Epiphytes

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As noted above, the only vascular epiphyte in the mesic climate, the fern Polypodium vulgare, is a facultative epiphyte. This means that there are gradations between the terrestrial and epiphytic habit (Gessner 1956; Richards 1996). In the tropics many species, e.g. among bromeliads and aroids, grow equally well terrestrially and epiphytically (Fig. 6.13; see also Table 6.4).

Benzing (1989a) has conceived five different schemes alternatively classifying epiphytes in categories based on:

I. relationships to the host (or "phorophyte"),

II. growth habit,

III. humidity,

IV. light

V. phorophyte-provided media

(Table 6.3). The epiphytic life form is effectively encompassed in categories I and II. The other three categories refer to the three major stress factors of epiphytic plant life (see Sect. 6.6). The entire system of five schemes is very useful as it offers a good summary of the great morphological and ecophysiological diversity among epiphytes and their associates. On the other hand, it suffers from the general problem of attempts of this kind of casting the diversity of life into schematic systems. Thus, the study of case stories may prove more appealing.

One of the most exciting case stories is offered by the Bromeliaceae (for monographs see Martin 1994; Benzing 2000). They operate with tanks and epidermal scales or trichomes. The tanks are made up by densely overlapping leaf-bases of the rosette-forming bromeliads and depending on life form there is a gradation in effectiveness of water storing capacity. The scales are epidermal structures which developed increasing complexity during the evolution of bromeliads. They consist of living basal or foot cells, stalk cells, which may be living or dead in the mature stage of the trichomes, and the actual scales comprised of dead cells (Fig. 6.14).

Fig. 6.13A,B The bromeliad Aechmea lingulata growing both terrestrially (A) and epiphytically (B). (St. John-Island, US Virgin Islands, Lesser Antilles)

Fig. 6.14A,B Schemes of scales of bromeliads. A Top view. B Cross-section the living cells of the scale dotted and with a nucleus. The black line along cells in B indicates cutinization of the epidermal cells and the outer walls of the trichome stalk cells which allows entry of solutes into the leaves only via a specific pathway enforcing membrane passage and cytoplasmic control over the solutes taken up. (After Sitte 1991 with permission of G. Fischer-Verlag)

Fig. 6.14A,B Schemes of scales of bromeliads. A Top view. B Cross-section the living cells of the scale dotted and with a nucleus. The black line along cells in B indicates cutinization of the epidermal cells and the outer walls of the trichome stalk cells which allows entry of solutes into the leaves only via a specific pathway enforcing membrane passage and cytoplasmic control over the solutes taken up. (After Sitte 1991 with permission of G. Fischer-Verlag)

Table 6.3 Five different schemes alternatively classifying epiphytes. (After Benzing 1989a,b)

I. Relationships to the phorophyte

1. Autotrophs, using the phorophyte only for support

1.1 Accidental

1.2 Facultative

1.3 Hemi-epiphytic

1.3.1 Primary

1.3.1.1 Strangling

1.3.1.2 Non-strangling

1.3.2 Secondary

1.4 Genuinely epiphytic

2. Parasites

II. Growth habit

1. Trees

2. Shrubs

3. Suffrutescent to herbaceous forms

3.1 Tuberous

3.1.1 Storage, woody and herbaceous

3.1.2 Myrmecophytic, mostly herbaceous

3.2 Broadly creeping: woody or herbaceous

3.3 Narrowly creeping: mostly herbaceous

3.4 Rosulate, herbaceous

3.5 Root/leaf tangle, herbaceous

3.6 Trash-basket, herbaceous

III. Humidity

1. Poikilohydrous (mostly lower plants)

2. Homoiohydrous

2.1 Hygrophytes

2.2 Mesophytes

2.3 Xerophytes

2.3.1 Drought-endurers

2.3.2 Drought-avoiders

2.4 Impounders

IV. Light

1. Exposure types

2. Sun types

3. Shade-tolerant types

V. Phorophyte-provided media

1. Relatively independent of rooting medium

1.1 Atmospheric forms

1.2 Twig and bark inhabitants

1.3 Forms creating substitute soils or attracting ant colonies

2. Utilizing preexisting specific rooting media

2.1 Humus-dependent

2.2.1 Shallow humus forms

2.2.2 Deep humus forms

2.2.3 Ant-nest garden and plant catchment inhabitants

2.2 Parasites

By the structure of tanks and scales we may distinguish four different life forms of bromeliads (Table 6.4):

Some bromeliads which are obligately terrestrial do not form tanks; often these forms are highly xeromorphic; they may be densely covered with scales; however the scales do not function in water and nutrient absorption but may rather serve reflection of light (see Sect. 5.2.2.1, Figs. 5.3 and 6.15A).

Other obligately terrestrial bromeliads have rudimentary tanks, which have limited water and litter collecting capacity; the scales make only minor contributions to water and solute uptake; however, in addition to the soil-roots, plants of this type develop stem-borne "tank-roots" growing up between the overlapping leaf bases into the tanks (Figs. 4.3 and 6.15B).

• Type III: Tank-Absorbing Trichome.

The roots are conditionally absorbent but mostly have only mechanical functions for holdfast in these epiphytic forms and may even secrete a cement-like lipopolysaccharide (Brighigna et al. 1990), the tanks effectively collect rain water and decomposing debris; scales are found most densely on the leaf bases in the tank, where they serve water and nutrient uptake (Figs. 6.13 and 6.15C).

• Type IV: Atmospheric-Absorbing Trichome.

Tanks in these forms are mostly absent and only occasionally poorly developed;

Table 6.4 Life-forms of Bromeliaceae, their characterization and distribution among the three subfamilies Pitcairnioideae, Bromelioideae and Tillandsioideae. (After Pittendrigh 1948; Smith et al. 1986a; Smith 1989; Benzing 2000; note that the latter author has recently separated two different groups out of type III and distinguishes five types)

Table 6.4 Life-forms of Bromeliaceae, their characterization and distribution among the three subfamilies Pitcairnioideae, Bromelioideae and Tillandsioideae. (After Pittendrigh 1948; Smith et al. 1986a; Smith 1989; Benzing 2000; note that the latter author has recently separated two different groups out of type III and distinguishes five types)

Designation of life-form

Root system

Tank

Epidermal trichomes

Growth habit

Taxa

Type I

Absorbent soil roots

Lacking

Unspecialized and non-absorbent

Obligately terrestrial

Great majority of Pictarnioideae/ many Bromel-ioideae

Type II

Absorbent soil roots and tank roots

Rudimentary

Relatively unspecialized

Obligately terrestrial

All terrestrial Bromelioideae

Type III

Usually only mechanical

Well developed

Specialized and absorbent; concentrated on leaf base

Most obligately (some facultatively) epiphytic

All the epiphytic

Bromelioideae, majority of Tillandsioideae

Type IV

Exclusively mechanical

Often entirely lacking

Specialized and absorbent; often cover entire shoot

Obligately epiphytic

(or saxicolous)

Tillandsioideae: Several species of Vriesea, otherwise species exclusively Tillandsia

Fig. 6.15A-D Life forms of bromeliads. A Type I, soil root, Pitcairnia integrifolia, Trinidad. B Type II, tank root, Bromelia humilis, Falcon, Venezuela. The basal leaves were removed and the rosette was turned upside down for photography, so that the tank roots growing upwards between the leaves can be seen. C Type III, tank-absorbing trichome, Tillandsia fasciculata, Cerro Santa Ana, Paraguana Peninsula, Falcon, Venzuela. D Type IV, atmospheric-absorbing trichome, Tillandsia usneoides, Merida, Venezuela

Fig. 6.15A-D Life forms of bromeliads. A Type I, soil root, Pitcairnia integrifolia, Trinidad. B Type II, tank root, Bromelia humilis, Falcon, Venezuela. The basal leaves were removed and the rosette was turned upside down for photography, so that the tank roots growing upwards between the leaves can be seen. C Type III, tank-absorbing trichome, Tillandsia fasciculata, Cerro Santa Ana, Paraguana Peninsula, Falcon, Venzuela. D Type IV, atmospheric-absorbing trichome, Tillandsia usneoides, Merida, Venezuela the entire leaf surface is covered by highly specialized scales, which provide the only route for uptake of water and minerals from rain and dust in the atmosphere; in some forms roots are lacking entirely (Figs. 6.4 and 6.15D).

These life forms of bromeliads provide an interesting example of how the vegetative plant form has been shaped by evolution towards epiphytism; and in this case, particularly driven by the need for water and nutrient acquisition in the epiphytic habitat.

6.5 Mistletoes

Mistletoes growing on bushes and trees are not literally epiphytes, which originally use the phorophytes only as a holdfast. Mistletoes are true parasites. They largely belong to two families of the Order Santalales, namely the Loranthaceae (~900 species and 65 genera) and the Viscaceae (~400 species). Mistletoes occur ubiquitously in the temperate zone, in arid regions as well as in the wet tropics (Sallé et al. 1993). The majority of mistletoe taxa occur in the tropics. Although ecophysiol-ogy of mistletoes is increasingly well studied (Popp and Richter 1997), apparently it is not known why they have such a particularly high diversity and biomass in the tropics (Benzing 1990).

When germinating on the host trees, haustoria of mistletoes penetrate through the bark and join the host cambium, where they form a cambium themselves, which keeps pace with that generating the secondary thickening of the host so that the haustoria gradually become incorporated in the host's wood (Sallé et al. 1993). Via the haustoria the mistletoes establish vascular contacts with the host. Very few mistletoes have phloem connections, since the contacts are predominantly apoplas-tic between the xylem elements of host and parasite. Thus, the standard view is that mistletoes are hemi-parasites on the xylem and transpiration-stream taking only water and nutrients from the host, while they are photosynthetically competent and capable of their own assimilation. The idea that mistletoes might have evolved from terrestrial root hemi-parasites sucking the xylem of host roots has been discussed (Benzing 1990).

In order to direct part of the transpiration stream from the host to their own shoot system for water and nutrient supply, mistletoes need to establish the required driving force. Indeed, it has been shown that they have a more negative leaf-water potential (see Box 6.1) and a larger leaf-conductance for water vapour and hence a higher transpiration rate, than the host leaves (Schulze et al. 1984; Ziegler 1986; Richter et al. 1995; Popp and Richter 1997). The difference between the leaf conductances in mistletoes and in their hosts respectively, can also be demonstrated by carbonisotope analysis, because in C3-plants the variable rate of CO2-diffusion via stomata primarily determines overall changes in 13C-discrimination during photosynthesis, i.e. more negative 813C-values indicate higher life time stomatal conductance, gH2O, higher average internal CO2-partial pressures pCi O , and lower water use efficiency, WUE (Sect. 2.5). Lüttge et al. (1998) measured 21 host mistletoe pairs in Brazil and found that consistently 813C-values were more negative in the mistletoes than in the host leaves documenting higher transpiration rates and gH2O of the mistletoes and their operation at higher pCO2 (Table 6.5). The higher transpiration rates also contributed to higher transpirational cooling of the mistletoe leaves as compared to the host leaves (Table 6.5), which certainly is an additional advantage of the parasites in hot and dry sites.

If mistletoes have similar or lower CO2-assimilation rates as compared to host leaves, this also implies that the mistletoes may have considerably lower water-use-efficiencies (WUE = CO2 assimilated : H2O transpired) at the expense of the host. Net CO2 uptake is generally considered to be lower in mistletoes as compared to their hosts (Popp and Richter 1997). However, in the study of Luttge et al. (1998) photosynthetic capacity assessed from measurements of chlorophyll fluorescence parameters (Box 4.6) in mistletoe leaves proved to be similar to that of host leaves. Only in very exposed open sites photosynthetic capacity of mistletoe leaves was inferior, but that was due to a pronounced sun-type expression of host plant leaves. Mistletoes may even grow on mangrove associates like Conocarpus erectus (Orozco et al. 1990; see Chap. 7) and true mangroves, where they must establish a water potential gradient large enough to allow movement of water downhill from the salt-loaded halophilic host to their own leaves, but it is also observed that increasing drought and salinity stress may make hosts less suitable for invasion by mistletoes (Miller et al. 2003).

The view that mistletoes exclusively are parasites for water and nutrients, needs to be modified since carbon gain of mistletoes from the host can be significant (Richter et al. 1995; Escher et al. 2004). Studies of partitioning of dry matter and mineral nutrients (Pate et al. 1991a,b), which included analyses of carbon-isotope ratios (see Sect. 2.5; Marshall and Ehleringer 1990), showed that 24% (Pate et al. 1991a) to 62% (Marshall and Ehleringer 1990) of the mistletoe carbon may be derived from the host, and a tabulation of Popp and Richter (1997) even lists values up to 87%. This is not yet the ceiling though, because a fully holoparasitic het-erotrophic Loranthaceae mistletoe, Tristerix aphyllus, has been discovered, which grows on the tissue of cactus stems (Kraus et al. 1995). First the transfer of carbon-compounds from the hosts to the mistletoes is partially due to the fact, that under various circumstances the xylem sap itself may also carry organic compounds. Second the mistletoe tissue may take up organic material from the host phloem by phloem unloading via apoplastic pathways and active membrane-transport, where the mistletoe becomes a sink for source substrates from the host. The involvement

Table 6.5 Comparison of 21 mistletoe/host-pairs in the cerrado belt of Brazil. Data are averages of differences between mistletoes and hosts leaves, i.e. mistletoe minus host values are shown. 8 13C and pCi O are derived from carbon isotope analyses of dried leaf material, leaf temperatures, Tleaf, are from instantaneous measurements in the field. (Averages ±SD(n)). (After data of Luttge et al. 1998)

of active, membrane-controlled transport can make acquisition of both mineral ions and organic compounds by the mistletoe from the host a highly selective process (Pate et al. 1991b; Rey et al. 1991; Escher et al. 2003). Phloem mobile mineral nutrients can also arrive in the mistletoes via the internal phloem-xylem-circulation of the host (Bannister et al. 2002).

An interesting morphological feature of mistletoes, related to parasite-host nutrient relations, is the often observed strong resemblance between parasite and host leaves (Ehleringer et al. 1986; Fig. 6.16). The mimicry of host leaves is common

Fig. 6.16 The Loranthaceae Phthirusa ovata (recognisable by its inflorescence) in a host tree, Brazil

since mistletoes often have higher nitrogen contents than their hosts, and hence reduces the likelihood of mistletoe herbivory. Mimicry is absent when mistletoes are poorer in N than the host.

6.6 Stressors Driving Ecophysiological Adaptation of Epiphytes and Hemi-Epiphytes

The major factors, which limit epiphytic life and thus may become stressors (Box 3.1), are:

• mineral nutrients.

Light (Sect. 6.6.1) is highly variable, similar to the situation of other species operating in different strata of the forest canopy (see Sect. 3.4.1). Water (Sect. 6.6.2) and mineral nutrients (Sect. 6.6.3) are particularly difficult resources to obtain by epiphytes having no roots in the soil and therefore availability of water may be considered the major constraint in the epiphytic habitat.

6.6.1 Light and the Evolution of Plants to Epiphytism

A generally encountered view assumes that climbing and epiphytism in plants is a struggle for light in an escape from the darkness of forest floors. This goes back to A.F.W. Schimper, who concluded his observations in forests of the American tropics with the hypothesis that epiphytic bromeliads evolved from shade adapted terrestrial forms (Schimper 1888). However, as we have already mentioned above (Sect. 6.1), lianas and vines are most frequently light demanding plants of pioneer successions. Epiphytes occupy sites of variable light exposure. Studies of the distribution of epiphytic orchids on phorophytes in a West African rainforest have shown that only a small percentage of the total number of species are found in the upper canopy, and most species dwell within the crowns of trees (Fig. 6.17).

Pittendrigh (1948) grouped the epiphytic bromeliads of Trinidad in three categories according to their light demand:

• a shade tolerant group.

Using stable carbon isotope analysis (Sect. 2.5), Griffiths and Smith (1983) have determined the distribution of C3-photosynthesis and CAM among these 40 species. They related the mode of photosynthesis to Pittendrigh's light-demanding categories and the annual precipitation at the sites where they occur in Trinidad. The result of

Fig. 6.17 Distribution of epiphytic orchids on trees in a West African rainforest. Numbers of species found in the different zones of the phorophyte related to total orchid species counted. The zones of the phorophyte are: A the basal part of the stem up to 3 m above ground level; B the stem up to the first ramifications; C , D and E the canopy divided into three equal parts along the length of the branches from inside to outside. (Goh and Kluge 1989, after Johansson 1975)

Fig. 6.17 Distribution of epiphytic orchids on trees in a West African rainforest. Numbers of species found in the different zones of the phorophyte related to total orchid species counted. The zones of the phorophyte are: A the basal part of the stem up to 3 m above ground level; B the stem up to the first ramifications; C , D and E the canopy divided into three equal parts along the length of the branches from inside to outside. (Goh and Kluge 1989, after Johansson 1975)

the survey is depicted in Fig. 6.18 (see also Fig. 6.23). At the wettest site (> 6.4 m precipitation per year) the shade group is not represented at all, with one species of the exposure group, six species of the sun group and only one CAM species being present. At somewhat lower annual precipitation, the sun group prevails with a total of eight species and three CAM species among them. Under intermediate precipitation, only C3 species comprise the exposure and shade groups. However, at the driest sites one finds only CAM plants of the exposure and sun groups. Thus, CAM among epiphytic bromeliads is clearly correlated with reduced water availability and sun exposure which exacerbates drought stress.

Together with the development and specialization of epidermal trichomes (see Sect. 6.4, Table 6.4, Fig. 6.14), which can be considered as an evolutionary trait (Mez 1904; Tietze 1906), Pittendrigh (1948) used the abundance and distribution of species in the three categories for consideration of the evolution of epiphytism among bromeliads. He suggested that epiphytic bromeliads did not evolve from shade demanding ancestors of the forest floor but rather were derived from terrestrial ancestors of open habitats originally adapted to sun exposure and at least temporary drought.

The observation that CAM occurs only among bromeliads of the sun and exposure groups supports this interpretation because phylogenetically CAM is considered to be rather an advanced physiological trait. The family of the Bromeliaceae

Fig. 6.18 Distribution of epiphytic Bromeliaceae of the exposure group (Ex), the sun group (Su) and the shade-tolerant group (Sh) with C3 photosynthesis (open parts of the bars) and CAM (closed parts of the bars) in Trinidad related to annual rainfall. (After Griffiths and Smith 1983)

Fig. 6.19 Scheme of putative phylogenetic relationships within the Bromeliaceae based on the tax-onomic distribution of CAM and C3 photosynthesis at the level of individual genera and molecular systematics. The scheme shows that both the epiphytic habit (E) and CAM must have arisen more than once during evolution of the present-day forms. Within the Bromelioideae there are indications of a progressive loss of CAM in some genera. T = terrestrial, E = epiphytic forms, C3 = plants with C3-photosynthesis. (Smith 1989; Crayn et al. 2000, 2004)

Fig. 6.19 Scheme of putative phylogenetic relationships within the Bromeliaceae based on the tax-onomic distribution of CAM and C3 photosynthesis at the level of individual genera and molecular systematics. The scheme shows that both the epiphytic habit (E) and CAM must have arisen more than once during evolution of the present-day forms. Within the Bromelioideae there are indications of a progressive loss of CAM in some genera. T = terrestrial, E = epiphytic forms, C3 = plants with C3-photosynthesis. (Smith 1989; Crayn et al. 2000, 2004)

is monophyletic. Among the three subfamilies most likely the Bromelioideae and possibly the Tillandsioideae are monophyletic while the Pitcarnioideae are poly-phyletic (Crayn et al. 2000, 2004; Horres et al. 2000). The family comprises about 2800 species (Luther and Sieff 1998). Martin (1994) has identified the photosyn-thetic pathway of 249 species of which 69% show CAM capacity with various degrees of CAM expression. CAM and epiphytism have evolved independently and polyphyletically several times within each subfamily (Fig. 6.19; Smith 1989; Crayn et al. 2000). From a terrestrial C3 ancestor epiphytism evolved first. In the branch giving rise to the Tillandsioideae from the epiphytic C3 plants CAM evolved, so that this subfamily is exclusively epiphytic with both C3 and CAM species. In the branch starting from the terrestrial C3 plants CAM evolved giving rise to the exclusively terrestrial Pitcairnioideae with both C3 and CAM plants. From the branch starting

Table 6.6 Cardinal points of light-response curves of various epiphytes compared to genuine sun and shade plants. Epiphytes were related to sun and shade plants respectively, by evaluating all of the three given criteria (light-compensation point, light-saturation of photosynthesis, rate of photosynthesis at saturation) together, because coordination is not simple when using single criteria individually. (After Luttge 1985)

Plant type or species Light compensa- Light saturation Rate of photosyn-

tion point mol of photosynthesis thesis at saturation photons m-2 s-1) (^mol photons (^mol CO2 or O2

Sun plantsa

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