What are characteristics of rainforests?

By Rhett A. Butler
April 1, 2019

Part I:

STRUCTURE OF THE TROPICAL RAINFOREST

Tropical rainforests vary significantly across the world but share key characteristics, including climate, precipitation patterns, layered canopy structures, intricate ecological relationships, and remarkable biodiversity. However, not all rainforests exhibit these traits uniformly, and many transition into adjacent ecosystems such as mangrove forests, montane forests, or tropical deciduous forests without clear boundaries.

GEOGRAPHY AND CLIMATE

Tropical rainforests are found within the tropics, the region between the Tropic of Capricorn and the Tropic of Cancer. Here, sunlight reaches Earth's surface at a near-90-degree angle, generating intense solar energy. This energy input is further amplified by the region’s consistent day length—approximately 12 hours of daylight year-round. In contrast, areas farther from the equator experience more seasonal variations in daylight. This steady supply of sunlight fuels the rainforest’s ecosystem through photosynthesis.

Due to high solar energy input, tropical rainforests tend to remain warm throughout the year, with temperatures typically ranging between 72–93°F (22–34°C). However, forests at higher elevations, such as cloud forests, can be considerably cooler. While some variation in temperature occurs seasonally, certain equatorial rainforests experience remarkably stable conditions, with annual temperature fluctuations as small as 0.5°F (0.3°C). Cloud cover and high humidity further moderate temperature extremes.

PRECIPITATION

Rainforests derive their name from their high levels of precipitation. Located within the intertropical convergence zone, these forests experience frequent and intense rainfall due to rising warm air that condenses into clouds and precipitation. Annual rainfall typically exceeds 80 inches (2,000 mm) and, in some areas, can surpass 430 inches (10,920 mm).

In equatorial regions, rainfall may occur year-round, with little distinction between "wet" and "dry" seasons. However, many rainforests do experience seasonal variations in precipitation. Even during drier periods, cloud cover often maintains high humidity levels, preventing the forest floor from drying out completely. Some neotropical rainforests rarely go a month without receiving at least 6 inches (150 mm) of rainfall. The relatively stable climate allows many rainforest trees to remain evergreen, continuously replacing leaves rather than shedding them all at once.

Rainforests located farther from the equator, such as those in Thailand, Sri Lanka, and parts of Central America, experience more distinct rainy and dry seasons. These forests are often classified as "semi-evergreen," as some tree species shed their leaves during the dry period, while others retain foliage year-round.

Rainforest humidity results from a combination of rainfall, cloud cover, and transpiration—the release of water vapor from plant leaves. A single canopy tree may transpire around 200 gallons (760 liters) of water annually, amounting to roughly 20,000 gallons (76,000 liters) per acre of dense canopy. This process helps generate local humidity and contributes to cloud formation, with some large rainforests producing up to 75% of their own precipitation. The Amazon rainforest alone is responsible for generating nearly half of its own rainfall.

Deforestation and climate change are altering rainforest hydrology. Since the mid-1990s, severe droughts have affected various rainforest regions, including Southeast Asia in 1997 and the Amazon in 2005 and 2010. Drier conditions, along with degradation from logging and agricultural expansion, have made forests more susceptible to wildfires.

Part II:

RAINFOREST CANOPY STRUCTURE

Rainforests have a distinct vertical structure composed of multiple layers, including the overstory, canopy, understory, shrub layer, and forest floor.

The canopy forms a dense ceiling of intertwined branches and leaves from closely spaced trees. This uppermost layer, typically 100–130 feet (30–40 meters) above the ground, is occasionally punctuated by towering emergent trees that extend even higher, creating the overstory. Beneath this, the understory consists of multiple tiers of foliage and branches. The lowest section of the understory, called the shrub layer, lies about 5–20 feet (1.5–6 meters) above the forest floor and consists mainly of saplings and smaller plants.

The dense foliage of the canopy blocks much of the sunlight from reaching the forest floor. In undisturbed (primary) rainforests, the limited light availability results in sparse ground-level vegetation. Rather than the tangled undergrowth often associated with "jungle" imagery, primary rainforest floors are relatively open, with scattered saplings, lianas (woody vines), and shade-tolerant plants.

A defining feature of the canopy is the abundance of epiphytes, plants that grow on trees rather than rooting in the soil. Unlike parasites, epiphytes do not extract nutrients from their host trees but instead use them for support. Their elevated position grants better access to sunlight, essential for photosynthesis. Epiphytes have evolved specialized adaptations to absorb moisture and nutrients from rain, mist, and organic debris accumulating in their root structures. These adaptations are explored in detail in the canopy section.

Another important plant type in the canopy system is the liana—a woody vine that begins its life on the forest floor before climbing tree trunks to reach the light-rich canopy. Similarly, hemiepiphytes, a distinct group of plants, start their growth in the canopy and later send aerial roots downward until they reach the soil. Once rooted, these plants can draw nutrients from the ground, reducing their reliance on rainwater and organic debris.

The canopy is home to an extraordinary array of plant and animal life, with many species uniquely adapted to this elevated environment. In tropical rainforests, a significant proportion of biodiversity exists within this layer. Given that rainforests are estimated to harbor around 50% of Earth's terrestrial species, their canopies represent one of the most biologically rich environments on the planet.

Part III:

INTERDEPENDENCE AND COMPLEX SYMBIOTIC RELATIONSHIPS

Interdependence is a fundamental characteristic of rainforest ecosystems, where species rely on each other in diverse ways—ranging from pollination and seed dispersal to predator-prey interactions and mutualistic partnerships.

These intricate relationships have developed over millions of years, shaping the stability of the ecosystem. The disappearance of a single species can have cascading effects, potentially impacting others that rely on it for survival. The loss of a keystone species—an organism that plays a disproportionately important role in maintaining ecosystem balance—can lead to significant disruptions.

For example, Brazil nut trees (Bertholletia excelsa) in the Amazon rainforest depend on multiple species for their reproduction. Their grapefruit-sized seed pods are too hard for most animals to crack, but the agouti, a small rodent with powerful teeth, can open them. While the agouti consumes some seeds, it also buries others, inadvertently planting new trees. Brazil

REVIEW QUESTIONS

Review questions - Part I

• Where are rainforests located?
• What are the tropics?
• Where/how does the rainforest get its energy?
• Rainforest are _________ since they are warm and have a lot of moisture in the air.
• How much rain do rainforests get?
• How do rainforests create their own rain?
• What is an evergreen tree?
• What are two things affecting the water cycle of tropical rainforests?
• Drought makes rainforests susceptible to what?

Review questions - Part II

• Most of the plant and animal species live in what level of the rainforest?
• What are epiphytes?
• What is an example of an epiphyte? (Hint: think of a popular kind of flower)
• What are lianas?

Review questions: - Part III

• What is a symbiotic relationship?
• What is a keystone species?
• Why are agoutis important in the rainforest ecosystem?

CITATIONS

Citations - Part I

• The opening quotation is found in One River (New York: Touchstone, 1996) by Wade Davis.
• Timme, S. ("Neotropical Plants and Ecology," Rainforest Workshop Packet 1994) pegs transpiration of individual canopy trees at 200 gallons of water per year - translating to 20,000 gallons transpired per acre of forest.
• Newman, A. in Tropical Rainforest, a world survey of our most valuable and endangered habitat with a blue print for its survival (New York: Facts on File, 1990) notes that large rainforests create as much as 75% of their own rain.
• The Woods Hole Research Center concluded in its RisQue98 (Risco de Queimada, or "Risk of Burning" in Amazonia - 1998) that as much as 50% of the Amazon rainforest was at risk of burning. The report was picked up by the popular media in Lewan, Todd. "Fears of a fiery Amazon nightmare-7-year study has implications for the global warming debate," Associated Press, 12/7/97.
• Two-thirds of the world's rainforests are fragmented according to M. McKloskey in "Note on the Fragmentation of Primary Rainforest," Ambio 22 (4), June: 250-51 1993.
• Paine, R. T. ("Food web complexity and species diversity" American Naturalist 100: 65-75, 1966) put forth the notion of a keystone species. Further discussion of the term can be found in Power, M. E., et al., "Challenges in the quest for keystones," BioScience 46: 609-620, 1996; and Khanina, L., "Determining keystone species," Conservation Ecology 2(2), 1998.