There are trucks and bare branches through which it is relatively easy to see somebody who may be listening. Because there are no obstructions between the source of sound and the listener, you cannot expect to achieve any meaningful sound reduction from these trees during the winter. Similarly, during the summer when the trees are in leaf, the sound still has small direct pathways through which it can travel. In addition, the leaves themselves do not absorb much sound except at very high frequencies. Therefore, we would not expect to achieve much sound absorption from deciduous tree barriers.
Pine trees are a little bit different.
Their leaves are much more dense and therefore there is more of an obstruction in the sound transmission path. However, must pine trees do not grow close to the ground therefore you can usually transmit sound to a listener along the open underside of the trees. For dense pines growing close to the ground, you might expect to achieve as much as 0.03 decibels of sound reduction for every foot of barrier. Thus, for a 100 foot wide tree barrier, you would only expect to achieve 3 decibels of sound reduction. This is not an effective method of noise control.