Trees and natural cycles

Plants are important in several key processes involved in the interacting systems of the Earth, including the hydrosphere, atmosphere and biosphere. Three of these processes are cycles – the water cycle, the nitrogen cycle and the carbon cycle. Plants play a key role in maintaining the balance of each of these cycles. As trees are larger than other plants, their contribution is significant.

The water cycle

The major influence of trees on the water cycle is through transpiration. Transpiration is where plants evaporate water vapour through tiny pores or openings in their leaves called stomata. The rate of transpiration depends on leaf shape and size, number of stomata and waxiness of the leaf surface as well as the local weather and climate conditions.

Dense rainforests can often be seen to have ‘steam’ arising from the canopy – this steam is transpired water vapour. The water vapour accumulates in the atmosphere before precipitating back to the Earth as rain, sleet or snow. The temperate rainforests of New Zealand, such as those on the West Coast, play a role in cycling huge amounts of water.

Deforestation in one area can affect the weather in another area, because if trees are cut down, less water evaporates into the atmosphere and there is subsequently less rain. At a local level, the land can become drier and less stable. When it rains, instead of the water being soaked up through tree roots, there is increased run-off and nutrient leaching. Deforested areas can become prone to both droughts and flooding.

The carbon cycle

Carbon moves through the Earth’s system in many different ways. This process is called the carbon cycle.

Plants take in carbon dioxide from the atmosphere during photosynthesis, converting it from inorganic carbon to organic carbon. Animals get carbon by eating plants or by eating other animals.

Carbon is returned to an inorganic state in a number of ways. Plant and animal cells produce carbon dioxide during cellular respiration. This carbon dioxide is then released to the atmosphere. Bacteria and fungi also release carbon dioxide into the atmosphere when they decompose dead organisms.

Carbon dioxide is an important gas in our atmosphere. It prevents heat from escaping, which helps warm up the Earth’s atmosphere. In a similar way to the glass of a greenhouse, it traps the heat from escaping. This is why it is called a greenhouse gas.

However, human activities such as deforestation and burning fossil fuels have greatly increased the levels of carbon dioxide in our atmosphere. This has enhanced the greenhouse effect and is contributing to climate change.

New Zealand’s One billion trees programme (2018–2027) will help offset some of the effects of climate change by growing trees to soak up carbon dioxide. Trees work as a carbon sink – the carbon becomes trapped in the trees as carbohydrates and other organic compounds.

The nitrogen cycle

Nitrogen is a crucial component for all life. It is an important part of many cells and life processes. As part of these life processes, nitrogen is transformed from one chemical form to another. For example, it is needed to make proteins, DNA, RNA and chlorophyll, which plants need to be able to carry out photosynthesis to make food. Nitrogen is also the most abundant element in our planet’s atmosphere.

The transformations that nitrogen undergoes as it moves between the atmosphere, water, the land and living organisms make up the nitrogen cycle.

Plants take up nitrogen compounds through their roots. Animals obtain these compounds when they eat the plants. When plants and animals die or when animals excrete wastes, the nitrogen compounds in the organic matter re-enter the soil.

Plant uptake of nitrogen compounds

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Once nitrogen is converted into compounds like ammonium and nitrate, these can be taken up from soils by plants and then the nitrogen can be used to form macromolecules like proteins and nucleic acids (DNA and RNA).

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A significant problem facing New Zealand is the leaching of nitrogen compounds from the land into our waterways. Because nitrogen supports plant and algal growth, the increased levels of nitrogen in the waterways can cause unexpected – and even harmful – effects in the ecosystem such as algal blooms.

Riparian planting of trees and other plants can have a positive effect on mitigating nitrogen run-off. This mitigation is achieved both through the increased uptake of nitrogen compounds by tree roots and the physical barrier that plants create – preventing the nitrogen from getting washed into the stream or river with sediments.