Epiphytes (Pronounced: Ep-ee-phytes) are the ferns, mosses, orchids, bromeliads and cacti that live suspended in the trees. They are one of the chief characteristics that make you feel like you are in rainforest. Although largely absent in my experience of the Andean rainforests, they are at home in the Amazonian. These plants cling to branches and trunks, even stacking upon each other and are a key factor in to contributing to the chaotic quantity of biomass.
How these have evolved is a curiosity at first thought for it stands to reason that there are less nutrients in the canopy and more struggle to hold on to be there. There are actually several advantages to living up high; the first being sunlight. In the rainforest the sheer quantity of plants with their large leafs absorbs a lot of the light, making the forest floor a dark place. The difficulty for seedlings to compete in the clustered mature trees is exacerbated further by this, but by starting in the canopy they can gain hope.
Seed dispersal by wind is uncommon in the tropics for not only is the likelihood low of the seed by chance finding a patch of suitable earth but also the aforementioned mass of vegitation shelters from the wind. In the canopy the gusts are stronger, the potential nooks and crannies in branches to get caught in are numerous and nutrients do not need to be wasted producing luscious fruits for animals to eat and disperse via excretion.
One of the major downsides however is the lack of water, which is why plants like bromeliads have evolved to tolerate thirst and why they can survive the neglect of a forgetful person as a house plant. They thrive most in the wettest forests to compensate for this shortage, the lack of available water of living on tree branches being similar to that on the ground of arid climates (which is why some cacti easily adapted to being epiphytes). This problem is worse still when you consider that a primary advantage of high-rise living is the ample sunlight, which evaporates what little water there is.
The plants though have produced many ways to solve this problem. New World orchids are primarily epiphytic, coping with the desiccation by developing bulbous stems to store water. Their system of carbon dioxide differs from their ground-dwelling counterparts as well; in the leaves are tiny pores called stomata, common to every plant to breath air. Most plants absorb the carbon dioxide necessary to turn photosynthesis into carbohydrates during the day, however this can be a major source of water loss through the open pours. Epiphytes orchids close them during the day and open them at night, when the temperature is lower and the humidity is higher. Convergent evolution, nature independently developing a solution to a problem more than once, has produced this in many canopy-dwelling plants.
Tank bromeliads have evolved a clump-like shape with long trough-like leaves that funnel toward a central stem. These form a watertight storage at the base and can hold up to two gallons of water. I have used this many a time to quench thirst however it is often advised to strain them of debris, insects and bromliadic tadpoles – as I’ve found out. These long rigid leaves also serve the purpose of collecting this debris and leaf litter as a source of nutrients. These leafs attracts the insects, that attract frogs to lay their spawn, and – assuming I do not swallow them – an ecosystem forms that excretes the nutrients the plant needs. A theory I have read states that there are a lot of similarities between bromeliads and carnivorous plants, with the shared factor between the two being they form in nitrogen poor areas. This suggests that it is not the lack of soil that has been the driving factor of bromeliads but the lack of nitrogen as the biggest problem, which insect can compensate for. Some epiphytes have taken the insect symbiosis further by growing especially designed tubes for colonies of ants to inhabit. These ants either excrete nutrients as the benefit or, if they are aggressive, protect against herbivores.
You may conclude that epiphytes are parasitic, which is not entirely true although they do blur the definition. A hundred water soaked plants clinging to a tree will cause that tree to invest more in support, but they do not affect the tree directly. An exception is the tropical miseltoes that send roots directly into the hosts circulatory system. Either way, many rainforest trees have developed means to counter the squatters with smooth bark to make it difficult for them to find purchase. Temperate forests with their oak, maple, beech, ash and conifers are varied in their leaf shape, yet in the tropics most of the leafs are fundamentally similar. The classic rain-drop leaf shape ends in a “drip tip” that sheds water as far away from the plant as possible so as to not feed the thirsty clingers. Some trees shed their bark at times to dislodge the unwanted guests, where others impregnate their bark with chemicals to prevent germination.
Vines are epiphytes and also a key aspect in our image of the forests. In fact 90% of the worlds vines live in these rainforests, the abundant moisture on the ground enabling them to bloat them selfs with water for rapid growth rather than plant tissues. The University of Delaware coined the phrase of “skototropism”, the growth of a plant towards darkness rather than light which vines demonstrate. If germinating vines creep towards the darkest areas, this not only implicates there is a tree to climb but also means it is the largest tree as well. When their host is found they switch behaviour to the normal sun-seeking for photosynthesis.
Lianas are woody vines that can be thigh-thick and have one kilometre of length. These vines wraps around a tree rather than just rely on tendrils, to support their weight. When they reach the canopy, the lower leafs used to provide the initial photosynthesis are dropped and massive leaves, two meters long grow at the top. Lianas can be tricked into thinking that they are in the canopy from just above the ground if there is ample light. They sprout huge leaves to look like shrubs, but should they be shaded they’re then transform into serpentine plants to snake their way up a host. Often the larger trees are difficult for all but lianas to climb up, as their large girth prevents wrapping. Once a liana establishes however it is disastrous for the host, for other vines can use the smaller diameter of the liana to climb up. Matapalo do the reverse of vines, by starting life in the canopy and sending roots down as fast as possible. In my opinion they are a particuarly interesting plant for which I will dedicate an article to them alone (Matapalo, Predatory Trees).
A botanist by the name of Francis Putz has suggested that trees have evolved to sway out of time with each other to snap vines. This is best achieved by growing a different architecture, inviting trees to be as diverse as possible or perish under vine constriction. Lianas have inturn evolved to counter this by adding twists and coils into their length so that they can facilitate the stretch.
Epiphytes provide an indication of diversity. Where ever they are, life must be abundant for them to evolve, to exploit niches otherwise considered insufficient by other plants. From these niches they have created miniature ecosystems and have been aggressivly forcing natural selection in other plants. The presences of these plants clinging for life not only indicates diversity, but drives it.