When we think of a garden, we think of plants and when we think of plants, we think of soil. It must come as a surprise to many of us that an entire group of plants has nothing to do with soil! They spend their entire life without any contact with soil. At least, the kind of soil that we find on the ground, which we dig, sow, and grow plants in. Collectively, these groups of plants are called epiphytes. They are found in the greatest abundance in the forest canopy, a three-dimensional world set nearly 40 metres above the ground. Forest canopies are called the eighth continent because so little is known about them. Here, everything from a tiny invertebrate, only a few millimetres long to a large mammal such as our relative, the Orangutan, thrives.
What are epiphytes?
Plants are among the most diverse groups of organisms on the planet, and they are the most abundant in the forest canopy as well. Epiphytes are a group of non-parasitic plants that germinate and grow on other plants. The best-known examples of epiphytes are orchids, ferns, lichens, mosses, cacti, and bromeliads. Currently, over 31,000 species of vascular epiphytes are known globally. Vascular epiphytes are plants that have distinct xylem and phloem. This group of epiphytes comprise nearly 10% of plant diversity. Epiphytic plant diversity is the highest in the tropical regions of the world such as Central and South America, Central Africa, and South and Southeast Asia. Despite being common and widespread, little is known about their biology.
The forest canopy is also a harsh place to live in. When it rains, it pours and the sun can be scorching hot. Unlike soil, there is not much of a substrate that holds water. Epiphytes have thus evolved unique physiology and morphology that are different from other plants. For example, they have specialised root structures, such as trichomes, that allow them to capture moisture and nutrients from the atmosphere. Many epiphytes also have thick and leathery leaves that reduce the loss of water. Epiphytes also tend to grow as dense clusters, typically with moss growing there as well. Moss acts as a sponge and retains some moisture. Plants cannot grow without nutrients and the canopy plants get them from the debris that accumulates on the moss or in the fork of a branch. The rich collection of animal faeces, fallen flowers, leaves, and fruits is collectively called Canopy Soil Organic Matter. Functionally, this organic matter is also where several invertebrates thrive.
Microhabitats for biodiversity
Epiphytic plants often also end up growing as clusters and these clusters are often a microcosm for other forms of biodiversity. While insects are an obvious group, there are other organisms such as birds that may visit these clusters or frogs that may take refuge in them. In parts of Central and South America, there are plants called bromeliads, which collect water. Poison dart frogs are known to deposit eggs within the leaf axil. The tadpoles mature into adults in the bromeliads and hop out to live on the forest floor, returning to the bromeliads to breed. Poison dart frogs also have an incredible parental care ritual, where the female revisits the bromeliads to deposit unfertilised eggs, ensuring there’s food for the tadpoles. Among the diverse forms of epiphytes are orchids, a charismatic group of plants that grow in the canopy. The Western Ghats has a rich diversity of orchids which have mesmerising flowers. Overall, epiphytes play a critical role in the forest canopy. Because of how sensitive they are and the functional roles they play, even small perturbations to the forest could affect them.
The lasting impact of logging
In the summer of 2008, fresh out of an undergraduate course, I ambled into the forests of Kalakad Mundanthurai Tiger Reserve (KMTR) in Tamil Nadu. I had joined Dr. Soubadra Devy’s team to climb up trees and learn the mysteries of the canopy. Nearly 40 yrs ago, the forests were logged for timber. The forests have been in recovery since then, but we knew little of what happened to epiphytes. My task was to understand how the epiphytes were doing in the logged area by comparing it with the unlogged sites. Globally, studies have shown that epiphytes have strong associations with being found on host trees compared to others and grow profusely in well-lit areas. I would start my day with a quick breakfast and catch a bus to the field site with my colleague, lugging a 20-kilo bag of ropes and other climbing gear. We would get down along the forest trail and start our hike into the forest where we ended up sampling 200 trees – 100 each in logged and unlogged forests. Climbing a 40 m tall tree is no easy task. We would first have to rig the tree with a powerful catapult and then haul over three different kinds of ropes and secure one of the solid core nylon ropes by tying it to a nearby tree. Once secure, we would slip on our harnesses and inch our way up the tree, like how a worm inches its way up a twig. Once in the canopy, we would look for epiphytes, and measure nearly 20 different characteristics of the tree and the epiphyte.
Often, we would be in a race against time. Some days, the catapult would get cut or the fishing line we used to rig the tree would snap or worse still, get entangled among the foliage. We would then have to either waste time unentangling it or cut it and start fresh and recover the entangled mess later. Once we finished the work, we would have to rush back to catch the only bus that would bring us back home. We thought using a motorbike would give us independence but as it turned out, I had to learn a new life skill – of fixing punctured tires!
We eventually completed the study and found 2,129 individuals of epiphytes belonging to 20 species, growing on 173 host trees belonging to 22 species. Among the plants, we found orchids to comprise the most diversity. Six of the orchids were endemic to the Western Ghats and the rest were either widespread in India or in parts of Southeast Asia. Contrary to what anyone would expect, we found a greater abundance as well as a diversity of epiphytes in the logged forest. Somehow, the logging activity appears to have led to a profusion of epiphytes! We suspect that logging created large gaps in the otherwise continuous canopy and allowed more light to enter. Of all the trees we sampled, only one tree had the most number of species as well as abundance. This tree is related to the Durian of Southeast Asia and is called Cullenia exarillata. This species was probably most suitable for germination owing to its commonness, large size, muddy bark and knobbed branch surface. Irrespective of which host tree the epiphytes grew on, Eria pauciflora was the most abundant in both logged and unlogged sites. This orchid grows as dense clusters and is rather pretty to look at.
Forest canopy and gaps in knowledge
Forest canopies, the world over, are an important part of the biosphere because they are the first layer to interact with the atmosphere. Climate change is an emerging threat to canopy life. In response to global warming, species distributions tend to go higher in altitude in search of cooler spots. However, species that live in the forest canopy do not have any further to go up. Because the forest canopy can be hot, one would observe a downward shift of species from the canopy as the forest floor is often much cooler than the canopy. How climate change may affect epiphytes is a key question and no one, in India at least, is really monitoring the forest canopies. Even if the effects of climate change are already being felt by plants, we are blissfully unaware of it. Many countries have set up a canopy crane which enables monitoring, but we are far from having such installations. Perhaps having canopy walkways like in other countries is a start to get the monkey in all of us, excited to be up among once close relatives.
Read more about the work here:
Seshadri KS, Ganesan R and Devy SM (2021) Persistent Effects of Historical Selective Logging on a Vascular Epiphyte Assemblage in the Forest Canopy of the Western Ghats, India. Front. For. Glob. Change 4:727422. doi: 10.3389/ffgc.2021.727422