| Nowadays biodiversity is one of the three hot points (global climate variety, biodiversity, sustainable ecosystem) of ecosystem and environment science .It is the basis for the sustainable and stable development of social economy. In this paper, the diversity of undergrowth of Pinus tabulaeformis forest and Populus davidana forest in Da Wo Pu ,Ping Quan was largely investigated using quadrat method and the relation between density and undergrowth diversity was studied systematically .The result showed:(1) Species richness of the undergrowth was different according to the kind of Pinus tabulaeformis forest and Populus davidana forest. Some indexes of species diversity , such as the number of all the species .the number of shrub species .the number of herb species and the corresponding Gleason index ,the number of individual .expressed a higher level of Pinus tabulaeformis forest than that of Populus davidana forest, especially shrub species.(2) The diversity of undergrowth was different between the Pinus tabulaeformis forest and Populus davidana forest. These indexes showed a same trend neatly ,in spite of the differences of species diversity index chased in this paper: Simpson index, Shannon-wiener index and PIE(Probability of Interspecific Encounter) of shrub of Pinus tabulaeformis forest was higher than that of Populus davidana forest . The diversity index of the total species was different because shrub and herb in two kinds of the forest are different and each index had its own emphasis. So Simpson index of Pinus tabulaeformis forest was higher than that of Populus davidana forest and the Shannon-wiener index of Pinus tabulaeformis forest was lower and PIE of the two kinds of forest were the same. Thus it also showed that the rare species of Pinus tabulaeformis forest were more than that of Populus davidana forest which had more common species. And the PIE of undergrowth of Pinus tabulaeformis forest was familiar with that of Populus davidana forest .(3) There was some difference between Pinus tabulaeformis and Populus davidana forests, so was the evenness index of the two kinds forest among the three diversity index . The Pielou evenness index based on the Shannon-wiener index and the Alatalo evenness index indicated the same results: There was nearly no difference in evenness index of the total species and shrub species of two kinds of forest.(4) The environment of Pinus tabulaeformis and Populus davidana forests were different, so was undergrowth .The endemic species in Pinus tabulaeformis forest was 29,belonged to 1 7 families, 28 genera. In Populus davidana forest, the number of endemic species was 36,belonged to 21 families, 31 genera. The number of common species of the two kinds of forests was 60, belonged to 29 families, 52 genera. The density and frequency of these common species were different in the two kinds of forests, especially the frequency, it was seriously significant.(5) There were many indexes which can indicate forest density such as the number of tree per hectare, crown density, degree of closing ,relative light, reasonable density index and so on , but the forest competition coefficient which came from reasonable density index was the most reasonable ,especially for the different age forest , it considered both the wood number of unit area and the volume of single wood ,and it proved that forest competition coefficient related to the crown density and the relative light and the related coefficient was above 0.80.(6) Forest competition coefficient deeply influenced the species richness, diversityand evenness . With upward of forest competition coefficient. the total trend of richness degree diversity and evenness of undergrowth went down.(7) Herb species occupied the absolute advantage both in Populus davidana forest and Pinus tabulaeformis forest. So the variety of herbs according to the changes of forest competition coefficient led to the trend of the variety of undergrowth. When forest competition coefficient went to a certain level, the varied trend...
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