As a general rule, it's best to avoid raising low branches more than a third of the total height of a tree. For instance, the lowest branches of a 30-foot tall tree should not be more than 10 feet above the ground. This is because older trees need as much photosynthesizing foliage as possible to stay healthy. Generally, a healthy, mature tree can only handle having about 10 to 15 percent of its foliage removed at once.
Older trees that don't have the energy reserves to recover can't overcome environmental stresses, such as drought or temporary humid conditions. If you have a particularly sick tree, any removal of foliage can cause it to die permanently. To safely remove a lower branch of a tree, you must make THREE cuts. First, make a ¼ cut at the bottom of the branch about 2-3 feet away from the trunk; this will prevent it from splitting and damaging the trunk.
Then, cut the entire branch a foot or less out of the notch cut; this will allow most of the branch's weight to fall safely. Make the final cut right at the neck of the branch (where it joins the tree). If these limbs are carefully removed, it won't adversely affect their lifespan and can usually be taken care of in an afternoon. Not only did he remove the branches from the bottom, but he also removed several branches in the middle of the trees. If you are pruning trees with heavy branches, you must be very careful not to damage the bark or interfere with the tree's natural healing response.
By making two cuts at the bottom and top of the branch, you allow the weight of the limb to cut through the canopy for you; this cut branch falls to the ground. And he removed all the branches of the fourth smallest tree in the group, basically leaving only the trunk. The only reason to cut a healthy limb is when it interferes with other things, such as traffic, power cables, buildings, or sometimes other limbs, in which case it must be completely removed using the techniques shown above. In those cases, it is often best to cut the lower branches so that the mass of the tree can grow above sight or signage. The tree will form a layer, like a crust; but unlike a human being or an animal, this crust does not disappear and protects the extracted branch for life. Large trees can be pruned at any time of year if there is a branch that poses an obvious danger of falling, but ideally pruning should take place sometime between late autumn and early spring.
It's a good rule of thumb to make it an annual practice to inspect trees in early spring, remove dead branches, and cut branches from houses, garages, and other structures to create several feet of free space. I want to cut that branch, but I don't want to weaken its overall strength because it holds up my swing. One lesson I learned is never to place your ladder under a branch you are cutting - even if it seems very safe for your tree - when making relief cuts. A tree that has too many of its lower branches removed may also have its immune system compromised, especially if those branches are cut incorrectly.