The upward movement of water in tall trees is due to transpiration.

Explanation:
Transpiration is a process wherein there is a loss of water vapour through the stomata (tiny pores) on the leaves.

As water evaporates from the leaves, it creates a negative pressure gradient, or tension, which creates a suction effect and pulls more water up from the roots to replace what was lost.

This process is facilitated by the cohesion-tension theory, which states that water molecules are cohesive and can stick together, forming a continuous column of water in the xylem, the specialized tissue responsible for water transport in plants.

Additionally, the adhesion of water molecules to the xylem walls helps to counteract the downward force of gravity and keep the water column moving upwards.