The rain forest canopy has trees of varying heights growing to reach the sunlight. What kind of interaction does this demonstrate?

This kind of interaction demonstrates competition. The forest canopy is one of the layers of plants and prevents sunlight from reaching the forest floor. The limited light forces plants to compete with each other for light by growing tall and straight towards the light. The competition causes most forest trees in the canopy to have long slender trunks supported by buttress roots.