Most people remember learning about photosynthesis during middle or high school.  They don’t necessarily remember any of the specifics of the process, especially how it relates to trees. The most important thing to know is that photosynthesis lets trees use the sun’s energy and convert it into the nutrients that they need.

What Is Photosynthesis?

As mentioned, photosynthesis is how trees take the energy from the sun, or light in general, and turn this into nutrients. Specifically, they turn this light into chemical energy in the form of sugar2. In order to photosynthesize, a tree needs more than just light; it also needs carbon dioxide and water. Air contains carbon dioxide. Water comes from rain, wet soil, or irrigation systems.

Where Does Photosynthesis Take Place?

The process of photosynthesis takes place within the tree’s leaves as this is where they keep all of the ingredients they need for the process to take place. The root system is responsible for bringing the water up from the ground and through to the leaves. The carbon dioxide gets inside the leaves via its stomata, or pores, which is how oxygen will exit after photosynthesis is over.

Understanding The Photosynthesis Equation

The following equation represents the process of photosynthesis3:
6 Carbon Dioxide Molecules + 12 Water Molecules + Sunlight → 6 Oxygen Molecules + Carbohydrates + Water
This essentially means that the tree uses carbon dioxide, water, and sunlight to create oxygen, carbohydrates, and water. It is these carbohydrates that the tree then uses. The chlorophyll molecules of the trees are responsible for giving it its green pigment and it is these cells that use the sun’s energy to separate some hydrogen from the water and add this to the carbon and oxygen molecules found in water.

This leads to the creation of soluble carbohydrates, or sugars, which the tree can turn into starch and store. Oxygen atoms from the water molecules are then freed. They combine and create free oxygen. Trees release this free oxygen into the atmosphere. Both protons and electrons are also freed during photosynthesis, and they produce ATP (Adenosine Triphosphate) as well as NAPD.

The Dark Reaction

Most people aren’t familiar with the “dark reactions” that are also part of photosynthesis. Their name is misleading as the reactions do not occur in the dark or at night in all cases; it simply indicates that they can occur whether or not light is present in the given moment.
The dark reaction of photosynthesis uses the chemical energy that was stored in the NAPD and ATP. This energy is used for the creation of carbohydrates with the leftover carbon dioxide from the air and hydrogen. Most of the time these carbohydrates will be in the form of the simple sugar known as glucose, but other compounds may also be produced.

Energy Production
Because energy is conserved, it cannot be created; instead it changes forms4. This means that when talking about the energy produced from photosynthesis, or any other process, it is measured by efficiency of the conversion. During photosynthesis, the energy from light becomes sugar glucose, which is chemical energy at a rate of between 3 and 6 percent. As a basis of comparison, an average solar panel will convert the light energy to electrical energy at 12 percent efficiency.

How Trees Use The Products

The trees use the carbohydrates resulting from photosynthesis as their food source. The phloem, which is a system made up of tubular cells, transports this food while bringing sugar from leaves to other areas of the tree. They do so at a rate of around 100 centimeters an hour. When the food reaches its destination, it is used to produce hormones, resins, and vitamins, each of which are necessary for growth as well as maintaining a resistance to pest infestations and disease.

Benefits Of Photosynthesis For Humans

Although trees developed photosynthesis as a way to grow and get their necessary nutrients and energy, the process benefits humans as well. The leftover oxygen which the trees release back into the air is a crucial part of this benefit as it enters the air that we breathe.
Humans also are producing an increasing amount of carbon dioxide as we burn fossil fuels. This is bad for the environment as a whole.  However, it does provide trees with the carbon dioxide they need for photosynthesis to occur. As such, trees help us to reduce the carbon dioxide levels in the atmosphere and experts encourage people to plant more trees and forests to counter the growing issue of this substance in the air.

Limits To Photosynthesis

All trees and other plants rely on photosynthesis to survive, but the process does have its limits. The most important are those concerning the tree’s access to carbon dioxide, water, and light. If for some reason the plant can’t access enough of any of these ingredients, it may become less effective at photosynthesis.
One example would be in weather that is hot or dry, such as in arid climates. In dry weather, the leaves of the tree may close their stomata, or pores. It is survival method of preventing water from evaporating.

Without the stomata open, the tree cannot take in the carbon dioxide necessary for photosynthesis. Instead, it will enter a metabolic process known as photorespiration which attempts to limit excess oxygenation.

Working With The Climate

You should consult an arborist before planting new trees on your property. This is especially important if you live in a dry or hot climate. There are some plants and trees which have evolved in a way that lets them live in this climate. Good examples of these are trees native to the area.  They keep a constant supply of large amounts of carbon dioxide in their cells. This allows them to easily undergo photosynthesis and prevent photorespiration.

http://forestry.about.com/od/Treebiology/g/Trees-And-The-Process-Of-Photosynthesis.htm
http://www.internationaltimber.com/news/trees/how-trees-eat–an-introductory-guide-to-photosynthesis