To the uninformed person, tree trimming is seemingly an easy task. Just grab a chainsaw remove the unwanted branches, and be done! But, unfortunately, it is more complex than that.

Trimming trees is both a science and an art in that Trees are living things with actual responses to the cuts that are made during trimming. Correct trimming helps the trees stay healthy by promoting growth, preventing branches from failing, and removing less healthy branches. Trimming is also helpful to keep the tree away from structures, buildings, and powerlines. Here are a few types of trimming that an arborist might recommend for your trees.

Crown cleaning
Crown cleaning is the most basic, least invasive type of trimming. It removes dying, diseased, crossed, crowded, and broken limbs. This trimming is beneficial to open up the tree to allow wind and light to pass through easily. It also enhances stability and removes unnecessary weight from the tree. Trees with sick limbs will improve once you trim the affected areas. Removing the affected areas allows the tree to grow back new, healthy branches and leaves instead of spending energy attempting to save the sick section. Crossed branches are a hazard to your property and your tree. They become damaged when the wind blows, and the two branches rub together; this creates scars in the tree and opens opportunities for insects and diseases to enter the wound. In addition, over time, the tree will develop thicker bark in these areas, which could cause pressure to build up that could break one or both of the crossed branches.
Crown Thinning:
Crown thinning removes smaller secondary branches from the crown of the tree. Thinning differs from crown cleaning because it is not focused on problem branches but on much smaller ones, often because they grow incorrectly. It is important to remove only what is needed when crown thinning because every cut you make on a tree is a wound that the tree will have to heal. Annual crown thinning removes branches from the tree that could become a hazard or look unsightly when they fully develop. It prevents the tree from wasting energy on these branches and promotes growth in more critical areas of the tree.
Crown raising:
Crown raising is the process of removing the lower branches of the tree. Usually, to provide better access for people, cars, and structures, that move underneath the tree. Take care when raising the crown of a tree because removing too much live growth from the tree can negatively impact the tree.
Crown raising can be beneficial to the tree as well, though. It helps remove weight from the tree and improves light on the ground below it for other plants. Crown raising also gives the tree a more manicured look, often increasing the property's value. A tree with low-hanging foliage blocks the view of the property from the street and makes the yard look unkempt. Having an arborist handle the crown raising is highly recommended because it can damage a tree significantly if done incorrectly. An arborist can immediately tell what branches to cut and which ones must stay until the following season.
Crown reduction:
A crown reduction is a type of pruning that removes tip-weight from the end of branches back where they are healthy and growing. From there, the tree will form a new crown. Crown reduction is beneficial when you need to lower the tree's height. Note that this is not the same as "tree topping," which is the process of completely removing the top of the tree by cutting the main trunk at the desired height. It is never a good idea to top your tree this way, as it can severely damage the tree, if not kill it. Crown reduction can be for clearance away from structures, or the tree has grown past a building that once provided the tree with shade. Some trees require some shade, or else they will become burned by the constant sunlight.
Crown restoration:
Crown Restoration is a term used to describe a specialized type of trimming. For example, if your large trees were improperly cut or topped, or if your tree suffered storm damage, your arborist would likely suggest this method:
He would select the most substantial branches possible and trim them to promote growth in the proper direction.
Once this new growth has come in, he can remove some of the other misplaced branches.
He will repeat this process until the entire tree has returned to a more normal, healthy shape.
Conclusion:
Taking care of your trees is one way to add value to your property. Doing it incorrectly, however, can damage or even kill your trees. Therefore, when trimming your trees, you should have an arborist out to inspect them first. The arborist can help you decide what needs removal and if it is the proper season to remove it. He will also be able to suggest how to cut if you want your tree to grow into a specific shape to provide a view or to add light to your yard.