The researchers studied the understory vegetation in mixed alder and cypress plantations to understand how it changes over time. They found that the abundance of plant species increased in the shrub and herb layers up to 20 years, but then declined. The height of shrubs and herbs increased initially, then decreased slowly. The coverage of shrubs and herbs increased before 15 years, then declined sharply. The biomass of shrubs increased before 15 years, then stabilized, while the biomass of herbs fluctuated and tended to decline with plantation age. Overall, the structure and functions of understory vegetation in these plantations tend to degrade as the plantations grow older.