Tropical rainforests, like those in the Amazon and Congo, help regulate Earth’s climate by storing carbon and cycling water. However, scientists still don’t fully understand how these forests respond to climate changes and human impacts. This study uses a computer model (ELM-FATES) to explore how differences in forest structure and plant traits affect carbon and water flows in these two major rainforests. The research focused on four rainforest sites, using data from recent studies and field observations. The team tested how variations in plant characteristics—such as leaf size, lifespan, and water use—affect forest productivity, water loss, and carbon storage. Results show that these traits strongly influence how much carbon and water the forests use and store, especially during different seasons and years. While the model matches some observed patterns in water and soil moisture, it still shows errors in how it simulates plant stress from dry conditions. To improve understanding, the study also ran tests focusing on the timing of leaf growth and how drought affects forest function. These insights can help make climate models more accurate and guide strategies to protect tropical forests and the global climate.