Habitats: Forest canopies

Geetha Iyer

Forests are lands covered by trees. But trees are not the only vegetation within a forest. Other kinds of plants populate them. The diversity and population of flora as well as the characteristic of vegetation in a forest differs and changes as one moves from the ground level to the topmost layer. In scientific parlance the differences are described as vegetative structures of a forest. In the April issue* I had described the forest floor, the lower most region (structure) in a tropical forest. In this issue I share with you the characteristics and biodiversity of the forest ceiling, i.e., the canopy. Starting with the forest floor the vegetative structures present are classified as shrub level, understory, canopy, and overstory. The overstory consists of the branches of the tall trees which emerge beyond the canopy, a feature seen in dense forests rather than open ones.

Canopies are common to all forests. But when canopies of tropical forests are considered there is something quite daunting to begin with. The canopy height can go up to around 100+ feet. Studying the characteristics or diversity at this height requires expertise, high-tech tools and needless to say, a lot of courage and stamina. Therefore, tropical canopies are not commonly explored parts of the forest, even globally. In India, Dr Soubadra Devi from ATREE has been a pioneer in this area of study. Her work, largely in the Southern Western Ghats and to a smaller extent in the Himalayas in Darjeeling has given us an inkling of life in the canopies in our country.¹ Although canopies are a feature in all forests, this article is about canopies of tropical forests where tree heights range from 60 to 120 ft or more.

The canopy is an almost closed structure made of branches of trees that are very closely placed. The crown of trees, with all its accompanying parts – branches, twigs, leaves, flowers, the epiphytes growing on it – form the canopy. Since there are epiphytes, flowers, fruits, besides the photosynthesizing leaves, it presents a habitat in which a variety of living organisms can thrive. Thousands – may be millions – of species of plants, microorganisms, insects, birds, and mammals that one does not encounter on the forest floor live exclusively in canopies. In fact, the presence of epiphytes is considered a characteristic of the canopy. This is therefore an ecologically significant part of the forest – considered by scientists to be a fully functional sub-system of the forest.

History of canopy study
Climbing to towering heights amid dangers posed by both biotic and abiotic factors, requires a lot of courage and commitment. In early 1800, Henry W Bates described arthropod fauna from canopy of tall trees that were felled in the forests. Some years down the line, work done by Collyer, Beebe, Chapman and others began to reveal that the canopy was rich in biodiversity especially that of arthropods. Smithsonian entomologist Terry Erwin’s work using fogging techniques to know about insects in the canopies set the ball rolling and kindled the curiosity of scientists to attempt some adventurous explorations. The insect diversity was found to be so high in the canopies that Erwin changed his original estimate of insect diversity from 5 million to 30 million. Amazed, he described the canopies as the “last biotic frontier”. Canopy study caught the interest of scientists. Back then the technique or funding for such projects was not available. Pioneering scientists used ladders to climb and explore tree tops. Some used the expertise of native climbers, while others trained monkeys to bring down samples (without eating them). Their discoveries excited and prompted others to search for ways and means to discover the canopy secrets. But they could access only a limited height. In the 1970s, mountaineering techniques were used to access canopies and build platforms for long-term surveillance. Today, a host of hi-tech equipment is used to study the canopy. They range from cranes, canopy walkways, ultra-lite planes or airships, dirigible balloons and balloon-rafts, ski-lift-style trams, and remote-controlled pulley systems to access and study canopies. What’s more, such systems are allowing for canopy adventure giving the general public, students and teachers an idea of what the canopy holds and does.

Significance of the canopy
Structural diversity: The canopy structure has two main components. The closeness of the crown due to the trees being close to each other results in the horizontal structure. The tree height determines the vertical structure. Adding to these are other components of plant diversity that also determine the characteristics of a canopy. These factors play a significant role in providing within this habitat, niches and micro-niches for diverse organisms to survive. Many arboreal fauna escape ground predators by staying and moving in the canopy. The lion-tailed macaque and Hoolock gibbon are canopy primates.

The canopy opens to the sky and therefore is the interface between atmosphere and biosphere. This in turn has a significant effect on processes such as photosynthesis, respiration, water and nutrient cycling and carbon flux². As the leaves get plenty of sunlight, photosynthesis rates are the highest in the canopy. The combination of environmental conditions at various heights creates a myriad of microclimates. Naturally this provides specialized niches for other plants and fauna to thrive in this habitat. Epiphytes grow in the nooks of branches and host of insects thrive in the tree crown. – In fact it was the discovery of insect diversity that fired the interest in canopy studies. Forest canopies act as a windbreak and intercept rainfall making the forest interior more hospitable.

Biologists have discovered new patterns of flowering, novel pollination systems and new role of epiphytes in nutrient cycling. The interactions between plants and their environment have immense significance for life not only in the canopy but other places on earth. Forest canopies account for at least 50 per cent of global CO₂ exchange between terrestrial ecosystems and the atmosphere, as well as a significant proportion of global net primary productivity (Encyclopeida of Forest Science, 2004). Would plant-atmosphere interactions affect global process? Even meteorologists have now begun to evince interest in the canopy.

Canopy facts

1. Thrips are generally infamous as plant sucking pests. Scientists found that flower thrips were the important pollinators of tall Dipterocarp trees in Malaysia.
2. Some canopy trees put forth adventitious roots. These then become places where epiphytes thrive. In fact, the striking feature of rainforest canopies is the abundant presence and diversity of epiphytes.
3. One study in the mountain forests of Ecuador found 109 distinct species of vascular epiphytes – succulents, ferns or orchids – growing on branches that covered an area no larger than a small courtyard.
4. The same study found that tree canopies also supported nonvascular epiphytes. Nearly 10,000 species of mosses, 7,200 species of liverworts, and thousands of types of lichens.
5. Cullenia exarillata in Southern Western Ghats hosts a high number of epiphytes. Its fruit is the favourite food for lion-tailed Macaques. Several other mammals visit this tree to feed on its flowers, inadvertently serving to disperse epiphytes growing on it³.
6. Records of evolution of new species of epiphytic groups support the hypothesis that the canopy is a site for speciation (evolution of new species).
7. Canopy is the primary site of gas exchange between the atmosphere and plants. So it sustains and promotes processes that permit diversity and forest health.
8. The soil like deposits and litter abundance provide a favourable habitat for arthropods. Arthropod diversity is highest in the canopy. Beetles have been found to flourish here.
9. 193 species of birds have been reported by Nadkarni and Matelson, which include among others, species of humming birds, flycatchers, tangers, macaws, toucans, parakeets, etc. Hornbills are canopy birds.

An understanding of the richness of the canopy can help in the conservation and sustainable management of forests. According to Dr Soubdra Devi, “Canopies buffer the earth’s surface from the vagaries of extreme weather patterns, and conservation of canopy surface can have a significant effect on soil and water conservation.” Dr. Margaret D. Lowman, popularly called Canopy Meg says that the canopy is “the mainframe computer for running the forest”.

References

1. https://news.mongabay.com/2006/11/canopy-research-is-key-to-understanding-rainforests/
2. M. S. Devy and T. Ganesh. Canopy science and its relevance in India. CURRENT SCIENCE, VOL. 85, NO. 5, 10 SEPTEMBER 2003
3. K. S. Seshadri, R. Ganesan and Soubadra M. Devy. Persistent Effects of Historical Selective Logging on a Vascular Epiphyte Assemblage in the Forest Canopy of the Western Ghats, India. Frontiers in Forests and Global Change. October 2021 | Volume 4 | Article 727422

*Forests part 1: An introduction appeared in the January 2022 issue of Teacher Plus.

The author is a consultant for science and environment education. She can be reached at scopsowl@gmail.com.