An Introduction to a Tropical Rainforest
Tropics
A tropical rainforest is a nonseasonal forest comprised of broad-leaved evergreen trees, some reaching heights above 100ft, and abundant rainfall, experiencing greater than 800cm of annual precipitation and a mean annual bio-temperature of above 24oC.
Rainforests are an incredibly biodiverse habitat, home to half the Earth’s fauna and flora species and are vital in providing an array of plant products that have been used for thousands of years, with millions of people relying on them for their livelihood, food, water, medicine, and fibres. Rainforests also play a vital role in mitigating climate change by absorbing carbon dioxide. Therefore, we all depend on them for our survival.
RAINFOREST PLANT CHARACTERISTICS
Most tropical tree species leaves have very similar shapes, resulting in difficult species identification. Characteristically, many leaves are waxy, oval, and unlobed, and many have sharp, pointed ends called drip-tips that allow fast runoff of rainwater.
TROPICAL FOREST STRUCTURE
The stratification of a tropical rainforest is very weak due to the structural complexity of the forest itself. Yet, five strata can be present.
EMERGENTS
Emergent trees are the tallest of them all and arise above the canopy, most in mushroom-shaped crowns that block sunlight to the layers below. They experience optimum sunlight but do have to withstand high temperatures, strong winds, and low humidity.
The tualang (Koompassia excelsa) is one of the tallest tropical tree species with the highest recorded height at 88m (289ft) and has large buttress roots to support its weight. Above the canopy, they grow branches with slippery trunks, protecting them from sun bears. This attracts the giant honeybee (Apis dorsata) that hangs its combs from the branches, also protecting the tree from loggers as the value of the honey is more than that of the timber.
"Rainforests are an incredibly biodiverse habitat, home to half the Earth's fauna and flora species."
CANOPY
The canopy is a tight, dense layer of trees with irregular, broad crowns 60-90 feet above the ground. Trees’ leaves and branches overlap as they desperately aim to obtain sunlight, resulting in epiphytes and other organisms covering the branches, and vines intertwining them. The canopy layer is estimated to contain 90% of rainforest life making it incredibly rich in biodiversity.
A range of vines are abundant throughout the rainforest, climbing trees and hanging from the canopy. Lianas are a woody, long-stemmed type of climbing vine found in tropical rainforests that can grow to lengths of up to 3,000 ft. Climbers are rooted in the soil of the forest floor and climb trees for support as they grow upwards towards the sunlight, with lianas attaching themselves using tendrils or sucker roots to a young sapling or by winding around a tree’s trunk. There are four types of climbers: tendril climbers, root climbers, scramblers, and twiners. The dense tropical undergrowth means plants compete for sunlight and in doing so need to grow up to the canopy.
In contrast, epiphytes are characteristically high up in the canopy. These are orchids, bromeliads, ferns, cacti, and other plant species that live on the surface of host tree branches and trunks, to optimise the amount of sunlight they can receive in the canopy. Despite living on the bark of these host plants, they are not parasitic.
UNDERSTORY
The canopy and understory are full of a range of different tree species with staggered heights. The variable light availability results in trees growing at different rates, as well as different tree species differing in height at maturity. Palms are usually present.
The understory is a shaded layer as it only receives 2-15% of the sunlight that shines on the canopy. Plants here have to be able to tolerate low light. As a moderately open layer, many young trees grow, and herbaceous plants have many large and thin leaves to optimise sunlight capture. If disturbance events occur leading to gaps in the forest, trees will grow rapidly to fill the space. Popular house plants are commonly from this layer.
FOREST FLOOR
The forest floor is renowned as the darker layer, receiving less than 2% of the overall sunlight. As a result, only plants adapted to low light conditions, like shrubs and other herbaceous plants, grow here. Ferns, palms, and growing seedling and sapling trees are also present.
SOIL & NUTRIENTS
Tropical rainforests generally have very poor soil nutrients as intense tropical weathering has washed them out, thus, have only a small layer of organic matter compared to temperate deciduous forests. Most available nutrients are in the living animal and plant material, and soil nutrients are usually in a form not accessible to plants. On the forest floor, organic matter like leaves, branches, fruit, and seeds decompose rapidly due to the moisture and warm temperature. After any tree or plant dies, living organisms are fast to obtain the nutrients before they can be washed away. However, when tropical forests are chopped down or burned, nutrients get quickly washed away by the substantial rain, and the soil is left deprived.