Life on the air or why epiphytic bromeliads matter.

The bromeliads (or Bromeliaceae) are a family of monocot flowering plants. Their leaves form rosettes (i.e. they are arranged in a circular manner, at a similar height). Depending on the species, the size and color of the leaves can vary widely as does the shape, although it is fair to say that most are triangular. Their inflorescences are usually upright spikes that vary widely in size from a few centimeters to 9-10 m tall. In some species the flowers grow at the base of the plant.

The best known member of the bromeliad family is the pineapple, whose uniquely shaped fruit can be found in markets all around the world and has become a synonym of the tropics. However, pineapple is hardly the only bromeliad: there are some 3,170 species, naturally distributed on the American Continent, from the southeast United States to southern Argentina and Chile. They live in a wide variety of habitats, from the exposed and dry environment of deserts and rocky canyons to temperate forests and the shady understory of tropical jungles (both rainy and dry) and even the high Andes. However, phylogenetic evidence suggests that bromeliads originated in relatively exposed but humid habitats, and then some lineages adapted to more shaded habitats.

 

Epiwhat?

Epiphytes, or epiphytic plants, are those that live on the branches or trunks of other plants, especially trees.  The host plants provide support and affect the microclimate within which the epiphytes grow. Some epiphytes can even live on powerlines and other man-made structures. It is estimated that 10% of all the flowering plant species are epiphytes. Approximately 1,144 bromeliad species are epiphytes. However it is interesting to note that this life form originated independently at least three times in the bromeliad family. Since the epiphytic environment usually lacks soil, epiphytic bromeliads rely solely on the atmosphere to obtain much needed water and nutrients. They have developed different strategies and adaptations both to obtain these resources and to minimize the loss of moisture. Some of these adaptations include waxy cuticles, succulent leaves, water impounding rosettes, long thin leaves that promote dew deposition and, most importantly, leaf trichomes – highly absorbent, scale-like structures on the surface of the leaves connected to the photosynthetic tissue inside the leaf.

Another important adaptation is crassulacean acid metabolism (CAM), a variant of photosynthesis that allows plants to open their stomata during the night when the air is cooler and more humid – thus reducing the evaporative loss of water. The absorbed CO₂ is stored in the form of malic acid and stored in the vacuoles of the cells. The next day, CO₂ is released and enters the photosynthetic pathway. This physiological adaptation is widely represented in epiphytes as well as desert plants, and has appeared independently several times.

So, what is their relevance?

Epiphytic bromeliads may not be the most conspicuous elements in forests. However they are very important to biodiversity; so called tank bromeliads can constitute ecosystems on their own because they can be the habitat of amphibians like the bromeliad frog Bromeliohyla bromeliacia and salamanders of the genus Dendrotriton and numerous insects and crustaceans like some ostracods. There is even a crab from Jamaica, Metopaulias depresus, that only lives on bromeliads.

Evidence also exists that coffee and palm plantations that retain epiphytes have higher bird species richness; the epiphytes can modify the microclimate at the canopy, making it cooler and more humid.

Epiphytic bromeliads even contribute ecosystem functions when they die; after falling to the forest floor they decompose and lead to the recycling nutrients like nitrogen and phosphorous.

They are of some use to humans too: in some regions they can be part of rituals or be consumed. In the Mexican state of Oaxaca, for example, some communities harvest them to use as Christmas decorations. Bromeliads also have scientific importance: their reliance on the atmosphere to obtain water and nutrients makes them good bioindicators of air pollution. Furthermore, it has been hypothesized that changes in their population size and recruitment rates can be early indicators of climate change because they depend on very specific microclimatic conditions, though this possibility should be studied more thoroughly.

While every species is different, it should be noted that epiphytic bromeliads are under pressure from habitat change. In some cases, they are illegally extracted as ornamental plants by collectors. Epiphytic bromeliads are an important component of ecosystems, but are also very vulnerable and great effort should be made to protect them.

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