Final answer:
Leaves fall from a shaken tree branch primarily due to gravity, which pulls the leaves towards the Earth's center. While wind, animal activity, and other factors can initiate the fall, gravity is the key force that causes the downward movement of the leaves.
Step-by-step explanation:
When the branch of a tree is shaken, leaves fall down primarily due to gravity. This natural phenomenon occurs because gravity acts on all objects, pulling them towards the center of the Earth. While wind may contribute to shaking the branch and causing the leaves to detach, it is gravity that ensures they fall downwards. Leaf fall can also be part of a plant's natural cycle, where leaves are shed to conserve resources and facilitate nutrient recycling, especially in deciduous plants that shed leaves seasonally. Photosynthesis is the process by which leaves produce food for the tree and is not a direct cause of leaves falling. Likewise, animal activity can cause leaves to fall if they interact with the tree, but it isn't the fundamental reason leaves fall when a branch is shaken.
Plants have evolved to shed leaves for various reasons including to reduce water loss during dry periods and to make efficient use of resources by reallocating nutrients from old leaves to other parts of the plant. Gravitropism is another concept related to gravity and plants, where plant growth is influenced by the direction of gravity. However, it is the force of gravity that is the key factor in why leaves fall straight down when a tree branch is shaken.