Abstract: This article takes natural secondary poplar-birch forests in Mengluan Forest Farm of Mulan Forestry administration as the research object.Species composition of forests of different ages(9 years,18 years and 28 years)and different layers(tree layer,shrub layer and grass layer) were investigated,biodiversity indexes were calculated with Excel and Forstat,and differences between different indexes were analyzed with SPSS17.0.The results show that there are 155 species,belonging to 50 families and 104 categories,and the number of species of 18-years-old forest is the largest,which is 120.Betula platyphylla,Populus davidiana Dode and Betula dahurica are dominant species of tree layer,Corylus mandshurica is dominant species of shrub layer,Phlomis umbrosa and Carex ussuriensis are dominant species of grass layer; Shannon-Wiener index,Simpson index and evenness index of tree layer all show that 9 year<28year<18year; indexes of shrub layer show that 18 year<9 year<28year; indexes of grass layer show that 28 year<9 year<18 year,moreover,they are all greater than indexes of tree layer and shrub layer.

Key words: Mengluan Forest Farm, poplar-birch forest, natural secondary forest, biodiversity