Abstract:

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 fire disturbance 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 main type, 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 increasing 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 cohort 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 of B. 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.

Key words: Daxing'anling Mountains, Virgin forest, Population, Betula platyphylla, Cohort structure, Fire disturbance, Site