Plants are in the air

We spent the last couple of days at the STRI Fortuna field station, a tiny building in the highlands of the Chiriquí province of Panama. Located near lake Fortuna, the station provides researchers with exceptional access to the beautiful mountain forests of the Reserva Forestal de Fortuna. As we hiked through the reserve, the beauty of the place struck me as well as one characteristic of the forest in particular: the large number of plants living above my head. Epiphytes were simply everywhere. Big, small, high, low, they seemed to thrive in the forests that we visited. In fact, it turns out that Fortuna has one of the highest percentage of trees with epiphytes on them of Panama’s lowland and mountain forests : no less than 74% of trees there carry epiphytes (Zotz 2016)!

 

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Epiphytes are abundant and diverse in Panama, and it is not rare for trees to be largely covered in epiphytic plants. Photo credits: Cyndy Sims Parr.

Epiphytes have always been fascinating to me. I think of epiphytic plants as true masters of frugal living. Indeed, most epiphytes establish, grow, and reproduce suspended in the forest canopy, a highly inhospitable environment. Like other plants, epiphytes use light to synthesize their foods via photosynthesis. Unlike most other plants, however, they do not have easy access to the pool of carbon dioxide, water, and nutrients that plants typically gather from soils. How do epiphytes make a living without touching the ground?

Epiphytes are not parasites, which means that they only use their host for physical support and their roots are solely there for attachment. As a consequence, epiphytes must obtain all the resources they need, including water and key nutrients, from the air surrounding them and the surface of their host’s bark. To do so, epiphytes have evolved extraordinary adaptations to maintaining water balance and mineral nutrition while living in the canopy (Benzing 2016). They therefore occupy a unique ecological space (Nieder et al. 2000), and though epiphytes are estimated to represent approximately 10% of all “higher and vascular” plant species (Kress 1986), most come from a handful of families only (Araceae, Bromeliaceae, Orchidaceae) (Benzing 2016).

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Epiphytes make a living far from the ground and its nutrient resources. Niphidium crassifolium epiphyte in Soberania National Park, Panama. Photo credits: Katja Schulz.

 

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A strangler fig surrounding its host. Photo credits: Col Ford and Natasha de Vere.

Some epiphytes, called hemi-epiphytes, start their life in the canopy but will progressively switch to a terrestrial way of life. Over time, hemi-epiphytes send roots to the ground and eventually make contact with the soil and its numerous resources. This behavior grants hemi-epiphytes greater independence from tree suitability compared to true epiphytes (Nieder et al. 2000), and even enables some to become large trees.  One of the most spectacular and well-known hemi-epiphytes is the strangler fig (Ficus species), which sends multiple shoots to the ground until it eventually surrounds its host tree. The latter often dies from girdling or light deprivation.

Beyond being impressive, acrobatic, and beautiful, epiphytes are also key ecological components of a tropical forest. Stanton et al. (2014) show that epiphytes have important consequences for the water relations of their host plants. They retain stemflow water and throughfall as well as provide hosts with an indirect buffer against canopy microclimatic conditions. Epiphytes also supply tropical fauna with vital resources, such as pollen, nectar, fruits, seeds, but also shelter and moisture. Many invertebrates and vertebrates (e.g. birds, frogs, or mammals) make use of these canopy resources (Benzing 2016). Moreover, epiphytes form mutualisms with specific groups, such as ants. The ant nest garden-phenomenon, whereby ants build composite nests in which they plant epiphytes, is likely the most impressive of these mutualisms (Yu 1994). As they render tropical forest canopies much more hospitable to a wide variety of other life-forms, epiphytes can be considered a keystone group (Benzing 2016).

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A bromeliad epiphyte in Poas Volcano National Park, Costa Rica. Photo credits: Flickr user _quintin_. URL:

However, epiphytes may also be viewed as a “weak link” of tropical forest ecoystems (Benzing 2016). Indeed, whereas epiphytes are essential biota, they are also extremely vulnerable to disturbance. Because of their strong dependence on the atmosphere for the acquisition of moisture and ions, epiphytes are particularly sensitive to changes in air quality (Benzing 2016). The distribution of epiphytes is also tightly dependent on microclimate, in particular moisture availability (Benzing 2016). Epiphytes might therefore be a “weak link” of tropical forests’ adaptation to climate change and other anthropogenic disturbances. Their vulnerability coupled to their ecological importance render them one of the key groups to be monitored in the decades to come, as their decline may have far-reaching ecological consequences for tropical forests around the world (Benzing 2016).

 

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The view into the valley from the montanes of Fortuna. A horse got in the way. Photo credits: Chloé Debyser.

REFERENCES

Benzing, D. H. 1998. Vulnerabilities of tropical forests to climate change: the significance of resident epiphytes. Pages 379-400  Potential impacts of climate change on tropical forest ecosystems. Springer.

Kress, W. J. 1986. A symposium: the biology of tropical epiphytes. Selbyana 9:1-22.

Nieder, J., S. Engwald, M. Klawun, and W. Barthlott. 2000. Spatial Distribution of Vascular Epiphytes (including Hemiepiphytes) in a Lowland Amazonian Rain Forest (Surumoni Crane Plot) of Southern Venezuela 1. Biotropica 32:385-396.

Stanton, D. E., J. Huallpa Chávez, L. Villegas, F. Villasante, J. Armesto, L. O. Hedin, and H. Horn. 2014. Epiphytes improve host plant water use by microenvironment modification. Functional ecology 28:1274-1283.

Yu, D. W. 1994. The Structural Role of Epiphytes in Ant Gardens. Biotropica 26:222-226.

Zotz, G. 2016. Plants on Plants – The Biology of Vascular Epiphytes. Springer, URL

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