Jeremy enters the bowels of Lighthouse Cave, San Salvador, Bahamas.
The neotropical forest is haven to an amazing variety of plant species. A most interesting group of plants found in this ecosystem is the epiphytes. The distinguishing and remarkable feature of the epiphytic plants is their ability, in many cases their requirement, to grow in the canopy of the forest, rather than from the forest floor. These plants exist on other plants, typically by wrapping their roots around the branches of trees (Kernan & Fowler, 1995 ) or by clustering with other epiphytes in mats (Matelson, Nadkarni, & Longino, 1993). This existence is more “challenging” for these plants than the forest floor would be, for in the canopy, these unique plants must face physical instability, reduced facilities for storage of nutrients and water, higher windspeeds, and great fluctuations in temperature and moisture levels (Matelson, Nadkarni, & Longino, 1993). Even with these challenging conditions, approximately 10% of all vascular plants exist as epiphytes (Matelson, Nadkarni, & Longino, 1993) and encompass more than eighty families (Benzing, 1995).
Vascular and Nonvascular Epiphytes
Many of the vascular epiphytes begin their lives on the forest floor, then through a variety of twisting and vining mechanisms make their way up standing trees to the forest canopy (Benzing, 1995). The true epiphytes, however, can live only in the canopy from the moment of seed dispersal, typically by wind or birds (Benzing, 1995); falling to the ground results in death ( Matelson, Nadkarni, & Longino, 1993). Most of the vascular epiphytes are monocots and angiosperms (Benzing, 1995). These plants typically engage in Crassulacean acid metabolism (CAM) due to the increased stress of variable water and sun availability indicated by their environment (Benzing, 1995). Many have a waxy layer over the leaves to protect against water loss during dry periods and have roots that absorb water from the humid atmosphere (www.rbgkew.org).
The most widely studied of the vascular epiphytes are the orchids and bromeliads. Members of the Orchidaceae comprise more than half of all vascular epiphytes (Benzing, 1995). Orchids are root dependent epiphytes, and are known for their varied and beautiful floral structures. Bromeliads ae characterized by flat leaves and central cup-like structures that allow for the capture of water (Benzing, 1995).
Of the nonvascuar epiphytes, the bryophytes are more discernable in the Neotropical forest. These plants are able to survive drought over long periods but initiate photosynthetic activity within moments of rehydration (Rhoades, 1995). These plants, like the vascular epiphytes, supply vital services for the tropical forests: providing habitats, cycling minerals, and participating in water relations (Rhoades, 1995). This group includes the hornworts and liverworts; while in most environments these plants are limited to the moist wood closer to or on the forest floor, in the tropical montane forests, they are often found in great number on tree branches (Rhoades, 1995). They are typically limited in their habitat, as are the vascular epiphytes, by water availability and light levels (Rhoades, 1995). Additional factors that limit or promote their survivability include competition from other species, urbanization by humans, and pollution levels (Rhoades, 1995). In fact, because they are so sensitive to pollutants, bryophytes are often used to monitor air quality (Rhoades, 1995).
Epiphytes and Their Friends
Epiphytes provide a great service to other organisms in the Neotropical forest. They provide protection, camouflage, food, water sources, and nesting sites and materials for both vertebrates and invertebrates (Rhoades, 1995). The interaction between epiphytes and various ant species has been extensively studied. These plants provide shelter and nesting sites for the ants, while the ants in return provide nutrients from excrement, seed dispersal, and defense against herbivores and competing plants (Benzing, 1995, Tobin, 1995). Some orchids attract ants with a nectar the ants consume (Benzing, 1995) while some species of ants consume plant material directly to obtain fluids and nutrients (Tobin, 1995). The nesting habits of ants can be quite varied in the ways they utilize the epiphyte and surrounding plants of the forest. Some build carton nests, some construct ant gardens, others take advantage of the domatia-bearing epiphytes (Tobin, 1995). These relationships are so vital and pervasive in the forest canopy that ants comprise 20 to 40% of arthropod biomass in this environment (Tobin, 1995).
Epiphytes and Ecology
Epiphytes, both vascular and nonvascular, play crucial roles in the water cycle of the Neotropical forests. They capture moisture as rain or fog and then release it, along with vital nutrients, during dry periods, thus maintaining the higher humidity levels of this environment. Further, they reduce the potentially disruptive effect on the forest of torrential rains, allowing the water to drip downward through the canopy (Rhoades, 1995).
Pollination of epiphytes is usually conducted by insects, birds, and bats (Benzing, 1995). Birds of the Corcovado Basin in Costa Rica are typically attracted by the bright red, blue, or orange berries which are covered with a sticky substance, allowing the berries to adhere to the substrate, regardless of its orientation (vertical or horizontal) (Kernan & Fowler, 1995). Upon dispersal of the seeds, the likelihood of germination is determined by wettability of the substrate, the texture and stability of the substrate (usually bark), and the water-holding capacity of the location of attachment of the seed (Benzing, 1995). The type of substrate on which the seed is located is crucial to determining the success of germination as well as the later success of the plant (Kernan & Fowler, 1995).
Epiphyte seeds typically adhere to bark in the lower 5 meters of the forest on tree trunks and branches with diameters between 5 and 10 centimeters (Kernan & Fowler, 1995). The angle, texture, and diameter of the substrate in many instances limit attachment by the epiphyte seed (Kernan & Fowler, 1995).
Epiphytes and Their Environment
Mature epiphyte success is more likely a factor of rainfall distribution during the year rather that total annual rainfall (Benzing, 1995). Shade tolerant species are more often found close to the forest floor than are those requiring sun. Those plants tolerant of full sun are often found near streams or other bodies of water which provide them with adequate humidity (Benzing, 1995). Those epiphytes requiring thick humic soils are typically found in very shady regions of the canopy, as this substrate is usually found on older trees with thick bark (Benizng, 1995, Kernan & Fowler, 1995).
Within a tropical forest canopy, many factors contribute to a plant’s ability to perform photosynthesis. Light levels may vary greatly and moisture levels may not remain constant, leading to moisture loss (Holbrook & Lund, 1995). Because of this environment, many epiphytes utilize Crassulacean acid metabolism (Holbrook & Lund, 1995).
When an epiphyte falls to the forest floor, it may die immediately, releasing valuable nutrients into the ecosystem, or may remain alive for weeks or even months or years (Matelson, Nadkarni, & Longino, 1993). When epiphytes fall as individuals, their survivability is less than if they fell as part of an intact mat composed of numerous epiphytes with intertwined roots (Matelson, Nadkarni, & Longino, 1993). It is likely that these mats retain enough water to ensure temporary survival of the plants (Matelson, Nadkarni, & Longino, 1993). The reasons for the death of the epiphytes that fall are not clearly determined; it is possible that changes in light and moisture from the canopy to the floor with the accompanying reduction in photosysnthesis leads to the cessation of these plants (Matelson, Nadkarni, & Longino, 1993). Furthermore, the lack of essential dry periods that persist in the open canopy may lead to the death of epiphytes that fall to the relatively wet forest floor (Matelson, Nadkarni, & Longino, 1993).
Epiphytes and You
Because of the specialized services to the ecosystem that the epiphytes provide, their continued presence in the neotropical forests is quite valuable. The intricate relationships these plants have with many species, particularly the ants, indicates that entire systems would suffer if their numbers were to be significantly reduced. Further, the great diversity of epiphytes contributes to the overall diversity of plant life on the planet. Their beauty alone indicates their value. Yet, because of the epiphytes propensity to grow on older trees, and because of their slow growth, they are quite vulnerable to environmental changes (www.iupac.org). Destruction of all, or even fragments of their habitat could result in severe reduction in their numbers or diversity. Because of their constant exposure to atmospheric conditions without the advantage of soil buffers afforded to ground-rooted plants, the epiphytes are particularly susceptibile to air pollution. They are used as indicators of air quality and for this additional service should be valued and exposed to conditions that are favorable, not only for the benefit of the epiphytes, but for all organisms.
Benzing, D. H. (1995). Vascular epiphytes. In Forest Canopies. Lowman, M. D. and Nadkarni, N. M. San Diego, CA: Academic Press.
Holbrook, N. M. and Lund, C. P. (1995). Photosynthesis in canopies. In Forest Canopies. Lowman, M. D. and Nadkarni, N. M. San Diego, CA: Academic Press.
Kernan, C. and Fowler, N. (1995). Differential substrate use by epiphytes in Corcovado National Park, Costa Rica: A source of guild structure. Journal of Ecology, 83, 65-73.
Matelson, T. J., Nadkarni, N. M., and Longino, J. T. (1993). Longevity of fallen epiphytes in a neotropical nomtane forest. Ecology, 74, 265-269.
Rhoades, F. M. (1995). Nonvascular epiphytes in forest canopies: Worldwide distribution, abundance, and ecological roles. In Forest Canopies. Lowman, M. D. and Nadkarni, N. M. San Diego, CA: Academic Press.
Tobin, J. E. (1995). Ecology and diversity of tropical forest canopy ants. In Forest Canopies. Lowman, M. D. and Nadkarni, N. M. San Diego, CA: Academic Press.
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