When a leaf falls, it is being accelerated by gravity to the ground. It is opposed, however, by air resistance, also known as the drag. The net force on a leaf is calculated by subtracting its weight from its drag.
The air resistance on a leaf is dependent on several physical factors of the leaf: area, mass and drag coefficient. Area is the size of the leaf. Mass is the weight of the leaf. Drag coefficient is calculated by the overall shape of the leaf as well as by its density and several other measurable factors. The fact that a leaf possesses a large area for its low mass means that it has a high drag factor and a relatively low downward speed.