I don't know what they were, but I saw a pool here in town a month or two ago where the back side of the pool was blocked off with a grove of small wispy trees that stood 15-20 feet high and were bushy all the way to the ground spaced about 2 feet apart and staggered 2 or 3 trees deep. Perhaps you could ask on a landscaping forum like gardenweb for some advice of identification help, they made me think of some sort of willow, but I really don't have a clue what they were.
Ike
ps After a bit of searching I think what they had was Sky Pencil Holly, even though most listings show the height at 8-10 feet that seems to be average height at 10 years of age with it being available already at 5-6 feet and a growth rate of up to a foot per year depending on climate. The house where I saw it was probably built in the mid 1990's, pool may be a couple of years newer. So 15-20 feet at 15 or so years in Louisiana may be about right if planted at about 6 ft height.