Final answer:
Competition for light and moisture in the layered structure of a tropical rainforest primarily explains the variation in tree heights, with taller trees in the canopy receiving more light.
Step-by-step explanation:
The variation in tree heights in the tropical rainforest is best explained by competition for light and moisture. In the densely vegetated layers of the tropical rainforest, there is a fierce competition among trees to reach sunlight, which is necessary for photosynthesis. The canopy layer, which includes the tallest trees, receives the most light, and as a result, these trees grow to greater heights. Trees in the understory have adapted to lower light conditions and generally do not reach the heights of those in the canopy. Furthermore, moisture is plentiful, but the nutrient-poor soil due to rapid leaching from high rainfall impacts tree growth as well. However, it is the competition for light that most directly influences the varying tree heights.