Step 1: Think about what limits photosynthesis in hot climates.
In warm, sunny regions, plants face high temperatures and strong light, conditions that normally push a lot of oxygen into the leaf tissue and waste energy through photorespiration.
Step 2: Look at how tropical plants get around this.
Many tropical species, especially C$_4$ plants, have evolved a way of fixing carbon that keeps oxygen away from the enzyme RuBisCO, so very little energy is lost to photorespiration.
Step 3: Connect this to productivity.
Because less energy is wasted, more of the captured sunlight goes directly into making sugars and biomass, so these plants grow faster and produce more overall.
\[ \boxed{\text{Minimize photorespiration}} \]