiphyllum, B Selenicereus inermis growing through a termite nest), and adaxial water storage tissue of a leaf succulent bromeliad (C)

• water saving, i.e. a high water use efficiency,

• provision of an osmotic driving force for water uptake by nocturnal acid accumulation,

• flexibility in the mode of carbon acquisition. See Sect. 5.2.2.2.

A census of epiphytic bromeliad species in Trinidad has related the frequency of bromeliad epiphytes and the relative number of CAM species to annual rainfall and the prevailing type of forest (Fig. 6.23). Very dry deciduous seasonal forest sustains low epiphytic bromeliad biomass and the small number of species are CAM plants. The abundance of the epiphytic bromeliad species is highest in the evergreen seasonal forest and the lower montane rainforest (Fig. 6.23). This is consistent with the general observation that epiphyte richness is highest at mid elevation, e.g. at 1,000 m a.s.l. in a Costa Rican study covering the altitudinal range from 0 to 2,500 m a.s.l., and strongly correlated to rainfall and not to temperature and light in the canopy (Cardelús et al. 2006). In the Trinidadian study the relative contribution of CAM species to the total number of species declines rapidly as forests get wetter and the water saving function of CAM becomes less important. A decrease of CAM epiphytes with increasing altitudes as seen in Fig. 6.23 is frequently described (Earnshaw et al. 1987; Hietz et al. 1999).

Table 6.7 summarizes some water-relation parameters (Box 6.1) of epiphytes. Since most of those studied to date are also CAM-species, it is difficult to decide

Seasonal Forest Rainforest semi- lower upper deciduous^ evergreen evergreen ¡ level J subalpine semi- lower upper deciduous^ evergreen evergreen ¡ level J subalpine

Fig. 6.23 Relations between total number of epiphytic bromeliad species, the relative number of CAM species among them, annual rainfall and prevailing forest types in Trinidad. (Smith 1989)

Fig. 6.23 Relations between total number of epiphytic bromeliad species, the relative number of CAM species among them, annual rainfall and prevailing forest types in Trinidad. (Smith 1989)

whether these are typical properties of epiphytes or general characteristics of CAM plants. The high relative water content of epiphytes as compared to various C3-crop plants and trees is noteworthy. A high relative water content is a typical feature of CAM-plants and is associated with high water-storage capacity and succulence (Fig. 6.24). For the epiphytic ferns and orchids of Australia, Winter et al. (1983) could demonstrate correlations between succulence and CAM expression (Fig. 6.24). The highest osmotic pressures (n) of epiphytes in Table 6.7 are somewhat above 20 bar; in C3-desert plants they may reach 100 bar. The lowest, i.e. most negative, water potentials (f) of epiphytes are at -10 bar, in C3-desert plants values below -150 bar may be found. Hence, the cell sap of epiphytes is diluted, i.e. osmotic pressures are relatively low (see also Martin et al. 2004), the water potential is high and the turgor pressure (P) is low. In this respect epiphytic C3- and CAM-bromeliads are little different, and also terrestrial CAM-plants show values in this range. The epiphytic C3/CAM intermediate Clusia uvitana in a rainforest in Panama has leaf water potentials in the same range, i.e. -7 to -9 bar (Zotz et al. 1994).

Clusia (see Sect. 6.6.2.3) and Ficus are genera of hemiepiphytes and stranglers, with very similar habits, but the latter has been studied much less in terms of physiological ecology of photosynthesis and water relations. This is astonishing because Ficus appears to be as successful in tropical forests as Clusia. Both have very different strategies though. Most species of Clusia have CAM-capacity but as far as it is known to date all species of Ficus are obligate C3-plants. Holbrook and Putz (1996b) have made interesting intraspecific comparisons of water relations in the life forms of epiphytes and terrestrial trees in the genera Ficus and Clusia. They found that in five species of Ficus the epiphytic life forms as compared to terrestrial trees had:

• several-fold higher specific leaf area (m2 g-1), which may also be taken as higher degree of "succulence",

• two- to fourfold lower stomatal densities, which may be discussed in relation to the need of reduced transpiration at lower availability of water in the epiphytic habitat,

Table 6.7 Water relation parameters of epiphytes as compared to terrestrial plants. (Simplified from Table 3 of Luttge 1985). Values were obtained by various authors with different methods as the Scholander pressure chamber technique, plasmolysis measurements and cryoscopy

Plants

Relative water contenta

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