Final answer:
The variation in tree heights in the tropical rainforest is primarily due to competition for light and moisture among the layers of vegetation.
Step-by-step explanation:
The primary factor that best explains the variation in tree heights in the tropical rainforest is competition for light and moisture. Tropical rainforests are characterized by a high net primary productivity due to ideal annual temperatures and abundant precipitation, which support rapid plant growth. However, the dense upper canopy limits light penetration to the understory and forest floor, compelling trees to grow taller to reach the light. The different layers, including a sparse forest floor, an understory of shrubbery, and a closed upper canopy of tall trees, create a fierce competition for light and moisture among the trees. Additionally, the high rainfall in these forests leads to nutrient-poor soils due to leaching, making competition for these scarce resources even more intense.