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Vegetation characteristics
figure 7.18 Tropical rainforest, Amazonia, Brazil
figure 7.19 Daintree Rainforest, Queensland, Australia
Tropical rainforest vegetation The tropical rainforest is the world’s most biodiverse ecosystem, however it is also incredibly fragile. This is because the species that live in rainforests have adapted to the constant high temperature and humidity and, due to the fact they are so specialised, their food sources are limited. This means that if deforestation occurs, it is very difficult for the rainforest to return to its original state. The Net Primary Productivity (NPP) is the amount of organic material produced by plants and available to herbivores. It varies dramatically between ecosystems and is determined by heat, water and nutrient availability, age and the health of the plant species. The NPP for the tropical rainforest is 2200 g/m²/year compared to the savanna, which has NPP of 990 g/m²/year. Due to the fact that there are no seasons in the tropical rainforest trees shed their leaves throughout the year. The trees are tall and thin with a crown of leaves at the top and buttress roots at the bottom. Buttress roots serve a dual purpose; they prevent the tree from falling over and they gather nutrients over a wider area. Trees in the rainforest broadly grow in three layers. The emergent layer is composed of those trees that have grown rapidly to reach the sunlight, thus overshadowing their competitors. Trees are typically between 45 and 50 m in height although they can reach 70 – 80 m on occasion. Species that live in the emergent layer include the harpy eagle, the sparrowhawk, the vampire bat and the gibbon. The canopy is composed of trees that reach 25–30 m in height. This layer is dense and almost continuous and so blocks out 70–80 per cent of the available light. The fauna is similar to that which is found in the emergent layer although it is more diverse; the canopy is estimated to be home to a quarter of all insect species. Epiphytes (plants that grow harmlessly upon other plants) are abundant in this layer and include ferns, orchids and bromeliads. There are also lianas (long-stemmed, woody vines) that are rooted in the soil but use the canopy trees as a vertical support to climb to access the sunlight. The final layer of trees is the understorey, which receives only about 5 per cent of the available sunlight, although where the canopy is broken by falling trees the understorey vegetation is much denser. Plants in the understorey tend to have large, colourful flowers so that the animals and insects that carry their pollen can find them in the dim light. The high humidity in this layer suits amphibians such as salamanders and frogs; tree frogs have evolved feet with tiny suction pads
figure 7.20 Cross section of rainforest layers
158 Tropical environments
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covered with sticky mucus that help them to hold onto branches. Below the tree layers the forest floor is covered with decaying vegetation, which decomposes rapidly due to the hot and humid climate. Savanna vegetation Savanna vegetation includes a mosaic of grasses, trees and shrubs that have adapted to survive the dry season. Species are xerophytic (adapted to drought) with adaptations that include deep tap roots to reach the water table and sunken stomata on the leaves to reduce moisture loss. Species are also pyrophytic (adapted to fire) meaning that many trees have a thick bark, a large amount of biomass (stored energy) below the ground and the ability to regrow rapidly after burning. Savanna grasses have their growth tissue located at the base of the shoot close to the soil surface so that burning or grazing actually encourages its growth. Examples of plant species in the savanna include the acacia, palm and baobab trees and elephant grass, which can grow over 5 m tall.
figure 7.21 Acacia savanna, Taita Hills, Kenya
Nutrient cycling: Gersmehl diagrams, soil fertility, energy flows and trophic levels Within ecosystems, nutrients are circulated and reused frequently. Macronutrients (those used in large quantities) such as oxygen, carbon, hydrogen and nitrogen and micronutrients (trace elements used in small quantities) such as magnesium, sulfur and phosphorus can all be absorbed by plants. Animals eat the plants and take on Precipitation the nutrients. The nutrients are eventually returned back to the Biomass soil when the plants and animals die and are broken down by Fallout Pathway decomposers. All nutrient cycles involve stores and flows between Litter the soil, litter and biomass, the proportions of which can be shown in the Gersmehl diagram (Figure 7.22). In the tropical rainforest the warm humid climate results in a high input of nutrients from Uptake weathering and precipitation. The continual growing season means Pathway that most of the nutrients are held in biomass storage; however, Runoff Decay Pathway there are few nutrients held in the litter layer as the breakdown of nutrients is rapid. Where trees have been removed, precipitation is Soil not intercepted and nutrients are leached out of the soil store; as a Circles - compartments size proportional to amounts of nutrient stored result, rainforest soils are relatively infertile despite appearances. In Leaching Arrows - width proportional to amounts contrast, the biomass and litter stores in the savanna are relatively Weathering of nutrient flow small as a result of the shorter growing season and fire respectively. figure 7.22 Gersmehl diagram to show nutrient cycling in the tropical rainforest The soils are quite fertile as most of the nutrients are found in this store so they are not burnt or leached out of the system. The main driving force in terms of energy within an ecosystem is the Sun – it provides the energy required for photosynthesis and drives the hydrological cycle. The flow of energy can be shown in food chains and webs, which are the natural systems in which organisms feed on Precipitation each other to survive. An organism’s place within that web is the trophic level. There are four trophic levels; primary producers, primary consumers, secondary consumers and tertiary consumers. Fallout Pathway Biomass Between each trophic level it is estimated that there is a 90 per cent loss of energy, which is a result of respiration and excreta. In Litter the tropical rainforest the primary producer trophic level consists of ferns, bamboo and moss. Insects, spiders, fish and parrots are Uptake Pathway the herbivores that eat the producers; they are known as primary consumers. The secondary consumers, which eat the primary Runoff Decay Pathway consumers, are animals such as bats, amphibians and reptiles whilst the tertiary consumers, which are at the top of the food Soil chain, include snakes and carnivorous mammals such as jaguars. In contrast, savannas are dominated by tall grasses, which are Circles - compartments size proportional to amounts of nutrient stored the main primary producers, however shrubs and sparse trees Leaching Arrows - width proportional to amounts such as the acacia are also found within this trophic level. Primary Weathering of nutrient flow consumers include giraffes, antelopes, wildebeests, elephants, figure 7.23 Gersmehl diagram to show nutrient cycling in the tropical rhinos and, in Australia, kangaroos. These herbivores frequently savanna Tropical environments 159
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