Why Plant Trees?

Reducing atmospheric carbon and supplying oxygen for all living things are just two of the many reasons for encouraging the planting of trees.

From an early age we learn that only plants absorb CO2 and give off life giving oxygen. We learn too, that plants are at the base of all food chains – without plants no animals would survive. Trees and other plants are fundamental to life on earth.

But the benefits are so much more far-reaching than this.

Carbon and Oxygen
Plants are the foundation of life on earth. Through the process of photosynthesis only plants sequester carbon to make carbohydrates (glucose) and these carbohydrates are the building blocks of all animal food.

Because of this plants, known as producers, are at the base of all food chains providing animals directly or indirectly with their food.

The process of photosynthesis also releases the oxygen necessary for respiration which is the process providing the energy that both plants and animals need in order to survive. Both photosynthesis and respiration are intimately involved in maintaining the carbon/oxygen cycle which is one of the natural cycles vital for the maintenance of life on earth.

In natural forests the carbon cycle is roughly in balance, that is, the amount of carbon dioxide taken up by plants during photosynthesis is equal to the amount released by respiration and decay.

However, because of the greater mass of living material in a forest, it has greater capacity to ‘lock up’ carbon in living tissue than other ecosystems such as pasture. Regenerating natural forests, create ‘carbon sinks’ by absorbing more carbon dioxide through photosynthesis than is released through respiration and decay.

Trees play an important part in mitigating the ‘greenhouse’ effect which is compounded by increasing emissions. It is estimated that that planting 23 trees per year will offset the carbon emissions of a medium sized vehicle travelling 15,000km per year.

Food Chains and Food Webs
Every organism needs to obtain food in order to live. Plants (producers) make their own food via photosynthesis, but animals do not and so  some animals eat plants (herbivores), other animals eat animals which have eaten plants (primary carnivores) and still other animals eat animals which have eaten animals (secondary carnivores).

A food chain is the sequence of who eats whom in a biological community in order to obtain food and follows the pattern
Producer    ——>    Herbivore    ——>    Primary Carnivore    ——>    Secondary Carnivore

However, a food chain is only part of a much more complex food web which exists in  natural ecosystems. This diagram prepared by the University of Waikato shows an example of a Mangrove Food Web. Click on the image to enlarge.

It is frequently forgotten that the removal of just one link in the food chain by either exploitation, competition from pests or disease will disrupt the food web and have a major effect on plants and/or animals above or below it in the food chain.

The removal of trees naturally has a severe impact on animals dependent on the trees for food and this further impacts on animals further up the food chain.


Natural Biogeochemical Cycles
Biogeochemical cycles can be broken down into two types: local cycles such as the phosphorus cycle, which involve elements with no mechanisms for long distance transfer; and global cycles, which involve an interchange between the atmosphere and the ecosystem. It is these global nutrient cycles, such as the nitrogen cycle and carbon cycle (see above), that unite the Earth and its living organisms.

Because there is generally only a small quantity of phosphorus available for uptake by plants, phosphorus is often a limiting factor for plant growth. This is why farmers often apply phosphate fertilisers to farmland to increase plant growth.  Runoff of phosphate fertilisers from farmland leads to eutrophication of waterways and can cause algal blooms in aquatic ecosystems, resulting in lower water quality. Poor water quality is one of the major environmental concerns in New Zealand today.

In farming a 525kg cow can produce  more than 23 litres of urine per day. Nitrate from the urine can enter waterways, either directly or via groundwater, with undesirable environmental outcomes. The amount of leaching of nitrates is also affected by when and how much fertiliser is applied to pasture.

Planting trees adjacent to lakes and waterways is a particularly effective way of reducing runoff of phosphates and nitrates thereby reducing lake and waterway eutrophication.

Balance of nature
We do not hear the term ‘balance of nature’ so much these days but it is a simple phrase to describe the fine equilibrium (homeostasis) which exists between populations in natural ecosystems. Because plants are at the base of all food chains they are integral to maintaining the balance essential to prevent the destruction of habitats.

For most native species, the destruction of their habitat is a worse threat than being hunted. Habitat loss in New Zealand has happened in three ways:

  • Whole ecosystems have been converted into farmland, exotic forests and settlements.
  • Ecosystems have been partially removed, creating ‘islands’ surrounded by farmland.
  • Ecosystems have been degraded by the loss of species and disruption of their ecological processes.

Tree removal not only effects local biodiversity (eg: insects, birds) but also impacts on the local environment (eg: slips, flooding, water quality) but also the global environment (eg: climate).

Planting trees provides the converse to the above – biodiversity is increased and impacts on the local environment is reduced.

Reducing Erosion
New Zealand loses between 200 and 300 million tonnes of soil every year to the oceans – a rate about 10 times the world average. Erosion is a natural process, but removing vegetation and using inappropriate land management practices simply accelerates the process.

The effects of erosion are not just confined to on-site loss of soil and nutrients and the subsequent reduction in farm or forestry production. Effects are pervasive, as sediment is redistributed from hill slopes to rivers and lakes, to flood plains and to the marine environment.

It is widely accepted that soil erosion under forests is much less than under other forms of vegetation cover such as pasture, as the trees have deeper stronger root systems.

Trees reduce the rate of erosion by:

  • protecting the soil from the impact of rain
  • transpiring large amounts of water, which counteracts very wet soil
  • binding soil to sloping land with their roots.

In 1997 the Ministry for the Environment noted that:

  • 50% of the country was affected by moderate to slight erosion
  • 10% had severe to extreme erosion (eastern North Island, parts of Taranaki, and the South Island high country)
  • only 31% of the total area could sustain pastoral farming without significant control of erosion
  • a further 28% could support restricted livestock grazing combined with erosion control.

Loss of productivity and the economic cost of erosion, sedimentation and flooding in New Zealand has been estimated to be around $159 million per year not including effects on water quality and biodiversity loss.

Stream flow and water quality
Factors effecting stream flow and water quality are silt from eroding land upstream and stream banks, fertiliser and nutrient run-off and industrial pollution.

Silting is bad for our waterways because:

  • it decreases the water clarity (less light for plants to photosynthesize)
  • fine sediments can suffocate fish and other water life
  • increased nutrients encourage undesirable plants and animals.

Eventually the water carries the soil particles downstream to the coast, causing increased sediment in our harbours. More sediment in our harbours means:

  • less habitat for kai moana (seafood)
  • poor water clarity
  • flooding problems upstream
  • infilling of harbours restricting recreation
  • changes in habitats.

Fertilisers and nutrients such as nitrate and phosphate that are washed into lakes and rivers upset the natural balance of nutrients in the water. Fish are harmed and some water plants (e.g. algae) grow too quickly. These plants clog lakes and rivers, reduce water clarity and starve fish of oxygen.

Riparian planting of trees in wetlands and alongside lakes and streams mitigates many of the problems caused by sedimentation, fertiliser and nutrient run-off and industrial pollution.







Climate Change and Global warming
The debate over what is causing global warming has over-shadowed the fact that it is actually happening. Since the beginning of last century the earth’s average surface temperature has increased by about 0.8 °C, with about two thirds of the increase occurring since 1980. This warming is almost certainly caused by increasing concentrations of greenhouse gases produced by human activities such as deforestation and the burning of fossil fuels.

Along with rising temperatures come sea-level rise, changing rainfall patterns, increased storminess and impacts on human health. Adaptation is the process of preparing for and adapting to the impact of these changes on our economy, environment, infrastructure and way of life, in order to minimise the risks and maximise the opportunities.

Planting more trees will help to reduce the effect of carbon emissions and reverse the effects of deforestation.

Click on the image to read a NZ Listener article on Global Warming.

Enhancing the built environment
Trees screen unsightly views, soften harsh outlines of buildings and decrease noise pollution in busy areas.  They also act as windbreaks, thereby decreasing heating costs while acting as shade in our cities’ hot spots. Trees and shrubs properly cared for can significantly add value to domestic or commercial properties.

Tree colour and form add beauty, enhance our landscape and contribute to the experience and quality of human lives.



Creating health benefits
Native trees provide many traditional maori medicines with proven healing value and today many of these trees are the source of many pharmaceutical products coming from plant extracts. And science has hardly scratched the surface of the potential provided by trees and plants!

Remember too that trees are wonderful food producers – with nuts, fruit, berries and in New Zealand the source of manuka honey – important components of many of our diets.

If this isn’t enough – then take in the wealth of recreational opportunities that trees offer. They have a role to play, too, in reducing workplace stress, helping speed the recovery of hospital patients, instilling community pride and reducing conflict. They are so much part of our spiritual and cultural heritage; at the end of the day, people just like seeing them grow!