High rainfall and temperature
Generally, most plants need water to survive. Hence, these two regions experience high rainfall, and so are highly productive. The high temperature allows for dead organisms to decay at a faster rate, thus returning nutrients back to the soil for the plants in the rainforest to absorb and grow. To summarize it, high temperatures allow for nutrients to return to the soil as well as photosynthesis, and the rainwater assists photosynthesis. All these contribute to the productivity of forests in these two regions.