About Trees

How plants are structured

The Philip´s Nature Encyclopedia explains that each part of a plant is designed to fulfil a particular function: to make food, to take up water and minerals, to transport and store food within the plant or to reproduce.

Above ground are the green stems and leaves, which capture light and produce food. The leaves of green plants grow in many shapes and sizes, but are generally arranged regularly along the stem. Such an arrangement minimizes overlap and ensures that each leaf catches a maximum amount of light. The leaf surface is sealed with a watertight, airtight layer of cells, the epidermis, which secretes a waxy cuticle. The leaf has pores, called stomata, through which air enters and waste gases leave. Water vapour also evaporates form the inside of the leaf through the stomata, creating the transpiration required to draw water upwards from the roots. To avoid excessive water loss on a hot sunny day, (stomata) are usually much more numerous on the lower surface of the leaf, the side not turned to the Sun.

Guard cells on leaf surfaces open and close the stomata to control, the rate of water loss.

The stem of the plant holds the leaves up and out towards the light, and holds the flowers out to catch the wind or to where the insects the birds that pollinate the planet can find them. A rigid column of wood supports the stems of arboreal plants. Herbaceous plants with slender stems and no woody tissue maintain a rigid stem by keeping the stem cells full of watery sap.

Plant Feeding

Roots absorb water and mineral salts, and anchor the plant in place. The surface epidermis cells of roots just behind the growing tip have long tubular extensions - the root hairs – increase the surface area of the root available for absorption. Plants continually take up water from the soil, for 90 percent or more of the water they take up is always lost through inevitable transpiration: the evaporation of water and water vapour into the atmosphere from the leaves.    

The way a plant grows is determined partly by its own heredity but also by its surroundings. Plants growing on the woodland floor are often tall and slender, competing for light, whereas the same plant growing in the open may be shorter and bushier. Plants growing in short grass will often develop ground – hugging rosettes of leaves to suppress the growth of grass around them and also to prevent the permanent parts of the plant from being eaten by grazing animals.

Shape and form

A fir tree, an oak and a date -palm illustrate three quite different types of plant architecture. The fir tree (a conifer) has a symmetric conical shape. Its branches come out at regular angles around the trunk, and bear small, stiff twigs along their length. The oak has branches dividing many times into successively smaller branches and finally into twigs that bear the leaves and flowers, creating its distinctive shape. The date – palm is quite different from either. Its trunk does not branch at all and carries much – divided leaves in a bunch at the top.

How Tropical rain forest plants live

In one hectare of rain forest there may be as many as 200 species of tree alone whereas in the same area of the richest temperate forest there might at best be about 25. The tops of the rain forest trees together form the vast, green canopy – a mass of branches, leaves, fruit and flowers creating an aerial world in which most of the forest animals live. Above the canopy tower are the forest giants. Way below are shrubs and climbers, the under storey, and on the shady floor are minute algae and delicate fungi.

Some rain forest may well have existed largely undisturbed for 60 million years. In these stable conditions the forests have become the richest habitats in the world.

They support half the total number of species of animals and plants on the planet.

Despite its diversity, the rain forest canopy looks from the air like a vast green carpet, spotted with the colours of flowers and birds, and frequently interrupted by the tall emergent. With its “air plants” that grow on other plants and its climbers a rain forest canopy tree may support over 30 other species of plants. 

Beneath the canopy forest is shady, with sparse undergrowth. To reach for the sunlight some plants climb towards the light, using established trees for support. Others are better adapted to survive in the shade.

The forest plant life supports its animals, providing shelter and sleeping places, as well as food in the form of fruit, flowers and foliage. Plants are also responsible for driving the forest ecosystem, especially in helping to recycle vital water and nutrients.

Rapid recycling.

On the forest floor fungi thread their way through the thin soil, helping to break down plant and animal remains into nutrients, which can then be used again by plants. A cubic centimetre of forest soil may hold several metres of fungal threads and as many bacteria as there are people on earth. Decomposition is so rapid on the floor that there is little accumulation of leaf litter. On this shallow soil, many trees grow buttress roots that spread out around the base of the trunk and help hold it up.

Every tree dies and falls sooner or later, but in the last few decades the forests have come under a much more serious threat: destruction by humans. Forest destruction does not just harm a rich ecosystem, it has huge environmental effects. Locally the removal of large areas of vegetation can lead to run-off and flooding. On the global scale, forest burning puts high additional amounts of carbon dioxide into the atmosphere, increasing the greenhouse gases and contributing to global warming.

Life is richest in the canopy, where the bulk of the foliage is and where most of the plant eaters live. Caterpillars of moths and butterflies feed on the leaves, and are in turn eaten by frogs and birds.  

Life in the canopy is very noisy with animals calling to keep in contact, to pass on alarms, or to issue territorial threats. Canopy animals are adapted to life in their tree top habitat. Parrots use strong claws and beaks to clamber among the twigs. Fruit bats move among the trees by flying, but other mammals –far from solid ground – have to cling on and climb around. Sloths hang from branches by their claws as they feed on leaves. Spider monkeys use their tails as a fifth limb to swing through the branches.

Floor dwellers

On the forest floor, small animals like ants, beetles, earthworms and termites feed on fallen leaves, dead wood and animal corpses. 

They are vital to the forest’s ecology, for together with fungi and bacteria they break down dead material into substances plants. This recycling of the forest’s nutrients is particularly important because many of the rain forest soils are very poor in minerals required for plant growth. Animals feed upon these plants and decomposers further up the food chain. Giant boars graze in clearings; pheasants scratch around for insects, seeds or fallen fruit; and anteaters use their long, sticky tongues to capture termites and ants.

At the top end of the food chain are the large predators. Leopards hunt birds and monkeys in the lower canopy and deer on the forest floor. Above the canopy float keen eyed forest eagles, fierce predators of the air.

The rain forest ecosystem is linked together by a complex food chain in which all the forest organisms depend upon each other.

The Northern forests

The dark, pointed profiles of conifer trees dominate the boreal forest. Species like pine, fir and spruce are ideally suited to the challenges of the environment: the shape of their branches allows them to shed weights of snow rather than break, and their slender trunks are flexible in the face of the strong winds. The root systems of these trees tend to be shallow, enabling them to obtain water from the surface soils.

The soils beneath the conifer canopy are poor, acid, and often thickly covered with needles. In these infertile conditions, the conifer’s growth is assisted by fungi, which help breakdown the needles and supply the trees with nutrients. In return, they probably gain carbohydrates from the tree.

Deciduous trees like birch and aspen also occur in the boreal forests, but the early spring growth of the evergreens allows them to overtop and dominate the deciduous trees. Plants like bilberry and sphagnum moss grow when enough light reaches the ground.

The great providers      

The conifer trees are fundamental to the boreal food chain. Moth and sawfly larvae eat the needles. Other insect larvae, including those of the wood wasp and pine sawfly, attack the wood itself. Capercaillies eat the conifer needles during winter, when food is to short. To crossbills and red squirrels, conifer seeds are important dietary elements. Caribou and reindeer, which migrate south from the tundra to spend the winter in the shelter of the forest, depend upon scraping away the snow beneath the trees to find lichens and other vegetation. Small rodents such as voles feed on bark, buds, fungi seeds and berries. During spring and summer in the boreal forests, insects are abundant, and they attract large numbers of migratory birds to the habitat to breed. The abundant voles and other small mammals are the prey of the red fox and the great grey owl. Other hunters include wolverines, wolves and brown bears.

The boreal forests are one of the least disturbed habitats in the world, but even so they have long been harvested for timber, and a local scale, berries and fungi. Large areas have been cleared for farmland. But many boreal forests are now at least protected by nature reserves and national parks.

Spreading their seeds

Flowering plants reproduce by seeds, formed in the female reproductive parts of a flower after fertilization. The developing seeds are enclosed in a protective case to form a fruit; conifers, on the other hand, bear their seeds on the surface of the scales of the cone, from which they are shed directly.

To ensure the best chance of germinating safely and have surviving to produce their own flowers, plants have evolved ingenious ways of protecting and dispersing seeds.

Wings and parachutes 

Many seeds are borne away on the wind. The small, single-seeded fruits of dandelions and thistles each have their own silky parachute that carries them some way from the parent plant. Conifers, such as pines and spruces, have light, papery- winged seeds. The wings of maple and sycamore fruits keep the nut-like seeds aloft for some time as they gently spiral to earth, allowing the wind to blow them beyond the shadow of the canopy of the parent tree. Orchids have minute seeds that can be blown away like a cloud of dust, and the wind shaking the ripe capsules of poppies scatters their tiny seeds.

Ocean crossings

The seeds of dandelions and thistles usually travel just a few yards, but some fruits are dispersed over hundreds of miles by water. Ocean currents carry the buoyant coconut across the Pacific to colonize newly formed coral atolls. Protected by a fibrous outer coat and a hard inner shell, the coconut embryo can survive these long journeys. The seeds of alder are equipped with a corky knob, which ensures they float if they are released over water. The stream carries them until they lodge against a muddy bank where they can germinate.

Animal carriers

Animals can disperse plant seeds and fruits in two ways. The hooked spines of the burrs of certain plants can become caught in the animal’s fur and dispersed as the animal grazers or, more commonly, the seed may be enclosed inside a tempting juicy fruit. Such fruits often change colour dramatically from green to red or purple when they are ripe, indicating to animals that they are now sweet and ready to eat. Red is the most common colour in fruits; pure red is invisible to insects, which would nibble the fruit without dispersing the seeds.

Making sunlight into food

In green plants and algae, photosynthesis takes place in the chloroplasts, miniature solar converters inside the plant’s cells. Chloroplasts contain the green pigment chlorophyll, which absorbs light. The energy of light is transferred to the chlorophyll molecule, which then passes it on through a complicated chain of reactions, and biochemical processes that finally result in the formation of simple organic (carbon containing) compounds such as sugars.

Carbon dioxide from the air provides the carbon atoms for these organic compounds. At the same time, water is split into its component atoms, producing oxygen gas: oxygen is thus a by-product of photosynthesis. Sugars are used by the plant as fuel for respiration, which generates chemical energy to power biochemical reactions essential for survival and growth. Respiration also produces carbon dioxide as a waste product, which can then be used again for photosynthesis.

The products of photosynthesis also represent the starting point for the formation of other simple organic molecules. These can then be combined into larger molecules such as proteins, nucleic acids, polysaccharides and lipids, from which all living material is made. Plants generally store food in the form of sucrose, a compound of the sugars glucose and fructose, and starch.

Minimizing energy loss

Energy is lost from the food chain at each upward step that is when a plant is eaten by an herbivore, or an herbivore by a carnivore. Without photosynthesis to tap the Sun’s virtually inexhaustible supply of energy, life would therefore rapidly run down.

Photosynthesis is not a particularly efficient way of converting the Sun’s energy into food.   

Only an average of about 1 percent of the light that hits a leaf is absorbed (up to 3 percent in the most productive cases) and even then the maximum possible level of photosynthesis is rarely achieved. Some desert plants, for example, that live in the conditions of intense light, high temperature and low humidity, keep their stomata closed during the day to prevent water loss, and are thus unable to take up the carbon dioxide that is available. Many tropical and desert plants have developed successful variations on the usual pathways of photosynthesis in order to deal with such conditions. Net photosynthetic rates of the tropical crops maize, sugar cane and sorghum can be two to three times those of wheat and rice.

The overall efficiency of photosynthesis is also affected by a metabolic peculiarity of many plants. A substantial amount of the photosynthetically fixed carbon is almost immediately converted back into carbon dioxide by the process of photorespiration, especially when carbon dioxide levels are low. The apparently wasteful process resembles normal respiration in that it consumes oxygen and produces carbon dioxide, but occurs only in the light, takes places in structures called peroxisomes, not in the mitochondria, and does not generate any usefu energy. It may however be involved in other functions of the plant, such as seed formation.

In “The Work of Nature “ Yvonne Baskin explains that in the tropics, keystone plants are often those that provide fruit for important seed-dispersing animals. The relationship between fruiting trees and fruit eating animals is called a mutualism because both parties benefit. Animals need the nourishment the nutrient-packed pulp and seeds, and the plant needs to have its seeds carried to new sites. Of course as animals eat fruit many of the seeds get digested or destroyed. But a fraction may be deposited later a good distance away from the parent tree. This service is valuable enough that 50 to 90 percent of the canopy trees and shrubs and small trees in the tropical forests of Central and South America bear fruits that are fleshy, brightly coloured, or otherwise adapted to attract animals.

The dependence of animals on fruit producers highlights the importance of protecting certain keystone plant species in the tropics.

Trees shelter the soil from the harsh actions of wind, sun and rain and also supply it with litter and detritus, replenishing its carbon and nutrient stocks. In addition, anywhere from 10 to more than 50 percent of the carbon that plants capture through photosynthesis eventually passes from roots and symbiotic root fungi into the soil, where it feeds the microbes that convert the plants detritus back into the reusable nutrients. 

Trees in most tropical forests, for instance, put out a dense mat of fine roots on or just below the surface to retrieve nutrients as soon as they are released from the litter and to capture rainwater quickly, before it can filter down to groundwater.

She explains that in addition, the forest canopy protects the soil from chemical weathering, which proceeds three to six times faster in the tropics than in temperate regions. The canopy also captures a large proportion of rainfall as stem flow. As the water runs across leaves and stems and down the trunk it picks up nutrients from the excrement from the hosts of animals living in the canopy, along with substances leached directly from the tree’s own issues. 

Trees and perennial shrubs “are the key to the site- restoring powers of fallow vegetation in the humid tropics because of their deep, permanent root systems”. Shrubs and trees, develop roots that thoroughly exploit soil resources, stretching into the subsoil to pump deeply leached nutrients back up to the surface. These roots also trap and recycle the nutrients that fall with the rain. In fact, the ability of these long-lived root systems to maintain soil fertility is a key reason why the most sustainable crop in the humid tropics are perennials: bananas, cocoa, rubber trees and oil palms.

When trees are dead they continue to be useful to man as they are used for furniture, wood and house construction.