The epiphytic habitat

Growing conditions in canopies - rooting media and microclimate - are diverse and often similar to those on the ground. Aerial substrata in arid woodlands, like dry sterile soils, probably impose comparable stresses on resident vegetation. Data presented later suggest that the sodden rotting trunks, ant nests, and continuously moist, debris-filled knotholes of an asea-sonal forest offer to certain epiphytes resources equal to those available in equable terrestrial habitats. In effect, the similarity between the two habitats complicates definition of the epiphytic biotope and leaves in doubt the reason why so many species root exclusively in tree crowns.

Within their broad milieu, canopy-adapted species seem to be no more versatile than soil-rooted plants; most will survive only under narrowly prescribed circumstances (e.g., rigid confinement to twigs vs. larger axes, humus as opposed to "unconditioned" bark, dim instead of brighter loci). Microsites are most diverse and abundant in multistratal forests. There, insolation is strong, and temperature and air saturation fluctuate in the upper canopy. Nearer the ground, photosynthetic photon flux density (PPFD; Fig. 1.1) is the major variable because sun flecks provide most of the irradiance (Chazdon and Fetcher 1984b). Survival at canopy margins so

<10 25 50 100 500 1000 >1000

Figure 1.1. The percentage of PPFD (determined from averaged 10-minute integrated readings) falling into seven classes on a wet-season (solid bars) and a dry-season (hatched bars) median day in a Costa Rican rain forest. PPFD (^mol m"2s_l) ranges of the classes were (1) <10; (2) 10-24; (3) 25-49; (4) 50-99; (5) 100-499; (6) 500-1000; and (7) >1000. (From Chazdon and Fetcher 1984a.)

<10 25 50 100 500 1000 >1000

Figure 1.1. The percentage of PPFD (determined from averaged 10-minute integrated readings) falling into seven classes on a wet-season (solid bars) and a dry-season (hatched bars) median day in a Costa Rican rain forest. PPFD (^mol m"2s_l) ranges of the classes were (1) <10; (2) 10-24; (3) 25-49; (4) 50-99; (5) 100-499; (6) 500-1000; and (7) >1000. (From Chazdon and Fetcher 1984a.)

requires considerable succulence, a capacity to impound precipitation (root balls, leaf cavities), or access to absorptive substrata (mats, ant nests). Dry, open woodlands are even harsher habitats for the epiphyte, but patchiness, disturbance, and gravity, as well as potentially adverse chemistry, must be overcome everywhere.

Fragmentation of substratum within and among tree crowns ensures that seed will probably land on the inhospitable ground rather than on a suitable canopy surface; mounting the necessary fecundity to accommodate these losses tests populations already hampered by aridity and nutrient scarcity. Moreover, successfully established progeny often suffer lethal disturbance: Bark fragments exfoliate, colonized twigs and branches fall, and infested phorophytes eventually collapse. Terrestrial microbes and adsorbents are not available to mitigate allelopathy between epiphyte and tree. On the positive side, epiphytes escape production of much of the mechanical and vascular tissue required by trees to secure light: It seems fair to label them mechanical parasites. They are also spared ground fires and the attention of flightless, weak-climbing predators.

Whatever the benefits and disadvantages, forest canopies offer diverse opportunities for varied life-styles. Just how fully tracheophytes have responded to these opportunities is reflected in the myriad habits and associated water-balance mechanisms, nutritional modes, and reproductive systems exhibited by arboreal species.

Classification of epiphytes

Segregation of epiphytes has been based on many parameters: the nature of adaptation and fidelity to supporting vegetation; growth habit; climatic tolerance; type of substratum; and mechanisms for securing basic resources. Because detailed comparisons of all these systems could alone fill several chapters, a synthesis is presented here that borrows heavily from worthy predecessors (Schimper 1888; Hosokawa 1943; Richards 1952, Wallace 1981). Wherever possible, terminology used in earlier accounts has been preserved. These reworked classifications emphasize function as well as form, using additional refinements in older categories that are made possible by recent developments in plant physiology. As in all earlier taxonomies, species assigned the same identity according to one set of criteria may fall into different categories when compared on other grounds. Scheme I groups the epiphytes according to fundamental types of phorophyte use and deals with peculiarities of that phenomenon in subcategories.

Scheme I. Categories based on relationship to the host A. Autotrophs: plants supported by woody vegetation; no nutrients extracted from host vasculature

1. Accidental

2. Facultative

3. Hemiepiphytic a. Primary

(1) Strangling

(2) Nonstrangling b. Secondary

4. "Truly" epiphytic (the "holoepiphytes" of Schimper)

B. Heterotrophs: plants subsisting on xylem contents and sometimes receiving a substantial part of their carbon supply from a host 1. Parasitic (mistletoes)

Accidental epiphytes possess no special modifications for canopy life, yet they occasionally grow to maturity in forests without ever rooting in the ground. Birds and wind promote colonization wherever moist cavities exist, be they in tree crowns, stone fences, derelict buildings, or rock cervices. Diversity is broadest and individuals are most abundant in humid forests. Autotrophs anchored in the crowns of north-temperate and Boreal trees always belong to this group.

Facultative epiphytes inhabit forest canopies and the ground interchangeably (Fig. 1.13). Depending on local conditions, a single species may anchor in earth or on bark or on both media in the same community. Group 2A is best represented on humid sites where tree branches and soil alike support thick, moisture-retaining mantles of bryophytes, lichens, vascular plants, and associated litter (Fig. 1.20). This group also occupies dry sites where, again, canopy and terrestrial media provide similar - in this instance, demanding - growing conditions. Locations featuring the greatest physical disparity between bark and soil substrata are least conducive to facultative epiphytism.

Primary hemiepiphytes, some of which are stranglers, have no access to the ground early on, but later, after elongate feeder roots grow down to the trunk's base, growth becomes more vigorous (Fig. 1.8). In time, the phoro-phyte can become enmeshed in anastomosing roots and may eventually die as a result of girdling and competition. Should a support decay, a strangling hemiepiphyte with its vigorous vascular cambium (e.g., large-leaf species of Ficus) becomes free-standing. Nonstranglers include small-leaf Ficus and most canopy-based members of Clusia. Secondary hemiepiphytes begin life rooted in earth near a phorophyte and become arboreal when attachment to the tree has been achieved and the vine's older stems and roots decay (Fig. 1.9). The common monocot pattern of steady basal dieback is conducive to secondary hemiepiphytism and explains the preponderance of Liliopsida in

Philodendron Epiphyte

Figures 1.2-1.7. Selected epiphyte types: (1.2) Tillandsia fasciculata with impounded debris in a South Florida swamp forest (X5i); (1.3) Ionopsis utricularioides growing on guava in Ecuador (XK); (1.4) Hydnophytum formicarium illustrating interior of tuberous hypocotyl (XK); (1.5) a trash-basket Anthurium in Amazonian rain forest (X'A); (1.6) the resurrection fern Polypodium polypodioides growing on Quercus virginiana in South Florida (X0.4); (1.7) Campylocentrum fasciola on Theobroma in Ecuador (X%).

this group. A capacity for vascular renewal via stem thickening favors the liana habit among vining dicot species.

True epiphytes routinely spend their entire lives without contacting either forest floor or host vasculature (Figs. 1.3, 1.5, 1.10, 1.11). This group contains the most specialized canopy dwellers, those whose supplies of water and mineral ions are often obtained through unusual plant form and physiology.

The heterotrophs are distinguished from all previous groups by parasitism via haustoria (Fig. 1.12). Mistletoes retain their photosynthetic capacity, although specialized Viscaceae (Figs. 6.1 J, 6.2E) draw substantial photosyn-thate from hosts; a few (e.g., Tristerix aphyllus) are entirely endophytic except for reproductive shoots.

A second scheme categorizes epiphytes by growth habit, a criterion that parallels such other plant characteristics as type of nutrient and water economy. The main distinction in this instance is secondary thickening which, in turn, often correlates with size and, to a lesser extent, longevity. Much more elaborate classifications based on gross form have been erected by others (e.g., Hosokawa 1943; Wallace 1981).

Scheme II

A. Trees

B. Shrubs

C. Suffrutescent to herbaceous forms

1. Tuberous a. Storage: woody and herbaceous b. Myrmecophytic: mostly herbaceous

2. Broadly creeping: woody or herbaceous

3. Narrowly creeping: mostly herbaceous

4. Rosulate: herbaceous

5. Root/leaf tangle: herbaceous

6. Trash-basket: herbaceous

Most forest dominants are trees which grow up from the ground without passing through an epiphytic phase, but some start out as primary strangling hemiepiphytes. A number of true and facultative epiphytes are shrubs, a few growing several meters tall (e.g., Blakea, Rhododendron). All but the most specialized mistletoes are shrubby. Occasional woody forms (e.g., Ficus as juveniles, Markea, some Ericaceae) and pseudobulbous orchids (Figs. 1.10, 1.15) produce storage tubers. Swollen hypocotyls of suffrutescent Myrme-codia and Hydnophytum (Fig. 1.4) also house ants that provide mineral ions (Huxley 1978). Thickened rhizomes of some ferns (Fig. 4.24E-G) perform

Figures 1.8-1.13. Selected epiphyte types: (1.8) Ficus aurea, a strangler fig growing on Taxodium distichum in South Florida; (1.9) an araceous secondary hemiepiphyte after it has lost contact with the soil in an Ecuadoran wet forest; (1.10) Encyclia tampensis (X '/s); (1.11) Tillandsia paucifolia shedding wind-dispersed seeds (X!4); (1.12) Phoradendron flavescens parasitizing Prunus avium in central Kentucky (XK); (1.13) Tillandsia fasciculaia growing on and beneath Quercus virginiana in South Florida.

Figures 1.14-1.19. Selected epiphyte types: (1.14) trap-bearing stems of Utricularia hiimboldtii in leaf axils of Brocchinia tatei on Cerro Neblina, Venezuela (X ยก4); (1.15) young leafy Catasetum growing on a rotten limb in a Mexican wet forest (X'/<); (1.16) Psygmorchis glossomystax anchored to guava twigs (XK); (1.17) an older ant nest-garden supporting luxuriant bromeliads, Peperomia, and gesneriads in Ecuadoran wet forests (X Mo); (1.18) a Lycopodium sp. (X>i); (1.19) Campyloneurum angustifolium with a well-developed root ball growing on Acer rubrum in a South Florida swamp forest (X 'A).

increasing similarity between epiphytic and terrestrial substrata

Figure 1.20. A graphic model depicting conditions most conducive to facultative epiphytism.

increasing similarity between epiphytic and terrestrial substrata

Figure 1.20. A graphic model depicting conditions most conducive to facultative epiphytism.

much the same way. Broadly creeping taxa with herbaceous or woody stems are well represented in Asclepiadaceae (Fig. 4.24C,D), Ericaceae, Gesneri-aceae (Fig. 1.17), and several fern families (Fig. 1.6). Rosulate shoots are best developed in Bromeliaceae (Fig. 1.2); in Anthurium (Fig. 1.5) and (fewer) Philodendron; in Liliaceae (Astelia and Collospermum); in Comme-linaceae (Cochliostema); in Gesneriaceae (Paradrymonia), and in the fern genera Platycerium (Fig. 4.19), Drynaria, and Asplenium. Root/leaf tangle epiphytes are illustrated by orchids with dangling roots (Fig. 1.7) and atmospheric bromeliads with numerous filiform leaves (Figs. 3.11, 7.2). Masses of upward-growing roots qualify various members of Anthurium (Fig. 1.5), Catasetum, Cyrtopodium, Dendrobium, and several other orchid genera for classic trash-basket status.

Humidity and light are the two most decisive factors governing epiphyte location; scheme III addresses the former variable.

Scheme III

A. Poikilohydrous: many bryophytes and lower plants; an unknown number of ferns; and very few, if any, angiosperms

B. Homoiohydrous

1. Hygrophytes

2. Mesophytes

3. Xerophytes a. Drought endurers b. Drought avoiders

4. Impounders

In this scheme, desiccation tolerance distinguishes two major groups: the "poikilohydrous" (desiccation-tolerant) and "homoiohydrous" (relatively desiccation-intolerant) species. Members of group A, sometimes called "resurrection" plants (Fig. 1.6), closely track changes in environmental humidity. The homoiohydrous plants of group B possess a progressively better-insulated water balance system from B1 through B3a. Subgroup B3 plants are further differentiated by leaf texture, longevity, and associated patterns of carbon gain and water use. These species either possess productive, ephemeral, desiccation-prone foliage suitable only for wet season activity (B3b; Fig. 1.15), or they are active year round by virtue of desiccation-resistant leaves or green stems with considerable water storage capacity (B3a; Figs. 1.10, 1.11, 3.1A-E). Members of sizable subgroup B4 maintain external moisture supplies in expanded leaf axils or root masses (Figs. 1.2, 1.5, 1.19). A more complete treatment of these water balance mechanisms and related characteristics of the epiphytes that possess them is presented later.

The second decisive habitat requirement - light - was studied by, among others, Colin Pittendrigh (1948) who segregated the bromeliads of Trinidad into three categories (scheme IV) based on apparent affinity for fully exposed, intermediate, and deeply shaded microsites.

Scheme IV

A. Exposure types: largely restricted to sites in full or nearly full sun

B. Sun types: tolerant of medium shade

C. Shade-tolerant types: tolerant of deep shade

More recent surveys of epiphytic floras elsewhere indicate that Pitten-drigh's three-tiered scheme has general applicability. Less clear is its stability and functional basis. Pittendrigh himself suspected that certain sciophytic species are restricted to shaded habitats more by moisture needs than by the damage strong irradiance might inflict. Acclimatization must occur to some degree among canopy-adapted plants as it does in terrestrial vegetation. But Tillandsia usneoides (Fig. 7.2) shows little physiological/anatomical response to growth in deep shade, a subject pursued further in the next chapter. There is no proof that any epiphyte is as shade-demanding as certain understory herbs in humid forests.

Epiphytes can be divided into main categories on the basis of how adaptable they are to media provided by, or associated with, their phorophyte (scheme V). In this sense, schemes V and I show a bit of overlap.

growing conditions

Figure 1.21. A graphic model depicting physical constraints in canopy habitats that influence epiphyte diversity.

growing conditions

Figure 1.21. A graphic model depicting physical constraints in canopy habitats that influence epiphyte diversity.

Scheme V

A. Relatively independent of rooting medium (obtain moisture and nutritive ions primarily from other sources)

1. Mist and atmospheric forms with minimal attachment to bark

2. Twig/bark inhabitants

3. Species that create substitute soils (impounders) or attract ant colonies

(ant-house epiphytes)

B. Tending to utilize a specific type of rooting medium for moisture and nutritive ions

1. Humus-adapted a. General types that root on shallow humus mats b. Deep humus types that penetrate knotholes or rotting wood c. Ant nest-garden and plant-catchment inhabitants (e.g., Platycerium nest endemics, Utricularia humboldtii in tanks of Brocchinia tatei, numerous species comprising ant nest-gardens)

2. Mistletoes

Group A species utilize phorophytes mainly for anchorage; there is little opportunity for influence from the supporting surface. Roots of certain aroids and orchids (Fig. 1.7) hang free (group Al) and intercept mist droplets or throughfall. Twig/bark forms (A2) draw most nutritive ions and moisture from flowing precipitation and leachates. Only naked twigs and unadorned bark are colonized, as if moisture-retaining media must be avoided (Figs. 1.3, 1.11). Indeed, prolonged contact with wet materials kills these species in culture; heavily trichomed tillandsioid bromeliads (Figs.

CONTINUOUSLY SUPPLIED PULSE SUPPLIED

VEGETATIVE STRUCTURE

( the condition experienced by ( the condition experienced by most everwet forest epiphytes ) most dry forest epiphytes )

herbaceous __

secondary hemiepiphyte _____ _

macroimpoundment____ _

vegetative reduction _combined vegetative functions

VEGETATIVE FUNCTION

humus-based precipitation-based myrmecophily carnivory slow growth antnest-garden occupants trophic myrmecophytes mycorrhiza ammonium user/tolerator water/carbon c3-----

deciduousness solute potential high poikilohydry

REPRODUCTION

pollination: diverse vectors diaspore wind dispersal: animal breeding system:

LIFE HISTORY

monocarpic polycarpic

Figure 1.22. Character states of vascular epiphytes dictated by moisture supply in their microhabitats. (From Benzing 1987a.)

1.11, 3.11) succumb if shoots remain moistened for more than a few days. Water and nutrient ions are drawn from root or shoot impoundments by members of group A3 (Figs. 1.2, 1.5, 1.19). Group B1 taps relatively continuous resource pools; possessing no impoundments, it roots in humus. Only knotholes or absorbent, rotten limbs (Fig. 1.15) will sustain the most transpiration-prone, drought-deciduous taxa. Inhabitants of ant nest-gardens (Figs. 1.17, 5.10, 5.11) and plant catchments (Fig. 1.14) are the most substratum-specific of this second group, as are the mistletoes that require connections to host vasculature.

Epiphytes may also be distinguished by additional, more subtle but nevertheless important, growth phenomena. Although members of no epiphytic species complete life cycles as quickly as do the ephemeral terrestrials, their maturation periods vary widely. Most precocious are some oncidioid orchids; Psygmorchis (Fig. 1.16) and Ionopsis (Fig. 1.3), and perhaps other miniatures as well, develop quickly enough to colonize, often exclusively, short-lived twigs and even individual leaves (Chase 1987). Larger orchids and other epiphytes in the same forests ripen first fruit only after many seasons. The atmospheric bromeliad Tillandsia paucifolia (Fig. 1.11) probably represents the typical pulse-supplied (PS) forms described below, requiring more than five years to mature (pers. obser.).

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  • bisirat
    What habitat does epiphytic habitat belongs to?
    2 years ago

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