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In biological systems, nutrients are cycled through the food chains. Bodies of plants and animals are broken down by billions of soil organisms, releasing the inorganic nutrients which can then be reutilised by plants. Both macronutrients and micronutrients are recycled constantly through plant and animal bodies, returned to the soil, broken down and then taken up into plants again.
Four basic nutrients are important to a tree:
• Carbon
• Phosphorous
• Nitrogen
• Water
Carbon
Carbon dioxide is taken into the tree leaf where it is used with water in the process of photosynthesis to create glucose.
The sugars are then used in the plant’s metabolic processes. Production of the polysaccharide cellulose is important to the carbon cycle because it is the most abundant polysaccharide in nature and accounts for much of the stored carbon. The polysaccharide cellulose and hemicellulose make up about 80% of the dry wood fibre in a tree. Once cellulose has been formed it is no longer available to the plant as an energy source.
Phosphorous
Phosphorous is used by trees for the formation of DNA molecules in chromosomes, and the ATP molecules that are used to transfer the energy in food into energy that can be used by the cell. The amount of phosphorous required by a plant is high in comparison to the levels of other minerals required. Phosphorous is found in the soil and taken up and stored. It is lost from an area when trees are harvested.
Nitrogen
Nitrogen is needed by plants and animals to produce protein. Protein is required to build and repair body tissues. Nitrogen makes up 78% of the atmosphere but the gas cannot be directly utilised by plants. Some kinds of bacteria and fungi can utilise elemental nitrogen and they live in the soil or in symbiotic relationships with plants (e.g. legumes). These organisms can ‘fix’ nitrogen from the air and produce nitrates that can be used by plants and animals. Only a small proportion of the nitrogen in the soil is available to plants. Most is bound up in the bodies of organisms. It is only when they die and the nitrates are released that other organisms can compete for the nutrients.
Water Conservation
Plants require large amounts of water. Water is used in many metabolic processes but particularly photosynthesis. However, much of the water taken up by trees can be lost to the atmosphere through transpiration – the evaporation of water from within the leaf. The water vapour escapes from holes in the leaf called stomata. A mature pine tree will take up to 100 litres of water from the soil on a warm day, most of which will be used in transpiration.
Forests return water to the atmosphere not only through root uptake. The large surface area provided by leaves and branches allows up to 60% of rain to evaporate before it reaches the ground. Only a small proportion of the rain that falls on forests reaches rivers, lakes and the ocean.
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