Our neighbor's tree is dropping leaves into our pool and he refuses to trim his trees.  Do we have a legal right to trim the trees are overhanging on our property?

Yes, at your expense, and with reasonableness and caution.  You may trim the tree back to the boundary line. You will be liable  if you trim the overhanging branches or cut the roots so severely the tree dies.  You may want to hire an arborist as tree-trimming is recommended at certain times of the year in the desert due to changes in the heat.