How forests in Panama and elsewhere grow, decline, and recover affects carbon dioxide levels in the atmosphere and human welfare. Scientists are investigating whether computational models of these forests could be improved by considering the amount of phosphorus found in the trees. Phosphorus is a key nutrient that is especially important in tropical forests. The team analyzed upper canopy leaves from 144 trees at two sites in Panama. The sites differed in species composition, rainfall, and soil fertility. They found that phosphorus was a useful predictor of photosynthesis, although improvements over existing models were modest.