Trees and understory plants in a subtropical forest in China were studied to see how they respond to added nitrogen. The researchers fertilized plots with different levels of nitrogen and measured plant growth over 3.4 years. They found that while larger trees were not affected, smaller trees and understory plants grew less in plots with added nitrogen. This suggests that nitrogen may not be a limiting factor in this forest, and adding more nitrogen could actually harm the ecosystem by worsening the shortage of other important nutrients.