Keywords: 
• forest fire intensity /
• species diversity /
• biomass /
• grey sequence /
• Daxingan Mountains

Abstract: The effects of forest fire on biomass and species diversity and soil physical and chemical properties in Betula platyphylla and Larix gmelinii forests were studied to analyze the correlation among them according to the sample survey and using grey sequence analysis. The results showed that the understory vegetation diversity index was descending with the increase of fire intensity: unburnedmoderate fireserious fire, although the annual variation obviously increased. And the increasing amount in burned area was more than that in unburned area. The total biomass of understory vegetation was descending with fire intensity increasing, but the annual variation increased, and the increasing amount was increased with the increase of fire intensity: serious firemoderate fireunburned. The root biomass was irregular in different fire intensities, but it was descending with distance increasing from the trunk: 1 m2 m3 m. The pH value and organic matter content of soil increased with fire intensity increasing, but nitrogen and potassium content was opposite. The unit weight and phosphorus of soil reached the maximum in moderately burned area. In the grey sequence analysis, the unit weight of soil, organic matter content and pH value played leading roles on biomass, and the unit weight of soil, organic matter, and nitrogen content played leading roles in diversity index. The fire did not destroy all the understory vegetation so that some vegetation died, and some forest gap formed, which creates living space and development conditions for understory vegetation. Some soil nutrient content had changed to provide beneficial nutrients for understory vegetation, so to some extent, the forest fire promots the recovery of understory vegetation.