We have Eucalyptus trees that are about 15 years old and about 25 feet tall. The bottom branches start at about 6 feet from the ground. The trees are a solid mass. How should we have them trimmed? Do the branches need to be thinned out? Should the lowest branches be cut off? The trees are either a slight Y shape or nearly vertical on all sides except at the top and bottom. We live in Long Beach, CA.