在景观火干扰历史重建的基础上，该文研究了大兴安岭北部地区原始林白桦(Betula platyphylla)种群的世代结构及其与火干扰、立地条件之间的关系。结果表明：白桦种群基于世代数的世代结构类型丰富，从一代型到五代型都有，空间分布呈斑块镶嵌状。以一代型与二代型为主要类型，空间分布集中，其它类型的世代结构空间异质性较高。世代数受火干扰频率、火干扰强度及两者的综合影响，相对来说以火干扰频率的影响更为显著。低频或高强类的火干扰易造成种群的世代数减少；高频或低强类的火干扰易造成世代数增多。世代数还随林型组类型而变 化，以溪旁林组>赤杨(Alnus mandshurica)林组>杜香(Ledum palustre)林组。白桦种群基于火干扰的世代结构类型也很丰富，存在3大类、5亚类和11小类，空间分布呈斑块镶嵌状。其中以火后优势火前劣势型和更新优势火前劣势型为主要类型，空间分布较聚集，其它类型的空间异质性较高。这种世代结构也受火干扰频率、火干扰强度及两者的综合影响。随着火频增高或者火强减弱，白桦种群的自我恢复性和抗火性增强，火后恢复性减弱。中高强火烧地上，世代结构为火后优势火前劣势型，低强火烧地上为更新优势火前劣势型。世代结构还随立地条件而变化。不同林型组之间，白桦种群的优势世代(火后代和更新代)存在显著的数量差异，但是世代结构保持为火后优势火前劣势型；偏东坡为火后优势火前劣势型，偏西坡及北坡为更新优势火后劣势型；224林班为火后优势型，218林班却为更新优势型 。
Background and Aims Natural fire is a frequent driving force of communities and populations. However, the cohort structure of Betula platyphylla in the virgin forest of the northern region of Daxing’anling Mountains, China, has rarely been studied. The aim of this paper is to explore the abundance, type and spatial distribution of the cohort structure of B. platyphylla, based on the number of cohorts, the fire regime and their relationship to site factors.
Methods We used an integrative approach to statistically analyze natural fired isturbance and population structure of B. platyphylla. Data on fire disturbance and population structure were collected from 96 plots distributed throughout the landscape. Fire history at the landscape scale was reconstructed based on tree regeneration and fire scars. Population cohort structure was recognized based on the number and regeneration type of cohorts identified.
Key Results Cohort structure based on the number of cohorts is diverse, consisting of one through five cohort groups with a patch_mosaic distribution in the landscape. The maintype, i.e., groups with one and two cohorts are aggregated in space, while the others have a heterogeneous distribution. The number of cohorts is influenced by fire frequency, fire severity and their interaction, but the effect of fire frequency is greatest. Fires of low frequency or high severity increase the number of population cohorts, while fires of high frequency or low severity decrease the number of cohorts. In addition, the number of cohorts
differs by forest type, with the number of cohorts decreasing from the forest type near streams to the type with Alnus mandshurica to the type with Ledum palustre. Cohort structure based on regeneration type is also diverse, with three groups, five sub_groups and 11 types distributed in patch_mosaic. The main sub_types, i.e., post_fire cohort dominance and pre_fire cohort inferiority and regeneration cohort dominance and pre_fire cohort inferiority, are distributed aggregately, while the others are spatially heterogeneous.
Cohort structure is strongly affected by fire frequency, fire severity and their interaction. With increa sing of frequency and decreasing of severity, the relative importance of both the regeneration cohort and the pre_fire cohort increases while that of the post_fire cohort decreases. The sub_group of post_fire cohort dominance and pre_fire cohort inferiority occurs on sites with mid_ and high_severity fires, while the sub_ group of regeneration cohort dominance and pre_fire cohort inferiority occurs on sites with low_severity fires. Both the group and the sub_group of cohort structure change with different types of sites. The cohort structure type is the same for sub_group of post_fire cohort dominance and pre_fire cohort inferiority on all sites, although there is significant
difference in the relative importance of the dominance cohorts (i.e., post_fire and regeneration cohorts) among different types of sites. The sub_group of post_fire cohort dominance and pre_fire cohor t inferiority is found on sites toward the east, while the sub_group of regeneration cohort dominance and pre_fire cohort!inferiority is on sites toward the west and north. The group of post_fire cohort is located on forest compartment 224, while the group of regeneration cohort dominates on forest compartment 218.
Conclusions This study suggests that the cohort structure ofB. platyphylla is highly diverse, with several dominant types of cohorts. B. platyphylla is weak in resistance to natural fires, but can maintain its role in the landscape after fire by strong resilience.