In most cases, wind turbines are located on high ground in open conditions. Planting trees that will exceed 30 feet in height anywhere in the prevailing wind path (fore or aft of the turbine) is likely to result in reduced wind speeds. Short trees (those that rarely exceed 30 feet at maturity) may not impact wind speeds at the critical heights if the turbine blades are well above that level.
Another factor in favor of this scenario is that high-ground areas where most turbines are located generally have shallow soils, which can significantly reduce the mature height of a given tree.
These trees are out of the turbine's wind path with open space in the direction of the prevailing winds.
Photo Credit: Raina Sheridan