Post-fire tree recruitment of a boreal larch forest in Northeast China

Boreal larch forests in eastern Siberia contain about half of the carbon accumulated in Eurasian forest communities, and fire is an important disturbance in this area, contributing to changes in forest composition and breaking the original succession trajectory. Assessing burn severity and environmental controls on post-fire larch recruitment is critical for understanding long-term effects of fire disturbance on forest succession in this region. A mega-fire that burned 8700. ha in the year 2000 in the Great Xing' an Mountains provided an opportunity to study the effects of mixed-severity fire disturbance in larch forests. We sampled tree recruitment in 83 burned sites to address the question of how burn severity and site environment interact to influence the species composition and density of post-fire tree recruitment, and hence the successional trajectory of forest. We explored the hypothesis that the larch forest was more likely to replace itself rapidly ("self-replacement succession") in areas of low-severity burn, but was more likely to be replaced by an early-seral community of broadleaf trees ("relay succession") in areas of high-severity burn. Our analysis showed that post-fire conifer and broadleaf recruit densities were both negatively related to burn severity and understory cover. Environmental conditions (e.g. elevation, slope, aspect) had weaker influences than burn severity on post-fire tree densities, but played a stronger role in determining the relative proportion of conifer recruits. Broadleaf trees recruited abundantly in low-severity burns in upland areas, even though they were absent from most of the pre-fire sites. In contrast, coniferous trees recruited more abundantly in low-severity burns in valley bottom areas. The comparison of pre- and post-fire tree composition in the burned patches indicated that self-replacement succession was likely to occur in valley bottom areas that burned with low severity. Burned upland areas are more likely to experience an alternative, broadleaf-dominated trajectory of relay succession. The increase in the severity and frequency of fires due to climate warming may prompt shifts from a larch dominated forest to an increasingly birch dominated landscape, substantially altering landscape dynamics and ecosystem services.