Factors Influencing Natural Regeneration Of Major Tree Species In Mixed Broad-leaved Forests In Maoershan Forest Farm,Heilongjiang Province

Mixed Broad-leaved forests was one of the major forests types of secondary forest in northeast China.The natural regeneration for component species were limited,while, it’s mechanism was still not well understood.Soil seed banks and population structure were surveyed by the methods of stratified sampling and layered counting to better understand the regeneration and succession in mixed broad-leaved forests. In addition,Combination of light and competition were used by the methods of understory removal and trenching treatments under different light intensity to better understand the survival and growth of common species: Fraxinus mandshurica, Juglans mandshurica and Quercus mongolica in mixed broad-leaved forests in Maoershan forest farm, Heilongjiang Province.(1) In the seed banks, the seed storage of F. mandshurica,Q. mongolica,J.mandshuricaand A. mono were quite different. In the seed banks, the seeds of F. mandshurica, Acer mono, Q. mongolica and J.mandshurica accounted for86.3%,7.3%,4.1%and2.4%, respectively. The seed storage of F. mandshurica and A. mono in decomposed organic layer were significantly higher than that in undecomposed organic layer and humus layer (P<0.05). The percentage of morphologically intact seeds reduced as the depth increased in the seed banks. The seed storage of Q. mongolica in decomposed organic layer were significantly higher than that in humus layer (P<0.05), and there was no obvious difference in seed storage between undecomposed organic layer and humus layer. An insignificant difference was observed in seed storage of J. mandshurica in different horizons.(2) The seedlings of F. mandshurica, Q. mongolica and J. mandshurica are hard to grow into successive layers, which is one of the reasons for limiting the nature regeneration. The ratios of the number of F.mandshurica and Q. mongolica in different layers were highest in regeneration layer, followed by upper layer and succession layer; J. mandshurica were highest in upper layer, followed by succession layer and regeneration layer; those of A. mono exhibited regeneration layer, successive layer, and upper layer in descending order.(3)Light intensity impact survival rate and biomass of F. mandshurica, J.mandshurica and Q. mongolica seedling.Under canopy the survival rate of three species trees were decreased2.9%,5.7%and19.4%compared with large gap,respectively. Q. mongolica seedling survival rate in large gap were significantly higher than canopy (P<0.05). Under canopy the biomass of three species trees were also decreased29.8%,35.8%and49.7%compared with large gap, respectively. All of them were in large gap significantly higher than canopy(P<0.05).(4) Overstory trees had inhibitory effect on foliar N content of three species trees.Understory vegetation had inhibitory effect on the biomass and foliar N content. Belowground competition from overstory trees were decreased8.0%,11.4%and2.9%to the foliar N content of three species trees, respectively. Belowground competition from understory vegetation had inhibitory effect on the biomass of F. mandshurica and J.mandshurica.They were decreased16.9%and4.2%. Belowground competition from understory vegetation were decreased6.9%,13.0%and0.9%to the foliar N content of three species trees, respectively. When absence of belowground competition from overstory trees and understory vegetation, the foliar N content of J.mandshurica and Q. mongolica were hightest, increased18.8%and7.3%,among them J.mandshurica were significantly higher(P<0.05).In summary, the seed storage of Q. mongolica and J. mandshurica appeared to limit natural regeneration. The seedlings of F. mandshurica, Q. mongolica and J.mandshurica are hard to grow into successive layers, which is one of the reasons for limiting the nature regeneration.The growth of three species trees seedling were inhibited by light.Exclude competition can raise seedling biomass and photosynthetic potential.