Sapling yield from oak and birch (others not tested) has decreased significantly. Whereas a 9x9 plot of oak or birch would yield close to or greater than 81 saplings in previous versions, the same size plot now consistently produces around 50% of the planted trees. Although it is purely conjecture, this may be related to decreased mob spawn rates in 1.7.4, which is already a reported bug.

Edit: This issue is much worse in 1.7.2, where yields are closer to 25% of the planted trees