Would like to add to my 22 year old stand of white spruce that surround our home. They form a protective wind break around the home that is three trees wide. The barrier is 300' on the West side and 200' on the North side. They are more than 40 foot tall now and beginning to thin out at the bottom. What type of understory trees could I plant within the stand to fill in and add color and texture to the stand?