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Benefits of Trees in the Urban Environment

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Gorgeous trees and beautiful shades

Gorgeous trees and beautiful shade - Flickr

Trees are all around us in the countryside but we are normally not far from a tree even when we are in a city. The benefits of trees are often ignored or simply not understood. Here are some of the many benefits that trees provide society.

Trees provide shade

Trees provide shade for us all. Yes, ok, we live in wet and cold Britain but the shade provided by trees helps to keep you and your car cool throughout the year. They also keep your house cool in the summer and warmer in the winter by trapping the heat under the canopy. On hot days the shade of a tree can be a welcome relief in the city which can be 5 degrees warmer than the neighbouring countryside. It is called the urban heat island effect but trees help more than just providing shade…

Trees cool the air

When light falls onto a black or dark surface, such as the roofs and tarmac roads in cities, the energy is absorbed and the surface is heated. This causes the surrounding air to be heated and we end up with hotter air in the city. This heating effect is the main cause of the urban heat island effect

When trees absorb light and use the energy in photosynthesis to produce oxygen and, as the energy is used, it does not turn to heat. Trees therefore intercept light which would otherwise fall on asphalt causing temperatures to rise. But it gets better, trees can actually cool the air.

Trees take water from the air and use it for photosynthesis (the hydrogen from water combines with the carbon dioxide, CO2, to form carbohydrates in the form of glucose, which the tree uses/stores for energy. As the water molecule, H2O, looses its hydrogen atoms it becomes a free oxygen molecule, O2) but trees take more water from the ground than they need, this water is lost through the leaves and evaporates. As the water evaporates and passes through the air it cools the air in a process known as evaporative cooling.

They absorb pollution

Some species of tress are chosen as street trees because they are very tolerant to the pollution in city air. They are also able to remove CO2, pollution and other particles from the air. They also produce oxygen giving us cleaner, more oxygenated air in the city making the air healthier for us all.

Carbon capture – carbon sink

As mentioned above, as the tree photosynthesises it captures carbon dioxide from the air and converts this into carbohydrates. This carbon is then used by the tree to grow and live. Although some of the carbon is released back into the air much of the carbon is ‘locked’ into the tree’s timber, branches and roots. Again this removes carbon directly from the air making the city air cleaner.

They provide a screen

Trees can be used to screen unsightly views but they can also be used to reduce noise and pollution, particularly evergreen species. The can also provide shelter from wind and rain and prying eyes.

Help to prevent flash floods

Trees slow rain as it falls through the air, the rain is held on leaves slowing its progress to the ground. This allows soil more time to absorb the water. More importantly for the urban environment trees slow the water from running down the road and down the drain. This reduces the surge of water entering our sewers and can prevent flash floods. On steeper ground the tree roots tie the ground together to prevent landslides and stops river banks from washing away.

Habitat

Trees provide homes and food for many animals from birds and squirrels to insects and bats. Trees also provide wildlife corridors allowing animals to move from one green area to another.

Happiness

Trees make people happy and give a can reduce stress. They can soften the harsh urban environment and provide a connection to natural habitat. Many people enjoy walking in our forests and parks, spending time amongst trees. This was particularly evident in early 2011 when over half a million people signed a petition to prevent the sell off of Forestry Commission woodland.

Increased property prices

As a result of all these benefits properties areas with a high number of trees can have a value upto 20% higher than equivalent properties in areas with fewer trees.

 

If you know of anymore benefits of trees please let us know by using the comment section below

References and further reading

The Importance of Soil pH

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Most people are aware that some plants and trees thrive in acidic soils and others will only grow in alkaline soils but why is this the case?

First lets touch on some basic chemistry. pH is a measurement of acidity or alkalinity on a scale of 1 to 14. A number below 7 indicates an acidic substance, above 7 indicates and alkaline and 7 represents a neutral substance. You can buy soil testing kits to find the pH of your soil from most garden centres and (in the UK) you are likely to have a pH from about 4 to 8.

The pH of your soil is determined by the amount of calcium or lime (as lime consists mainly of calcium in various forms) contained within the soil. Calcium is an alkaline element and it is usually lost from the soil over time as it is washed out of the soil. In some environments, such as soil over a bedrock of limestone, the calcium is replaced on an ongoing basis. Other soils, such as sand, are unable to hold on to the calcium and slowly turn more acidic over time.

At this point I have to take you on a tangent and look more closely at what plants need from soil. Soil is full of nutrients which plants need to grow and particles of nutrients are ‘stuck’ to the soil by adhesion at the atomic level. The nutrients stick to both inorganic matter (clay particles) and organic matter (highly decayed plant matter) within the soil known as colloids. The other inorganic parts of the soil such as sand and silt are chemically inactive so nutrients are unable to stick to them. This is why calcium is easily washed from sand and is why clay is so important in healthy soil.

So what does this mean to my plants?

Different nutrients are available at different soil pH levels. This affects the way plants grow in different soils.

Some plants are very sensitive to soil pH, the most well know example are Rhododendrons and heathers which will not tolerate lime in the soil. On the other hand there are plants which greatly prefer a lime soil.  This is because of the nutrients available to the plant at a particular pH.

Soil pH affects which nutrients are available to a plants because minerals will be solid at a certain pH and soluble (able to dissolve into the water) at another pH. If the nutrient is solid it is unavailable to to plant. The nutrient needs to be dissolved before it can be absorbed by the plant or stick to the colloids (clay particles or decayed plant matter) in the soil.

Aluminium is not a plant nutrient and if the soil pH is over 5 it remains as a solid and is washed out the soil by water. However, if the pH drops below 5 aluminium becomes available to be ‘stuck’ to the colloids. If the pH were to continue to fall below pH 4.5 the amount of aluminium becomes toxic to some plants and the aluminium particles are in such great numbers they displace the other nutrients from the colloids. This reduces the amount of other nutrients available to plants.

So acidic soils are damaging because of the amount of toxic aluminium where as alkaline soils affect plants because the nutrients needed by the plant are kept lock up in solid form at the higher pH. For example plants in a high pH soil can suffer from manganese deficiency where as manganese would be in excess in low pH soils. If a plant is suffering from a Iron, Boron or Manganese deficiency in a high pH soil you are unable to correct the deficiency by adding more of the nutrient as it will remain ‘lock up’ in solid form until the pH is lowered.

The pH of acidic soils can be increased by adding lime whereas alkaline soils can have their pH lowered by adding peat or other decayed plant matter.

Soil pH also affects the organisms found within the soil. Earthworms are important as they aerate  the soil and help bind it together but they prefer non-acidic soils and need high amounts of calcium available. Beneficial bacteria will generally tolerate a wide pH but most fungi prefer acidic soil.

So as you can see soil pH has a complex relationship with plants and organism within the soil and that the wrong soil pH could starve your plant (or tree) of essential nutrients or poison it with excess aluminium.